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Russell McMahon and David Leatham talked about lower cost alternatives for heat pumps:

What do you think about using any of the following as heat pumps for residential air, water or other heating? Any and all should work. Mainly they are quite limited in volume, and each must have some modification of the condenser or some device added to effectively use the heat.
Domestic refrigerator
Lowish power, Designed to run from approx 2C in to 25C out. If it's one with the condenser rack put the on the back... put that side in the area to be heated and the other side would be exposed to the heatsource; maybe outside with the door open.
Chest freezer
Similar to fridge but colder input (-10 or lower). Higher power. Most freezers use their outside covering as the condenser, so would you immerse the whole freezer in a waterbath? Use a fan to direct outside air into the freezer cavity? How would you defrost the cavity?
Domestic Dehumidifier
Designed to cool to around freezing point. Outlet temperature is "free" in normal use - ie they don't care AND they cool it with the cold air. Convert to heatpump by closing air path from inlet to outlet and supplying a second fan to cool hot side or some other means of taking air from coil. Range of powers available - none super powerful. Portable. Easily mounted as required. Heat exchanging hot side with eg domestic hot water cold supply should be easy. Available new from $NZ150 = $US100 up. IF a $150 unit gave 500 watts out and had a COP of 2 it would save $NZ0.06/hour run and pay for itself (dehumidifier cost only) in $150/.06 = about 2400 hours. As such a device would run continuously on hot water heating at that power level it would pay for itself in under 4 months. Once installed it would save $NZ500 or so a year - until it died. They may be unhappy about a 100% 24/7 duty cycle :-)

I'd go with what's cheapest to get, if you want to play around with this idea. The dehydrator (and the others) should run many years full duty. They do that anyway. The hottish air could be blown thru a 'boiler'-tubes thru a water jacket.

It's just amazing how cold the air near a tea kettle on a stove is, at least untill the water gets fairly hot. Seems like the heat all just migrates to the water.


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