My feedback on main drains in an above ground pool...

Hey guys, I figured since I have had a pool for a few months now and since it is now winterized, it was time to give my feedback in regards to main-drains in an above ground pool.

I know there is tons of people saying, "Don't do it!", but I ignored them all and did it anyways.

As a result I have found all the following major plusses:

- Increased circulation within the pool
- Collection of debris at the bottom near the drains
- I always have circulation, regardless of how "full" the skimmer gets (I have them both cut in full at the pump)
- Draining of the pool (excess water from rain, or draining to winterize) is easy

Now, I also have dual returns (one near the skimmer, like everyone else, and another I installed directly across the pool from the original one) and I have played quite a bit with the valving I installed to run on skimmer/one return like most above ground pools have, as well as combinations of all sorts (main drains only, both returns equal, skimmer only, etc) and as well as using my pool heater quite a bit (the kids were swimming in 45 degree weather for a few weeks).

I can say that the main drains are more of a necessity more than a "really don't need" kind of item. in heating the pool without the main drains the bottom was always cooler (and yes, I tried aiming the returns, using a single to "stir more" with a stronger stream, etc) and without dual returns you always had a "dumping spot" for debris to end up (which meant more vacuuming). I also found that without main drains, if the skimmer got full it would choke off the pump and returns. That just doesn't happen with the main drains also being cut in.

I will say, the skimmer works best without the main drains being cut in, but that is also dependent on a single return being used (seemed to force the largest amount of floating garbage to the outside quicker).

But, with both returns, the main drain and the skimmer all operating full, the pool had the best circulation, the surface always stayed clear, the garbage collected near the main drains as much as possible (which means vacuuming was kept to minimum, since all you do it shut off the skimmer and the main drains sucked in as much as possible) and in general the pool was as low-maintenance as possible.

Now, I will admit, my setup is pretty stout, in that I have 2" piping from the skimmer as well as 2" piping at the main drains (dual main drains, connected via 2" and tee'd to 2") with all 2" lines for the suction and the return side (I did use 1.5" line to go to the second return, but it was 2" up until it tee'd off). I used a Pentair Dynamo 1.5hp dual speed pump and ran it on low for 10 hours a day and on high for two shifts of two hours each, so 10 hours on low and 4 hours on high. I have timers for pump actuation as well as low/high modes (with a firemans switch on the high mode) as well as a manual override built into the pump that allows me to switch to high-mode and activate the firemans relay so I can command heat for longer periods off-timer. I need to figure out a way of adding another relay so that I can just set the pump to low-mode and if heat is needed it will kick the pump to high-mode when the heat turns on. I know I can do that with an external automation system, but I am trying to avoid that for now.

The main drains are also installed with a 50lb bag of fast-setting concrete used as a base for the drains as well as encasing the tee'd section in concrete as well to make sure no-one can move anything.

My advice to anyone considering a main drain setup, go for it. You won't regret it and entrapment isn't an issue with the dual-drain setups they sell today. Yes, you have those that complain it is just another possible leak, but really it all comes down to assembling it properly. I would also argue that those that have leaks at skimmers/returns and main drains are probably in need of a new liner anyways. Good time to use new gaskets everywhere and make sure the pool itself is in good shape. Nothing is built to last forever, and a set of main drains is not going to accelerate how quickly things need to be replaced.

Very interesting! I have an in-ground with a main drain and often switch off/on the skimmer and main drain valves depending on what I'm doing also. Very handy. I would agree that DIY'ers would find benefit to adding a main drain.

Agreed. :)

Hey Dennis, Dennis here. While I agree with most everything you said, I still say no main drain, in the majority of above ground pool installations.

You have no idea how many times I have heard, "my pool is leaking, I think it's my main drain." It seldom is but always the first place suspected. That is if it is properly installed, if not, then it probably always is the cause. Thing is, there is no way of knowing.

Automatic pool cleaners do exactly the same thing, they draw water from the bottom, plus they clean the entire pool floor. They replace the need for a center drain.

There is a lot that can be done with an above ground pool, huge filter systems, multiple returns, main drains, dedicated cleaner ports and so on. It's all good if done right and maintained. But if you are at the mercy of others when it comes to keeping all this stuff working, you may want to keep it simple.:)

Thanks for the reply Dennis.

And, from your perspective, being a pool installation professional, I can understand your motivations to keep the "common owners" from going with a main drain.

I am far from a "common owner", and I have noticed that a large number of those on the web posting about pools seem to be those are are far removed from being engineering types, handyman types and/or those with mechanical proficiency.

So, as you said, for the majority of people, no main drain is common sense. But, for those of us "with a clue and ability" a main drain is essential.

Without a main drain winterizing is a PITA. Without a main drain you can't circulate water while heating a pool and melting the ice the formed over winter for an early spring opening (which I did this year, which required using only the main drain to pull water out, circulate it through the heater and then send heated water back to the pool while also allowing me to fill the pool with my dedicated fill bib and line setup)... My kids were swimming the first week of March when we hit 70 degrees for a day or two. Without a main drain that would have been impossible as I would have had to fill the pool first to use the skimmer, then I would have had the issue of ice shards plugging the skimmer (which would have prevented flow through the system to use the heater)...

I just want to be sure that those that are true DIY types that are extremely detail oriented seriously consider a main drain setup for it's many advantages. Those hiring others to do everything need not consider it.

Great post Dennis, no argument here. It is more of an advanced thing, but more than worth it in many cases. Enjoy the "long" summer with your kids. I raised four daughters in Arizona, never short of swimming season, and we always had a pool to enjoy together.

Dennis

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Originally Posted by DennisP

As a result I have found all the following major plusses:

- Increased circulation within the pool
- Collection of debris at the bottom near the drains
- I always have circulation, regardless of how "full" the skimmer gets (I have them both cut in full at the pump)
- Draining of the pool (excess water from rain, or draining to winterize) is easy


I will say, the skimmer works best without the main drains being cut in, but that is also dependent on a single return being used (seemed to force the largest amount of floating garbage to the outside quicker).

But, with both returns, the main drain and the skimmer all operating full, the pool had the best circulation, the surface always stayed clear, the garbage collected near the main drains as much as possible (which means vacuuming was kept to minimum, since all you do it shut off the skimmer and the main drains sucked in as much as possible) and in general the pool was as low-maintenance as possible.

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Dennis, Can I post and this may wind you up a bit but I don't mean to, just trying to go nearer to what really takes place?
Increased circulation of mainly clean water from midway down the pool?
Collection of debris at the bottom near the drains, if the skimming action was working well there wouldn't be debris on the bottom? Dirt enters from the surface first? Near, near the main drains, not into the main drains so not clearing the debris unless you push it in, so misses the dirt and just circulates cleaner water from above the dirt layer.
I always have circulation regardless of how full the skimmer gets, A second skimmer? for the reasons stated above?
Draining the pool, well yes that's why they were fitted but a submersible pump or opening a return?

I can see the heating point though.
Please don't take this the wrong way, I am just discussing.

Oh, no big deal on the discussion... I take no offense...

Well, without going into too much detail, I think you are oversimplifying the way water currents work and the way any body of water reacts to not only currents, but how debris within those currents get filtered out...

Honestly, I think two skimmers AND main drains would be beneficial in a pool where you have debris making it's way in (such as trees/excessive winds/etc)... Now is a second skimmer going to make a 100% difference, no. Will it make a 30-50% difference, for a pool with lots of floating debris, sure.

Now, for me, the big issue for main drains is not for just the ability to drain the pool. It is for all the reasons I stated above. My pool stays cleaner, there is always good flow regardless of how full the skimmer gets, and I have great equalization of temp throughout the depth because of the constant pull of water from all areas in regards to heating.

