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RagnarHairybreeks-
By the (Carnot efficiency) numbers:
Stove TH ~600K (exhaust gas temperature), TC ~290K (intake air temperature)
1-(290/600)=0.51
If you're not going to steal from the "house" side (which can't work, because you'd have to increase the refrigerant compressor power consumption more than the work you could get out of a refrigerant expansion turbine, to avoid violating the 2nd law), the temperature differential available from a ground source heat pump is at maximum
TH ~280K (soil temperature), TC ~260K (refrigerant compressor intake)1-(260/280)=0.07
gbj_tester
I'm guessing it's because electricity on tap is cheap, while making a generator to drive the compressor and fan in a heat pump is expensive. It's not easy to get motive power out of a system with such a low temperature differential, since the maximum theoretical efficiency of a heat engine is governed by the difference in temperature between the hot and cold sides.