High Efficiencty Heat Pump Water Heaters

Overview

A high efficiency heat pump water heater uses a heat pump (essentially an air conditioner) to pull heat from the air and transfer it to the water. To supplement heating, elecric heating elements are also installed for periods of high demand or when the surrounding air is too cool for efficient water heating (below 45°F). As a secondary result, the air in the surrounding room is also cooled and dehumidified. This unit is especially effective in hot humid enviroments. Even though we may normally think of air conditioners as large power users, these units are still over 60% more efficient than a conventional electric water heater, because the unit (operation in heat-pump mode) is mearly moving heat from the room to the water instead of creating it.

While energy costs are lower than any other water heater type (except solar), initial costs are higher than most other water heaters (also, except solar - which has considerably higher installation costs), but the heat pump's efficiency generally pays for itself in a few years and over a course of 13 years (the typical life span of a water heater) the high efficiency heat pump units provide the lowest total cost (considering installation and operation costs) of any type (including solar). For a cost summary see this page: http://www.aceee.org/Consumerguide/waterheating.htm provided by the American Council for an Energy-Efficient Economy.

These units are probably not practical replacements for a gas fueled water heater because they require a 240v power supply (which is probably not available near a previous gas water heater installation - as there would have been no need for it), and the hot water recovery rate for a gas water heater is greater than the same size electric water heater.

How does a Heat Pump Water Heater Work?

The heat pump system contains a fan that forces air through an evaporator (1). The evaporator contains a liquid refrigerant. This refrigerant evaporates and extracts heat from the ambient air.

The refrigerant then passes through the condenser (3), which in this case is wrapped around the water tank. The hot refrigerant loses its heat which goes into the DHW.

The now warm gaseous refrigerant is then compressed (2) by the compressor which is driven by an electric motor. As it goes through the compressor the pressure and temperature rises. The refrigerant turns back into a liquid which is now hot.

The now cooler refrigerant then passes through an expansion valve (4), where it goes back into a gaseous state and the process begins anew.

The US Department of Energy has this to say about the energy efficiency of these units:

The energy factor (EF) indicates a water heater's overall energy efficiency based on the amount of hot water produced per unit of fuel consumed over a typical day. This includes the following:

  • Recovery efficiency – how efficiently the heat from the energy source is transferred to the water
  • Standby losses – the percentage of heat loss per hour from the stored water compared to the heat content of the water (water heaters with storage tanks)
  • Cycling losses – the loss of heat as the water circulates through a water heater tank, and/or inlet and outlet pipes.

The higher the energy factor, the more efficient the water heater. However, higher energy factor values don't always mean lower annual operating costs, especially when you compare fuel sources.

Product literature from a manufacturer usually provides a water heater model's energy factor. Also, see the Product Information resources listed on the right side of this page (or below if you've printed it out).

Don't choose a water heater model based solely on its energy factor. When selecting a water heater, it's also important to consider the following: Size, Fuel type, Overall cost, First hour rating

Products

There are two competing products that I recommend for this application that have the highest energy factor ratings": Others are also available.

GE Geospring Hybrid Water Heater

Stiebel Eltron Accelera 300 heat pump water heater

Additional Resources

US Department of Energy publication: Residential Heat Pump Water Heaters (PDF document)

Californial Energy Commission report on water heaters