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

[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.

[mas985]

Don't go by a pool with an Intex pump. As I mentioned before they are rather pathetic. The Dynamo would have solved all those issues. But exactly how did you know for a fact that you weren't getting full circulation? Did you take multiple FC readings around the perimeter of the pool and did you see a large variation? That would be surprising to me.

As for multiple skimmers. Having two skimmers splits the flow so skimming action is reduced in each skimmer by 50%. Combined with low speed, it can be almost non-existent. I would only recommend a second skimmer for a very large pool or one with a potential dead spot (e.g. L shaped). AG pools don't tend to have dead spots. Also, if you have prevailing winds for the most part in a single direction, all debris will tend to flow toward one end of the pool and that is where the skimmer should be located. In this case, a second skimmer won't do you much good and you are better off with a single skimmer.

[DennisP]

That was exactly how I found out the mix sucked, by test readings and putting a dye in the pool... I could put it in on the far end and it would slowly sink and dissipate, but a half day later you had a blue blob directly under where you dumped it in. And that was without a filter in the bigger pump, so it was really sad...

Oh, I gotcha in regards to the skimming flow, in these discussions I always "assume" there is valving in place (I never do anything "half-way" and everything will be able to be turned on or shut off), so even with multiple skimmers, each skimmer would have a valve on it. The only item I considered not going that far is with dual main drains, and that is having them tee'd together and a single 2" run from the tee to a valve...

Ok, well the drains will be here tomorrow and I will have to decide if those are going in or not... I do believe I will put in the extra returns though...