Depending on air for a heat source is always going to be a problem in cold weather. Drilling in deep rock for geothermal is supposed to be easier because the sides don't have to be lined. The heat pump for GSHP can be smaller nominal capacity. I had a 5 kW ASHP which was replaced by a 3 kW GSHP. The electricity use is almost the same but the GSHP produces about three times more heat in cold weather.
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