In regards to debris, only some debris will float. Only what floats will get sucked into the skimmer. Anything else that falls to the bottom will stay there, unless it gets sucked into a main drain. Now, does that mean that all debris that falls to the bottom goes into the main drains, no. But, from experience, a substantial portion does make it's way either into the main drains, or close enough that while vacuuming it is easy enough to push whatever will go into the main drains and whatever is left actually gets sucked up by the vacuum. ie, the pool is much cleaner with less work with a main drain compared to not having one. I must also point out the folly of assuming that you have full circulation if you only have a skimmer and the skimmer basket gets full. That is far from my experience with large leaves that get sucked up in my installation, as it is obvious that when the skimmer gets full the majority of water gets pulled from the main drains by way of the amount of floating debris that still exists on the surface, yet when I cut to only pull from the skimmer in such cases I get a huge reduction of output from the returns...

as far as draining the pool, there is a huge difference from using a main drain with 2" piping and a submersible pump. The main drain allows for full pipe volume in draining, with the pump only allowing for the max hose size you can connect in draining. Apples and oranges in comparison, since there isn't any. It took me all of 30 minutes to drain the pool enough with the pump running and the main drains cut in exclusively. With a submersible pump I would have been lucky to do the same in 12 hours. There is a massive difference in volume between the two. Opening a return is only valid if you wish to flood the immediate area with chlorinated water, and even then you wouldn't have the speed of using main drains with the pump running.

I do agree that a lot of dirt settles to the bottom and stays there, regardless of how many returns or skimmers/drains, so you will always have the need to vacuum the pool at times (or use an automated pool cleaner), but I can attest to the fact that the pool stays way cleaner with main drains running along with the skimmer overall, so cleaning via vacuum/automated pool cleaner is reduced in all cases, especially in mine.

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Originally Posted by DennisP

Thanks for the reply Dennis.

And, from your perspective, being a pool installation professional, I can understand your motivations to keep the "common owners" from going with a main drain.

I am far from a "common owner", and I have noticed that a large number of those on the web posting about pools seem to be those are are far removed from being engineering types, handyman types and/or those with mechanical proficiency.

So, as you said, for the majority of people, no main drain is common sense. But, for those of us "with a clue and ability" a main drain is essential.

Without a main drain winterizing is a PITA. Without a main drain you can't circulate water while heating a pool and melting the ice the formed over winter for an early spring opening (which I did this year, which required using only the main drain to pull water out, circulate it through the heater and then send heated water back to the pool while also allowing me to fill the pool with my dedicated fill bib and line setup)... My kids were swimming the first week of March when we hit 70 degrees for a day or two. Without a main drain that would have been impossible as I would have had to fill the pool first to use the skimmer, then I would have had the issue of ice shards plugging the skimmer (which would have prevented flow through the system to use the heater)...

I just want to be sure that those that are true DIY types that are extremely detail oriented seriously consider a main drain setup for it's many advantages. Those hiring others to do everything need not consider it.

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I must say that I have never attempted to heat a pool while there was still ice on the surface. Kind of a luxury heating an outdoor pool while it is still "winter". Kind of hard to remove the pool cover when there is ice and snow on it!

That being said, I would never put a main drain in the bottom of an above pool liner. Too much risk of a leak for my taste, with very little benefit. I also prefer an easy way to winterize the pool. Having water lines below grade are not for me. The only time I have ever drained the pool was when the liner was replaced, so I don't need to worry about drain time.

I have a Fanta Sea 16' x 32' x 4' flat bottom pool that is now on its third vinyl liner. The pool has one skimmer, one pump outlet, and one lower drain/pump feed on the sidewall about 5" from the bottom of the pool. Circulation is fine. However, over time debris does settle in the center of the pool. That's pretty much to be expected due the circulation pattern. Then again, I only vacuum the pool once a month.

Many PBs are building pools now without main drains but not because they don't have a clue or because they are trying to save costs but because studies have shown that they really don't contribute much to the sanitation and cleanliness of a pool. Here is one such study done by the APSP: http://apsp.org/portals/0/PDFs/Case%...n%20Drains.pdf

The video at the end shows how ineffective a main drain really is even at very high flow rates. I have confirmed this myself with my own pool since I have a MD and have operated with it on and with it off and I saw absolutely no difference in water quality or bottom cleanliness. I once dumped a bucket of small debris around the MD and even with the pump running over 8 hours with ONLY the MD running, only the first couple of inches around the MD were free of debris. Because of the VGBA, MDs are now designed to have very low water velocity to reduce the risk of entrapment which makes these MDs even worse at drawing in debris. While you may have seen debris setting around the MDs, that could be due to the slope in the floor rather than the MD itself.

But there is another significant reason not use MDs. Skimmers require a certain flow rate in order to effectively draw in debris. Some manufactures recommend at least 15 GPM. With the addition of an MD, it steals water from the skimmer and there may not be enough suction on low speed of a two speed pump so running on high speed may be necessary which is about 4x the energy use. Same is true with a VS pump. More suction ports require higher RPM settings and higher energy costs.

Sorry but I really think you are fooling yourself into thinking that the MD is actually doing anything at all. The evidence simply doesn't support it.

Wow! The YouTube video in the last citation at the end of that APSP study is pretty damning. With only 11" of water above the drain, to see the blue dye just float past it is pretty impressive. I think there's probably some effect on the residence time of the dye over the drain by how far you position the main drain from the skimmer but the drain definitely seems to provide very little suction.

The only good I ever see my MD possibly doing would be to act as a secondary source of water to the pump should the skimmer become clogged. However, I have yet to find a dead raccoon plugging up my skimmer basket :)

A wall port is more effective for equalization purposes because it is less likely to get clogged with leaves as would a MD.

mas985, I was going to lead Dennis slowly to the same direction but you just slam dunked the discussion! ;-)

If you take a kitchen sink and do lots of washing up the dirt is everywhere. pull out the plug and let the water run away, it will even form a vortex at high speed but only when the last 1/4" of water drains away to 0 does dirt on the base of the sink flow down the drain otherwise it stays put and you have to wash it away afterwards. same in a pool. you have to push the debris down the holes so the rest of the time it's circulating pretty much the clean stuff.

If the circulation via the M/D improved Dennis then your skimmer can't handle the flow, a second or third skimmer would take the strain off the pump and dirt enters from the top so taking it out before it can sink to the bottom is the best answer.

Leaves etc do stick to the walls of the pool, this is due to the boundary effect, water at the edge maybe stationary or even going in the opposite direct to the flow created by the return jets. Increase this flow with your big ol pumps and the boundary layer gets bigger so more debris becomes trapped at the perimeter.

This can again be seen in a dirty bowl of washing up water, go mad winding it around in a circle and you may see the perimeter dirt stationary or going the opposite way. Moving the water gently produces a smaller boundary layer.

It's true skimmers don't work very well on low flow, but that's again because the water needs to push the debris into the skimmer rather than trying to pull the water in. It would help a lot if there was a really good skimmer on the market but pool equipment has never needed to be efficient because if you bolt on a big enough pump, no matter how bad the plumbing water will arrive at the other end so bad design is the normal with pool equipment. I am working on a better design of skimmer as a project, it takes time and I could used some CFD software to shorten down my experimental stage.

The other point I would like to make is what is the point of the turnover rate? or what is the point of better circulation?

Chlorine kills most harmful things in seconds, they are dead and that happens if the chlorine is available and well mixed which doesn't mean high turnover or circulation. What chlorine doesn't kill needs to be filtered out but regular high rate sand filters and element filters and even DE if the rate is too high doesn't filter these out because many are too small so they just get recirculated around and around, just a waste of electricity because you are not achieving anything. Chlorine has done it's job and the rest of the dirt and skin cells etc gets broken up in the rough treatment moving through the filters that are too course to remove the other stuff.

Slower filtration is better filtration, high rate is just straining the big stuff and using lots more expensive electricity to do it.

If a Chef wants a clear consomme they let it flow through the filter slowly to produce an almost clear stock, if they applied 1 bar pressure to push it through it would come out dirty.

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Originally Posted by DennisP

In regards to debris, only some debris will float. Only what floats will get sucked into the skimmer. Anything else that falls to the bottom will stay there,

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Most debris floats at least for a while, the surface tension on the water supports a fair amount of dust, leaves, pollen etc until it gets water logged and then sinks, that is the optimum time to remove it but you can only do this if the system is running. That means if you switch off your pump at night and there is a storm/strong winds or even mild wind, by morning you'll be out there with the cleaner or robot.

