Heat Pump Not Working in Cold Weather — What to Do
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Post a job — we'll find you an engineer →Do not pour boiling water on the outdoor unit — thermal shock can damage the heat exchanger fins. Use warm water only.
Heat pumps are designed to work efficiently in cold weather — it's one of their key selling points — but when the thermometer drops below freezing, many homeowners notice their system seems to struggle or underperform. This is actually normal to a degree, though it shouldn't mean your house stays cold. The efficiency of an air source heat pump does decline in extreme cold, and if defrost cycles aren't working properly or the outdoor unit gets blocked by ice and snow, performance can suffer noticeably. This guide walks through the most common reasons why a heat pump might seem to fail during a cold snap, and what you can safely do about it before calling your installer. If you're seeing fault codes, the outdoor unit is completely iced up, or your home simply won't reach temperature despite the system running, it's time to get a professional involved.
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Most likely cause & what to check
Air source heat pumps can extract heat from outdoor air down to around -15°C to -25°C depending on the model — they do not stop working when it gets cold. However, efficiency (COP) reduces as the outdoor temperature drops, and they may struggle to maintain the set flow temperature on extreme cold days.
Check for ice build-up: if the outdoor unit has a thick coating of ice that hasn't cleared after a defrost cycle, the defrost system may have failed. Carefully pour warm (not boiling) water over the unit body to assist melting. Ensure the area beneath the unit can drain freely.
Check the outdoor unit is not obstructed — snow drifts or leaves blocking airflow around the unit will cause poor performance and prolonged defrost. Clear any obstructions and ensure at least 30cm clearance on all sides.
Check the flow temperature setting — in very cold weather the heat pump may need a higher flow temperature to meet the heat demand. If the system has weather compensation, check it is correctly calibrated for the outdoor sensor reading.
Check for an electric backup or immersion heater — most heat pump systems have a backup electric element that kicks in at very low outdoor temperatures. Check it is enabled in the settings. If the backup is disabled and outdoor temp drops below -5°C, the house may not reach temperature.
If the heat pump is showing a fault code and will not run at all, note the code and contact your installer or the manufacturer's helpline. Running in very cold weather is a known stress test for the system — any underlying faults often surface in cold snaps.
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Frequently asked questions
Do heat pumps stop working when it freezes?
No — heat pumps are engineered to extract heat from air well below freezing, typically down to -15°C or -25°C depending on the model. What does change is their efficiency; the colder it gets, the harder the pump has to work to deliver heat at the temperature your system needs. You may notice longer run times or the backup electric heater kicking in more often, but this is normal operation, not a failure.
Why is my heat pump iced up and not defrosting?
Ice build-up is normal on an air source unit in cold, damp weather — the defrost cycle should melt it automatically. If ice persists thickly after the system has cycled several times, the defrost control may have failed. You can help by carefully pouring warm (never boiling) water over the unit body to assist melting, but if ice returns within hours, you'll need the defrost sensor or valve checked by a qualified engineer.
Can I pour hot water on my heat pump to defrost it?
Absolutely not — boiling or very hot water can cause thermal shock and damage the delicate aluminium fins on the heat exchanger. Always use warm water only (around 40°C), apply it gently, and allow time for natural drainage. If you're regularly needing to manually defrost the unit, this suggests the automatic defrost isn't working and needs professional attention.
My heat pump won't heat the house in winter — what should I check first?
First, check that the outdoor unit isn't blocked by snow, ice drifts, or leaves — poor airflow is a common culprit. Next, verify the flow temperature setting isn't too low; in very cold weather, you may need to raise it or ensure weather compensation is calibrated correctly. Finally, confirm that any electric backup heater is enabled in the system settings — without it, the pump alone may not meet demand below -5°C outdoors.