HPWHs should be installed in an indoor space—a garage, utility room, or basement—of at least 1,000 cubic feet (equivalent to a 10- by 12.5-foot room with an 8-foot ceiling). Since the HPWH is extracting heat energy from that space, the room will get cooler. The larger the space, the less impact the HPWH will have on the room’s temperature. The warmer the space (up to about 120°F), the better the HPWH’s performance will be. HPWHs are great at scavenging waste heat from washing machines and dryers, furnaces and boilers, and/or wood stoves.
However, if the space is too small, the HPWH will cool a room too much and the unit will operate at a reduced COP. Forced-air circulation can make it possible to use an HPWH in a smaller space, but if you are operating a blower to feed the HPWH, that energy use will reduce the overall energy savings of the HPWH. Circulating cooled air from an HPWH to cool the rest of your home in the summer can be a beneficial strategy. Some manufacturers will offer ducting hardware that allows you to install a small amount of ducting to move the heat-pump air around. If the ducting runs are too long, however, an additional blower would be needed to avoid overwhelming the unit’s blower. (The Geyser —see below—offers a ducting takeoff.)
An HPWH’s lowest operational temperature varies depending on the room’s relative humidity and temperature. HPWHs can operate in unconditioned spaces, like garages and back porches, with some caveats. The coils of the HPWH can become frosted depending on the air temperature and humidity. This energizes a defrost cycle, as in most modern refrigerators, but will compromise the HPWH’s efficiency.
Since the heat pump also dehumidifies the air, it needs a drain for the condensate produced. You can run it into a floor drain or save the condensate, which is basically distilled water.
HPWHs are factory-sealed units and are similar in complexity to room air conditioners. Most lower-cost units carry a one-year full warranty. Beyond that, warranties vary by model and manufacturer. HPWHs are complex mechanical devices that need to be serviced by an experienced professional, and most limited warranties do not cover labor expenses.
HPWHs can be larger than typical electric water heaters. Before you buy, make sure the unit will fit in the space where you plan to install it. Stand-alone HPWHs, with their separate storage tanks, require more floor space than all-in-one units.
All commonly available residential HPWHs deliver less heat per hour than a conventional electric DHW tank. This can be a disadvantage for high-volume hot water users. However, having a larger storage tank can minimize this problem. The amount of hot water from any tank-style water heater is limited to the tank’s volume and the heater’s recovery time. Once the storage is used up, expect a longer recovery time for an HPWH compared to a conventional tank-style water heater. Most HPWHs can deliver 6,000 to 8,000 Btu per hour. Recovery time is contingent on tank size and temperature settings. In general, HPWHs take twice as long to recover as a tank-style electric water heater. If you want to supply hot water to a washing machine and a dishwasher at the same time you’re taking a half-hour shower, an on-demand gas- or oil-fired system will likely be necessary—or you’ll need to have a very large storage tank.
Some companies, such as Nyle Systems, offer higher-output commercial HPWHs, which extract more heat from the room. This works well for commercial laundry rooms or laundromats that have rather high room temperatures and humidity for the HPWH to extract heat from.
HPWHs are not silent operators. They emit up to the same sound levels as a room air conditioner or dehumidifier. If you have an HPWH installed in a small house, expect to hear it running.