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]

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.

[Anonymous [GDPR] European in the UK]

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.

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.

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.

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.