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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.
The drawing shows specifically how the heat transfer is done. Outdoor air passes over an evaporator (heat exchanger), transferring heat to a liquid refrigerant. This vapourises the refrigerant, which then enters the compressor. The compressor uses electrical energy to increase the refrigerant pressure (and temperature) and circulate it around the condenser (heat exchanger) where the refrigerant cools as it heats the water. The condensed liquid refrigerant then passes through an expansion valve, and the cycle continues.
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. 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.
To enhance efficiency, the heat pump unit could be operated when:
- outdoor temperatures are warmer (such as during the afternoon), which improves the performance of the system
- the price of electricity is lower (such as a night-rate tariff)
- surplus electricity is available from a photovoltaic system that would otherwise be exported to the electricity grid.
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.
A building consent is generally required when a new heat pump water heating system is installed or a new split heat pump is added to an existing water storage heater.
A building consent is not required where a certifying plumber:
- replaces an existing water storage heater with a heat pump water storage heater
- replaces and repositions an existing internal water storage heater with an external heat pump water storage heater
- replaces an existing external water storage heater with an external heat pump water storage heater.
The outdoor components must be positioned to allow unimpeded airflow through the system. Ideally, the location should be warm. The outdoor components may switch on at irregular times, so to avoid them becoming a nuisance at night they should not be located close to bedrooms.
The hot water tank, whether internal or external, should be restrained with straps to suitable anchoring points on the building. Acceptable Solution G12/AS1 gives restraint methods suitable for tanks up to 360 litres. Straps at top and bottom should be no more than 100 mm from the end of the tank. Tanks over 200 litres should have a third strap in the centre.
An isolation switch must be mounted on the exterior of the house. If the cable breaks away, the electricity supply to the cable can be shut off from the isolation switch.
All hard wiring must be carried out by a registered electrician and have an electrical certificate of compliance.
CO2 heat pumps
While heat pumps are an energy-efficient option for water heating, there is a side to them that is less than environmentally friendly – many of them slowly leak refrigerant. In practical terms, just 1 kilo of a commonly-used hydrofluorocarbon refrigerant released into the atmosphere has the same effect as 1.3 tonnes of CO2.
Air-to-water heat pumps are now available with technology that vastly reduces the problem – they use CO2 as the refrigerant. CO2 has a global warming potential (GWP) of 1, while a traditional refrigerant used in some heat pump water heaters in New Zealand – R-134a (tetrafluoroethane) – has a GWP of 1300.
- www.smarterhomes.org.nz – water heating
- www.genless.govt.nz – water heating systems
- www.building.govt.nz – heat pump water heaters
Updated: 14 September 2020