Energy

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

Water Heating: Heat Pumps

In heat pump water heating systems, a heat exchanger on the outside of the building absorbs heat and transfers it to water in a storage cylinder.

Heat pump water heating is among the most energy efficient water heating options available. Recent BRANZ research has shown that heat pump water heaters can use environmental sources of heat for about 50% of a household’s annual hot water demand, which is comparable to solar water heating.

Heat pump water heating technology has been available for some time, but only recently has become affordable, reliable, compact and quiet enough to be viable.

Advantages of heat pump water heating include:

  • very efficient conversion of energy to heat
  • the energy source is available 24 hours a day, 12 months of the year
  • it can provide energy efficient heating for larger amounts of water, for space heating (eg underfloor) and for use in hot tubs and swimming pools
  • more even energy use throughout the year.

Drawbacks of air-to-water heat pumps are:

  • loss of efficiency as temperature decreases below 6–7 ºC (but some models can still provide heating in temperatures down to —10ºC)
  • 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 weatherskin of the building
  • fan noise in outdoor units can be a problem.

Key design decisions

Key design decisions for heat pump water heating will include:

  • whether to use an all-in-one system or a split system
  • the size and location of the heat pump system.

Booster systems

Some heat pump water systems do not require boosters – as some use an electric element to get water temperature up to 60oC.

Heat pumps can be used to boost other water heating systems, and can be combined with solar water heating to allow maximum energy efficiency in high sunshine periods.

Energy efficiency

As heat pumps only move heat, and do not actually generate it, they have a very high ratio of energy input to heat output. This heating energy efficiency is expressed as a coefficient of performance (COP). Typical domestic heat pumps used for water heating have a COP of 2.0 to 3.0 – which means they create two to three times the energy for water heating then they use in electricity to run the pump and fan (under typical conditions).

As outside temperatures drop, the heat pump’s energy efficiency reduces.

Heat pump water heating capacity

The rate of heat transfer decreases as the outside air temperature drops:

  • at around 6°C outside temperature, the refrigerant in the exterior coils will be below water freezing point – if there is high humidity, then water vapour can condense and freeze on the exterior coils, reducing heat absorption
  • to defrost the coil, the refrigerant flow is reversed for 4 to 10 minutes, the outdoor fan is switched off, and the water is not heated further during this time
  • at very low temperatures, the heating capacity of the heat pump is seriously affected, as it is reversing heat transfer to defrost the coils more often, and so transferring significantly less heat to the water
  • at around –15°C, most heat pump will stop working altogether; some heat pumps cannot function with exterior temperatures below —10°C.

Some systems will reduce outdoor fan speed to delay or even prevent freezing, but heating capacity is significantly reduced.

The Government’s Energy Efficiency and Conservation Agency (EECA) is looking at setting up a standards scheme for heat pumps used for water heating, to match the Energy Rating and Energy Star labels used on other appliances.

Heat pump configuration

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

  • all-in-one systems: these have the heat exchanger and the storage cylinder in a single unit, usually located outside – the noise of the unit may need insulating from occupants and neighbours (see under ‘Heat pump outdoor unit’ in the Space Heating: Heat Pumps section)
  • split systems: these have the heat exchanger set up separate from the storage cylinder.

For your clients

To help your clients make decisions about water heating options, and to help them find ways of reducing their hot water bills, refer them to www.smarterhomes.org.nz/energy/water-heating.