Best way to draw heat from under a heated floor?

What I have is a 5' X 6' area of electrically-heated bathroom floor, and I would like to draw heat from its underside to pre-warm the water going to my water heater. The core of the 2-1/2"-thick floor (with ceramic tile on top) is right around 100 degrees F, and it is definitely warm to the touch from below. My thought is to install a 3/4" or 1" copper "U" about 12" wide and 6' long as an insulated heat exchanger between two floor joists near the center of that floor area.

Assuming an occasional flow of less that 5 gpm with only one or two fixtures running, how much heat am I likely to add to cold well water? And, would it be okay to use aluminum strips to fasten the copper pipe to the wooden bottom of the floor?

You'd get some heat transfer, but probably not as much as you think - the water would travel through too fast. It tends to work best with high differences in temperature. What do you heat your water with now? Note, the floor gets warm because the heaters are on for awhile, especially at that thickness. How long does the floor take to reach that steady state temp - my guess would be over a few hours, not instantly.

jadnashua said:

You'd get some heat transfer, but probably not as much as you think - the water would travel through too fast.

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That is why I am thinking about using 1", but I have yet to figure the volume. Also, some kind of saw-tooth or zig-zag configuration could make it longer.

jadnashua said:

It tends to work best with high differences in temperature. What do you heat your water with now?

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We presently have an electric, dual-element, 40-gallon water heater. Also, I taped a thermometer on the underside of the floor a few hours ago, and it is steady at 80 degrees F. So, that gives me a differential of a little over 20.

jadnashua said:

Note, the floor gets warm because the heaters are on for awhile, especially at that thickness. How long does the floor take to reach that steady state temp - my guess would be over a few hours, not instantly.

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I only got everything hooked up and working a couple of days ago, but yes, the heating wire is on for a good share of the time. However, the thermostat somehow "remembers" how long it takes to reach the set temperature, and it also adjusts run-time (via spurts of power) to maintain.

After leaving it off all night to begin doing some observing and adjusting the thermostat's programming yesterday morning, it took only 1-1/2 hours for the floor surface to get from 68 to 96 degrees ... and after another hour or so, it stayed over 100 all day in spite of the breeze blowing through the house and the bathroom's unfinished walls! I tell you, this little system will definitely add some noticable heat in our new bathroom!

I think you are wasting your time...the floor is typically set at around maybe 10W per square foot. Your water heater is probably in the order of 3500W, maybe more. It probably takes an hour to reheat the water once drained. What you do expect from a floor with a total of around maybe 300 watts? Not having intimate contact, a small delta T, and moving the water through the pipe won't do much. The cost of the copper piping doesn't justify the very small gain, IMHO.

You are using an electric heater to heat the floor. If heat is escaping below, you are better off to insulate it and use the water heater to heat the water heater.

jadnashua said:

I think you are wasting your time...the floor is typically set at around maybe 10W per square foot. Your water heater is probably in the order of 3500W, maybe more. It probably takes an hour to reheat the water once drained. What you do expect from a floor with a total of around maybe 300 watts?

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Well, you sure put that one in perspective, eh?!

I had first thought about running tubing all around between the original floor and the concrete board, but I suspected that would be a lot of work for very little gain ... and now it looks like this alternative would not be much different.

Oh well.

Bob NH said:

You are using an electric heater to heat the floor. If heat is escaping below, you are better off to insulate it and use the water heater to heat the water heater.

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Yes, I understand, and I do intend to insulate at least that section of floor even though the basement below is also heated.

Hey! Did anybody ever think about trying to capture and use the heat behind a refrigerator?!

That is potentially more efficient. Trying to grab heat from the floor is counterproductive, since it is essentially equivalent. Well, probably not, in moving that water from one place to another, you'd probably reradiate it back into the heated space. At least the coils behind the frig are probably hotter, and cooling them off makes that operation more efficient. Still, at the cost of copper, it would take a long time to recoup the costs, and you'd have a lot of soldered fittings that could leak.

jadnashua said:

... coils behind the frig are probably hotter, and cooling them off makes that operation more efficient....

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One day someone will run the cold water supply line zigzag back and forth on the wall in the back of the fridge niche, using Pex-al-pex which needs no elbows. Works best with a fridge that has exposed cooling coils.

Then the cold water tap will give slightly warm water for 30 to 90 seconds.

So far I have only gotten 5 seconds of lukewarm water out of my cold water line since it only crosses that space once.

david

pipe

You are thinking about it the wrong way. The only heat transfer is through the pipe wall. The greater the disparity between the pipe's surface area and its volume the less the heat buildup. (doubling the pipe size quadruples the volume, but only doubles teh surface area) You need the smallest tubing, usually paralleled with a manifold, so you have a lot of area and a small volume in each run, which is why solar heaters have a multitude of 3/8" tubes to absorb the heat, and your radiant floor has a grid rather than just a few large heat strips.