“Do heat pumps actually work in cold weather?” is the single most common question we get from prospective heat pump customers in the North East. The short answer: yes, they work fine — but the long answer involves how heat pumps lose efficiency as outdoor temperature drops, what “fine” actually means in real running costs, and the specific 2-3 cold weeks a year where the question matters.
Here’s the honest picture.
How heat pumps lose efficiency in cold weather
A heat pump’s Coefficient of Performance (COP) — units of heat delivered per unit of electricity consumed — depends on the temperature difference between outdoor air (the heat source) and water in the central heating system (the heat destination). The bigger the gap, the harder the heat pump works and the lower the COP.
Typical figures for a well-designed air source heat pump in the UK:
| Outdoor temp | Flow temp | Typical COP | Compared to “advertised” |
|---|---|---|---|
| 10°C (mild autumn) | 35°C | 4.5-5.0 | Above SCOP |
| 7°C (UK standard test) | 35°C | 4.0-4.5 | At SCOP |
| 0°C (frost) | 40°C | 3.0-3.5 | Below SCOP |
| -5°C (cold snap) | 45°C | 2.5-3.0 | Well below SCOP |
| -10°C (rare UK extreme) | 50°C | 1.8-2.3 | Performance-degraded |
The headline SCOP (Seasonal Coefficient of Performance) figures manufacturers quote — typically 3.5-4.5 — are weighted across a typical heating season. They include the mild months when COP is 4-5 and the cold weeks when COP is 2-3, averaged by heating demand. The real seasonal experience matches the SCOP figure for a typical North East home.
What “below 3” means in practice
When the outdoor temperature drops to -5°C and the heat pump’s COP falls to 2.5, the system is still delivering heat. It’s just delivering 2.5 units of heat per unit of electricity, vs the 4-unit ratio in mild weather.
For a typical North East home using 12,000 kWh of heat per year:
- A SCOP 3.5 heat pump uses ~3,400 kWh of electricity over a year
- A modern gas combi at 92% efficiency uses ~13,000 kWh of gas
Even on the worst day of the year — say -8°C in mid-January at COP 2.0 — a heat pump still uses meaningfully less primary energy per unit of heat than the gas boiler it replaces. The cost comparison gets closer (because UK electricity is more expensive per kWh than gas) but doesn’t flip — running costs stay broadly comparable on a swap-only basis, and a heat pump still wins decisively on running cost when replacing oil or LPG.
How often does the cold-weather question actually matter?
Looking at Met Office data for the North East over 2020-2025:
- Days at -5°C or below: typically 4-12 per year
- Days at -10°C or below: typically 0-2 per year
- Hours at -5°C or below in an average winter: typically 40-150 hours total
In other words, the “heat pumps don’t work in cold weather” concern applies to roughly 0.5-2% of the year. The other 98-99% the heat pump runs at moderate-to-good COP and delivers comfortable heat.
For the cold-week edge cases, modern heat pumps cope by running for longer hours (heat output is steadier rather than the gas-boiler pattern of intense bursts and idle periods). The house stays at design temperature; running cost on those specific days is higher than annual average. The effect on annual bills is modest.
Cold-weather specific design considerations
A heat pump system that performs in cold weather is one that was designed for cold weather. The specific design decisions:
1. Designed flow temperature. Most heat pumps achieve their advertised COP at 35-40°C flow. If your home’s radiators can’t deliver enough heat at 40°C, the designer has to push flow temperature up to 45-50°C to compensate — which drops COP across the entire heating season, not just cold weeks. Heat Geek methodology specifically addresses this.
2. Emitter sizing. A radiator sized for a 70°C gas boiler delivers roughly half the heat at 45°C heat pump flow. If the radiator was already marginal for the room, it’s now significantly undersized. Heat-loss surveys identify this; sympathetic radiator upgrades (K2 to K3, deeper convector, or occasional underfloor) fix it.
3. Weather compensation controls. Modern heat pump controls vary the flow temperature with outdoor temperature — flow rises as outdoor drops, maintaining indoor design temperature without running flat-out 24/7. Weather compensation is the single most important control feature for cold-weather performance. We commission it specifically on every install.
4. Outdoor unit defrost cycles. When outdoor temperature is around 0-5°C with high humidity, frost forms on the outdoor unit’s evaporator coils. Modern units detect this and run automatic defrost cycles (typically 5-15 minutes every 1-2 hours). During defrost, heat output briefly drops; a well-designed system uses the heating distribution to ride through this seamlessly.