We don't run our pools 24/7 because of cost. I have gone to the opposite end of the rainbow with testing this. My pool runs 24/7 the water is the cleanest it has ever been. The little dust cloud that gets pushed up when manual vacuuming, the one where the pool is clearer before you start cleaning? That dust cloud is no more because the water is moving constantly so more goes into the skimmer (downside is you have to empty the skimmer more often)
Now many people say my water is crystal clear but eyesight is bad, plenty more to come out of the water to make it really clear when tested with a turbidity meter. Now the electricity I use on my pool is 30 watts when no one is swimming, that is still enough to have the water moving and skimmer working and when swimmers are in the pool I push it to 69 watts. I can afford to run my pool 24/7 on that low level of electricity and moving water is better than stationary. The flow rate is slower but that means the filter catches more although I do use better filter media than sand (4 micron filtration but with flocculation as well to remove sub micron particles) so more of what the filter catches is not recirculated and is back washed away leaving cleaner water and using less chlorine although that is cheap anyway in the big scheme of things.

I sold my pool robot 3 seasons ago, it's not needed any more.

The pools I work on with main drains get re plumbed and I put a 3 port on them to use them as inlets unless draining or cleaning is taking place. Putting the heated water back through the main drains does wonderful things for warming up and de stratifying the water.

Honestly, most of this is about actual experience and what you have to deal with. There is a huge push away from main drains, primarily from the safety aspect of it. It is just like people disliking guns. Most say that because they fear it. Main drains are due all the bad press with people getting hurt over the ages, but with dual main drains that concern is all but eliminated in a residential pool.

I understand the science of boundaries, skin effect, wall adhesion, etc, etc... I keep my pool covered at all times with a solar cover when not in use. A skimmer in that aspect doesn't do much and is prone to clogging by the cover and anything else. If anything with the pump on low I notice mostly small stuff accumulating in there. The main drains are cut in full along with the skimmer.

If anyone would care to re-read my first post on this, I gave all the pertinent information as to what I am experiencing and why I am doing it and what the advantages are to me, personally. To argue any of that is just stupid. I never said it only picks up this and if it keeps the floor clean or not. I know it doesn't. It pulls in anything that is suspended in the water it pulls through it. As I stated I can usually cut the main drains in full, stir the water up around them and have them pull in anything small enough to work through the grills. Everything else gets vacuumed (which isn't much once the spring cleanup and fall leaves are done).

In regards to using the main drains as a feed for heated pool water, it also works in reverse, pulling cool water from the bottom center of the pool and allowing circulating warm water form the returns to work towards the center. Simple water flow dynamics dictate that. And with a strong circular current in the pool (using dual returns and pointing them at a 45 degree angle down and to the right creates quite a bit of current in the pool), which is always present since you heat with the pump on high, you have the best case of allowing for the cool water to be sucked in the main drains.

Again, do it or not, that is all personal choice. I understand pool professionals not wanting to put them in. I understand why they tell their clientele they aren't worth it. But for people like myself that operate a pool the way I do, it isn't even a matter of "should I or shouldn't I?", it is a matter of it is the best solution to deal with draining, heating, cleaning and overall circulation and flow. It is just amazing how people try to say it doesn't do anything when it is obviously pulling water from the lowest point in the pool in the center. Plus it is hilarious when most people that argue you shouldn't put one in are the ones that have them in their own pools already. I could care less if you don't find a value or a need and if you have yours cut off all the time, it makes zero difference to the benefits I am personally experiencing in the operation of my own pool.

If anyone cares to have the same advantages, they are welcome to install them as well. To argue the majority of the above posts is beyond futile and ignorant, since none of what was said applies to anything I stated. I appreciate the argument on the academic level, but in an applied sense, in my pool, for my purposes, none of it matters.

Oh, and in fall when the leaves are flying, it makes a huge difference to still be able to filter the water via the mains drains if the skimmer clogs with leaves... I had it happen last fall when the kids were swimming when it was 50 degrees outside... The pump was on high, the pool heater was running heating the pool and the skimmer was totally filled with leaves... If the main drains would not have been able to supply the water properly the heater would have shut off due to either low flow or too high of heater temps and the kids wouldn't have been able to enjoy it as long as they did. Without main drains that situation would have been ruined since the pool would have started to get cold quickly (and a 3-4 degree drop with the kids playing is a big deal to them being able to enjoy themselves).

OK then let's address each of your original points:

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- Increased circulation within the pool

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How do you know? Did you do a dye test with a MD on vs off and see how long it takes to get a full mixture of color. If not, you might be surprised that is not all that different. The MD may have a slight edge but by the time the pump shuts off, there is no difference. Plus properly placement of high velocity returns is far more effective at improving circulation than a low velocity MD.

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- Collection of debris at the bottom near the drains

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Ok, I guess if you don't have a floor cleaner, this might be useful. But again, this could be due to the slope of the floor or other factors. Have you tried turning off the MD to see if anything changes? As I mentioned before when I did this with my pool, I saw no difference in where debris collected except within a few inches of the MD. In fact with the MD on, it was more prone to getting clogged with leaves that fell near it.

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- I always have circulation, regardless of how "full" the skimmer gets (I have them both cut in full at the pump)

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As I mentioned before, a wall port is a better solution because as you just said in the above quote, debris collects around the MD and if you have leaves, they will clog the MD as well.

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- Draining of the pool (excess water from rain, or draining to winterize) is easy

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I can see this with an IG pool but with an AG pool you can just use gravity to drain the pool (e.g. siphon or side plug).


I just don't see these points as justification for adding a MD because for the most part each of your points can be accomplished in other ways without the downsides of having a bottom MD. To each his own as they say.

What, if any is the "down side" to a main drain???

Sorry, I already stated the rest above everywhere... You have yet to address that single point.

As far as it being a possible "leak point", again, no more than any other hole put into the liner for any other purpose (such as the skimmer/returns/etc). As far as it being an entrapment, again, dual drains far apart = no entrapment hazard, especially when cut in with the skimmer.

Mark, I fully respect the engineering side you champion, and I am usually the first and foremost to do function over form. In this case it is just common sense based upon the usage I have and the fact that it is drawing water in from the center of the pool at the bottom. That in of itself overrides any other "noise" from arguing against it. It allows for complete and total turnover of the water in the pool constantly with the best mix possible within the shortest timeframe. Which is primary for heating a pool, cleaning a pool and even sanitizing a pool as far as I am concerned. Oh, and I may add, that yes the main drains to cause enough suction to pull the solar cover down in the center if there in rainwater sitting on top of it. In those cases we go out and roll the solar cover up and refloat it after to get the extra rainwater to mix with the pool water.

Outside of entrapment with older single drains, I fail to see any sort of downside of using a main drain. It offers far more options and flexibility with absolutely no downside whatsoever if used with proper valving. I still fail to see the argument against them (outside of a pool builder/maintainer not wanting to deal with customers with them). If I don't want to use the main drain and want max surface cleaning effect, I can just switch it off (and I have valving at the pool and at the equipment pad, so I can do it where-ever it is convenient). Conversely I can also switch off the skimmer if I want the main drains to do all the work (and I sometimes do that to optimize the pool heating).

And, if you consider the installation of a main drain as the big reason not to have one (which for the majority of people the reason they put up above ground pools is because they are easier to do and a main drain negates some of that "ease"), in my case that is not the case at all. I have autistic children that I would worry about constantly if I had an in-ground pool. For me it wasn't even an option, so I put all the piping for my pool in-ground, hard piped everything with 2" PVC and located the equipment pad at the back of the garage and built a 1200sq/ft paver patio around the pool, so putting in a main drain was trivial in comparison. This way I have zero concerns about the kids and the pool piping and anything else concerned in the backyard. It is fenced in entire with wrought-iron fencing (real wrought iron that is welded and solid) so I can let the kids out in the backyard with no concerns at all. I did all the work, including the patio, so an in-ground would have been a cakewalk to do at the time, but the worry with the children didn't make sense, so an above ground was the only option.

