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Can I heat water using hot air trapped in roof?

  1. Aug 26, 2009 #1
    Is it possible for me to heat water with some type of system that can use the heat trapped in my ceiling cavity? If so what would be the best way of doing this?
  2. jcsd
  3. Aug 26, 2009 #2


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    Welcome to PF, Big Red.
    You can warm it, but it even with perfect efficiency it won't get any hotter than the air around it. When you factor in the inevitable losses, it's probably not worth the effort and expense of attempting it.
  4. Aug 26, 2009 #3
    Thank you for your reply Danger.
    I have a couple of further questions if anyone could please assist me. I live in an area with a warmer climate (ranging between 25-40 deg celcius for nine months of the year). I believe that with additional heat gains from inside the house (through running of appliances and lighting etc..) that the roof cavity temp would get even hotter than the outside temp. What I envisage ( I don't know if this type of system is possible or not - hopefully someone can inform me) is running water directly from the mains water supply to the roof cavity and setting up some type of system where the water can be heated to the roof cavity air temp (for example 50 deg celcius), and from there feeding the water back down to my electric hot water system where the temp can be controlled and topped up to the desired temp of between 60-70 deg celcius. If the mains water temp is 20 deg celcius and I can heat the water to 50 deg celcius by utilizing my roof cavity heat, then I imagine I can reduce my household energy consumption as the electric hot water system will only have to heat the water between 10-20 deg celcius rather than 40-50 deg celcius.
    Question 1. Does what I have suggested above sound achievable and if so what would be the most effective type of design and materials to use for its production?
    Question 2. Is there perhaps a better way to utilize the heat trapped in my roof cavity for the purpose of lowering my households overall energy consumption?
    I don't have a great deal of physics or engineering knowledge but I have the energy and enthusiam to try and develop a system that will lower my dependance on coal fired electricity, so anyones assistance would be greatly appreciated!
  5. Aug 26, 2009 #4


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    There are folks here, such as Russ Watters, who are experts in this sort of thing. I don't really know very much about it, so I'll leave it in their hands. Good to have you aboard.
  6. Sep 1, 2009 #5
  7. Sep 3, 2009 #6
    Have you measured the temperature inside the roof cavity to verify that it is in fact hotter than the ambient air?

    What about days when it is cooler outside, or even at night time it might get cooler up there. Then you may actually be losing energy.

    I would also imagine that pumping the water up to the roof would cost more energy than you would gain. But I dont know for sure, thats just what I would assume.
  8. Sep 3, 2009 #7
    Hi Big Red,
    You certainly can utilise the heat in your roof cavity.
    First bear in mind that, to transfer heat from one medium to another, you need a temperature differential between the two and you need time. To make the excercise more effective you also need a large surface area over which heat transfer will take place.
    I assume you have a cold water storage tank in your roof and this is fed directly from the mains water supply; also that the stored water is gravity-fed to the outlet points (baths, wash basins, toilets, etc.).

    The following is my suggestion:

    Lay lots of flexible piping throughout the roof area. connect one end to bottom of the storage tank and fit a small circulating pump at that end. Connection the other end to the top of the tank.
    When the pump is operated, cool water will be drawn from the tank, warmed during circulation and the warm water will be returned to the top of the tank.
    The pump should be switched off when ambient temperatures drop at night time. This can be done manually or by time switch or thermostatic switch.
    The piping should be layed such that the hot air can circulate around it and contact the entire surface area. The piping should preferably rise constantly from cool end to warm end to avoid air locks or pockets and to assist flow.
    The circulating pump may be a standard unit as used in central heating systems; probably about 60 or 100 Watt.
    The selection of piping may need a little research, depending on what is available in you locality. I said flexible piping for ease of installation. The colour should be black or dark to assist heat absorbtion. The pipe wall should be thin as possible to minimise resistance to heat transfer. 1/2" or 3/4" HDPE (high density polypropylene) is probably the most practical.
    The above should give you water at about 10 degrees C below the roof space air temperature. I would expect the savings in water heating bills to pay for the materials cost in the first year of operation.
    Black mild steel piping is good but tends to be more expensive and requires greater pipe-fitting skills.
    Please be aware that, while this system is in operation, you will not have cold water in your taps. i.e. no cold showers on a hot day! You should still have drinking water at the kitchen sink which is usually connected to the mains supply.

    Caution: Whereas the efficiency of the system increases in proportion to the lingth of circulation piping installed, you should be careful not to overload the ceiling structure.

    Hope this helps.
  9. Sep 3, 2009 #8
    First of all, the energy savings from this application are higher than just the water heating. You save on heating your water and you save on cooling your home.

    Couple of questions. How much domestic hot water do you use? Is your above ceiling space large, like an attic?

    This is best done as a pool heater. If you are preheating your domestic water. You are preheating say 100 gallons per day of domestic water just 20 deg F you save 6.2 kW at 80% efficiency per day, there is also simultaneously 20,825 btu per day of cooling available in that 100 gallons. If you are heating a pool, it gives you nearly 9 tons of cooling for 1000 gallons per day at 10 deg F rise at 80% efficiency, saving 30.5 kW of pool heating per day.

    http://www.solarattic.com/pcs1.htm" [Broken]
    Last edited by a moderator: May 4, 2017
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