Does a Heat Pump Water Heater Use More Electricity in Winter?

Does a Heat Pump Water Heater Use More Electricity in Winter? Yes, a heat pump water heater can use more electricity in winter, particularly if it takes its heat from cold outside air or an unheated space. As the air temperature falls, the heat pump has less readily available thermal energy to extract and must work harder to raise the water to the required temperature. This increases the temperature lift and generally reduces the heat pump’s Coefficient of Performance (COP). However, the size of the winter increase depends heavily on where the appliance gets its air from, the hot water temperature, household demand and the design of the heat pump water heater.

Why does cold weather affect heat pump efficiency?

An air source heat pump water heater works by extracting thermal energy from air.

Air passes across an evaporator containing cold refrigerant. The refrigerant absorbs heat before being compressed, increasing its pressure and temperature. This energy is then transferred through the condenser into the domestic hot water.

When the incoming air is relatively warm, the compressor needs less electrical energy to achieve the required water temperature.

When the incoming air becomes colder, the difference between the source temperature and required hot water temperature increases. This is known as the temperature lift.

A greater temperature lift generally means a lower COP and increased electricity consumption.

Our heat pump water heater range starts with a compact 100 litre wall mounted air source heat pump for smaller domestic hot water requirements.

For larger households, our floor-standing options include a 200 litre domestic air source hot water heater and 300 litre domestic air source hot water heater.

For commercial and high-demand residential applications, explore the 400 litre commercial domestic water heater or 500 litre commercial domestic water heater.

How does COP affect winter electricity consumption?

COP tells you how much useful heat a heat pump produces relative to the electricity it consumes.

For example, imagine that 6 kWh of heat is required to produce the necessary hot water.

If the heat pump operates at a COP of 3:

6 kWh ÷ 3 = 2 kWh of electricity

If colder operating conditions reduce the COP to 2:

6 kWh ÷ 2 = 3 kWh of electricity

The household receives the same 6 kWh of useful heat, but the heat pump requires 50% more electricity in the second example.

These figures are illustrative, but they demonstrate why the seasonal efficiency of a heat pump is more informative than looking at a single headline COP figure.

Does every heat pump water heater use more electricity in winter?

Not necessarily.

The effect depends significantly on where the heat pump obtains its air.

A heat pump water heater taking air directly from outside will be more exposed to seasonal temperature changes. In the UK, winter air can be considerably colder than summer air, so COP can decrease.

An integrated heat pump installed inside a utility room, plant room, or other suitable space may experience much smaller changes if the room remains at a relatively stable temperature.

However, there is an important consideration. If the heat pump extracts heat from a room that is being heated by the property’s space heating system, some of the energy transferred into the hot water has indirectly come from the heating system.

This should be considered when assessing the overall efficiency of the property rather than looking only at the heat pump water heater’s electricity meter.

Does the hot water temperature matter more in winter?

Yes. Required water temperature and air temperature work together to determine temperature lift.

Producing domestic hot water at 50°C from 20°C air is a much easier operating condition than producing 60°C water from air close to 0°C.

Running a heat pump water heater at unnecessarily high storage temperatures can therefore have a particularly noticeable effect on winter efficiency.

The appropriate setpoint should balance energy efficiency, usable hot water capacity, recovery time and Legionella control, while following the manufacturer’s instructions.

What about defrosting?

Depending on its design and air source, a heat pump may also need to manage frost on the evaporator when operating in cold and humid conditions.

Moisture in the air can freeze on a sufficiently cold evaporator surface, restricting airflow and reducing heat transfer. Heat pumps designed to operate under these conditions may use a defrost cycle to remove the ice.

Energy used during defrosting can contribute to lower overall efficiency during colder conditions.

Not every heat pump water heater will experience the same level of defrost operation, particularly units taking air from warmer indoor environments.

Will the immersion heater run more during winter?

Potentially.

Some heat pump water heaters use an electric immersion heater or auxiliary heating element when the compressor cannot recover the cylinder quickly enough, when operating conditions fall outside the heat pump’s normal range or when a high-temperature cycle is required.

Because direct electric resistance heating effectively has a COP close to 1 at the point of use, frequent immersion heater operation can significantly increase electricity consumption.

Modern high-temperature R290 heat pump water heaters may be able to maintain higher water temperatures over a wider operating range using the compressor, although their COP will still change with operating conditions.

Are heat pump water heaters still efficient during winter?

Yes, a correctly selected heat pump water heater can remain an efficient method of producing domestic hot water during winter. The important point is that its efficiency is not constant throughout the year.

For good winter performance, consider the manufacturer’s operating temperature range, the COP at different air temperatures, the hot water setpoint, the ducting arrangement, and the auxiliary heater controls.

A properly sized system that allows the heat pump to perform most of the water heating should still use substantially less electricity than producing the same amount of hot water entirely with direct electric resistance heating, even though its electricity consumption may increase during colder conditions.