Do Heat Pumps Work in Cold Weather? A Landlord's View

The short answer

Heat pumps work in cold weather, including in a British cold snap, provided the system was designed for it. The MCS design standard, MIS 3005-D, requires the heat pump to supply 100% of the home’s heat load at a design outdoor temperature for its location, without help from any backup electric heater.

What changes in the cold is efficiency. The colder the outside air, the more electricity an air source heat pump uses for each unit of heat. For a landlord, the question is not “will it work” but “was it designed properly, and what will the tenant notice”.

What happens to COP in a cold snap

COP is the heat delivered for each unit of electricity at a given moment. The best UK field data comes from the Electrification of Heat demonstration project, analysed by Energy Systems Catapult for the government. It measured hundreds of heat pumps in real homes every 30 minutes.

Outdoor temperature	Median COP, air source	Median COP, ground source
10°C	3.37
minus 3°C	2.37
minus 5°C	2.30	3.10

These are 30-minute medians on the report’s widest boundary, which counts backup and immersion heaters and pumps. The report found air source efficiency peaked at 10°C and fell as the temperature dropped. The effect was much smaller for ground source, though that sample was small. Our guide to ground source heat pumps for rental properties covers when that option fits.

The other lever is flow temperature. In the same report, median COP across all heat pumps was 3.50 at a 35°C flow, 2.38 at 55°C and 2.02 above 65°C. A heat pump pushed to run hot in a cold spell, because the radiators are too small, loses on both counts.

Design outdoor temperature: the MCS rule

MIS 3005-D Issue 3.0 sets out how an MCS installer sizes the system:

• A room-by-room heat load calculation to BS EN 12831-1:2017.
• Internal design temperatures of 21°C for living rooms, 18°C for bedrooms and 22°C for bathrooms.
• External design temperatures from CIBSE Guide A. The installer chooses column A (the temperature met or exceeded 99% of hours in a year) or column B (99.6%) based on its assessment of the location.
• Full output without backup. The heat pump must meet at least 100% of the heat load at the design flow temperature with no input from any supplementary electric heater, and any such heater must be designed not to operate above the design outside temperature.

The standard’s table gives examples:

Location	Column A (99%)	Column B (99.6%)
London	minus 1.7°C	minus 3.0°C
Birmingham	minus 3.2°C	minus 5.1°C
Manchester	minus 2.7°C	minus 4.5°C
Cardiff	minus 1.5°C	minus 3.1°C
Plymouth	minus 0.2°C	minus 1.5°C

The Energy Systems Catapult report calls minus 3°C and minus 5°C the common GB design conditions. By definition, the outside temperature is colder than the column A figure for about 1% of the hours in a typical year, and colder than column B for about 0.4%. In those hours a well designed system may fall slightly short of full output.

Why this matters for a rental

The landlord standard is close to the MCS one. The government’s HHSRS guide for landlords in England (updated 16 September 2026) lists as a baseline indicator a heating system able to heat the main living room to 21°C and other habitable rooms to 18°C when it is minus 1°C outside. The same guide says difficulty keeping a property warm contributes to the hazard of excess cold. A properly sized heat pump designed to the MCS temperatures should cover this. An undersized one, or one relying on an immersion heater or plug-in heaters, is a risk.

Defrost cycles

In cold, damp weather, frost can form on the outdoor unit of an air source heat pump. The unit clears it with a defrost cycle. MIS 3005-D says an air source system should be able to maintain the internal design temperatures across multiple defrost cycles.

For a landlord, the practical step is simple: ask the installer to explain, in writing, what a defrost looks and sounds like on the chosen model, and pass that to the tenant. A tenant who has been told what to expect is less likely to report a working system as a fault.

What tenants notice in cold weather

• Radiators feel cooler to the touch. A heat pump sends cooler water round the system than a typical gas boiler. In the Electrification of Heat project the mean operating flow temperature was 39.7°C. The room can still reach temperature with a radiator that is warm rather than hot.
• It runs for longer. Lower temperature water means longer running to deliver the same heat. The same report suggests intermittent operation is less efficient, so short bursts of heating are a poor fit.
• Bills rise. More heat is needed in cold weather and, as the table above shows, each unit of heat takes more electricity. Set this out at the start of the tenancy.
• Controls matter more. Weather compensation adjusts the flow temperature to the outside temperature. Overriding it to force a quick warm-up can raise the flow temperature, and higher flow temperatures mean a lower COP.

A one-page guide for tenants, covering the controls, the defrost and why radiators feel cooler, does more for winter complaints than any spec sheet.

Worked example (modelled, not a real project)

Inputs: Ofgem cap unit rates for 1 October to 31 December 2026 (electricity 26.32p per kWh, gas 7.97p per kWh); gas boiler at 90% efficiency; the field data median COPs above. Cost per kWh of heat is the unit rate divided by efficiency. Standing charges left out.

Heat source and conditions	Cost per kWh of heat
Gas boiler, 90%	8.86p
Air source, median COP 3.37 (10°C)	7.81p
Air source, median COP 2.37 (minus 3°C)	11.11p
Air source, median COP 2.30 (minus 5°C)	11.44p
Ground source, median COP 3.10 (minus 5°C)	8.49p

In this model, an air source heat pump is cheaper per unit of heat than gas on a mild day and dearer in a hard frost. What the tenant pays over a year depends on the seasonal average (SCOP), not the coldest week. See COP and SCOP explained.

When cold weather is a reason to pause

• Very high heat loss. If the house needs a large unit and hot radiators to stay warm at the design temperature, insulation first will usually give a better result.
• No room for bigger radiators. If emitters cannot be enlarged, the flow temperature stays high and winter efficiency suffers. MIS 3005-D says where a design proposes a flow temperature above 55°C, an alternative at 55°C or lower should also be offered.
• A quote with no design temperatures. If the paperwork does not state the external design temperature, the heat loss and the flow temperature, ask for them before signing. You can check the firm is certified on the MCS register.

Next step

Test winter running costs against your own figures in the heat pump landlord calculator. If you want a design for your rental, send an enquiry through the quote page and relevant independent firms contact you, with no charge to you and no obligation.

Figures were read from the sources listed on 8 October 2026. This is general information, not financial advice.