Heat pump servicing and winter maintenance for air-to-air units: what to check, what it costs, and common winter faults
A wall split used for heating needs one professional service a year, ideally before the cold arrives, at £60 to £150 per unit. Here is the checklist you can do yourself, what the engineer does, and how to tell normal frost from a real fault.
If your wall-mounted air conditioning unit is going to be your heating this winter, it needs one proper service a year, and autumn is the time to book it. Heat pump servicing for an air-to-air unit is a smaller job than the £150 to £300 boiler-style service quoted for air-to-water systems: expect £60 to £150 per indoor unit from a UK installer, an hour or so of work, and a clean, tested system going into the cold months. There is no legal requirement for an annual refrigerant leak check on a domestic split, but Daikin and Panasonic warranties do require yearly maintenance, so skipping it can cost you more than the service.
Most of what keeps a unit healthy in winter is in your hands and takes twenty minutes. This guide covers the pre-winter checklist, what the engineer does, what it costs, and the winter behaviour that worries people for no reason: frost on the outdoor unit, steam, puddles, and a blast of cool air while the unit defrosts. Our guides to air conditioning servicing and air conditioning maintenance cover the cooling side; this is the heating sibling. Still deciding whether to buy? Start with the air-to-air heat pumps guide.
How often does an air-to-air heat pump need servicing?
Once a year, by an F-Gas certified engineer. Daikin's user reference guide for its Perfera range says "the installer has to perform a yearly maintenance", and every UK installer and cost guide we checked recommends the same interval for a domestic split. A unit that cools in July and heats in January works ten months of the year, so if heating is the main job, the autumn visit is the one that matters.
Book before the first cold snap rather than during it. Engineers are quietest in September and October, and you would rather learn that a unit is not heating properly on a mild day than at 8pm in December.
The pre-winter checklist you can do yourself
None of this needs tools or touches the refrigerant circuit, which is the line you must not cross.
Clean the indoor filters. Slide them out, vacuum or rinse them, let them dry. Daikin and Mitsubishi Electric both say every two weeks in use. A clogged filter is the most common reason a unit "does not heat like it used to".
Look at the indoor coil. With the filters out, shine a torch on the aluminium fins. Grey fur means the unit needs a professional coil clean.
Clear the outdoor unit. Remove leaves, bins, planters, and overgrown shrubs from around the inlet at the back and the fan at the front. Both manufacturers list blocked inlets and outlets as a "does not heat" cause.
Check the defrost drainage. In heating mode the outdoor unit drips water every time it defrosts. If that water runs onto a path it freezes into a slip hazard; if it pools under the unit it refreezes into an ice block. A drain elbow and hose to a gully, or a bed of gravel, fixes both.
Run heat mode for half an hour. On a cool evening set the unit to heat at 24°C. Warm air should arrive within a few minutes (Daikin says about three) and keep coming. If it never warms up, report the fault while it is still cheap to fix.
Replace the remote batteries. Daikin says they last about a year. A flat remote is a surprisingly common "the unit is broken" call-out.
Check the app schedule. A cooling timer set in summer may still be running. Reset it for heating, and dig out the warranty paperwork while you are at it.
What a professional heat pump service includes
A domestic split service takes 45 minutes to two hours. Installer service pages and the manufacturers' guidance agree on the contents:
Wash the filters; clean the indoor coil, fan barrel, and drain tray, with an anti-bacterial treatment if needed (some firms charge £15 to £30 extra).
Flush the condensate drain so it does not block or freeze.
Clean the outdoor coil and check the fan runs freely.
Check refrigerant pressures and running temperatures in heating mode. A sealed split does not use up refrigerant, so a low charge means a leak to find, not a routine top-up. If a firm quotes a regas as standard, ask why.
Inspect electrical connections, the isolator, and the pipe insulation, which UV and birds destroy within a couple of summers.
Run the unit in both modes, check the temperature rise across the indoor coil, read the fault history, and confirm the defrost cycle triggers and clears.
Write it up. Daikin's warranty policy says "maintenance records must be kept that can be inspected on request".
Heat pump service cost for an air-to-air unit
Prices correct as of September 2026. Installers price per indoor unit; ask for the per-unit rate before agreeing a multi-split job.
| Service | Typical UK price | Notes |
|---|---|---|
| Standard service, one wall split | £60 to £120 | Checkatrade and MyJobQuote quote £70 to £120; RightAir Solutions £60 to £90 outside London |
| Standard service, London and South East | £85 to £150 | RightAir £85 to £140; Efficient Home Energy £99 basic, £150 standard |
| Deep clean (indoor and outdoor coils washed) | £95 to £220 | Every two or three years, or if the unit smells |
| Maintenance plan | £8 to £40 a month | One service plus priority call-out; check what a call-out actually costs |
| Call-out for a fault | £130 to £180 weekday, £170 to £220 weekend | Checkatrade figures, first hour included |
| Air-to-water heat pump service (comparison) | £150 to £300 | Checkatrade average £200; British Gas £189. A different product and job |
The comparison row matters because almost everything written about "heat pump servicing" in the UK is about air-to-water systems. British Gas's £189 checklist covers the expansion vessel, radiators, hot water cylinder, and magnetic filter, none of which your wall split has. Do not let a boiler-style price be quoted for an air conditioning service.
