Energy

Designing homes to conserve energy and use it efficiently, from sources that cause least environmental harm.

Heat pump water heating

Heat pumps provide a very efficient method of heating water.

On this page:

  • Advantages and disadvantages
  • Energy efficiency of heat pumps for water heating
  • Heat pump configuration
  • Key design decisions

An air-to-water heat pump takes heat energy from the outdoor air and transfers it to the stored water in a cylinder. It can also use a ground or water (e.g. a river or lake) heat source, but these sources are less common for New Zealand residential use.

 
Heat pump water heater cycle

         

   

Advantages and disadvantages

Advantages of heat pump water heating include:

  • efficient conversion of energy to heat
  • the energy source is always available
  • it can provide energy-efficient heating for large amounts of water for use for space heating (e.g. underfloor), hot tubs and swimming pools
  • it can be used to boost other water heating systems
  • government subsidies may be available.

Disadvantages of air-to-water heat pumps include:

  • loss of efficiency as temperature decreases below 6–7ºC although some will continue to provide heating in temperatures down to -10–15ºC
  • purchase and installation costs are higher than for standard electric storage cylinder systems
  • space is needed to install the exterior unit
  • penetrations have to be made through the building cladding
  • outdoor units may be noisy
  • less efficient with low water use (1 or 2 person households)
  • independent testing has shown that some systems perform significantly better than others, so seek advice from suppliers or independent agencies such as Consumer NZ.

Energy efficiency of heat pumps for water heating

Heat pumps are a highly efficient method of water heating and offer the most efficient electricity-based option, as the electricity is used only to move the heat, not to create it. They can have a coefficient of performance (COP) of between 2.0 and 3.0, which means they create two to three times the energy for water heating than they use in electricity to run the pump and fan. Consequently, they also have lower levels of greenhouse gas emissions than other electricity-based water heating. Some heat pump water heaters include an electrical heating element to supplement the heat pump output in colder conditions.

As noted above, heat pump efficiency, particularly with air-to-water heat pumps, is reduced by low outdoor temperatures.

When considered throughout the year, the energy efficiency of air-to-water heat pumps is comparable to solar water heating. Heat pump water heating is therefore particularly suited to sites with poor solar gain.

Ground-to-water heat pumps have a less variable heat source, but with their high set-up costs and ground area required, they are more suitable to multi-residential developments or very cold climates.

Heat pump configuration

There are two different types of heat pump water heating systems:

  • all-in-one integrated systems, which have the heat exchanger and the storage cylinder in a single unit, located outside
  • split systems, which have the heat exchanger set up separately from the storage cylinder. In this case, the cylinder can be either inside or outside.

Key design decisions

Key design decisions for heat pump water heating will include:

  • whether to use an integrated system or a split system
  • the size and location of the heat pump system.

More information

Updated: 15 August 2014