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.