Heat pump water heating

Heat pumps are a popular way to heat New Zealand homes and offices, and the same technology can also be used to heat water. Heat pump water heaters are generally a more efficient way to heat water than standard electric cylinders.

Heat pump water heaters use energy from the outdoor air or ground to heat water, which is stored in an insulated hot water tank.

What's good about heat pump water heaters

• A well-designed and installed system uses less energy and produces fewer greenhouse gas emissions than standard electric cylinders.

• If you're also considering solar water heating, heat pump water heaters can work in places that aren't good for solar water heaters - places that get less sun, like the south side of hills.

• If cheaper night-rate electricity is available in your area, some models can take advantage of it. Check with your electricity supplier and, if it's available, get your cylinder sized so you can use it.

What to be aware of

• Some systems are better designed for operating in cold climates than others - check with your supplier or manufacturer.
• Some systems use an electric booster or backup element in cold conditions. These can be more expensive to run.
• The upfront cost of heat pump water heating is relatively expensive.
• In a minority of cases you may need a building consent to install a heat pump water heater. Check with your council. [link to local council directory]

On this page:

• How heat pump water heaters work
• Considerations when buying a heat pump water heater
• Consumer NZ heat pump water heating report

How heat pump water heaters work

A refrigerator moves heat from inside the fridge to the outside - keeping the inside cold. A heat pump water heater uses the same principle in reverse. It transfers heat from outside air into a hot water tank.

The outside air gets heated up by energy from the sun all year round. So even when the sun is not shining brightly, or it is the middle of winter, there is still solar heat energy in the air that is available for use.

Heat pump systems only use electrical energy to ‘move' the heat - they don't actually ‘make' heat. This makes heat pump systems much more efficient than traditional electric or gas water heaters.

Some systems incorporate an electric element booster, for when the air is too cold for their heat pump to extract heat efficiently, while other systems can operate at very low temperatures and still produce enough hot water at good efficiencies. There are also systems that are designed to be used only in warmer areas.

The actual efficiency that you would get from a heat pump water heater depends on the make of the system, the quality of the installation, the average temperatures where you live and the location of the compressor unit.

Considerations when buying a heat pump water heater

Types of heat pump water heaters

There are two different types of heat pump water heater:

• split systems; and
• all-in-one units.

Split systems have the compressor unit outside and the hot water tank generally inside although the tank can also be located outside the house separate from the compressor. In some cases you can use your existing tank.

All-in-one units have the compressor and tank together and the whole system usually sits outside or in a utility space, like a garage.

Optimal conditions for heat pumps

Generally, the hotter the outside air, the better the heat pump runs. Heat pump water heaters tend to work best in areas with average air temperatures above about 7° Celsius. But most will still run more efficiently than a traditional electric or gas water heater in below-zero temperatures.

If you live in a colder part of the country, ask your manufacturer if their system is designed to work at low temperatures. Ask to see performance results at different temperatures for the model you are thinking of buying. If the heat pump won't operate in colder temperatures, then the system may rely on the standard electric element so you won't get the energy savings you've invested in or it may not be suitable for your location.

Other uses for heat pump water heaters

Some heat pump water heating systems can also give you ‘central heating'. They do this by pumping warm water from the hot water tank around a system of under-floor heating pipes.

Some other systems can be used to heat a hot tub, spa or swimming pool more cheaply than using an electric heater.

Consumer NZ heat pump water heating report

In April 2009 Consumer NZ (with the support of EECA) carried out tests on the performance of six heat pump water heaters suitable for household use.
The complete report can be accessed free of charge from the Consumer NZ website.

More information

Choose the right water heating system
Simple ways to save on hot water