Besides, I can always pull the pool and fill in the area with dirt, a fire pit, gardens and sitting walls when we no longer want to have a pool. I don't have to touch the paver patio or the rest of the landscaping when we get to that point and it all still "fits"...

Hi Dennis, hope your not getting upset? I did note a change in your post as it seems we are challenging you. I am not, I am just discussing. The point of discussing is to see if I have missed anything on the main drain topic.

In your previous post you say "Honestly, most of this is about actual experience and what you have to deal with".
It's the same for me. I started off in swimming pools just doing what I was told, believing everything the man in charge said.
My background is engineering, hydraulics and pneumatics so I do know a little more than the average pool builder. For me the greatest influence and most astounding turnaround was 4 years ago from the report on main drains and pools without main drains documents and links that mas985 posted. That was something that stuck in my mind and I thought over it for a long time. It's not rhetoric and the flow from the computer fluid dynamics is something used on aircraft design, space craft design, in fact anywhere where a gas or liquid has to flow.

I have played with pools similar to yours and noted the effects of opening and closing various parts and the quite frankly stupid ideas that the industry has as facts that we all follow. The key difference is I also have used the version without main drains and over large pumps and I couldn't believe the results. My pool is now a bit of a wreck as it is my test bed. I am fortunate that some of my customers have become friends and if I ask them can I experiment a little (no harm will come to their pools) they indulge me to play.

That main drain in the video, it's flowing at a massive 795 gallons per minute, 795! yet unless the dye gets within 2"-3 " of the grill it doesn't get sucked in it flows right on past. That fact is probably the single most important point on main drains.

The CPO teaches that 90% of the contaminants are in the surface of the water so that's where we should maximise the removal of dirt.

Now in your first post you describe the situation standard and the situation as you have modified it. Your sensible ideas have improved the situation (note I do not say common sense as I doubt what is "common" these days)

Water is incompressible so if you have a skimmer it should be matched with the same size in square inches of returns so 1 return for a skimmer is not rational, of course it will still work but the flow restriction from 1 return limits by definition the flow into the skimmer. I note you have 2 but you also have a big pump with a potential on high speed to still cause a restriction even with the M/D open because what come out the returns is what governs what flows into the pump. What kind of flow rate have you got?
I don't go with small openings in returns to speed up the water flow at the exit as this is simply physics, and for every action there is an equal and opposite reaction so the harder and faster you push the water in the pool is trying to stop it and the restriction in the return opening is having a bigger effect up stream with a slow down in water flow and more strain and pressure on the filter gauge.

If you do use the main drain to pull water for heating (what heater have you got that requires high speed?) and you send this out of the returns it still doesn't got to the bottom because it hot and less dense, it will float, sending the hot water into the pool from the M/D as a return does warm the bottom of the pool, does mix quicker and can cause any dirt to be lifted up into the mix to be skimmed out, you cannot get stuck onto the returning M/D and as you have stated this better flow with a main drain position (only returning the water will aid the skimmer performance and not reduce it. There should always be more than one skimmer although plenty of pools with only one do work, they work better with multiple skimmers and that can also handle the leaf load.

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Originally Posted by DennisP

What, if any is the "down side" to a main drain???

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This forum and others are peppered with testaments about MD clogs and leaks. It is much more serious in an IG pool than an AG pool but still an issue that seems to be quite frequent. Given that every attribute that you assign to a MD can be accomplished in other ways, why have it? A MD is a little like an appendix, it serves no real purpose but it can be a real PITA when it goes bad.

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Originally Posted by Teapot

so if you have a skimmer it should be matched with the same size in square inches of returns so 1 return for a skimmer is not rational, of course it will still work but the flow restriction from 1 return limits by definition the flow into the skimmer.

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I have a little bit of a problem with this statement. You actually don't want the returns to have the same area as the suction ports. Pool plumbing is not the same as HVAC. If the return head loss gets too low when compared to the suction side head loss, the pump has a tendency to leak air into the pump basket and even cavitate. In general, you want the return head loss to be several times that of the suction head loss and restricting the returns with eyeballs is a good way to ensure that while still providing useful work.

Plus return eyeballs serve another useful purpose. With higher exit velocity the water reaches more distant parts of the pool faster so mixing and circulation are a little better with smaller eyeballs. It does reduce flow rate but not by much and studies have shown (see sig) that flow rate and turnover are not all that important to pool sanitation and circulation. Also, the higher pressure does not cause more stress on the pump. In fact it causes less stress on the pump motor. Higher pressure means more head loss which in turn means lower flow rates and since a pump draws less power the further to left on the head curve (i.e. higher head loss), this also means there is less energy consumption and less heat to cause stress on the pump motor. It is also true that the efficiency drops a bit but if the run time is kept the same, the energy draw is still less, albeit less water is filtered per minute but again, I don't think that matters much.

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Originally Posted by mas985

I have a little bit of a problem with this statement. You actually don't want the returns to have the same area as the suction ports. Pool plumbing is not the same as HVAC. If the return head loss gets too low when compared to the suction side head loss, the pump has a tendency to leak air into the pump basket and even cavitate. In general, you want the return head loss to be several times that of the suction head loss and restricting the returns with eyeballs is a good way to ensure that while still providing useful work.

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I can attest to this point with a example in my pool. I have a 1-1/2HP single speed pump (not happy about that) which drives my waterfall. The suction side is plumbed with 2" PVC and gets water from two wall ports in the shallow end covered with standard drain caps. The pressure side of the pump is split between two 2" PVC lines using a three-way Jandy valve. One line is 2" pipe all the way to the water fall (~75' or so) which is totally open at the water fall (no flow restriction, just an open PVC pipe). The other leg of the pressure side is 2" PVC to a wall return with an eyeball (I have plans to turn this into a pressure-side cleaner port someday). So, I can send 100% of the flow to the waterfall or 100% of the flow to the wall return or fractionally split it between the two.

If I have more than ~ 30% of the pump flow going to the open pipe in the waterfall, the pump makes a horrible rattling noise (no air bubbles). I take this to be cavitation or close to it. I absolutely need the back pressure caused by the eyeball return to keep the pump quiet. I considered at one point in time installing a back-pressure regulator on the pressure side of the pump to regulate the flow better but a high quality 2" BPR in PVC costs almost $2000. So for now, since I'm not dropping money on a BPR valve, I just keep the Jandy turned mostly towards the wall return. I'll have to come up with a fancy manifold at the waterfall to restrict the orifice a bit and add more back-pressure to the pump. I consider it a pool builder design flaw...

So I agree with mas985 that you must have properly designed head loss on both sides of the pump or else you risk premature equipment failure.

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Originally Posted by DennisP

Oh, and in fall when the leaves are flying, it makes a huge difference to still be able to filter the water via the mains drains if the skimmer clogs with leaves... I had it happen last fall when the kids were swimming when it was 50 degrees outside... The pump was on high, the pool heater was running heating the pool and the skimmer was totally filled with leaves... If the main drains would not have been able to supply the water properly the heater would have shut off due to either low flow or too high of heater temps and the kids wouldn't have been able to enjoy it as long as they did. Without main drains that situation would have been ruined since the pool would have started to get cold quickly (and a 3-4 degree drop with the kids playing is a big deal to them being able to enjoy themselves).

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If you have a skimmer and a "main drain" located on the pool sidewall (AKA two inputs), a clogged skimmer will have no effect on circulation and flow rate. Over thirty years experience with that setup.

As far as mixing heated return water with pool water, all you need is an eyeball fitting connected to the heated water return. Aim it down a touch and the heated water mixes with the lower level cooler pool water.

I prefer that the "main drain" be located on the pool sidewall rather than on the pool bottom.

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Originally Posted by mas985

I have a little bit of a problem with this statement. You actually don't want the returns to have the same area as the suction ports. Pool plumbing is not the same as HVAC. If the return head loss gets too low when compared to the suction side head loss, the pump has a tendency to leak air into the pump basket and even cavitate. In general, you want the return head loss to be several times that of the suction head loss and restricting the returns with eyeballs is a good way to ensure that while still providing useful work.

