Stop Winter Performance Dips in Air Source Heat Pumps
Why Heat Pumps Dip in Winter — and How to Fix It
As outdoor temperatures fall, the heat pump must lift heat across a larger temperature difference. COP will dip and defrosts will increase slightly — that’s expected. What isn’t normal are big drops from poor setup. With the right weather-compensation curve, flow temperature and controls, performance remains stable.
In UK cold snaps, a well‑set radiator system should hold a COP around 2.5–3.0; underfloor heating often achieves 3.0–3.5. You’ll keep bills predictable by avoiding high flow temperatures and constant thermostat overrides. New to heat pumps? Start with our explainer on how heat pumps work and read our heat pump myths.
Set Up Weather Compensation Correctly
Confirm your home’s design heat loss and emitter outputs first. Begin with a conservative slope, then use a small parallel shift to fine‑tune comfort without making the curve too aggressive. Typical starting points for UK homes:
- Radiators: ~45°C flow at 0°C outdoors; ~35°C at 10°C.
- Underfloor: ~35°C at 0°C; ~28–30°C at 10°C.
Set “room influence” to low–medium so the room stat trims the curve rather than fighting it. Avoid big thermostat jumps; they override weather compensation and force higher flows. If design data is missing, book a whole-home heat loss survey.
- Curve too steep – rooms overshoot, short cycles, high flow temps.
- Curve too flat – rooms underheat as it gets colder, long run times.
- Good curve – steady room temp, stable delta T, modest compressor cycling.
- Adjust methodically – change parallel shift by 0.5–1.0°C, then observe for 24 hours before altering slope.
Manage Flow Temperature and Flow Rate for Peak COP
Lower flow temperature improves COP. Keep circuits open and maintain healthy flow: target delta T 5–7°C for radiators, 3–5°C for underfloor. Too little volumetric flow raises head pressure and can increase defrost frequency.
- Open TRVs fully in reference rooms; avoid throttling the main loop.
- Purge air, clean strainers, and confirm primary pump speed and direction.
- Check blending valve settings on UFH manifolds; avoid unnecessary mixing.
- Use buffers/headers only when hydraulically necessary; oversizing or high buffer temps can harm COP.
- Verify minimum and recommended flow (L/min) against the manufacturer’s data.
- Common errors: fixed high flow temps, closed TRVs, oversized secondary pumps.
For larger emitter area and gentler flow temperatures, see our underfloor heating systems.
This image was generated with AI and may not always represent the product or service exactly.
Defrost Strategies That Don’t Waste Energy
Frost is inevitable in damp, near‑freezing weather, but heavy icing is avoidable. Site the outdoor unit with clearances on all faces, prevent recirculation in alcoves, and shield from prevailing winds without blocking airflow. Ensure the condensate path is free‑draining and cannot freeze.
Understand your model’s defrost logic. Avoid hot water calls during defrost so stored heat isn’t robbed from the cylinder. In exposed sites, a small base‑pan heater or heat‑traced drain prevents ice dams. Before winter, run a forced defrost and confirm the drain flows freely.
- Clear leaves and debris from coils and base pan.
- Check drain gradient and outlet; add heat trace if needed.
- Verify outdoor sensor placement is not sun‑ or wind‑biased.
- Confirm firmware and controller settings match the installation.
- Typical clearances: rear ≥300 mm, sides ≥300 mm, front ≥1 m (check your manufacturer’s guidance).
Homeowner Controls That Improve Winter Efficiency
Run the heat pump steadily. Use a mild night set‑back of 0.5–1.5°C rather than switching off; deep setbacks force recovery at higher flow temperatures. Keep TRVs fully open in the reference zone and avoid third‑party smart thermostats that disable weather compensation.
- Weekday example: 06:00–22:30 at 20–21°C; 22:30–06:00 at 19–20°C.
- Schedule DHW outside peak heating periods; run legionella cycles at midday.
- Leave the indoor controller in automatic weather‑comp mode.
- Make one change at a time; log outdoor temp, flow temp and room temp to see cause and effect.
For simple care tips that protect efficiency, see our maintenance guide for homeowners.