Is an annual F-Gas leak check a legal requirement?
For almost every home unit, no, and several installer websites get this wrong. The UK's retained F-Gas regulation requires leak checks only on equipment holding 5 tonnes of CO2 equivalent or more (every 12 months at that level, more often for larger systems). CO2 equivalent is the refrigerant charge multiplied by its global warming potential.
Do the sum for a typical split. Daikin's Perfera FTXM35R holds 0.76kg of R32, and R32 has a GWP of 675. That is 0.51 tonnes of CO2 equivalent, a tenth of the threshold. A large multi-split holding 2kg is still only 1.35 tonnes. The figure is printed on the outdoor unit's data plate if you want to check yours.
So the annual service is a warranty condition and good practice, not a legal duty on you as the owner. What is legal and non-negotiable is who touches the refrigerant: under the same regulation only a certified person may leak-check, recover, charge, or repair the circuit. Confirm the engineer's company is REFCOM registered (or equivalent) before they open a valve.
Will skipping a service void the warranty?
Usually yes, and the wording is clear once you find it.
Daikin offers 3 years through a Certified Installer (5 years purchasable) and 5 years through D1 and D1+ partners, with up to 7 years quoted on its installer page. The policy requires the equipment to be "installed, commissioned and maintained throughout the warranty period by the same contractor, and in accordance with the manufacturer's recommendations". Its air-to-water page is blunter: "a warranty is only valid if annual maintenance is carried out."
Panasonic offers 3 years as standard, 5 through an accredited installer, and 7 through a PRO Partner, registered within 30 days. Its warranty key facts say the products "shall be required to undergo maintenance every year and in compliance with the manufacturer's guidelines", by a suitably qualified party, and exclude "defects resulting from failure to use or maintain the Products in accordance with the operating instructions".
Mitsubishi Electric does not publish its residential air conditioning warranty terms on a public page. Its service page promotes regular servicing by REFCOM Elite certified engineers, and the condition will be on the paperwork your installer gave you. If the extended warranty is tied to the installing firm, stay with them for the warranty period.
A £90 service is cheap insurance on a £3,000 unit with a seven-year warranty that depends on it.
Winter behaviour that is normal
Most winter "faults" on an air-to-air unit are the unit doing its job. In heating mode the outdoor coil runs colder than the outside air, so on damp days near freezing it frosts up. Daikin's user guide: "frost might occur on the outdoor unit and decrease the heating capacity. In that case, the system automatically switches to defrosting operation to remove the frost. During defrosting operation, hot air is NOT blown from the indoor unit."
What that looks like from the sofa:
Frost on the outdoor unit. An even white coating on the fins is normal. Daikin's UK advice is "Yes, that is perfectly normal and you certainly do not have to worry", and not to chip or scrape it off.
Heating stops and you hear water. Defrost reverses the refrigerant flow to melt the frost. Daikin quotes 3 to 10 minutes and Mitsubishi about 10; some installers say allow 20 before worrying. The indoor fan slows or stops and you may get a short blast of cool air. Do not switch the unit off mid-defrost; you only restart the cycle.
Steam from the outdoor unit. The melted frost flashes to vapour as heating resumes. Daikin: "Moisture generated by defrost becomes steam and is exhausted."
Water on the ground. The same melt runs out of the base, and Daikin notes "a small amount of water is also discharged during a heating operation" even without defrost. Manage where it goes; do not try to stop it.
More defrosts in cold, wet weather. A raw 2°C drizzle causes more frosting than a dry minus 5°C, and it is one reason output drops on the worst days. Our guide to air-to-air heat pumps in cold weather has the numbers.
