Answer first
In the Electrification of Heat field trial, the analysed systems continued operating during cold conditions; the mean whole-system COPH4 on the coldest analysed day was 2.44 at a mean outdoor temperature of −0.4°C. That is measured project evidence, not a guarantee for every installation.
A Scottish quote should state the design outdoor temperature, room heat losses, proposed flow temperature, unit capacity at that condition, treatment of defrost and any backup heater. A mild-weather catalogue COP is not enough.
Mean whole-system COPH4 on the project’s coldest analysed day
View sourceMean outdoor temperature in that coldest-day analysis
View sourceCapacity is checked against the property heat loss at local winter conditions
View sourceGet a system sized for your home’s Scottish winter load
Tell FLO about the property, current heating and the point you need resolved. A £0 home survey can turn the general answer into a room-by-room design and written quote.
Cold air still contains usable heat
The refrigerant circuit can absorb heat from cold outdoor air, but the compressor works across a larger temperature lift as conditions become colder. Efficiency normally falls and the building simultaneously loses more heat.
Manufacturers publish capacity and performance at defined air and water temperatures. The designer selects from those tables rather than relying on the model’s nominal name.
Defrost is normal and must be allowed for
Moisture can freeze on the outdoor coil in cold, damp weather. The unit periodically reverses operation to clear it, temporarily using energy and changing output.
Correct siting, drainage, clearance and system volume support reliable defrost. Persistent ice or repeated abnormal cycles deserve investigation, but occasional steam and water can be normal.
The home and emitters set the winter test
An adequate outdoor unit cannot compensate for undersized radiators, restricted flow or a heating curve set too low. Conversely, simply raising water temperature can reduce efficiency.
Room-by-room heat loss and emitter output establish a balanced design. Planned fabric work can lower the load, but only if it is included and completed.
Backup heat should be deliberate
Some designs include an immersion or electrical backup for exceptional conditions, hot water or fault resilience. The controls and cost assumptions should explain when it is expected to operate.
A backup heater running routinely can conceal a sizing, flow, setting or fault issue. Monitoring electricity and controller history helps distinguish planned support from a problem.
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Questions people also ask
Do air-source heat pumps stop below zero?+
No. Suitable models operate below zero, but capacity and efficiency must be checked at the actual design temperatures.
Is defrost a fault?+
Normally no. It is a standard process in cold, damp conditions; persistent ice or unusual cycling should be checked.
Do I need a second boiler for winter?+
Not automatically. A correctly designed heat pump can meet the design load; any backup should be explicit and justified.
Evidence used for this answer
Figures are explained in context and links go to the independent source. Prices, tariffs, scheme rules and standards can change, so check the source again before making a financial or technical decision.
- Energy Systems Catapult: heat-pump performance analysisMeasured seasonal performance and cold-weather operation from the Electrification of Heat demonstration project.
- MCS heat-pump design standardTechnical requirements for heat loss, equipment selection, domestic hot water and system design.
- MCS domestic heat-pump guideConsumer guidance covering design, installation, controls, handover and maintenance.
- Energy Saving Trust: air source heat pumpsIndependent guidance on installation costs, running costs, suitability, radiators and household savings.