Q. A customer wants me to install one of the new heat-pump water heaters in the ground-floor utility room of a house built on a slab. I don’t have any direct experience with them, but since they work by drawing heat from the surrounding air, won’t any energy savings in the winter be at least partly offset by the cost of heating that indoor air to begin with? And could the outflow of cool air from the utility room cause comfort problems?
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.
2. The HPWH is in the thermal envelope of a direct-gain passive-solar house with a wood-stove backup. Again, the heat pump is operating in a favorable temperature regime, and the source of the heat is either the sun or firewood. If the space would otherwise be overheated in winter – a common problem in passive-solar spaces on sunny winter days – incidental cooling may not be objectionable. This is a good application.
3. The HPWH is in the thermal envelope of an electrically heated house. Each unit of energy removed from the air is replaced by electric resistance heat. This would not be a good choice.
4. The HPWH is in the thermal envelope of a house heated with a minisplit heat pump that itself operates at a COP of 2.5. The COP of the HPWH is effectively reduced from 2 to 1.4 because the minisplit must work harder to offset the cooling effect of the HPWH. This improves to 1.7 if the house is in heating mode for six months of the year and the cooling effect of the heat pump is negligible or welcome during the remaining six months. In general, the more the climate shifts toward being cooling-dominated, the better the HPWH looks in this application.