Geothermal COP Calculator

In Heating mode, the heat pump extracts heat from the relatively cold ground loop and delivers it to the building at a higher temperature, so the ground loop plays the cold role and the delivered air/water plays the hot role in the Carnot formula. In Cooling mode, the heat pump instead extracts heat from the building and rejects it into the ground loop, which now runs warmer as the heat-rejection sink -- so the ground loop becomes the hot reservoir and the delivered cold air/water becomes the cold reservoir.

Carnot COP represents the theoretical maximum possible for a given temperature gap under ideal, reversible thermodynamics -- no real machine reaches it. This calculator applies a commonly cited rule of thumb that real ground-source heat pump compressors achieve roughly 40-60% of their Carnot COP due to compressor inefficiency, heat-exchanger approach temperature losses, and other real-world irreversibilities, and uses 50% as a representative midpoint estimate.

Compressor COP only accounts for the compressor's own power draw relative to heating or cooling output, while System COP divides the same output by TOTAL system power, including the electricity the ground-loop circulating pump consumes. Since the pump adds power draw without adding heating or cooling capacity, folding it in always pulls System COP below Compressor COP alone -- this is why System COP is the more realistic figure for estimating actual operating cost.

Source and Delivery Temperature are independent inputs, so it is possible to enter a combination where the two reservoirs are in the wrong physical order for the mode you selected -- for example, a Delivery Temperature at or below Source Temperature while in Heating mode, which is not a real operating point for a heat pump extracting heat from the ground. Rather than compute a Carnot COP from an invalid temperature order and display a plausible-looking but meaningless number, this calculator reports that the result is not computable for that combination.

Yes -- published figures for ground-source heat pump systems commonly cite a typical System COP in roughly that 3-5 range under normal operating conditions, though actual performance varies by ground loop design, climate, delivered temperature, and equipment quality. This calculator's default source and delivery temperatures land within that commonly cited range, which is a useful sanity check when entering your own system's numbers.