How Do Heat Pumps Work in Winter?

Heat pumps do work in winter. They can extract heat from outdoor air or the ground even when it feels cold outside, then raise that heat to a useful temperature for the home. The important caveat is that winter performance depends on correct design, suitable radiators or underfloor heating, good controls and a home that does not lose heat too quickly.

The common myth is that there is no heat available when the air temperature drops below freezing. That is not how heat pumps work. There is still heat energy in cold air. A heat pump uses refrigerant, compression and a heat exchanger to collect that low-grade heat and move it indoors.

Key Takeaways

• Modern air source heat pumps can work below freezing, and many are designed to operate in temperatures far colder than typical UK winter conditions.
• Efficiency usually falls as outdoor temperatures drop because the heat pump has to work harder.
• Defrost cycles are normal in cold, damp weather and stop ice building up on the outdoor coil.
• Winter comfort depends heavily on heat loss calculations, radiator sizing, flow temperature and controls.
• A heat pump usually works best with steady operation rather than short high-temperature bursts like a boiler.

How Can A Heat Pump Get Heat From Cold Air?

Cold air still contains heat energy unless it reaches absolute zero, which is far below any weather conditions on Earth. An air source heat pump uses a refrigerant that can absorb heat at very low temperatures. The refrigerant evaporates, is compressed to raise its temperature, then releases heat into the home’s heating system.

Ground source heat pumps use the same broad principle but collect heat from buried pipework. Because ground temperature is steadier than air temperature, ground source systems can be less affected by cold snaps, although they cost more to install.

What Happens To Efficiency In Winter?

Heat pump efficiency is often described using COP, or coefficient of performance. A COP of 3 means the heat pump delivers three units of heat for each unit of electricity used. In colder weather, COP usually drops because the system has to lift heat from a colder source to the required heating temperature.

That does not mean the heat pump stops working. It means running costs can rise during the coldest periods. The whole-season performance is what matters, not one cold hour.

The Guardian has reported on research showing that heat pumps can perform effectively in freezing conditions, and countries with colder winters than the UK use them widely. The UK problem is rarely “winter is too cold”. It is more often poor design, high flow temperatures or weak insulation.

Why Defrost Cycles Happen

In cold damp weather, moisture can freeze on the outdoor coil of an air source heat pump. Ice blocks airflow, so the unit runs a defrost cycle. During defrost, the system briefly reverses or redirects heat to melt ice from the coil.

Defrost is normal. You may notice steam, water draining from the unit or a temporary change in operation. What is not normal is constant icing that never clears, water pooling dangerously around the unit, or repeated fault codes.

Grant UK notes that air source heat pumps can continue operating in cold weather and use features such as defrost cycles and backup elements to maintain operation. That is why outdoor unit location and drainage matter during installation.

Defrost should be occasional and controlled. If the unit is repeatedly icing over, surrounded by standing water or struggling to clear itself, the installer should check airflow, drainage, refrigerant operation, sensor readings and whether the unit has enough clearance around it.

Will Radiators Feel As Hot As A Boiler?

Often, no. Heat pumps usually run at lower flow temperatures than gas boilers. Instead of heating radiators very hot for short periods, they tend to run more steadily at lower temperatures.

This can feel different at first. A radiator may be warm rather than scorching, yet the room can still reach the desired temperature if the radiator is correctly sized. If radiators are too small for low-temperature operation, the heat pump may have to run hotter and less efficiently.

Our guide to radiator sizing explains why emitter output matters so much with low-temperature systems.

Do Heat Pumps Need Backup Heating?

Many systems include backup immersion or electric support, especially for hot water or extreme conditions. Backup heat should not be the main heat source for normal winter operation. If it runs often, the system may be undersized, poorly controlled, badly commissioned or struggling with the building’s heat loss.

For domestic hot water, immersion support may also be used for periodic high-temperature cycles. That is different from relying on direct electric heating every day because the heat pump cannot keep up.