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mas985, I totally agree with you, it's a lot of your information that I have used in the past, you filter comparison chart saved me hours of work but and this is the big one! Why can't we run pools like HVAC system? I do, I have for 4 years. It's a paradigm shift away from the old idea of big power guzzling pump which when you take the head away will cavitate, complete agreement on that. Now I gravity flood my pump with below the water level placement so suction side headloss is low. the system pressure on the tank gauge is around .25 PSI when I am running slow (no swimmers and over night) and 0.75 PSI at swimming daytime running normal. I might get 2.5 psi when backwashing. The biggest difference is the power my setup consumes, slow running I move 1320 US gallons per hour at 30 watts of electricity. Daytime running 1981 US gallons per hour at 69 watts. this give me 4 turnovers per day or there abouts. The pump runs 24/7 to ensure any debris falling into the pool still gets skimmed out rather than sinking and the slower flow has meant water quality has increased, Previously without flocculation the water was around 5 NTU's still really clear to the eye but slower filtration brought this down to 1 NTU.

On a bigger pool, my customers we have a bigger setup, that runs at 50 watts over night pushing 2588 US gallons per hour and 125 watts during normal times and pushing 3777 US gallons per hour. Technically he should run more to get 4 turnovers in 24 hours but he prefers to save the money and the pool water is still exceptional.

In reality the old pool way is power hungry, wasteful and inefficient on filtration (the whole point of what we are doing)
We really just want to move water from the pool, through a filter and back to the pool and that is simply what I do. The idea of producing excess friction to the point of loading the filter to 15 PSI normally is just some old dinosaur idea that has become the track pool builders run on but it's not required. The the big pumps we use are redundant and as you quite rightly say will cause cavitation if used on a highly efficient setup. Don't build the plumbing to suit the pump issue build the pump to suit the plumbing around the same as HVAC It will save $1000's.

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Originally Posted by mas985

Plus return eyeballs serve another useful purpose. With higher exit velocity the water reaches more distant parts of the pool faster so mixing and circulation are a little better with smaller eyeballs. It does reduce flow rate but not by much and studies have shown (see sig) that flow rate and turnover are not all that important to pool sanitation and circulation.

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Do they? I have an issue with that statement and here is why. There are a few guys on Youtube (The slow Mo Guys) they fire an AK47 assault rifle under water in a pool. The muzzle velocity of an AK47 is 715 m/s (2,350 ft/s) yet in water the bullet travel 5ft. Instead I prefer to balance the size of the outlets to the plumbing and use the larger flow of water to move more water rather than a smaller faster (temporary) flow, It's like a locomotive at slow speed coming into the station, is bigger and moving slower but will destroy the station if it hits the buffer without the brakes.
https://www.youtube.com/watch?v=cp5gdUHFGIQ

I do agree with turnover rates etc, what is the point? you can turnover the water as many times as you want to pass some regulation etc but if the filtration isn't fine enough you are really just re circulating the small stuff. Chlorine has already killed the bacteria and those that are resistant and oocytes are too small to get caught in most filters so you may as well not bother and just fit a strainer at 30 microns. My filtration removes stuff to 4 microns so I am having an effect, I can use flocculation to remove even more. The need to run slower but keep moving is to ensure floating debris goes into the skimmer not to the bottom, that is why I sold my pool robot 3 years ago.

Again, I fail to see any downside to a main drain... Clogs??? Really??? that does not make sense... I also fail to see why you would have any more leaks there than with any other fitting anywhere else...

It all comes down to installation, plain and simple. In my case the pool is sitting on almost 2ft deep worth of crushed limestone that extends 3 feet at least that over the edge of the pool in all directions. The main drain piping is buried in compacted crushed limestone along with the main drains themselves and the connecting tee section being buried in quick set concrete too. There is no way the main drains are moving, settling, changing, etc...

So, yeah, go ahead and bemoan the whole main drain issue all you guys like, but it is fact that cooler water gets pulled into the main drain thus causing quicker heating and overall better mixing within a shorter timeframe. To say otherwise is being obtuse. Sure you can argue that you can aim returns, use lower on-wall suction points, etc, etc... fact of the matter is water in on the sides aimed to swirl the pool and water going out the skimmer as well as the main drains will maximize the mixture way better than using a skimmer alone. To argue anything else is just being foolish and goes to show a bias against main drains for a whole host of reasons other than what I have stated.

Honestly, I am sure there are a bunch of people that have had loved ones hurt or killed by main drains, and I don't hold it against them one bit to try to get them removed from everyones pools because they truly don't want anyone else to go through the pain and heartache, and it is almost fanatically obvious that is the big slant against main drains in general. But, with dual drains, and keeping them cut in along with the skimmer, the possibility of entrapment is nil.

As I said before, from a pool professional standpoint they are a nightmare to install, to repair, to deal with. As a installer/maintainer I would much rather deal with stuff that is affixed to the sides of a pool, honestly ANY pool (be it above ground or in-ground) instead of having to deal with the floor and a main drain. But, personally, as the person that installed it, and the one that will maintain it, it just makes far more sense to have it in the bottom than to have other crap drilled into the walls. Heck, I am kinda sorry I put an extra return in. It works well, but looks just as you would expect, with it being an eyesore. I am probably going to try to cover it up at some point with a statue, or plants or something... And that return is a solid white 1.5" PVC pipe coming up from underground to the elbow and into the pool, so it isn't like it is ugly hoses or anything else that would detract, but it still looks unsightly.

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Originally Posted by DennisP

Again, I fail to see any downside to a main drain... Clogs??? Really??? that does not make sense... I also fail to see why you would have any more leaks there than with any other fitting anywhere else...

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You don't necessarily have more incidents with a MD than a skimmer but it is far easier to fix a skimmer leak or clog than it is a MD. I have seen many POs that think it is a good idea to use their MD for spring clean up and create a massive clog.

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So, yeah, go ahead and bemoan the whole main drain issue all you guys like, but it is fact that cooler water gets pulled into the main drain thus causing quicker heating and overall better mixing within a shorter timeframe. To say otherwise is being obtuse. Sure you can argue that you can aim returns, use lower on-wall suction points, etc, etc... fact of the matter is water in on the sides aimed to swirl the pool and water going out the skimmer as well as the main drains will maximize the mixture way better than using a skimmer alone. To argue anything else is just being foolish and goes to show a bias against main drains for a whole host of reasons other than what I have stated.

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So I guess I am going to have to be foolish again as I have solar and have measured temperature differences from the surface to the deepest end (9') and saw no difference in temperatures with or without the main drain (I have one return that is pointed into the deep end). It really doesn't take much to mix up the water and almost any return is sufficient unless the pool is really deep.

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Originally Posted by Teapot

Why can't we run pools like HVAC system? I do, I have for 4 years.

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What I really meant by that is that an HVAC fan will do fine with equal friction loss on both side but a pool pump will not. But I bet if I did a head loss calc on your setup, the head loss is not exactly the same on both sides of the pump and therefore the two sides are not exactly equal. If they were, you would probably have cavitation at higher RPM. Low RPM doesn't matter because the pump cannot cavitate on lower speeds.

Also, a PO on another forum thought he was being smart and energy efficient by running a separate line for every return (i.e. keep the pipe size AND ports the same on both sides of the pump). Combining that with a low head loss cartridge filter and the pump was cavitating severely. The only options for him was to either throttle the return with a valve or down size the eyeballs. At least with the latter, the higher exit velocity does some useful work while throttling just adds head loss.

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Do they? I have an issue with that statement and here is why. There are a few guys on Youtube (The slow Mo Guys) they fire an AK47 assault rifle under water in a pool. The muzzle velocity of an AK47 is 715 m/s (2,350 ft/s) yet in water the bullet travel 5ft.