5. Hot water priority. Modern heat pumps prioritise hot water when needed (typically scheduled cycles, not real-time on-demand). On a cold day, that means a 30-minute hot water cycle followed by space heating reverting to lower flow temperature for ongoing operation.
Manufacturer cold-weather performance
The brands we install all perform competently in cold weather, with measurable differences at the edges:
Mitsubishi Ecodan — Industry-standard cold-weather performance. Specifies operation down to -25°C outdoor; in practice we see customers’ systems running fine at -10°C. Strong defrost handling.
Vaillant aroTHERM Plus — Uses R290 refrigerant which actually delivers slightly higher COP than R32 at low outdoor temperatures (-5°C to -15°C range). Particularly attractive for rural Northumberland or Tyne Valley properties where extended cold spells happen more often.
Daikin Altherma 3 — Specifies operation to -25°C. Daikin’s “Hybrid” variants (Altherma H Hybrid) include a backup gas boiler that kicks in only at very low outdoor temperatures — a niche product but useful where extreme reliability matters (rural off-grid for example).
Cold-weather running cost
Worked example: a typical 3-bed semi in Hexham (which has more cold days than Middlesbrough or Newcastle) with a 10kW air source heat pump.
| Period | Days | Avg COP | Heat delivered | Electricity used | Cost @ 28p/kWh |
|---|---|---|---|---|---|
| Sept-Nov (mild autumn) | 90 | 4.0 | 3,000 kWh | 750 kWh | £210 |
| Dec-Feb mild days (5°C+) | 60 | 3.5 | 3,000 kWh | 860 kWh | £241 |
| Dec-Feb cold days (-2°C to 5°C) | 25 | 2.8 | 1,500 kWh | 535 kWh | £150 |
| Dec-Feb cold snaps (-5°C to -2°C) | 5 | 2.3 | 400 kWh | 175 kWh | £49 |
| Mar-May (mild spring) | 90 | 4.2 | 3,000 kWh | 715 kWh | £200 |
| Summer DHW only | 95 | 3.5 | 1,100 kWh | 315 kWh | £88 |
| Total | 365 | 3.5 SCOP | 12,000 kWh | 3,350 kWh | £938 |
On a time-of-use tariff like Octopus Cosy at 8p/kWh off-peak (60% of usage shifted), the same household pays around £550-£600 a year. With paired solar PV (say a 6kW array generating 5,000 kWh/year, of which 30-40% offsets heat pump demand), the heat pump’s net cost drops further to £400-£500/year.
For comparison: the same 3-bed semi heating with oil at 2025-2026 prices typically costs £2,400-£3,200/year. With LPG, £2,800-£3,600/year. Heat pump cold-weather running cost is a fraction of what off-gas-grid customers currently pay.
When cold weather is a real problem
Worth being honest: there are situations where cold weather genuinely is a heat pump problem, not just a perception issue.
1. Poorly insulated solid-wall properties without realistic insulation retrofit options. Heat loss is too high; the heat pump runs at maximum capacity continuously through cold spells and still struggles to maintain temperature. The honest answer here is fabric improvement first, heat pump second.
2. Properties where emitter upgrades aren’t consented. Listed buildings, conservation-area constraints, or budget constraints that prevent radiator changes. The heat pump can’t deliver enough heat at low flow temperatures, leading to cold rooms in winter.
3. Properties where the install wasn’t designed properly. The single most common cause of “the heat pump doesn’t work” complaints. Boxed-in installs, undersized radiators, no weather compensation, flow temperature set too high — all design failures rather than technology failures. Heat Geek methodology specifically addresses this.
4. Backup hot water for periodic Legionella cycles. Heat pumps deliver hot water at 50°C; statutory Legionella prevention typically requires 60°C+ cycles. Most modern heat pump systems handle this via a small electric immersion top-up; some installers skip this, leading to compliance issues in care homes and HMOs.
Bottom line
UK air source heat pumps work well in UK cold weather when they’re designed properly. The cold-weather efficiency drop is real but small in annual terms — typically 5-10% of total heating cost is at the cold-snap end of the range. Running costs at -5°C are higher than the annual average but still meaningfully below the gas / oil / LPG alternatives most heat pump customers are replacing.
The honest variable isn’t temperature; it’s installer competence. A Heat Geek-trained installer with a properly designed system delivers what the manufacturer claims. A boxed-in install with rule-of-thumb sizing delivers disappointment. We use the former approach as standard.
Book a free heat pump survey → and we’ll model your specific home’s cold-weather performance based on actual heat loss, current insulation, and the specific Northumberland / County Durham microclimate. No commitment.
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