How Does a Heat Pump Work in Cold Weather?

What changes when outdoor air gets colder

A heat pump moves heat from outside air into the heating system. Colder air holds less useful heat, so the compressor works harder for each unit of heat delivered. Two things move at once:

At the same time the building’s heat loss rises because the indoor–outdoor temperature gap is larger. So the machine is less efficient and the house needs more heat on the same day. That is why winter dominates the annual bill (in our running-cost work, roughly 45–55% of annual cost falls in December–February for a typical 3-bed).

None of this is a fault code. It is why heat loss calculations and manufacturer capacity tables at low outdoor temperatures exist.

COP, SCOP and capacity — stop mixing the labels

From our published SCOP examples (11,000 kWh heat demand, 26p/kWh electricity): moving from SCOP 3.5 to 2.8 is roughly £200 a year on that house. Cold-weather design is where SCOP is won or lost — especially flow temperature and whether the unit is big enough to avoid electric backup as a habit.

Full tables: Heat pump running costs explained.

Flow temperature: the lever you control

Efficiency depends heavily on the gap between outdoor air and the water temperature you ask for. High flow temperatures in cold weather stack two penalties: worse COP and harder work for a unit whose capacity is already reduced.

Heat pumps prefer much of the season in the mid-30s to low-40s °C where emitters allow it. Running 50–55°C “to be safe” because radiators are small is how winter performance and the vs-gas comparison both degrade.

Practical reading: flow temperature and efficiency · radiator upgrades · underfloor heating as a low-temperature partner.

Defrost cycles — normal vs problem

In damp cold weather moisture freezes on the outdoor coil. The unit runs a defrost cycle: sound changes, heating may pause briefly, then normal operation returns. That is expected — especially in coastal and humid South West air, which can mean more frosting than a dry inland cold snap at the same temperature.

Persistent problems often track back to airflow (leaves, enclosures, tight alcoves), installation quality, or a system that is simply undersized for the load so it never “gets ahead.”

Devon & Cornwall winters — local climate, not generic UK

National heat-pump arguments often lean on Scottish or northern design days. That is the wrong reference for most of this peninsula. The South West is one of the mildest parts of the UK in winter, with strong maritime influence — especially on the coasts of Cornwall and south Devon.

What “mild” actually means here

• •Coastal strip — winter night minima often average in the low single digits °C; hard freezes happen but are usually short.
• •Inland Devon and higher ground (including towards Dartmoor and upland valleys) — colder nights, more frost, occasional sharper snaps than the sea-front.
• •Cornwall’s far west and Isles influence — among the mildest winter minima in England; snow settling is relatively rare on low coastal ground.
• •February effect — sea temperatures bottom out late winter, so some coastal sites are coldest in February rather than January.

Cold snaps still occur — including ice and wintry showers in recent winters — but multi-week deep freezes that define some inland UK design stories are not the normal baseline for Exeter, Plymouth, Truro or the south coast.

Design outdoor temperature (why it matters for sizing)

Heat loss calculations use a local external design temperature, not “average January.” MCS-oriented design tools pick a reference weather location near the postcode. For the Plymouth area, published reference design values sit around −1.5°C (and a milder secondary figure near 0°C depending on the dataset column) — clearly kinder than many northern locations.

Your exact figure should come from the designer’s weather location for your postcode (north Devon inland is not the same as Falmouth seafront). The point for homeowners:

• •Ask what outdoor design temperature was used
• •Ask what capacity the heat pump has at that temperature and your flow temperature
• •Do not accept a mild-weather nameplate kW as proof it covers a cold inland night

Damp air and defrost on the peninsula

Maritime air is moist. That means outdoor coils can frost even when the thermometer is only around freezing — not only in hard continental cold. Coastal and estuary sites (Plymouth Sound, Exe, Tamar, Fal, north Cornish coast) often see regular defrost behaviour in damp cold spells.

That is normal climate physics. It becomes a problem when the unit is boxed in, starved of airflow, or so undersized it never recovers between cycles. Siting on windy cliff plots also needs care: wind can help or hinder depending on exposure and how the unit is placed relative to walls and courtyards.

