Radiant tube in a pool floor, how realistic??

Robert O'Connor_12

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Robert O'Connor/NJ

hr

I think a local ICF salesman

is telling customers that this is a do-able way to heat an in ground pool.

This customer is building a 39X16 foot pool 4-1/2 feet deep.The pool is built with ICF forms 5" of insulation in the walls and 2" on the bottom. It will have an insulated cover.

So how much output could I expect from 624 square feet of floor? If I supplied 140 to the floor and the pool temperature was 80 for instance, could I realize 140-80=60X 2 BTU/ ft/ degree difference. Similar to a radiant floor.

So an output of 120 btu/ square foot x 624 for an output of 74,880 BTU/ hr.

Seems most of the pool heater sizing charts are more concerned with surface area than actual gallons. This being a shallow, very well insulated pool should cut down on loss to the ground.

I want to be up front with the owner, and provide some answers based on engineering not a sales spin :)He is willing to front the cost of the tube to try it, I'm inclined to give it a go.

Any thoughts or experiences?

I connected a Weil Ultra to a DIY system like this last year (same ICF salesman's job) but the owner ran out of cash before he finished the project, so no actual operating data yet.

hot rod

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Mike T., Swampeast MO

Hey Hot Rod. Hope you can deal with some more of my "imagineering"...

As long as you're talking about square feet of TUBE AREA in your question, "If I supplied 140 to the floor and the pool temperature was 80 for instance, could I realize 140-80=60X 2 BTU/ ft/ degree difference? Similar to a radiant floor." you might be close.

If you were talking about square feet of pool bottom, I believe you're WAY off the mark.

You can raise the temp of a concrete slab heating a room quite easily because heating by way of convection through the air is quite inefficient. The slab raises in temperature and radiates because it can't convect rapidly enough.

A pool is totally different. It's filled with water--a very good conductor of heat--and that water is free to move so convection increases the speed of conduction. The convection to the water will be happening VERY fast--so fast that the concrete will only have a small rise in temperature. I know you know the effect of moving water underneath a slab that's trying to heat a house...

Think of a big pot of water on a stove. Put in cold water and put the pot on a burner on "high" for a minute or so. Remove the pan and touch the bottom. You won't get burned as it will be nearly the temp of the water in the pan.

140-80=60X 2 BTU/ ft/ degree difference might therefore produce a somewhat reasonable estimate if you use the square footage of the tube itself--not the square footage of the bottom of the pool. Would be MUCH better to find some resistance value for the tube involved and compute conduction to the concrete as that may be very different. Unless there's a chance of ground water moving below the pool bottom, you're probably safe removing only a very small portion of this output for "downward" loss.

If the idea of 140-80=60X 2 BTU/ sqft TUBE area is reasonable then you get 120 btu/hr/sqft.

Say you need 20 mbh (quite low for any pool I've ever heard of). That's 167 btu/hr for each square foot of tube area. You'll need 120 square feet of tube area to meet the load. Say you're using 3/4" tube with 0.875" OD.

It takes 4.36' of the tube for 1 square foot of surface area so you'd need 523' of tube. That's roughly 12" tube spacing. If you need much more than 40 mbh output (what you would get by this method if you interleaved two such loops) I'd suggest that it's impractical.

Again, this could be way off as the conduction value of tube to concrete may be quite different. I'm not sure and I've yet to find any published resistance value for PEX itself.

George Peteya_3

Pool Surface Area ...

... is used for sizing because most of a pool's heat loss is due to evaporation. It's 70% or more. If, for example, you lose half an inch of water out of a 16'x 39' pool in a day (not uncommon), that's 1,624 lbs. Multiply by the latent heat, 970 Btu/lb, and you need almost 1.6 million Btu just to maintain the water temperature.

Unless, of course, you can convince the Customer to use a pool cover whenever the pool is not in use (yeah, right.).

George Peteya_3

Pool cover

Just saw that you mentioned an insulated cover. Sizing charts assume no cover. Kind of like when I do a heat loss, I use 2 for window R-values unless I can steal the sticker off the live, installed window.

