Do I Still Need an Immersion Heater With a Heat Pump Water Heater?

Yes, it is generally advisable for a heat pump water heater to have an immersion heater, even though it should not normally be the main source of heat. The heat pump typically supplies most hot water energy, while the immersion heater serves as backup during high demand and aids in high-temperature or Legionella control cycles. Some modern high-temperature heat pump water heaters can reach 60°C or above using the compressor alone, but an immersion heater can still provide useful resilience.

What does the immersion heater do?

An immersion heater is an electric resistance heating element installed directly into the hot water cylinder.

Unlike an air source heat pump, it does not extract renewable heat energy from the surrounding air. Electricity passes through the heating element and is converted directly into heat, which is transferred to the stored water.

A heat pump water heater operates differently. Its evaporator, compressor, refrigerant circuit and condenser transfer heat from the air into the domestic hot water.

Because the heat pump transfers existing thermal energy rather than producing all its heat through electrical resistance, it can deliver considerably more useful heat per kWh of electricity consumed.

The immersion heater should therefore normally be considered a supplementary heat source rather than the primary method of heating the cylinder.

For details of the different air source heat pump water heaters we offer, visit our dedicated range pages:

Wall hung air source water heaters
Residential air source heat pump water heaters
Commercial air source water heaters

Is an immersion heater needed for Legionella cycles?

It depends on the heat pump.

Many heat pump water heaters operate most efficiently at normal domestic hot water temperatures of around 50°C to 55°C. Where the system’s water hygiene strategy requires a periodic higher-temperature cycle, an immersion heater may be used to raise the stored water temperature further.

For example, the compressor might heat the cylinder to 50°C or 55°C before the immersion heater raises it to approximately 60°C or another temperature specified by the manufacturer.

However, this is not necessary with every appliance.

Some modern R290 heat pump water heaters can achieve 60°C or higher using the refrigeration cycle alone. In these systems, the immersion heater may not normally be required to achieve the high-temperature cycle.

The manufacturer’s instructions and the appropriate Legionella control strategy for the installation should determine how this function operates.

Can the immersion heater provide backup hot water?

This is one of its biggest advantages.

If the heat pump develops a fault, the compressor cannot operate, or environmental conditions prevent normal heat pump operation, the immersion heater can provide an alternative means of heating the cylinder.

This can be particularly useful in a domestic property because a refrigeration circuit fault may require a specialist engineer. With a functioning immersion heater, the household can potentially retain a hot water supply while the heat pump is inspected or repaired.

The immersion heater can therefore provide valuable backup and redundancy.

Can an immersion heater improve hot water recovery?

Yes. Some heat pump water heaters have a boost or rapid heating mode that operates the heat pump and immersion heater together.

A heat pump may have a relatively modest heating output compared with a large gas boiler or powerful electric heater. Consequently, reheating a large volume of stored water can take longer.

If a household unexpectedly uses most of its stored hot water, activating the immersion heater can reduce the recovery time.

However, frequent reliance on boost mode may indicate that the hot water cylinder is undersized, the temperature settings need adjustment or the heat pump has not been correctly matched to the household’s hot water demand.

Does using an immersion heater make a heat pump less efficient?

The immersion heater itself does not reduce the efficiency of the refrigeration cycle, but excessive use can significantly increase the overall electricity consumption of the hot water system.

An electric immersion heater provides approximately 1 kWh of heat from 1 kWh of electrical energy at the point of use.

By comparison, a heat pump operating at a Coefficient of Performance (COP) of 3 could provide approximately 3 kWh of useful heat for 1 kWh of electricity under the specified operating conditions.

It therefore makes economic sense to allow the heat pump to perform as much of the everyday water heating as practical.

Do high-temperature R290 heat pumps still need immersion heaters?

A high-temperature R290 heat pump may be capable of producing domestic hot water at 60°C or considerably higher without electric resistance heating.

That reduces the need for immersion heater assistance, particularly during high-temperature cycles.

However, high-temperature capability does not remove the benefits of having an independent backup heat source. An immersion heater can still provide emergency hot water if the compressor or refrigerant circuit is unavailable.

Should you remove the immersion heater?

In most installations, there is little advantage in removing an immersion heater simply because a heat pump is capable of heating the water on its own.

The preferred arrangement is normally to use the heat pump as the primary heat source and retain the immersion heater for backup, boost heating or high-temperature operation where required.

Correct controls are particularly important. A poorly configured system that unnecessarily operates the immersion heater every day can undermine much of the energy-saving benefit of a heat pump water heater.

A well-designed system therefore prioritises the heat pump for normal domestic hot water production while keeping the immersion heater available when it is genuinely needed.