Heat pumps use electricity far more efficiently than ordinary electric water heaters. They are usually used to heat water.

They work by extracting heat from the air outside, using a process that's like a refrigerator working in reverse.

Heat pumps provide a very efficient method of heating water.

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 Indian 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 vaporises 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 of heat pump water heating include:

• efficient conversion of energy to heat (Upto 70% Electricity Savings)
• the energy source is always available
• it can provide energy-efficient heating for Residential or 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

Disadvantages of air-to-water heat pumps include:

• loss of efficiency as temperature decreases below 4ºC although some will continue to provide heating in temperatures down to -10–15ºC

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 3.0 and 5.0, which means they create multiple 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 slightly 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
• 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.

Heat pump configuration / Types

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.
• However Splits systems are more efficient and durable as compared to all-inone integrated systems.

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.

Installation

A building consent (Centralise hot water Plumbing Provisions) 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 centralise water storage heater with a 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.