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A bullet has far higher density than water and so it will sink before reaching the other side. Higher exit velocity of water will simply move in a straight line and the faster it moves, the faster it will get to the other side. One way to make a spa jet feel stronger is to use a smaller orifice spa jet which increase the force per unit area. When I changed my return eyeballs from 1" to 3/4" and I saw a noticeable difference in debris movement to the skimmer on low speed. However, there was only about a 1/2 PSI rise on pressure but I do have 4 returns. The smaller returns make the flow more directional but it also helps to move the surface water better at lower RPM for better skimming action. If the larger eyeballs are working for you then fine, use them. I was only saying that the smaller eyeballs can have benefits for certain situations.

And here we have it... You are talking apples and I am talking oranges...

Basically you are in a warmer climate, I am in a much colder one. I am looking to warm a pool from 45 to 90+ degrees in the shortest time possible. I don't have time to allow solar to do the work. I am using a 250k BTU heater and paying for natural gas. I am needing the pool heated the quickest, without exception.

You are heating a pool via solar, probably with the pump on low, but even on high, no big difference. Solar will not generate the heat a 250k BTU heater will unless you have a huge solar unit and/or it is already pretty warm out were you are at. I have neither convenience. I am in a worst case scenario.

Like I stated, I know main drains work much better. I have ran the tests last year. There is a huge difference in general with the pool running with and without the main drains, in all scenarios. I outlined everything above. These are all real-world observations in my setup. Like I said it is amazing you guys will all argue it when I have posted exactly what I have experienced.

I am also above ground, you are in ground. Outside air temp affects my pool not only on the surface, but on the sides as well when the temps get below 60 degrees, even worse below 50 degrees.

Again, the arguements here are based on a "study" that is obviously a campaign to get rid of main drains because someone got hurt and the people performing the study have are on a crusade. I could care less how much debris they pull in. I care about all the advantages I have seen personally.

It is amazing how polarizing a single topic like this is. And again, usually the ones advocating not having one, actually have one. How ironic. It is also amazing how far you guys go to try to "prove a point" when you ignore anything else anyone has to say. This isn't like arguing about water flow in piping (which is 100% purely engineering on all facets), there are tons of factors that get involved that are specific to any given installation. I already said many times I understand why the "pros" don't want to deal with them. If it is that, then just say it is that and move on. The rest is so unbelievably ignorant it is hard not to tell you guys you are all being stupid, since you are. I already have the main drains in. They are not going anywhere. Take into account EVERYTHING I have stated and it is quite obvious I am beyond happy I went this way. Just come right out and tell everyone that you are advocating they DO NOT go with one because it is YOUR OPINION. And, please, be adult enough to explain why that opinion exists, regardless if it is a matter of, "I think they are a hazard" (the fear factor), or, "I don't see how they can help" (the ignorance factor), or, "in MY case they don't do anything for ME" (the apples and oranges factor), or at the least, "I don't ever want to deal with the massive PITA a main drain problem is because I maintain others pools" (the pool professional factor)...

But again, it is more a matter of everyone trying to convince everyone their opinion is better, since it is just a matter of fact that when you are pulling water from the lowest, coolest location that is the furthest point away from a given return that is located nearer the surface and aimed to the side, that you are maximizing circulation. To argue that is just showing how ignorant you are being.

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To argue that is just showing how ignorant you are being.

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Really? You have to hurl personal insults to prove your point? I guess you have to attack the messenger because you can't argue the message.

Not at all... It is a very simple principle that you have been arguing against aimlessly for this entire thread. Simple, cool water sits at the bottom. Warm water at top. Cool water gets sucked in, warm water gets injected. Complete turnover with the shortest amount of time. You will NOT have that without pulling water from the furthest, lowest point of the pool while returning water at a higher further point away. C'mon, how hard is that to understand??!?!?!?

Basically you are trying to say the sky is not blue because you are color blind and a study by color blind people is stating that the sky is mauve... I am not color blind, I know the sky is blue and I know what happens in my own pool. I pay my heating bill. I have a HUGE financial motivation to make sure I get the quickest result in the shortest time.

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Originally Posted by DennisP

Not at all... It is a very simple principle that you have been arguing against aimlessly for this entire thread. Simple, cool water sits at the bottom. Warm water at top. Cool water gets sucked in, warm water gets injected. Complete turnover with the shortest amount of time. You will NOT have that without pulling water from the furthest, lowest point of the pool while returning water at a higher further point away. C'mon, how hard is that to understand??!?!?!?

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I understand that very well and was not arguing that point at all. But the effect only occurs as long as there is a temperature difference between the surface and the bottom. My main point is that after the pump turns on, any temperature difference disappears very quickly (< 30 min) because the returns mix the water pretty effectively. So I fail to see how the heating efficiency is going to be affected that much by a main drain especially when the pool is only 4' deep. I maintain that it will still take about the same amount of time to heat a pool 1 degree with or without a main drain (~40 min) and if you measure the temperature difference of the top vs bottom of the pool, it will be about the same after about 30 min.

Remember too that this site and other pool sites recommend a 30 min run time to fully mix added chemicals throughout the pool so there is precedence to the fact that mixture of chemicals, and by proxy heat, throughout the water usually occurs within about 30 min.

And I thoroughly disagree on your "conclusions"... Even with just over 4ft of water I can tell a difference in temp between the upper levels and the bottom without the main drains. With them on it is drastically different.

I would argue your assumption that since chemicals mix that temperature variances are also nil. That is far from the case.

Again, believe what you want. I have direct observation and quantifiable results to backup what I have posted. And unless you have the exact same pool setup, in the exact same environment, you have no point of reference other than flawed studies to fall back upon. I am an engineer, but far from blinded enough to think that those are the only answers and fully realize there are environmental factors that change everything compared to a set of "fixed examples". Your diatribe on pump dynamics show that you understand those aspects of engineering, so why be so obtuse on this???

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Even with just over 4ft of water I can tell a difference in temp between the upper levels and the bottom without the main drains. With them on it is drastically different.

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Within the first few minutes of the pump turning on, I would agree but I think you would have to agree that at some point even without a MD, the water temps would eventually equalize. It is illogical to think that the water temps would remain different at the top and bottom of the pool forever while the pump is running and constantly mixing the water. Even with a single return pointed horizontally, you would see with a dye test, return water mixing with the pool water at all levels of the pool. I really don't think it would take long at all for the temperatures to equalize.


But let me be clear, I am not disputing that MDs help to equalize the temperature difference. What I am saying is that there are other ways to accomplish same thing and they are equally as effective. As I mentioned before, I have one of my returns pointed down to my deep end and the deep end temperature at 9' equalizes very quickly, within minutes. So instead of a MD, you could have added a lower return and had the same results.

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I would argue your assumption that since chemicals mix that temperature variances are also nil. That is far from the case.

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Why would it not be the case? Both heat and chemicals move along with the water currents as they are both part of the water. Seems logical to me.

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you have no point of reference other than flawed studies to fall back upon.

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Actually I do, my own measurements and observations as well as a few posts from other POs that have pools without MDs and have no such problem with temperature difference. But why do you think these studies are flawed? They were performed by well respected universities and organizations. If you have any other contradictory scientific evidence then please share it. My point with the studies is that somebody actually went to the trouble of running experiments and simulations and documenting the results. This is what engineers and scientist do to confirm hypotheses.


I understand your reluctance to accept this evidence because you went to the expense and trouble of installing the MD and you really want it to have a benefit that you could not possibly have achieved in any other way but to me, the evidence is indisputable.

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Originally Posted by mas985

What I really meant by that is that an HVAC fan will do fine with equal friction loss on both side but a pool pump will not. But I bet if I did a head loss calc on your setup, the head loss is not exactly the same on both sides of the pump and therefore the two sides are not exactly equal. If they were, you would probably have cavitation at higher RPM. Low RPM doesn't matter because the pump cannot cavitate on lower speeds.

Also, a PO on another forum thought he was being smart and energy efficient by running a separate line for every return (i.e. keep the pipe size AND ports the same on both sides of the pump). Combining that with a low head loss cartridge filter and the pump was cavitating severely. The only options for him was to either throttle the return with a valve or down size the eyeballs. At least with the latter, the higher exit velocity does some useful work while throttling just adds head loss.

