Heat pumps are a well-established technology for space heating. The same principle of transferring heat is at work in heat pump water heaters (HPWHs) except that the heat they extract from air is delivered to water. An ENERGY STAR qualified HPWH uses about half as much electricity as a conventional electric resistance water heater.

Heat Pump Water Heater Efficiency
ENERGY STAR qualified heat pump water heaters can save the average household almost $300 per year on its electric bills compared to standard electric water heaters. HPWHs typically have more of an up-front cost, but the savings will usually pay for the unit within 5 to 7 years.

Two terms are used to describe the performance of HPWHs:

• The heating coefficient of performance (COP), which is the ratio of heat energy of the HPWH to the electrical energy input when both are in consistent units.
• The Energy Factor (EF), which is the ratio of heat output to energy input as measured during a specific 24-hour test procedure. A conventional electric resistance water heater EF would ideally be 1, but is slightly lower because of tank standby losses (heat loss through piping and tank walls). Testing procedure and calculations to determine EF are the same for all heater manufacturers.

HPWH performance varies, but in typical usage, HPWHs tend to operate with an EF around 2 to 2.5. This means that they heat over twice as much hot water as you would get from an electric resistance water heater with the same electrical energy input. Most HPWHs on the market are hybrid models that primarily operate as a heat pump, but also have a heating element that can be cycled on to heat water very quickly if needed.

Example

Water heating in a residence varies with family size, but a family of four may use an average of 60 gallons per day. This would require 12 kWh per day of electric resistance (EF of 0.86) water heating (from 55°F to 120°F).

An exhaust air heat pump water heater rated with an EF of 2.5 would provide the same amount of hot water for 4.8 kWh/day – a savings of 2,628 kWh/year.

Who Can Benefit Most from
Heat Pump Water Heaters?

HPWHs are best considered for the following situations:

• Buildings with large, steady requirements for hot water, such as residences occupied by larger families (4 or more persons)
• Buildings with a fairly steady need for air conditioning most of the year, such as in warm climates
• Where electric resistance water heaters are presently used
• In mild/cool climates when heat pumps are used for space conditioning
• In mild/cool climates where there is a need for continual mechanical ventilation, such as for humidity/moisture control
• In mild/cool climates where the water heater can extract heat from a large, unconditioned basement or crawl space

Cautions:

• HPWHs should not be installed in un-ventilated closets or small rooms inside a residence unless a ducted air supply can be provided.
• Exhaust air design HPWHs should not be used near open-flame devices (gas stoves, fireplaces) unless those devices have dedicated makeup air supply.
• HPWHs are not recommended for installation in outside or unconditioned spaces where the potential for freezing conditions exists.