Outdoor Unit Position In Bad Weather

The outdoor unit needs space to breathe. Snow, leaves, bins, fencing, dense shrubs and poorly designed covers can all restrict airflow. If air cannot move through the coil properly, the heat pump works harder and may defrost more often.

Drainage also matters. Defrost water needs somewhere safe to go. In freezing weather, water dripping onto a path can create an ice hazard. A good installation considers winter behaviour before the first cold snap, not after someone slips on frozen condensate.

Noise can also be more noticeable in winter because the unit may run for longer periods. Correct siting, proper mounting and sensible distance from bedrooms and neighbours help avoid unnecessary irritation.

How To Run A Heat Pump Well In Winter

Avoid treating a heat pump like a boiler. Large setbacks and sudden boosts can make it work harder. Many homes are better with steady temperatures, weather compensation and small adjustments.

Keep the outdoor unit clear of leaves, snow, bins and overgrown planting. Make sure defrost water drains safely. Do not switch the system off repeatedly unless the installer has advised it.

If the home feels cold, do not only turn the flow temperature up. Ask whether the issue is radiator sizing, weather compensation settings, insulation, balancing, hot water priority or a fault. A higher flow temperature may improve comfort but reduce efficiency.

Weather compensation is particularly important. It adjusts flow temperature based on outdoor conditions, so the system can run warmer when the weather is colder and cooler when the weather is milder. When set properly, this helps balance comfort and efficiency without constant manual changes.

Winter Checklist For Homeowners

Before the coldest weather arrives, check that the outdoor unit is clear, the controller is set correctly, radiators or underfloor heating zones are open where needed, and filters or strainers have been serviced according to the installer’s guidance.

During a cold snap, watch for patterns rather than reacting to one cold morning. A short defrost cycle, gentle radiator temperature or longer running time can all be normal. Warning signs include rooms never reaching temperature, frequent backup heater use, repeated fault codes, unusual noise or ice that does not clear.

If you are planning a system rather than already living with one, our guide to how heat pumps are installed explains why survey quality and commissioning affect winter performance so heavily.

What Makes Winter Performance Poor?

The main causes are poor heat loss calculations, undersized radiators, high heat loss from the building, incorrect controls, blocked filters, poor water flow, bad outdoor unit siting and weak commissioning.

Heat pumps are not magic. They reward careful design. A properly installed system can be comfortable and efficient. A rushed one can reinforce every myth about heat pumps in winter.

Case Study: A Cold House With Warm Radiators

Background

A homeowner complained that their heat pump was running constantly in January and the house was not reaching target temperature.

Project Overview

The engineer checked flow temperature, weather compensation, radiator sizes and heat loss. The heat pump itself was operating, but two key rooms had radiators too small for the design temperature.

Implementation

The radiators were upgraded, the system balanced and weather compensation adjusted. The homeowner was shown how to use steady setpoints instead of repeated boosts.

Results

Comfort improved and backup heater use reduced. The issue was not that heat pumps cannot work in winter. It was that the emitters were not matched to the building.

Expert Insights From Our Heating Engineers

One of our senior heating engineers with over 20 years of experience says winter heat pump complaints usually come down to design or controls. The machine is only one part of the system.

He recommends checking three things before blaming the heat pump: room-by-room heat loss, radiator output at the chosen flow temperature, and whether the homeowner has been shown how steady low-temperature heating behaves.

He also says the best winter systems are often quiet from a customer-service point of view. There are no dramatic boosts or constant interventions because the design temperature, emitter sizing and controls have already been matched to the building.

Frequently Asked Questions

Summing Up

Heat pumps can work well in winter, including during freezing weather. They do not create heat like a boiler. They move heat from outside or the ground into the home, then work best when the building and emitters are designed around lower flow temperatures.

If you want reliable winter performance, focus on survey quality, radiator sizing, controls, insulation and commissioning. That is where comfort and efficiency are won.

Updated May 2026