J.C.A._3

Maybe HR's on to something....

That would be one hell of a buffer tank(for a while) as the days get shorter, and the cover lets the sun shine in.

Maybe a reverse flow through a flat plate and radiation of a super size to take the chill off on the sun porch at the end of season? (heck, you spend all summer heating it...why not get some back? Chris

Unknown

Anyone heating a pool without a cover needs to be slapped. Losses without evaporation are quite low from my reading. Cover would pay for itself in very short order indeed.

Pool heater sizing is also based on certain temperature rise speeds. That's the major issue is heating it up from nothing. If the client is willing to turn it on, leave it on, and use a cover, the actual operating heat needed seems to be quite low. Cut the rise from 1 degree an hour to 1/2 degree an hour and you've really knocked a big chunk out of your required output. But, it will be slower to change temps, obviously.

George Peteya_3

(whisper) solar

... Don't tell anyone, it's a secret. Solar pool heating is the most efficient solar heating there is. But without a cover, it's dope slap time. A tennis court green concrete deck about the same size as the pool, with 3/4" poly pipe 8" oc ...

BTW, if the Man Upstairs made all those Maine forests green, why do solar collectors have to be black?

Unknown

Excellent points George. Heck, if you can heat these pumps with some panels, you can transfer the heat easily through PEX.

I ran calcs on this sort of problem myself recently HR.. not sure they would stand up in a court of law, but I saw no reasons to be shy.

Gordy

pool heaters

I have a 24' dia x 4'6" deep above ground. My heater is a hayward 120,000 btu. It will give 1/2* rise in temp. per hour. So when I fire up the pool upon opening and the water is say 65* it takes 34 hours to get it up to 82* COVERED. I had the kids messing with it and it got up to 100*.

George is right evaporation is the big factor in Btu loss for pools 70% figure is the norm above or below ground.

I think you need to find out what the customer EXPECTS out of their pool heater first Hot Rod. Plus where is the geographic location, is there shade trees, or better question how much sun will the pool get through out the day.
Will they cover when not in use faithfully. When do they want to open the pool, and when will they close the pool (Time of year)
I think radiant may be very slow getting up to temp. but once it got there I think it would be way more efficient. than your standard pool heater.

Also what temp. does the customer care to have their pool.
For us 85 is way to warm, 82 seems to please the women, but is none refreshing to me after working outside all day. Lotsa factors when it comes to pools.


Gordy

Unknown

well, I wonder about the efficiency. For efficient heat transfer, you can't get more efficient than heating the pool water directly.

However if the pool heating equipment doesn't stack up to a mod/con, then it may shift.

Of course, if you're buying a mod/con just to heat the pool... ah, who knows.

Made sense for us since we had a huge Vito beast sitting there already with nothing to do for the whole summer, and a pool that needed heat..

George Peteya_3

OK, how about this ...

... if you have any snowmelt jobs where the Customer has a pool, set up a heat exchanger to take all that summertime "waste heat" out of the driveway and stick it in the pool where it belongs.

IMHO, we have reached the point where heating pools with fossil fuels is bloody ridiculous, and there's no going back. Oops, sorry for the microrant.

Unknown

Two steps ahead of you george

we'll report how it goes, of course.

I'm with you. Unfortunately, we're not yet in a position to say no to a paycheck. But we do fight for efficiency and sanity as much as possible. Frankly, I discourage snowmelt every chance I get. But, some people just don't care about the economics..

George Peteya_3

Yankee ingenuity ...

... alive and well. There's hope for us yet.

But what's a dude to do? He lives in Florida, wants to come home with no notice in the dead of winter and get up his driveway (pavers) that rises 65 feet in a 350' run with 2 switchbacks ... feel his pain, he needs to visit the Porsches.

Mike T., Swampeast MO

re: Tube Output

At the temperature conditions you mentioned 140F heating water; 80F pool water and with the tube about 3" below the surface 300 btu/hr per square foot of TUBE area to the water in the pool [seems] reasonable.

With such high heat transfer potential maintaining 140F average with "safe" flow conditions will obviously require significantly shorter loops than with floors heating rooms.