Anyone heard of, or had experience with flow-reversal?

[DennisP]

I had a whole reply typed and lost it due to a browser error...

Long story short, I cannot see how having extra suction inlets are ever a negative. If a single skimmer is sufficient, then adding a few extra drains cannot hurt. I have 5 kids and have zero concern of entrapment due to dual drains tee'd together, and a 30% skimmer limit at least fixed in. Even with a 2hp pump on high, there would not be enough suction to cause anyone to be stuck to a drain. I also am a maintenance freak, so if a drain cap is broken or something I guarantee no-one will use the pool until I replace it.

I figure on leaving the pump on low almost 24/7... At that I am concerned about proper mixing, thus I figure a single skimmer, two returns across from each other and dual drains about 6 feet apart should give me the best possibility for proper and complete turnover throughout the pool all the time.

I read the entire article and watched the video, and I have little issue with any of that... But, from a "what is ideal" situation where entrapment isn't a concern, I don't see an issue.

[JimK]

I had a whole reply typed and lost it due to a browser error...

Long story short, I cannot see how having extra suction inlets are ever a negative. If a single skimmer is sufficient, then adding a few extra drains cannot hurt. I have 5 kids and have zero concern of entrapment due to dual drains tee'd together, and a 30% skimmer limit at least fixed in. Even with a 2hp pump on high, there would not be enough suction to cause anyone to be stuck to a drain. I also am a maintenance freak, so if a drain cap is broken or something I guarantee no-one will use the pool until I replace it.

I figure on leaving the pump on low almost 24/7... At that I am concerned about proper mixing, thus I figure a single skimmer, two returns across from each other and dual drains about 6 feet apart should give me the best possibility for proper and complete turnover throughout the pool all the time.

I read the entire article and watched the video, and I have little issue with any of that... But, from a "what is ideal" situation where entrapment isn't a concern, I don't see an issue.

I'm also not concerned about entrapment in my pool (no kids btw). The main drain shares a pipe with two skimmers and there's no valve to shut off the skimmers (there is a valve to shut off the main drain). That combined with the main drain cover (checked on a regular bases to ensure its secure) there's simply not enough suction to entrap someone.

[mas985]

I have 5 kids and have zero concern of entrapment due to dual drains tee'd together, and a 30% skimmer limit at least fixed in.

Dual drains only eliminate one type of entrapment due to strong suction. However, they do not eliminate the chance of long hair getting stuck in the main drain. Nothing can prevent that (except short hair) so don't get a false sense of security.

As to other problems, main drains are the hardest items to fix but on a AG pool, they are a little easier to fix although you will still need to drain the pool so that is not a huge issue. But knowing what I do now, I would never put a MD in residential IG pool again let alone an AG pool which makes even less sense.

I figure on leaving the pump on low almost 24/7

That really isn't required either. There is an article in my signature on run time that you should read.

[DennisP]

mas985, so, your whole issue on main drains have everything to do with entrapment, and nothing to do with circulating water?

I have seen almost a "fanatical" take on stuff, and for being a hydraulics guy, I can't see why you are so against them. If it is fully because of the entrapment issue, fine. But, I am almost getting a feel it is like the "right to life" issue... Either you are pro-choice or pro-life and there is little middle-ground. You can definitely tell when reading certain technical articles on subjects like the main drain issue that there is a hard "slant" towards "NO MAIN DRAIN!" in some of them. But, just like people against high-hp cars because of the high chance of killing yourself/others with it, or even those that are "green" about energy efficiency vs those of us with gas-guzzlers. (I have a 1000+ HP car along with a 750hp car and a large conversion van, so I am far from the "tree-hugger" type, yet I am far from wasteful in regards to saving energy where it makes sense, but with 8 people in my household, we are major consumers of energy, food, gas, etc, etc) I just want the best available options for the best outcome. I hate being told that just because someone got hurt with something that I shouldn't do it, since in a lot of cases people get hurt due to ignorance, stupidity, lack of parental responsibility, etc, etc... All things that are avoidable if people use some reasonable thought processes.

In regards to having the pump on low 24/7, I am referring to the fact that I figure we will have high bather loads in the pool during the summer, thus with it running on low longer it would be better overall. But, that is just an initial thought and I will modify the low/high speed schedules as needed with a outlook towards shutting it off for as long as possible when it makes sense.

[mas985]

mas985, so, your whole issue on main drains have everything to do with entrapment, and nothing to do with circulating water?

Not quite. I just think there needs to be a clear benefit before making a modification. MDs don't seem to add any circulation benefit (see below) and they will reduce the flow rate to the skimmer (split flow). The entrapment issue just adds to the argument against them. Let me simplify.

PROS - NONE

CONS - Entrapment, leaks PLUS reduced skimmer action (flow rate is split)

If you were to put two identical pools side by side, one with a MD and one without, you would see absolutely no difference in water quality although because of the skimmer issue, the pool without the MD would have a cleaner surface than the pool with a MD. So what is the point? Keep it simple.

