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

[DennisP]

Well, I had an idea today in dealing with heating the pool more efficiently using the natural rise of warmer medium through colder ones... Basically, what about using the main drain as a hot-water return instead of the return fittings?

I did some diagrams and using simple 3-way and 2-way valves to accomplish this, I figured a 4-way vane-type valve would be needed to simplify this and started looking it up to find it just doesn't exist. What I did find is a methodology of exactly what I was thinking and it was a patented set of valves and design that used the main drain as a return, with the ball-returns and skimmer used as the intake (basically it takes warmer water from the top of the pool, heats it via the heater and returns it via the main drain allowing for better mixing and overall better thermal balance throughout the substrate) and as long as you are using a solar cover, the idea is you can achieve max-efficiency which would mean a total expenditure of about only 1/3 of what it costs doing it "normally"...

Now, I am not sure you can achieve an almost 70% increase in efficiency, but overall I have no doubt you cause a better mix and an increase of a certain amount of efficiency (probably less than 10-15% overall), but in general I am wondering if it is worth the expense of implementing this type of setup?

I would have to imagine using the main drain as the source of water would gain at least half the efficiency increase (purely by causing a better overall mix), but is there anyone with any valid input on the pros/cons???

[CarlD]

Why not just use solar panels?

It occurs to me that you are trying to pump against a column of water 4, 5, 6 or 7 feet high, and each cubic foot of water is 65lbs, so you're moving at least 260lbs of water (yeah, I know it's not a square foot, but you get the drift).

[mas985]

First, I think you are assuming too much. Normal returns mix the water pretty well and while the pump is running, I doubt there is much of a temperature difference between the top and bottom of the pool if you took the time to measure it. And if there is, you can just point the returns downward and solve that.



total expenditure of about only 1/3 of what it costs doing it "normally"...How do you get that?




It occurs to me that you are trying to pump against a column of water 4, 5, 6 or 7 feet high, and each cubic foot of water is 65lbs, so you're moving at least 260lbs of water (yeah, I know it's not a square foot, but you get the drift).After leaving the pump, the water starts near water level, travels downward to the main drain and then back up so you really aren't pumping against a column of water because there is head gain on the way down and head loss on the way up so net static head is 0.

[DennisP]

Actually, the weight of water doesn't matter at all... Since it is an above ground pool and the equipment is above ground on the same plane, you have a net of zero in regards to head of pressure (water in the pool looking to get out) and the water you are pumping back in (within that same height of the water column that is the source)...

But, that isn't the issue... The issue comes down to the efficiency to be gained by pumping hot water into the coldest area of the pool (the bottom) and letting it mix and rise through the rest of the pool, compared to pulling water from the top of the pool (the warmest area, especially with a pool cover on) and returning heated water within a few inches of that area (at the eye-ball return). With multiple returns, all pointed to the deepest area, you will get a fairly decent mix as well, but ideally it wouldn't be "as good" as returning through the main drain (which is the lowest point in the pool).

In regards to solar heat, that is a completely different discussion, and in my case, not viable, as I have a cedar shake, heavily pitched roof in an area that would complain up and down if I put something so unsightly on my roof... So, solar is out, and the gas heater I have I figure will only be used at the beginning and at the end of the season, so it's overall cost isn't a big deal.

[mas985]

I have measured mine and I get less than 1 degree difference between the top and bottom of my 8' deep pool (i.e. I can't measure a difference) and that is with solar heat running. I think you are wasting your time.

Also AG or IG, the weight of the water does not matter because water source and destination are at the same elevation. Net static head is zero in both cases.

[DennisP]

mas985, the 1/3 is the figure they calculated and derived after putting the flow-reversal system into practice many years ago... It assumes a pool cover and an in-ground pool installation with a deep end...

You do bring up a good point, is a single return sufficient on a 27ft above ground pool or larger?

I would imagine on a 21ft or smaller pool a single return with good flow would be just fine, but larger pools of 27-33ft I would have to imagine a single return would be a stretch to cause proper mixing...

So, would two returns work better for the larger pools? Maybe 3 or even 4?

I just don't get the HUGE difference in pool flow designs between in-ground and above ground pools. I understand that in-ground pools have varied layouts and designs and some of that requires multiple returns/skimmers/etc to effectively clean and mix the water, but it just seems that in-ground pools are wayyy overbuilt and complicated compared to above ground pools and if the in-ground are clearer with better, more effective turnover, why not apply a lot of that to an above ground pool for the same benefits... And yes, I know, a lot of complex plumbing is for spas and integrated water features and such, but take those out of the equation and a lot of in-ground pools of simplistic designs have at least double the returns/skimmers of their above-ground brethren for the same or even less volume at times...

[DennisP]

I forgot to add, that even for a "simple" in-ground pool, ie, no spa, no water features, just say a 16x32 in-ground with a deep end, around 18-19k gallons, you will have huge filter systems (300+ sq/ft), big pumps (1.5-2hp+), at least 2 returns, 2 skimmers and a main drain arrangement. Why would you want the same setup on a 27ft x 54" 20k gallon pool???

I know, I know, a lot of equipment is oversized (such as filters and pumps) for their app, but it seems that above ground pools are viewed as "cheapie, who cares, doesn't matter, it is throw-away after-all" type of attitude... And in the end, there is little difference except for the up-front expenditure... Same size body of water and same chemicals needed to maintain them regardless of in-ground and above ground...

Just like it seems "viable" to have a booster pump and pressure side cleaner for an in-ground, but seems absurd for an above-ground. Again, why?

[mas985]

AG pools are only 3-4' deep so you will get even less temp difference. I really don't think it is going to matter. Also a second return probably won't give you much unless the pool is really big.