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Yes that person's idea was floored, not really from the plumbing point of view except the single lines for the returns is way too much as any flow calculator and headloss calculator would show but the issue is the wrong pump. We think pool, we think pool pump we buy off the shelf without thinking the issue through. Baring in mind I don't run the pump fast except for back washing cavitation isn't an issue. I have a slow start mechanism for the pump to build up to speed which is better than 0-2800 rpm but in the main the filter provides sufficient reduction in flow to prevent cavitation with the slow start and reduce power I use. At 30 watts the motor RPM is 1400, at normal pool speed 1800 RPM (69 watts). I agree they are probably not exactly the same but close and no cavitation as the power being used is controlled, unlike so many of the variable speed pumps that start with a burst of maximum RPM to prime presumably, I try to work with nature and not against it.

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Originally Posted by mas985

A bullet has far higher density than water and so it will sink before reaching the other side. Higher exit velocity of water will simply move in a straight line and the faster it moves, the faster it will get to the other side. One way to make a spa jet feel stronger is to use a smaller orifice spa jet which increase the force per unit area. When I changed my return eyeballs from 1" to 3/4" and I saw a noticeable difference in debris movement to the skimmer on low speed. However, there was only about a 1/2 PSI rise on pressure but I do have 4 returns. The smaller returns make the flow more directional but it also helps to move the surface water better at lower RPM for better skimming action. If the larger eyeballs are working for you then fine, use them. I was only saying that the smaller eyeballs can have benefits for certain situations.

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Yes the bullet does but it also has a way higher velocity, it's path decays so quickly as the water put's up an opposing force (Newton's 3rd law of motion). I agree the directional flow does help so adjusting the size to suit is necessary. The flow doesn't move in a straight line though, it forms a turbulent street shedding it's power as it goes. It may move in the general direction but the turbulent flow helps mix the chemicals rather than pushing a jet of freshly chlorinated water into the skimmer which could upset the chemical redox probe. I too have 4 returns on my pool, another much bigger had 7 and 3 skimmers, that really got the water mixing and near no dead spots. My point about the eyeballs is they are crap from a fluid design perspective as they don't smoothly transfer from one diameter to another, there are shelves and steps everywhere so more back pressure is applied to the flow. I too use the smaller eyeballs where necessary but the total area of all the holes is near equal to the area of the pipe/s delivering it and the eyeballs are taper machined to the right size to achieve the best result.

My spend on electricity per season is around $32 at the current euro/dollar exchange rate. I refuse to go back to old ideas by old pool builders who say I have built pools for 30-40 years but really they haven't learnt a thing in that time they just repeat 1 years experience 30-40 times.

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Originally Posted by DennisP

And I thoroughly disagree on your "conclusions"... Even with just over 4ft of water I can tell a difference in temp between the upper levels and the bottom without the main drains. With them on it is drastically different.

I would argue your assumption that since chemicals mix that temperature variances are also nil. That is far from the case.

Again, believe what you want. I have direct observation and quantifiable results to backup what I have posted. And unless you have the exact same pool setup, in the exact same environment, you have no point of reference other than flawed studies to fall back upon. I am an engineer, but far from blinded enough to think that those are the only answers and fully realize there are environmental factors that change everything compared to a set of "fixed examples". Your diatribe on pump dynamics show that you understand those aspects of engineering, so why be so obtuse on this???

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Oh, I don't follow your ideas Dennis and your Facts are not really facts they are as you see it but not facts.
Of course you get better flow with you M/D's as you don't have more than the 1 skimmer? how can you possible get the same flow with 1 skimmer compared to when the M/D's are involved?
As to the M/D's getting your pool up to temperature, it depends where your thermometer is. I have the pleasure of playing with pools set up like yours and also ones I have put a 3 port valve on so I can switch the M/D's from inlet to outlet.
Now you believe taking cold water from the bottom and feeding it into the top via returns is quickest. I don't, the water at the surface is warmer (fact) take the water from there and it takes less energy and therefore less time to heat that up than the colder water from the bottom that's a fact to. feed that into the bottom of the pool via the M/D's and the lower part of the pool gets warmer quicker as you said it causes things to mix quicker, yes it does whether it's used as an inlet or an outlet is mixes more that's a fact, you've agreed, I have tested it, it follows physics and you can't mess with physics.

The daft thing you said back a few posts about taking out your other return and wishing you hadn't put it? Go for it Mr engineer! then you'll loose performance as the friction and restriction you'll place on the system will create a great big headloss. You cannot compress a liquid so you can only get into the pump via skimmer and M/D's what the return line will allow to flow, the headloss and friction is just wasted energy.

I am not bemoaning M/D's, I was amazed that pools without them worked at first, the guy who taught me thought as you did. My pool doesn't have a M/D but I don't have a robot cleaner either as I don't need it a net and a pool Blaster Max is all I have and the Max is only for shear quickness as there is bound to be one leaf that misses the skimmers but my debris is taken out before it sinks in most cases but you are right I have to empty my skimmers more often.

Dennis where is the floor in the studies? they use the same software that keeps planes in the air and the maths behind it is very very accurate, the same is used in films to generate waves and fluid movements?
You need a study to prove it's floored, is there one?

Regardless of the so called benefits of having a pair of master drains on the pool floor, they would be of little use to me. I do heat the pool with solar deck panels.

First, it is hard enough to properly place a pool liner exactly where it belongs when you only have to locate sidewall drains, skimmers and the return. Floor drains just makes the liner installation difficult.

Second, I don't want to have to deal with winterizing the piping that feeds the floor drains. Not quite sure how I could winterize the master drain plumbing with my pool setup either.

My pool liner fits like a glove, and I doubt that it would fit that well if I had used floor mounted master drains.

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Yes the bullet does but it also has a way higher velocity, it's path decays so quickly as the water put's up an opposing force (Newton's 3rd law of motion).

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But you are neglecting the movement of the water itself. When the pump first turns on, there is a lot of resistance but as time goes on and the pool currents are setup, the resistive force is reduced and the directional flow moves further and further away from the return reaching farther areas of the pool.

Also, I didn't mean to imply that the currents are only in one straight direction. I was mainly referring the fact that water does not drop like the bullet does. I understand that the water travels more or less in a cone and the cone angle is dependent upon the orifice size and shape.

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except the single lines for the returns is way too much

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You mentioned that you try to keep the port size the same between the suction and return sides but the above quote would indicate that you do not bring those same ports sizes (i.e diameter) all the way to the pump for at least the return side. If that is the case, the return side would definitely have more head loss due to less parallel pipe paths than the suction side. So I have to ask, isn't that defeating the purpose of the whole "port size" objective? Pipe, fittings and the exit orifice all add head loss to varying degrees so I don't see any benefit having the port sizes equal other than they have slightly less head loss than a smaller orifice eyeball. Is that the main objective?

Also, I am curious about your pump. Can you give more details about it (make/model)?

Hi Mark, It was really for illustrative purposes more than what takes place over time, I admit to still learning and it's observational data, happy to have input from yourself, it's marvelous as you are one of the few to have such an understanding, so here goes.
The force of the bullet, or water dissipates, as we know energy can only be transferred so it begins to move more of the water, and more and more. Does the smaller faster orifice impart more or less energy than the slower moving but larger orifice? I think the restriction in the smaller fitting is robbing some of the energy. The fact that the water is nearer the surface means the air being less dense than water will allow as you describe allow a passage of water to be directed where you want, it's that rippling path on the surface. Does the larger less restrictive orifice moving more water, do the same, yes but a little slower, this I have observed to be actually more accurate at finding the path than the faster smaller orifice as that creates a more turbulent street so diverges more into the surrounding pool.

