Problem: Use excess exhaust air heat from industry to heat outdoor pool. What temperature and at what pressure would air have to be to heat volume of water, say 375 m3 to 25 °C. Is there a way of calculating a smaller volume, then scaling it up?

  • The physics involved in this question might be better addressed in physics.stackexchange.com . It would also help if you knew the temperature and volume of the exhaust air, as that will make a difference. Another significant factor for the amount of heat required to sustain the pool at 25 degs is the ongoing heat loss from the pool, which will differ depending on ambient temperature, humidity, and wind speed, as well as the size of the pool (more surface area = more heat loss to evaporation). Without those thing defined, you can't get an exact answer. Jan 26, 2022 at 4:13
  • Mechanical engineers who deal with heat exchange can deal with this. Whether what you want to achieve is possible with also be dependent on: the temperature of the exhaust air, the temperature you would like the pool water to be, the distance between the heated air & the pool - how long do the heating pipes have to be, diameter of the pipes, available insulation for the pipes & its heat retaining, the properties, the production rate of heated air (cubic meters per hour), humidity of the heated air, what will be the heat transfer medium: air, water or refrigerant.
    – Fred
    Jan 26, 2022 at 22:57
  • Thank you, I'll have to measure the volume of air and temperature of air that is coming off from the exhaust gas. The surface area would me 250m² at a maximum. The temperature of the water would be able to fluctuate between 22deg & 28. The next problem is to work out what mechanical process to use to getting the air to heat water...
    – Hawkeyewoo
    Jan 27, 2022 at 12:31
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    As @blacksmith37 notes, the source of the exhaust is important - can you provide that? Corrosion can be incredibly fast if acidic liquids condense on the heat exchanger.
    – Mark
    Jan 27, 2022 at 22:20
  • Having just thought about this, the best way to get warm/hot air to heat water is to direct the air into pipes, duct or flue & to submerge the pipes, duct or flue in the water. However, do not exhaust the into the water in case the hot air contains something that could contaminate the water. Let the end of the flue emerge from the water so the cooled air can exhaust to the atmosphere.
    – Fred
    Jun 26, 2022 at 19:26

1 Answer 1


It is primarily a matter of cost. Nat gas heater is likely the lowest cost. No pressure would be required for the air/flue gas, only movement. Any heat is good, it does not need to be a specific temperature except possibly for a commercial pool somewhere like a hotel. Our subdivision pool is heated by the sun to around 29C for 4 months; some say too warm, some say too cool , some say it is fine. I am sure all would consider 25C too low. " Industry exhaust" would need to be defined to be considered. Industrial flue gas can a very corrosive material requiring expensive alloys. I had a plan to heat my home pool with heat from the house attic; Use a home furnace fan (free) to blow air through an auto radiator ( cheap, from junkyard) using a side stream from the pool filter pump ( only cost only some plastic pipe). But like many good intentions it never happened.

  • Natural gas is not low cost, it's literally cooking the planet. Avoiding that is a big part of the point of Sustainable Living.
    – Móż
    Jun 25, 2022 at 23:33
  • Must admit I am off, my excuse is old age. I have mostly paid 1 to 2 $ US: I see now it has jumped to over $ 6. In perspective I remember 0.10 $ ( price to oil company from gas pipeline). Still cheaper than any renewable when full cost basis is used. Jun 26, 2022 at 17:19
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    This does not seem to be an answer to the question. Jun 27, 2022 at 1:13
  • @blacksmith37 you're assuming we can just buy another planet when we wreck this one? "full cost" has to include that or it's just a lie.
    – Móż
    Jun 27, 2022 at 10:45

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