A rule of thumb for GSHP is 2 gallons per minute per ton of heat. Before my heat pump was installed, the installer ran all the water sources in my house (faucets, showers, etc) for half an house to see if pressure dropped at all. It didn't, so I have a 3.5 ton unit, uses about 7 gals per minute when running, which can be more than 50% of the time on a cold day.
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