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

[SunnyOptimism]

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

[mas985]

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

[Anonymous [GDPR] European in the UK]

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.

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,

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.

[DennisP]

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).

[mas985]

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

- Increased circulation within the pool

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.

- Collection of debris at the bottom near the drains

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.

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

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.

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

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.

[DennisP]

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"...

[Anonymous [GDPR] European in the UK]

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.

[mas985]

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

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.

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 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.

[SunnyOptimism]

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.

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.

[J_Palmer_Cass]

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).

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.