Common winter faults: symptom, likely cause, what to do
| Symptom | Likely cause | What to do |
|---|---|---|
| Cool air in heat mode for a few minutes | Warm-up (about 3 minutes) or defrost (3 to 10 minutes) | Wait 20 minutes. Look for frost or steam outside. Do not switch off |
| Cold air continuously | Mode set to cool, fan, or dry; on a multi-split, another indoor unit set to cooling | Check the heating symbol on the controller and app. All indoor units on a multi-split must share a mode |
| Nothing running at all | Tripped breaker or isolator, timer or schedule, flat remote batteries | Check all four before calling anyone |
| Heats weakly, never reaches set point | Dirty filters or coil, blocked outdoor unit, or a cold snap at the unit's limit | Clean filters, clear the outdoor unit. Still poor above about minus 5°C? Book a service |
| Solid ice over the coil, fan iced in, ice still there hours after a defrost | Defrost sensor fault, low refrigerant from a leak, blocked airflow, or melt refreezing at the base | Switch off at the controller and call an engineer. Never pour hot water on it or chip at it |
| Ice building from the ground up | Defrost water pooling and refreezing | Fix the drainage: hose to a gully, gravel bed, or raise the unit on its brackets |
| Gurgling from the indoor unit | Refrigerant flow in defrost, or outside air entering the drain | Normal. If constant, ask about a drain check valve at the next service |
| New rattle, hum, or grinding outside | Fan catching ice or debris, loose panel, failing fan motor | Switch off, look for debris. If it persists, call an engineer |
| Burning smell, breaker tripping, water from the indoor unit | Electrical fault or blocked condensate drain | Switch off at the isolator and call an engineer. Daikin lists all three as "contact your dealer" faults |
| Error code or flashing lamp | Sensor, communication, or protection fault; a flashing lamp with dirty filters can be the filter reminder | Photograph the code, clean the filters, restart once at the isolator. If it returns, report the code |
If the unit is cooling badly rather than heating badly, the causes overlap but the fixes differ; see our guide to an air conditioner not cooling.
When to call an engineer
Call one when the unit is frozen solid, when an error code returns after a restart, when output is poor on a mild day with clean filters, or when anything electrical looks or smells wrong. Call immediately for a burning smell or a tripping breaker. You can find a local engineer in our directory.
Never try to fix a refrigerant problem yourself. Opening the circuit, charging it, or repairing a leak without F-Gas certification is a criminal offence, and low refrigerant always means a leak that a proper engineer needs to find first. Low charge also makes the outdoor coil run abnormally cold and ice up, so "my heat pump is frozen" is often "my heat pump is leaking".
When you book, say the unit is used for heating and you want it tested in heat mode, including a defrost. A summer-style service on a cool day can miss a defrost sensor fault that only shows up in January.
Frequently asked questions
How often should an air-to-air heat pump be serviced? +
Once a year by an F-Gas certified engineer, plus filter cleaning every two weeks in use. If you heat with it, book the service in September or October so faults show up before the cold does.
How much does a heat pump service cost in the UK? +
For a wall-mounted air-to-air unit, £60 to £150 per indoor unit as of September 2026, with London at the top of the band and deep cleans up to about £220. The £150 to £300 figures quoted elsewhere are for air-to-water heat pumps with radiators and a cylinder.
Is an air source heat pump service the same as an air conditioning service? +
Not if "air source heat pump" means an air-to-water unit. Your wall split is an air-to-air heat pump, and its service is an air conditioning service run in both modes: filters, coils, drain, pressures, electrics, and a defrost test. Book an air conditioning company, not a heating engineer.
Is ice on the outdoor unit normal? +
An even coating of frost that clears every so often is normal in heating mode. A solid block of ice, ice on the fan blades, or ice still there hours after a defrost means a fault: blocked airflow, a defrost sensor, a refrigerant leak, or drainage refreezing. Switch off and call an engineer rather than chipping at it.
Why is my heat pump not heating? +
In order of likelihood: the mode is not set to heat (or another indoor unit on a multi-split is cooling), the unit is defrosting, the filters or outdoor unit are blocked, the remote batteries are flat, a timer has switched it off, or the breaker has tripped. If all of those are fine and output is still weak on a mild day, the refrigerant charge or a sensor is the next suspect.
Do I legally need an annual F-Gas leak check? +
Not for a normal home unit. The retained F-Gas regulation requires leak checks from 5 tonnes of CO2 equivalent, and a typical R32 split holds around 0.5 tonnes. The service is still a warranty condition with Daikin and Panasonic, but not a legal duty on you.
Can I service the unit myself? +
You can clean the filters, clear the outdoor unit, sort the drainage, and change the remote batteries. You cannot open the refrigerant circuit, check pressures, or add refrigerant; that needs F-Gas certification and is illegal without it, and most warranties require the annual maintenance to be done by a qualified engineer.
Book a pre-winter service
A heating-mode service by an F-Gas certified engineer costs £60 to £150 per unit and keeps your warranty valid. Compare local air conditioning companies before the cold arrives.
For the rest of the winter cluster, see the heating guides hub and our other running and maintenance guides.
