A.Marc Rosenbaum, an energy consultant in West Tisbury, Mass., responds: If the heat-pump water heater (HPWH) is operating with a coefficient of performance (COP) of 2 - meaning that it takes one unit of energy from the air for every unit of electrical energy it consumes - then half of the energy that goes into the water comes from the heated air in the living space. Whether this makes economic sense in a given situation will depend on where that energy came from in the first place. Here are some possible scenarios in heating-dominated climates:

1. The HPWH is in a basement with a gas furnace and leaky uninsulated ducts that keep the basement at 70°F. The HPWH is operating efficiently because it is taking heat from nice warm air, and that heat is only indirectly getting to the living space. This is probably a good application.

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