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It has bugged me for a while. If the earth is a constant 50-60-odd degrees several tens of feet beneath ground, and houses (I assume; ours at least does) don't drop down to lower than those temperatures often, how come a geothermal unit isn't just using supplemental electrical resistance heating (and not the ground loop heat source?) all the time?

Is it because the above ground equipment is actually a heat pump (extracting more heat out of the circulated liquid than the temperature it comes out of the ground at)?

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    $\begingroup$ Exactly. If you understand how an air heat pump works, then you understand how a geothermal heat pump works. The only difference is that in the former, the working medium (where the heat is being extracted from) is outside sir, while in the later, the working medium is a fluid that is circulated through the ground. $\endgroup$ – SteveSh Apr 16 at 15:27
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    $\begingroup$ Quite agree @depperm. This is a "how things work" question not a home improvement problem. $\endgroup$ – Harper - Reinstate Monica Apr 16 at 16:01
  • $\begingroup$ @depperm & SteveSh & Reinstate Monica - Wouldn't be opposed to the move. I believe flagged the question with notes to this effect will get this moved, no? $\endgroup$ – user66001 Apr 17 at 4:37
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Barring a convenient "hot spring" location, "geothermal heat" is, indeed, shorthand for "geothermal heat-pump heat" and typically takes in water at common ground temperatures in the 50F/10C range, drops that water a few degrees to extract heat and sends it on its way, whether back through a loop, out to a pond, or down a different well than it was pumped from.

This is much less difficult than the job of cold-climate air-source heat pumps, which have to work over a much wider input temperature range, and need the most heat output just when it's hardest to extract heat from very cold air.

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  • $\begingroup$ Some have a water loop buried underground so the water just circulates continuously. The temperature 2 metres below ground level is fairly constant all year round compared to the surface. So this can provide heating in winter and cooling in summer. Also check out "puit canadienne" where incoming air for ventilation is taken below ground for heating / cooling depending on the season. $\endgroup$ – Solar Mike Apr 17 at 10:22
  • $\begingroup$ @SolarMike - Thanks for your reply. Think Ecnerwal covered the water (think it might actually be glycol, or some substance that has a lower freezing temperature than water) loop in his answer "...whether back through a loop...". Thanks for introducing me to puit canadienne. Interesting concept. Wonder if there is anyone in the DC tri-state area of America that could quote on installation of such a system, though I guess even if cheaper upfront it wouldn't be as efficient as a geothermal heat pump system, so would cost more over time. $\endgroup$ – user66001 Apr 17 at 13:58

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