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

[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.

[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...

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.

[mas985]

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...

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.

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.

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.

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

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.

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.

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.

[DennisP]

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.

[mas985]

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

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.

[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??!?!?!?

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.

[mas985]

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??!?!?!?

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.

[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???

[mas985]

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.

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.

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

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.

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

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.

[Anonymous [GDPR] European in the UK]

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.

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.

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.

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.