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

[Anonymous [GDPR] European in the UK]

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.

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.

[DennisP]

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.

[J_Palmer_Cass]

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.

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.

[mas985]

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.

[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.