An increasing percentage of green builders are choosing to install heat-pump water heaters — appliances that use an air-source heat pump to extract heat from the air, transferring that heat to water stored in an insulated tank. Most heat-pump water heaters incorporate both of these components — the heat pump and the insulated tank — into a single unit, with the heat pump compressor mounted above the insulated tank. This type of water heater is typically installed in a basement, mechanical room, or (in warm climates) in an attached garage.
If you look at the heat pump compressor mounted on top of a typical heat-pump water heater, you’ll notice that there are two openings for air flow. The unit has a fan that pulls ambient air through the intake. Inside the unit, this air passes over a copper heat-exchange coil and transfers some of its heat to the refrigerant flowing through the coil. Then the air, now cooled, is expelled from the exhaust side of the appliance.
This process raises the temperature of the refrigerant and, eventually, the water in the tank. The process also cools the air in the room where the heat-pump water heater is located. If a heat-pump water heater is installed in a very small room — a closet or a tiny mechanical room, for example — the room can get so cold that the heat pump can’t operate properly. That’s why the manufacturers of heat-pump water heaters dictate the minimum volume (generally, 1,000 cubic feet) of the room in which this type of appliance can be installed.
The larger the room, the lower the chance that the room’s air temperature will sink to a level that is too low for efficient operation of the water heater.
Some manufacturers of heat-pump water heaters allow installers to connect these…