Maybe you missed this in the Main Drain article:

Myth
A main drain is essential in a pool to maintain healthy water.
Science does not support this conclusion. In fact, science concludes just the opposite. In a 2006
issue of Fluent News, the leading manufacturer of computational fluid dynamics software, an
article ran that compared pools with and without drains. The conclusion: there were no
significant differences between the circulations of the two pools. In fact, the skimmer-only pool
was slightly better.iv What is even more disturbing is that on page 50 of this same 2006 issue, the
software was used to model a revolutionary new swim skin technology that was sure to dominate
the upcoming 2008 Olympic Games. Pictured in that review: Michael Phelps, Olympic swimming gold medalist. The swim-speed technology was embraced, but the swim-safety
technology was essentially overlooked by the mainstream media

[DennisP]

Actually, I didn't miss that at all... But, that is all static condition stuff. My point is a large body of water is a dynamic environment and what is wrong with the option of being able to draw from the bottom if you want?

Ideally that is the way I am viewing it, another option for mixing the water. Sure, for static condition it doesn't matter much, but what if I want the main drains to do all the work any no skimmer action? What if I want to heat the pool and pull only water from the bottom to accomplish that?

I mean what is the point of having all these valves unless you can pick and choose the configuration you want at a given time?

From a "I maintain other peoples pools" I can FULLY see why a main drain setup is unnecessary. But, from a "I maintain my own" perspective I can see all the options alowing you to experiment and determine the best possibe methodology that works for you.

Having all this data is great, but only what you put into practice really matters.

Could I skip the main drains and have a nice pool? Of course.

Could I even say forget it and put in no extra returns as well and rock a pool with only a singe skimmer and return? Of course.

But, the whole point is, why not add the rest?

Again, I fall back to the inground vs above ground issue. You will never find an in-ground pool of even the most simplistic design that is 10k gallons or bigger with only one return and one skimmer. Just does not happen. So, why is it the "standard" for above ground pools even double that size or more? (and yes, I understand the "simplicity" factor of above ground vs inground, but that does not apply in my case, obviously)

I am viewing main drains as the ability to pull water, when you want, from another location in the pool. Yes the returns can be looked at to "mix" the water, but with the pump on low that ability is greatly compromised. The studies say nothing about that kind of situation. And you cannot tell me that pulling from the bottom of the pool and returning to the side, on low wouldn't provide a better mix than a return and skimmer within a couple of feet each other. That is my whole point. It is not about the effectiveness of the skimmer or the drain as individuals, it is purely about being able to provide a better mix at a lower pump speed.

[mas985]

Low speed is only 1/2 the flow rate as high speed so circulation is not really compromised, it is just slower. So if you get full circulation in 1 hour on high speed, then low speed should take 2 hours. But AG pools achieve full circulation much faster than IG pools due to just the shape (oval or circular). So even on low speed, I suspect that you get full circulation in less than an hour even without a MD.

But keep in mind that with low speed, skimming action is greatly reduced so having another suction port will only make that worse. That is why I don't use my main drain anymore because I want 100% of the flow going through the skimmer when on low speed. I think it is more important to remove debris before it hits the pool floor.

But you keep asking the editorial question of why not put it in even if you don't need it or use it. My answer is because it increases the risk of a leak down the road and that is enough for me not to put it in. Every time you penetrate the wall of a pool, especially an AG pool, you increase the risk that at some point in time, that interface will leak. I am of the mindset to avoid potential problems when possible.

Also, I didn't mention this before but a return is about 10x more effective for circulation than a MD because the return exit velocity is much higher (turbulent flow) than that into a MD (laminar flow) so the return mixes the water much better than a MD plus the return has newly chlorinated water (if SWG or inline is used). If you are really concerned about circulation near the bottom of the pool, it makes much more sense to simply add the second or even third return closer to the bottom of the pool. That way circulation is better AND you don't compromise skimming ability AND should a leak occur, it would be much easier to fix.

[DennisP]

Mark, thanks for the reply. And I really do appreciate the breakdown of the thought process, since, as you can tell, I do pretty much the same exact thing.

And I am going off the 15ft pool we had prior where the pump was anemic and undersized and I know for fact that there was not full circulation within even 4 hours. I am equating that with a 27ft pool and the pump I have on low basically having the same effect. I found that pool very easy to maintain, but it also was a fraction of the water the bigger pool will have, thus my concern.

You did mention exactly what I was thinking, adding in the extra returns closer to the bottom. Having the extra returns 1/3 of the way around the pool from the original and having one pointed straight out and angled to the side, and the last one pointed upward and angled to the side should cause enough turbulence and flow to provide for proper mixing in all situations, regardless of pump speed.

Now, what are the thoughts on multiple skimmers? I have no trees directly above and don't anticipate a ton of leaves, just a few from the other side of the yard that has a big oak above the neighbors yard, as well as possibly from the burning bushes that are about 5 ft away from the pool. I expect many more leaves on the ground than what would be in the pool.