As to your last comment, AG pools are built for low cost and easy assembly, for the most part. Also, they tend to be smaller so don't need much extra circulation because they are round and avoid dead spots.

An IG pool can be built with a single skimmer and return (have seen some like this) but it doesn't take much more effort to add more returns and it can have large benefit because IG pools tend to be much more irregular so it is easier for dead spots to occur which can be alleviated by multiple returns. Usually they are done on the same side of the pool which makes it easier. Also, most IG pools only have 1 skimmer and maybe 1 main drain although the trend in the industry is to eliminate the MD. IG pools with two skimmers tend to be much larger pools >20k where the surface area is large and a single skimmer may not be enough to keep the surface clean.

So in general I don't think IG pools are over built because of the differences between the two types of pools. If anything AG pools tend to be under built because some of the equipment (e.g. Intex) is not really sufficient to keep the pool clean.

[DennisP]

Oh, I agree... I think above ground pools are way minimized compared to in-ground pools. The depth doesn't matter much, since it isn't like you see multiple returns in the deep-end alone. And average wise, it still is the same amount of water for a large above ground pool and a "normal" in-ground pool. Of course extremely large in-ground pools are much bigger, but realistically water volume is water volume...

I have been looking at pool system hydraulic designs for a few months now and it is just amazing that water turnover and circulation is taken way more seriously for in-ground pools...

I would have to imagine an above ground at 20k gallons would need a mimimum of the same type of plumbing for a comparable in-ground to be just as effective. Sure, the majority of the above-ground industry gets by with a single return and a single skimmer. But, "accepted" is far from "ideal".

That is why I am going with a dual main drain, at least two returns and 1 skimmer. I don't have trees above the pool, so a single skimmer should do just fine. The main drains are ideal to reduce the amount of time vacuuming or using an auxillary pool cleaner as well as affording for better overall "mixing". Multiple returns just make sense for the size of the pool (27ft x 54").

I mean in regards to equipment, the same argument goes for the pumps. I understand why in-ground pumps need to pull a head of water, but even for say a 1.5hp pump, the in-ground variants at the same HP rating have way more flow. Again, outside of the head of water, it doesn't make sense. It does when you figure in extra water features, multiple returns, etc, etc... So, does that mean with my setup, I should be looking at in-ground pumps and I have a way "undersized" pump at this point? (for reference I have a Pentair Dynamo 1.5hp 2-speed pump)

[mas985]

IMHO main drains are worthless and are more of a risk than benefit. They don't add much at all to circulation and just add an entrapment hazard. They do not substitute for vacuuming and have very little influence to dirt around them. In fact, if you pour a bunch of dirt directly on a main drain, it will only clean about 1-2 inches around the drain and that is it. They also easily clog with leaves. People give MDs way way too much credit. I would skip it if I were you.

Dynamo is a great pump for an AG pool and a 1.5 HP is likely over sized if anything but it depends on the filter size. You don't need an IG pump. IG pools need IG pumps because IG pumps are self priming and are above water level. Although you can use a Dynamo on a IG pool because it just so happens to be also self priming.

[DennisP]

Here is my equipment setup:

- Pentair Dynamo 1.5hp 2-speed pump
- Sta-Rite System3 cartridge filter (300sq/ft)
- Jandy Lite2 250k BTU NG pool hater (millivolt unit)
- Jandy check and 3-way valves
- all 2" plumbing from the pool to the equipment pad and at the pad
- three 2" lines installed and one 3/4" line installed for configurability

I plan on adding a 15gallon chlorine tank with Stenner pump in the future along with possibly a booster pump and pressure cleaner or a robotic or vacuum cleaner. I am installing dual main drains to comply with code and to remove any concern about "entrapment".

I talked to a few people and if you have good circulation in the pool with plenty of turbulence at the bottom the main drains are supposed to do quite a good job of minimizing vacuuming. I figure for the minimal expense (I am doing the complete install myself) it would probably be worthwhile.

[DennisP]

Pics of the equipment pad can be found in this thread: http://www.poolforum.com/pf2/showthread.php/25702-Question-in-regards-to-pool-plumbing?p=126065#post126065

[nefretrameses]

I heard of this with respect to solar heating panels. I don't recall where. I think their premise was the same as yours - warm water introduced at the bottom will push colder water up where it will be picked up for solar heating or something like that. But I think solar introduces a temperature delta irrespective of the input water temperature (within a relevant range). It sounds like there's not a significant difference anyway. I'd trust Mark's analysis and opinion.

[mas985]

I talked to a few people and if you have good circulation in the pool with plenty of turbulence at the bottom the main drains are supposed to do quite a good job of minimizing vacuuming.Sorry but I think these "people" are just misguided and do not understand hydraulics very well. I have main drains and I can tell you they don't do anything. This is especially true of dual main drains with safety covers. The reduction in entrapment also reduces the force of the water in any one direction (done on purpose) so they do move anything very well. That is why they lower entrapment.

Perhaps you should read this article and watch the video if you still do not believe me:

http://www.poolinspections.com/manuals/drains/pools-without-drains.pdf

https://www.youtube.com/watch?v=0IOUGhuKkSM

[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]

Interesting stuff.

When I was shopping for an inground pool back in 2003 I got quotes from 3 different pool companies. Two offered designs that included a main drain and one offered a design that had a return at the bottom of the deep end rather than a main drain.

I'd never heard of a pool without a main drain so I thought the company that didn't include one was cutting corners to increase profit (their pool was a little more expensive than the others). So we ended up with a design that included a main drain.

Based on the links provided it seems the company who didn't install main drains was on to something.

[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 24/7That 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.

[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...