Most pool builders are obsessed with the circular path (Dennis demonstrated this several times) that swirling brings debris out of suspension into the dead spot in the middle of the pool where it sinks to the bottom. The same ideas of skimmer at one end and returns at the other is equally obsessed over. The directed return jet has control over leaves for a short distance only (10-12 feet) after which the flow decays and the energy previously put into the pool making it swirl takes over from the now diminished jet and the leaf can miss the skimmer, it then circulates once more and if it misses again arrives near the middle, partially water logged and beginning to sink. The pool robot salesman rubs their hands with glee as another willing customer arrives $$$.

Some pool builders do it slightly differently, returns one side and skimmers the other, the shortest distance, so much more obvious as the return jet, fast or slow will still have control over the leaf at that distance so it's easier to direct the leaf towards a skimmer (should almost always be more than one) add then that the return placed near the skimmer, facing downwards to action a horizontal rolling action creating lift and more debris is going into the skimmer without a main drain in sight.

To the other point:
If they ran say two single 1.5" lines but had a 2" to the pump from the skimmer that would be over the top unless as you said the return path was longer and more torturous, that's why we try to calculate the headloss in any pipe run. Depending on the path, number of fittings etc I would most likely run the same size pipe and separate the flows via Y's not T's to the returns but obviously each pool has it's own specific solution. As you said earlier the smaller orifice only adds 0.5 PSI, as my system runs between 0.25 and 0.75 PSI I really couldn't add 0.5 just to squirt water a little faster. LOL. I calculate the area of the pipe work and make the returns taper smoothly to the orifice dimensions that add up to total of the supplied pipe, I may vary that if I do want a directional faster flow and may also balance that out with the others.

I try to arrange the pad very accurately, pump inflow below the water line, return path above the water line but flowing down hill using the water in the pool as the minimum restriction possible, yes really obsessive, I want to try and run a system as a siphon one day. For now though, more like the HVAC just the minimal push from the pump but achieving reasonable flow. If I had a normal pool pump on there I would imagine it could cavitate especially with the pump going 0 to full RPM in under a second, I am surprised more pumps in the USA don't suffer cavitation because they are so powerful and the headloss in some cases is so high the water must be boiling inside the volute?

I the EU we have several pool companies that market monoblock filtration units in their pools, these pools have far the worst flow of any pool I work on, especially at the far end as they have a single return mounted very near the skimmer so water takes the short route. They do eventually (well sometimes) manage a bulk water movement but it's really poor and that is utilising a pump twice the power of a conventional pool (most USA pools are already using twice the power LOL) than we would in Europe but you do love add ons but even Sunny Optimism has a large pump for just a water fall but I digress. These pools still work when everything is within range so from that i deduced that we can really reduce the flow tremendously and likewise the power and still have a good pool. When the monoblock units have an algae issue then you really see how bad they are it can take more than a week to get them clear (bad course bag or pleated filters placed on the pump intakes rather than the outflow) I experimented on my own pool and even running the low power I do it still out performs the monoblock filtered pools but i have a lot more in reserve so if I need to get the water moving I can up the flow.

For commercial reasons I don't want to name the pumps I use but it's a lot more than just the pump, the whole circuit is improved where possible. I used to make many of the parts myself but as I realised that was more the icing on the cake I now adapt other parts to suit.

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Does the smaller faster orifice impart more or less energy than the slower moving but larger orifice? I think the restriction in the smaller fitting is robbing some of the energy.

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Yes, the total energy is less for a smaller orifice but the energy per unit area is higher directly in front so it is more focused and tends to push the water farther than the larger orifice. Same reason smaller spa jets feel more powerful.

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As you said earlier the smaller orifice only adds 0.5 PSI, as my system runs between 0.25 and 0.75 PSI I really couldn't add 0.5 just to squirt water a little faster.

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Not quite. The 0.5 PSI is at 70 GPM (full speed). Pressure loss is proportional to GPM^2. Given your RPM and wattage, I suspect your flow rates are much much lower. For 10 GPM the PSI loss is only 0.01 PSI.

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I am surprised more pumps in the USA don't suffer cavitation because they are so powerful and the headloss in some cases is so high the water must be boiling inside the volute?

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First, cavitation only occurs in the impeller inlet. As water travels outward in the impeller pressure rises and the vapor bubbles burst. So all the damage occurs in the impeller and fairly close to the inlet. The volute has the highest pressure in the system so water could not boil there. But the reason most pools do not have caviation problems is because PBs tend to install plumbing systems that have much higher head loss in the return plumbing than in the suction plumbing and this keeps the pump from cavitating.

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Originally Posted by mas985

Yes, the total energy is less for a smaller orifice but the energy per unit area is higher directly in front so it is more focused and tends to push the water farther than the larger orifice. Same reason smaller spa jets feel more powerful.

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Up close this is true, how about at a distance? The less overall power dissipates easier, the path decays. Because it's closer to the surface or pointing to the surface would help. It's like laminar flow jets all the rage but they are better than a conventional nozzle as that spray breaks up into the air because it's too turbulent to stay together. Just my observation.

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Originally Posted by mas985

Not quite. The 0.5 PSI is at 70 GPM (full speed). Pressure loss is proportional to GPM^2. Given your RPM and wattage, I suspect your flow rates are much much lower. For 10 GPM the PSI loss is only 0.01 PSI.

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Yes true, my flow rate being around 31 US GPM would still be around 0.2 PSI (lazy guess, off to work) although my taper machined eyeballs probably less than the blunt end with smaller hole common to pool manufacturers.

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Originally Posted by mas985

First, cavitation only occurs in the impeller inlet. As water travels outward in the impeller pressure rises and the vapor bubbles burst. So all the damage occurs in the impeller and fairly close to the inlet. The volute has the highest pressure in the system so water could not boil there. But the reason most pools do not have caviation problems is because PBs tend to install plumbing systems that have much higher head loss in the return plumbing than in the suction plumbing and this keeps the pump from cavitating.

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Yes, I wrote that badly, I meant the boiling temperature can be felt on the volute it's cavitation on the back of the impeller blades I was meaning, you put that so much better than me. I had to watch a YouTube video to re think what I had written. It was interesting to see the cavitation start so far ahead of the pump caused by the pressure drop in the valve the throttled down. https://www.youtube.com/watch?v=eMDAw0TXvUo
Because I have the pumps below the water line I don't need a self priming pump so the impeller design is different to a conventional pool pump even when the owners have the pump below water level they still use the "pool pump" cos that's what the label says......

Even running flat out at 2850 RPM I do not get cavitation, maybe because the pump increases rpm slowly it doesn't trigger the sudden pressure drop but also of course the dynamic head increases in proportion to the amount of water being moved
The massive savings in electricity, and less cleaning work with improved water clarity speak for themselves, Ben set out the Best guess idea and changed a lot of things, this is the first forum I have gone anywhere near explaining what I have done on pools but there would be a lot of very happy pool owners if they had the same system, running pools on a light bulbs worth of electricity! It's good for the co2 footprint as well, saving around 2 tons of co2 per season compared to a 3/4 HP motor and 1.5 HP probably double that.

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Up close this is true, how about at a distance?

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From my observations, it is true at a distance as well. The currents are stronger in the middle of the stream with the smaller orifice but off to the sides, the larger orifice has more flow. But I would expect this behavior since the more force you have in a particular direction, the further the flow should travel. You can actually prove this to yourself with a hose and an adjustable nozzle. The smaller the nozzle the further the water travels both above water level and below water level just to different extents due to the differences in density of air and water.

Thanks Mark, something to pay a bit more attention to with the dye in the water. I did have some flow figures somewhere and there was also a slight reduction in the flow with the smaller orifice but that due to only a tiny amount of energy being used at the pump, icing on the cake really but good to examine.

By way of comparison, the other larger pool I mentioned runs at 125 watts, 1400 RPM moving 63 US GPM, 0.5 PSI on tank gauge. When needed ( to pass regulation) we can crank that to 100 US GPM, 464 watts 2150 RPM, gauge reading 2.7 PSI but the pump could do more, sadly the builders fitted 3 skimmers but on to one pipe so the max we can draw is 100 GPM. That is a standard setup but the M/D is plumbed on a 3 port to do inlet and outlet for heating or cleaning if required.