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My family has a property in Portugal with a broken old solar water heater mounted on the roof. We've been using the immersion heater instead of the solar panel for a while now, but rising energy costs mean we're now looking into replacing the solar panel.

So, my question is: would it be possible (or is there already some precedent here) to install piping under the baking hot terrace on the roof of the building with the dual purpose of heating water and cooling the property below? I'm thinking of something like underfloor heating in reverse - using hot flooring to heat water pipes.

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Yes, that could work if the roof surface really gets that hot. As you say, essentially take the approach used for "underfloor heating" with PEX tubing and turn it around. You would probably want to keep the tubing pretty close to the surface, and you might want to actually measure the temperature on the roof deck to be sure it will be adequate, but in a reliably sunny climate it probably will be.

It would be a low-efficiency heater as compared to a "proper solar panel" but it also has the advantage of being much larger than a "proper solar heater." The main place where a solar panel would outdo it would be in the morning, where a solar panel should heat up quickly as soon as the sun hits it, but the terrace probably takes several hours to heat up due to thermal mass.

  • I was actually thinking it might work in tandem with a proper solar panel - pre-heat the water with low efficiency under-terrace piping and hopefully that would allow the solar panel to get more water up to temperature in a shorter time. I guess the major potential pitfall with this idea is that any problems with the underfloor pipes might require taking up the entire terrace, vs doing a simple swap out job for problems with a standalone solar panel... – user29742 Jul 21 '14 at 12:33
  • Good logic. Just arrange it so you can bypass the terrace pipe if needed, and do a careful job (and avoid drilling holes in the terrace) to avoid problems there. Depending how much manual fiddling you care to do, you might use the bypass in the morning and cut the terrace back in after it warms up, or you might only use the bypass if there was a problem with the terrace pipes. – Ecnerwal Jul 21 '14 at 12:40
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Ecnerwal gave you a VERY good answer, and I'd like to underscore one point in a separate answer ("accept" Ecnerwal's answer, mine's just supplemental).

I have a neighbor who did something similar, laid pipes into a well-sunlit concrete floor. He learned a strong lesson from doing that: while heat RISES through masonry very very well, heat does NOT travel downward through masonry very well at all; you can't count on the blazing-hot upper surface to heat any pipes below it if the pipes are more than one or two inches below the surface. Experiment by (reversibly) peeling up a little of the terrace surface on a blazing hot sunny day, and feel its underside. I think you'll find that if that stone is more than a couple of inches thick, its underside will be easily 50*F cooler than the top surface, and possibly as much as 100*F cooler.

My neighbor never DID get any benefit to his under-floor pipe, so he abandoned it. You, too, may get no measurable benefit. Too, you should know that the long run of pipe may slow down the hot-water flow by virtue of friction.

  • Exactly, the masonry's thermal mass is working against you. Thermal mass is great for storing heat and slowing down its transfer, but not so great for collecting it. There's a reason solar heat collectors are made out of metal, glass, and black plastic. – iLikeDirt Oct 18 '14 at 16:31

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