Do Heat Pumps Work in Cold Weather? A UK & Ireland Winter Guide
Do Heat Pumps Work in Cold Weather? A UK & Ireland Winter Guide
One of the most common concerns about air source heat pumps is whether they can continue heating a home when outdoor temperatures fall.
The answer is yes. Modern air source heat pumps are designed to work during cold weather and can continue operating when temperatures fall below freezing.
Cold outdoor air still contains thermal energy. A heat pump extracts this energy, increases its temperature and transfers it into the water used by your heating system. As the outdoor temperature falls, the heat pump may need to work harder, but a correctly sized and properly designed system can continue supplying heating and hot water throughout winter.
How does a heat pump extract heat from cold air?
An air source heat pump does not create heat by burning fuel. Instead, it transfers heat that is already present in the outdoor air.
The process works in four main stages:
- Outdoor air passes across an evaporator containing refrigerant.
- The refrigerant absorbs thermal energy from the air.
- A compressor increases the pressure and temperature of the refrigerant.
- The collected heat is transferred into the water supplying radiators, underfloor heating or a hot-water cylinder.
Even when the air feels cold to us, it still contains usable thermal energy. This is why air source heat pumps can continue operating at temperatures below 0°C.
Do air source heat pumps lose efficiency in cold weather?
Air source heat pumps continue to work in winter, but they do not necessarily operate at exactly the same efficiency at every outdoor temperature.
As the temperature outside falls, the difference between the outdoor air temperature and the required heating-water temperature becomes greater. The compressor must therefore work harder to raise the temperature of the refrigerant.
This can result in:
- Increased electricity consumption during colder periods
- A reduction in the heat pump’s coefficient of performance
- More frequent defrost cycles in cold and damp conditions
- Longer operating periods to maintain the required indoor temperature
This does not mean the heat pump has stopped working. It means the system is using more energy to collect and transfer heat from colder air.
Government guidance states that heat pumps can typically produce around three units of heat for every unit of electricity used, although actual performance varies according to outdoor temperature, flow temperature, property heat loss and system design.
When comparing heat pumps, it is useful to look at the Seasonal Coefficient of Performance, or SCOP. This represents the system’s estimated efficiency across an entire heating season rather than under one set of laboratory conditions.
What happens to a heat pump in freezing conditions?
During cold and humid weather, moisture in the air can freeze on the outdoor heat exchanger.
A light coating of frost is normal. However, if too much frost builds up, it can restrict airflow and reduce the amount of heat the unit can extract.
Modern air source heat pumps manage this using an automatic defrost cycle.
During defrost mode, the system temporarily directs heat towards the outdoor heat exchanger. This melts the frost, allowing the water to drain away before the unit returns to normal heating operation.
You may notice the outdoor fan stop temporarily, water draining beneath the unit or what appears to be steam rising from it. These are generally normal signs that the defrost cycle is working. Mitsubishi Electric explains that frost can form even when temperatures are slightly above freezing, particularly during cold and damp conditions.
How often does a heat pump defrost?
There is no fixed defrost schedule.
The frequency can be affected by:
- Outdoor temperature
- Humidity levels
- The amount of moisture in the air
- Airflow through the unit
- Current heating demand
- The design and controls of the selected heat pump
Cold, damp weather around freezing can sometimes produce more frost than considerably colder but drier conditions. The system should automatically monitor the heat exchanger and begin a defrost cycle when required.
At what temperature does a heat pump stop working?
There is no single temperature limit that applies to every heat pump. The permitted operating range depends on the manufacturer, model, refrigerant and system design.
Mitsubishi Electric states that its Ecodan R290 range has a guaranteed operating range down to -25°C ambient temperature. Its 14kW R32 Zubadan model is designed for low-temperature operation. Exact heating output and operating limits should always be checked against the technical data for the individual model.
This distinction is important: a heat pump’s minimum operating temperature is not necessarily the temperature at which it maintains its full rated output or highest efficiency.