Building stock that fights the mild climate

Mild outdoor air does not help much if the building leaks heat. Across Devon and Cornwall that often means:

• •Solid stone or cob walls with little or no external insulation
• •Suspended timber floors over cold voids
• •Single glazing or old secondary glazing in conservation areas
• •Exposed ridges and coastal cottages with high wind-driven infiltration

A well-insulated new build near the coast can run low flow temperatures and look excellent on seasonal efficiency. The same outdoor climate on an unimproved inland longhouse is a different design job — more capacity, more attention to emitters, more honesty about fabric.

Rural oil-heated homes in these stock types are common. Their running-cost comparison is often against oil fills, not mains gas — covered in heat pump vs gas (and oil) running costs.

Dartmoor and high ground — when “Devon is mild” does not apply

Dartmoor and similar upland edges are the clearest reminder that the county is not one climate. Altitude, wind and open exposure push heating loads up compared with sheltered south-coast towns — even when the postcode still says Devon.

That means both a higher peak kW (size the plant for the real design day) and often higher annual heat demand (more cold hours). A unit that is comfortable on a mild coastal plot can be short on capacity on an exposed moor-edge longhouse of the same floor area.

Design habits for Dartmoor-edge and high inland sites:

• •Use the correct weather location for the postcode — do not default to a seafront reference if the site is high and open
• •Check manufacturer capacity at that outdoor temperature and the intended flow temperature
• •Treat wind exposure and solid-wall fabric as first-class inputs, not footnotes
• •Site the outdoor unit for airflow and weather — driving rain and wind need a deliberate position
• •Be honest about emitters: high heat loss plus tiny radiators forces high flow temperatures and poor winter SCOP

Microclimate pockets matter even a few miles apart: open plateau versus leeward of high ground, frost hollow versus ridge. Site notes beat a single “Dartmoor” stereotype — but as a planning rule, never size an upland or moor-edge home as if it were Exmouth or Falmouth in January.

Many of these homes are still on oil or LPG. The running-cost case against tank fuels is often strong; the engineering case still needs local design temperatures and fabric honesty. See rural oil comparison and what size heat pump.

Sizing for the cold day — not the brochure day

Undersizing is still one of the most common reasons houses feel cold in a snap. A unit that looks fine in October can fall behind when outdoor temperature drops and capacity derates.

Competent design uses:

• •Room-by-room or whole-house heat loss at a stated outdoor design temperature
• •Manufacturer capacity tables at that outdoor temperature and the intended flow temperature
• •Emitters that can deliver the load at that flow temperature

In this region that often means checking capacity around local design temperatures only a little below freezing — not assuming a −10°C northern scenario, and not assuming “Devon is mild so any unit will do.” Inland postcodes and exposed sites still need their own numbers.

Rule-of-thumb “11 kW for a 3-bed” without heat loss is how January complaints are born. More: what size heat pump do I need · heat loss calculations.

How to keep performance up when it turns cold

1. 1.Size from heat loss and capacity tables — not a marketing kW.
2. 2.Protect low flow temperatures with adequate radiators or underfloor heating.
3. 3.Commission weather compensation — outdoor temperature should drive flow temperature.
4. 4.Prefer steady running over deep overnight setbacks and hard morning recovery.
5. 5.Keep the outdoor unit clear — leaves, decking cages and tight alcoves hurt airflow and encourage icing issues.
6. 6.Improve fabric where you can — less load on the coldest days is the simplest efficiency upgrade.
7. 7.Use a sensible tariff — does not change COP, but changes what winter kWh cost.

Do you need a hybrid boiler for cold snaps?

Usually not in Devon and Cornwall if the heat pump is sized and the emitters match the flow temperature. Hybrids add cost, controls complexity and another fossil appliance to maintain.

They can be a deliberate design choice when:

• •Heat loss and design outdoor temperature demand more output than a practical ASHP install can provide
• •Planning, noise or space block a larger outdoor unit
• •A boiler is retained as engineered backup — not as a fix for skipped heat loss

“I’m worried about one cold week” without a heat loss calculation is not a hybrid specification. It is an unfinished design.

If the house will not get warm in a cold snap

Work through causes in order:

1. Is the outdoor unit actually running and free of ice/debris?
2. What are flow and return temperatures?
3. Is weather compensation asking for something the emitters can deliver?
4. Was capacity checked at design outdoor temperature?
5. Has heat loss changed (extension, loft stripped, new open-plan void)?

Longer checklist: why is my heat pump not heating the house properly.

Honest bottom line

Frequently asked questions