Is air conditioning the same thing as an air-to-air heat pump?

In heating mode, yes — a reverse-cycle split system is an air-to-air heat pump, and MCS lists it as one of the technologies it certifies. It is a different machine from the air-to-water heat pump that feeds radiators and a cylinder, which is a heating engineer's work.

Why does it stop blowing warm every so often in cold weather?

That is the defrost cycle. The outdoor coil gets colder than the outside air in heating, so frost builds on it. The system briefly reverses to melt that frost — you will see steam outside — and heating resumes when it finishes.

Does it still work when it is freezing outside?

Modern units continue to produce heat well below zero, but output and efficiency fall as it gets colder, and each model has a stated operating range in its manual. A system that has always struggled in a cold snap may be undersized for the room rather than faulty.

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Updated 30 August 2026

Air Conditioning Not Heating In Winter

The system cools perfectly in summer but blows cold, or nothing at all, when you switch it to heat.

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Air conditioning work, illustrative image

What's happening?

Most split systems sold in the UK are reverse-cycle: a valve flips the direction of the refrigerant so the outdoor unit collects heat from the outside air and the indoor head releases it. Run that way it is an air-to-air heat pump, and it is usually the cheapest heat in the house. Heating faults split into three groups. The system may not be in heat mode at all — the mode button is easy to nudge, and heating on many remotes is a sun symbol rather than the word. The reversing valve may have failed or stuck, so it stays in cooling. Or the machine is behaving exactly as designed and you are watching a defrost cycle: below about 5°C outside, frost forms on the outdoor coil, the system periodically reverses to melt it, and the indoor head stops blowing warm for several minutes while it does.

What usually causes it?

Remote or controller left in cool, dry or fan mode rather than heat
Normal defrost cycle running, with steam coming off the outdoor unit and no heat indoors for a few minutes
Reversing valve stuck or its solenoid coil failed, so the system will not switch out of cooling
Refrigerant charge low, which shows up in heating before it shows up in cooling
Outdoor coil iced solid because the defrost cycle or its sensor has failed
Model is cooling-only, with no heating capability at all
Outdoor temperature below the model's stated operating range, so output has fallen away

Is it dangerous?

No. Losing heating from a split system is a comfort and cost problem rather than a safety one, and it is worth checking what else heats the house before it becomes urgent. The one thing to avoid is chipping ice off an outdoor coil that has frozen solid — the fins puncture easily and a punctured coil turns a defrost fault into a refrigerant leak.

Can I fix it myself?

Check the mode first, because it is genuinely the most common answer: press mode until the display shows the heating symbol, then set the target temperature well above the room temperature. Give it fifteen minutes — reverse-cycle systems take longer to feel warm than a boiler does. If the outdoor unit is steaming and the indoor head has gone quiet, wait it out; that is a defrost cycle finishing. Check the outdoor coil for a solid block of ice, and if you find one, switch the system off and leave it to thaw.

When should you call an air conditioning engineer?

Call an air conditioning engineer if the system cools normally but never heats, if the outdoor coil ices up and does not clear itself, or if heating output has faded compared to previous winters. Cooling that works while heating does not is a strong pointer at the reversing valve, and that is a sealed-system job for an F gas certified engineer.

What will the air conditioning engineer actually do?

Confirm the model is reverse-cycle and check the controller configuration and any installer settings
Test the reversing valve solenoid electrically and verify the valve actually shifts under running conditions
Watch a full defrost cycle and check the defrost sensor and its termination logic
Take pressures and superheat in heating mode, where a low charge shows more clearly than in cooling
Clear the outdoor coil and check the drain path beneath it, which blocks with ice on many UK installs
Replace the valve, coil or sensor and recommission the system in both modes

Frequently asked questions

We’re happy to answer all your questions.

Sources & References

Ice On The Pipework Or Indoor UnitCaution System Keeps Needing RegassingCaution Remote Or Wall Controller Not RespondingLow risk

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