Are heat pumps suitable for winters in the UK and Ireland?
Modern air source heat pumps can be suitable for typical winter conditions across the UK and Ireland when they are correctly sized and designed for the property and local winter conditions.
However, the outdoor temperature is only one part of the calculation. The quality of the system design can have a greater effect on comfort and running costs than the weather alone.
A heat pump should be selected using an accurate room-by-room heat-loss calculation. This establishes how much heat the property requires during the expected winter design temperature.
If a heat pump is undersized, it may struggle to meet demand during colder periods. An oversized system may cycle unnecessarily and operate less efficiently. Correct sizing is therefore essential.
What affects heat pump performance during winter?
Property heat loss
A property that loses heat quickly requires more energy to maintain a comfortable temperature.
Insulation, glazing, airtightness, exposed walls and property size all affect the heating demand. A property does not need to be perfectly insulated before using a heat pump, but reducing unnecessary heat loss can improve comfort and lower running costs.
Heating flow temperature
Heat pumps generally operate more efficiently when producing lower-temperature water.
Underfloor heating is particularly well suited to heat pumps because it distributes heat across a large floor area. Appropriately sized radiators can also provide the required output using lower flow temperatures. Larger or higher-output radiators may be recommended in some rooms.
System sizing
The heat pump must be selected according to the calculated heating load, rather than property floor area alone.
Floor area can provide an initial indication, but construction type, insulation, glazing and room temperatures must also be considered when completing a detailed design.
Weather compensation
Weather compensation adjusts the heating-water temperature according to the outdoor conditions.
During milder weather, the system can reduce the flow temperature to improve efficiency. When temperatures fall, it can increase the flow temperature to meet the higher heating demand.
Outdoor unit position
The outdoor unit requires suitable clearances so that air can move freely through the heat exchanger.
The installation should also allow defrost water to drain safely. Restricted airflow, poor drainage or an unsuitable location can reduce winter performance.
Heating controls
Heat pumps usually work best by maintaining a steady indoor temperature rather than repeatedly heating a property from cold.
The control strategy should be configured for the property, heating emitters and household routine.
Do heat pumps need backup heating in winter?
Some heat pump systems incorporate an electric backup or boost heater. This can support the system during particularly demanding conditions or assist with hot-water operation.
Whether backup heating is required depends on the selected unit, system design and property heat loss. A properly designed system should be assessed against the expected winter heating demand rather than relying on backup heat to compensate for poor sizing.
How can you support heat pump performance in cold weather?
To help an air source heat pump operate effectively through winter:
- Keep the outdoor unit’s required airflow clear.
- Ensure snow, leaves or other debris do not obstruct the heat exchanger.
- Allow defrost water to drain away correctly.
- Do not attempt to scrape frost from the heat exchanger.
- Use the recommended weather-compensation settings.
- Avoid making frequent, extreme changes to the thermostat.
- Arrange servicing and maintenance in accordance with the manufacturer’s guidance.
- Contact a qualified heating professional if heavy ice remains after the defrost cycle or the system displays a fault.
A temporary coating of frost, water beneath the unit or a short plume of vapour during defrosting does not normally indicate a problem.
Choosing an air source heat pump for cold weather
The right heat pump cannot be selected from outdoor temperature alone.
The design should consider:
- The property’s calculated heat loss
- The required outdoor design temperature
- Heating flow and return temperatures
- Radiator or underfloor heating outputs
- Domestic hot-water demand
- Available electrical supply
- Outdoor unit position
- Manufacturer performance data at low temperatures
BPC Heat supplies Mitsubishi Electric air source heat pumps, cylinders, controls and heating accessories for projects across the UK and Ireland. Its current range includes R32 monobloc units and Mitsubishi Zubadan models.
For initial sizing and pricing, the BPC Heat Instant ASHP Quote tool provides an indicative pump and cylinder recommendation. For a project-specific system, property plans and heating requirements can be submitted through the BPC Heat design service.