Design Checks for Cold‑Weather Resilience
Right‑sizing is critical in winter. Confirm design heat loss (EN 12831) and ensure radiators or UFH can deliver output at low flow temperatures. Insulate all primary pipework, cylinder connections and external lines. Use the correct glycol concentration and inhibitor for freeze and corrosion protection.
- Verify radiator outputs at 35–45°C flow; add emitter area if margins are tight.
- Seal and insulate outdoor refrigerant/water lines to cut losses.
- Plan for wind exposure and acoustic limits when siting the unit.
- Electrical readiness: correct breaker/radial, RCD/AFDD, surge protection, and load management with EV/PV.
- Design outdoor temperature: check your local −2 to −5°C figure and set the curve accordingly.
Troubleshooting Winter Performance Drops
Start with quick checks: confirm system pressure, bleed air, clean strainers, and read the flow/return delta T. Check outdoor and flow sensors are seated correctly and reading sensibly. Review the weather‑comp curve and TRV positions.
- Short cycling – curve too steep, closed TRVs, or oversized secondary pumps.
- Frequent defrosts – airflow recirculation, blocked coil, low flow rate.
- High bills – fixed high flow temp, DHW heating at peak times, failed or mis‑placed sensor.
- Low room temps – curve too flat, insufficient emitter area, or mixing valve set too low.
Electrical or refrigerant faults are not DIY. If symptoms persist, book an MCS‑certified engineer from Clima Tech.
This image was generated with AI and may not always represent the product or service exactly.
Maintenance, Monitoring and Our Accredited Service
A winter‑ready service protects efficiency and reliability. We clean coils, check refrigerant circuits (REFCOM‑qualified), test electrics (NICEIC), verify safety controls, calibrate sensors, and sample system water for glycol and chemistry.
- Controller optimisation: weather‑comp curve, room influence and DHW timing.
- Flow health: pump settings, air removal and delta T tuning.
- Optional monitoring: add a gateway to track COP, flow temps and defrost counts.
Clima Tech brings 20+ years of renewable HVAC experience and holds MCS, REFCOM, NICEIC and Gas Safe accreditations for full peace of mind.
Boost Winter Performance with PV, Cylinders and Smart Tariffs
Pair your heat pump with solar PV and off‑peak tariffs to cut winter running costs. Use a cylinder sized for low‑temperature DHW and set daily store temps around 48–50°C, with periodic pasteurisation to a higher setpoint as required.
- Top up DHW with solar diverters on bright days.
- Use gentle pre‑heating before peak times to flatten load.
- Align controls with smart tariffs and solar PV for best value.
Grants, Upgrades and How Clima Tech Can Help This Winter
Book a winter optimisation visit. We’ll set your weather‑comp curve, verify flow, balance emitters, and test defrost and controls. If upgrades are sensible — such as cylinder replacement, emitter swaps or electrical protections — we’ll specify what delivers the biggest gain.
Funding support is available via the Boiler Upgrade Scheme, with current grants up to £7,500. Ready to get started? Share a few details using our estimation form and we’ll schedule a visit at a time that suits you.
FAQs
What winter COP should I expect?
Radiator systems typically see 2.5–3.0 in cold snaps; UFH often holds 3.0–3.5. With correct setup, performance should stay consistent across the season.
How do I know my weather‑compensation curve is right?
Rooms feel steady without overshoot, delta T is consistent, and the unit doesn’t short cycle. If rooms underheat as it gets colder, raise the parallel shift slightly before changing slope.
Is night set‑back better than switching off?
Yes. Use a mild 0.5–1.5°C set‑back. Turning off forces a high‑temperature recovery and wastes energy the next morning.
How often should a heat pump defrost?
It depends on humidity and temperature. Several defrosts per hour in damp, near‑freezing weather can be normal; persistent icing suggests airflow or drain issues.
Do I need glycol in my system?
Most exposed or low‑temperature systems benefit from glycol for freeze protection. Concentration must match manufacturer guidance and be checked at service.
When should I call an engineer?
If you see electrical faults, refrigerant errors, rapid cycling, or icing that won’t clear after checks. An MCS‑certified technician will diagnose safely and correctly.