Air Conditioning Not Heating: Causes, Checks and Repair Costs
Modern air conditioning heats as well as it cools, so a unit that blows cold in heat mode is a genuine fault — not a limitation. Here are the causes in the order engineers find them, what is safe to check yourself, and what each repair costs in 2026.
Why your air conditioning is not heating
An air-to-air unit in heat mode is a reversible heat pump: it moves heat from outside into the room. If it runs but stays cold, the usual causes — in order — are the wrong mode or set-point, dirty filters, an iced-up outdoor unit or a stuck defrost cycle, low refrigerant from a leak, or a stuck reversing valve. The first three you can often sort yourself; refrigerant and the reversing valve need an F-Gas certified engineer. Cold air for a few minutes while the outdoor unit steams is usually a normal defrost, not a breakdown.
The causes, in the order engineers find them
Heating faults split between settings and airflow (most call-outs), a winter-specific defrost or icing problem, and — less often — refrigerant or the four-way reversing valve that lets the system heat at all.
1. Wrong mode, set-point or a normal defrost
Check the remote is in heat (sun symbol) rather than cool, dry, fan or auto, and that the set temperature is above room temperature. On a multi-split every indoor head must be in the same mode — one left in cool will block the others from heating. If the unit blows cool for a few minutes while the outdoor coil hisses and steams, that is a defrost cycle clearing frost from the outdoor unit and is completely normal in cold, damp weather.
2. Dirty filters or a blocked coil
Clogged filters starve the indoor coil of air, so even when the system is making heat, little reaches the room. Lift the front panel, slide the filters out, wash in lukewarm water, dry and refit. If it is over a year since a service, the coil behind them may be furred up and need a professional clean.
3. Outdoor unit iced up or defrost stuck
In heat mode the outdoor coil runs cold and naturally gathers frost; the unit should melt it automatically. If the whole outdoor unit is caked in ice, or it defrosts constantly and never settles into heating, the defrost sensor, control board or outdoor fan may have failed. Switch off and let any ice thaw before an engineer looks at it.
4. Low refrigerant — which always means a leak
Heating actually shows a low charge sooner than cooling does, because the system has to work harder to pull heat from cold outdoor air. Splits are sealed and do not use up gas, so a low charge means refrigerant has escaped through a flare joint, corroded coil or damaged pipe. Only an F-Gas certified engineer may find and fix the leak, pressure-test and re-gas — see our re-gas cost guide for why a top-up alone is not an option.
5. Stuck reversing (four-way) valve
The reversing valve is the part that flips the refrigerant flow so the same unit can heat as well as cool. If it sticks or its solenoid coil fails, the classic symptom is a unit that cools fine but will not heat (or the reverse). It is a specialist repair: the refrigerant must be recovered, the valve replaced and the circuit re-charged, so it is one of the pricier fixes short of a compressor.
6. Capacitor, fan motor, PCB or compressor
If the outdoor fan will not turn, or the unit hums and clicks but never starts, a run capacitor or fan motor has usually failed — routine, same-day repairs. A dead control board (PCB) or a worn compressor is rarer and, on an older unit, a moment to weigh up repair or replace.
Can air conditioning really heat a UK winter?
Yes. Air-to-air heat pumps deliver useful heat well below freezing, though capacity tapers as it gets colder. These are indicative figures for a typical R32 wall-split — your unit's data sheet is the authority.
Indicative only, expressed against rated output at 7°C outdoor. Real capacity depends on the model, its SCOP and how it is sized — a well-sized R32 split typically returns a SCOP of 3.5–4+, meaning 3.5–4 units of heat per unit of electricity.
What you can check yourself — and what needs an F-Gas engineer
Anything behind the filter panel or inside the outdoor casing involves mains voltage, a pressurised refrigerant circuit or both. Clean, look and reset — never open, connect or top up.
| Check | Safe to do yourself? | Why |
|---|---|---|
| Confirm the remote is in heat mode with the set-point above room temperature | Yes | The single most common cause of a "no heat" call-out |
| Clean or replace the indoor filters | Yes | No tools, no electrics — just the front panel and a sink |
| Let a fully iced outdoor unit thaw, keep 30 cm clear around it | Yes | Restores airflow; never chip ice off the coil or fins |
| Switch off at the isolator, wait ten minutes, restart | Yes | Clears many protection lock-outs and some error codes |
| Open the outdoor unit, test the reversing valve, capacitors or wiring | No | Capacitors hold a charge after power is off; live terminals inside |
| Add refrigerant, open valves or fit gauges | No — illegal without F-Gas certification | Venting refrigerant is an offence and a top-up without a leak repair is wasted |
If a check on the left fixes it, book a service anyway — neglect brings the fault back and cuts winter efficiency.
Typical repair cost by cause (2026)
Call-outs typically run £80–£150 and include diagnosis plus minor work; parts are extra and refrigerant work is priced separately. Ranges are UK baselines — get quotes for your unit.
| Cause | Typical fix | Typical cost |
|---|---|---|
| Wrong mode / set-point / normal defrost | Settings check — no repair needed | £0 DIY |
| Dirty filters / blocked coil | DIY filter wash, or engineer coil clean as part of a service | £0 DIY · £75–£150 service |
| Iced outdoor unit / defrost sensor or fan | Thaw, then replace sensor or fan — call-out plus part | £80–£250 |
| Low refrigerant (leak) | Find and fix leak, pressure test, re-gas | £100–£300 re-gas + leak repair |
| Stuck reversing (four-way) valve | Recover gas, replace valve, re-charge | £350–£600+ |
| Capacitor, fan motor or PCB | Replace on the day — call-out plus the part | £80–£150 + part |
| Compressor | Quote basis — compare with a new unit | Often uneconomic vs £1,500–£3,000 new |
Baselines: call-out £80–£150; re-gas £100–£300; single split installed £1,500–£3,000. Full breakdown in air conditioning repair costs.
When "not heating" means it is time for a new unit
A reversing valve or compressor failure on an older system is the tipping point. Age: past 10–15 years, efficiency drops and spares get scarce. Refrigerant: R22 cannot legally be re-gassed and R410A is being phased down under F-Gas quotas — today's units run on R32 and heat more efficiently. The 50% rule: if the repair quote passes half the £1,500–£3,000 price of a new single split, a new, warrantied unit usually wins — and a modern one will heat your room far more cheaply. Daikin and Mitsubishi Electric carry strong cold-climate heating performance and 5–7 year warranties through accredited installers. Filter, capacitor or fan: repair. Reversing valve or compressor on an old R410A system: read repair or replace? and get both quotes. Still cooling but not heating? Compare notes with our not cooling guide — the reversing valve links the two.
Sources & methodology
- Fault order and the reversing-valve symptom reflect the diagnostic sequence in manufacturer service manuals (Daikin, Mitsubishi Electric, Fujitsu, Panasonic, LG, Samsung).
- Refrigerant handling: GB F-Gas Regulation (retained Regulation (EU) 517/2014) — only certified persons may recover, charge or check for leaks; venting is an offence.
- Running-cost context: Ofgem price cap 1 Jul–30 Sep 2026, electricity ~26.11p/kWh; heat-pump SCOP 3.5–4+ per manufacturer data.
- Price baselines are 2026 UK figures used across this site: call-out £80–£150, re-gas £100–£300, service £75–£150, single split installed £1,500–£3,000.
FAQs
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