Heat pump faults: what you can check, and who can touch it

Two circuits, two trades

Every air source heat pump is two systems bolted together. On the water side — the primary circuit, the cylinder, the pump, the valves, the controller — it is a heating system like any other, installed and serviced by your installer, and there is a good deal an owner can see and report. On the refrigerant side — the compressor, the coil behind the fins, the pipework between the units, the charge — it is a sealed pressurised circuit that in the UK only an F-Gas certified engineer may open, recover from or recharge, as a statutory requirement rather than a manufacturer preference. Most of the value in reading a heat pump code is working out which side it is on, because that decides who you call, and the manufacturers give you the means.

Reading the code

Mitsubishi Electric print the rule themselves. The first character of an Ecodan code tells you where the fault is: L codes are the water circuit and cylinder, P codes are thermistors, J codes are the wireless controller link, E codes are wired communications, and U, F and A codes all mean outdoor unit failure, in Mitsubishi's own wording. That last group is the refrigerant and compressor side, and it is the group no owner and no ordinary electrician should be opening.

Daikin codes come in two forms. Older Altherma controllers show two characters — U4, E7, H3. Controllers from roughly 2021 add a suffix — U4-00, E3-24, 7H-08 — and the suffix narrows the cause, so 7H-01, 7H-07 and 7H-08 are three different situations and looking up only the first half of a modern code against an old table gives you the wrong component. Where the description begins "OU:", the fault is in the outdoor unit and therefore, in most cases, on the refrigerant or inverter side.

Vaillant run four families and only one is a fault. F codes are faults and take priority on the display, alternating every two seconds if several occur; S codes are normal running states; M codes are maintenance messages with the unit still running. The same fault is printed F.22 in owner-facing text and F.022 in the appendix, so search for both.

What you can check on the water side

These are observations, not repairs, and every one of them shortens the installer's visit.

• The pressure gauge. Read it and note the figure before you ring anyone. On a Vaillant, below 0.5 bar produces F.22 and below 0.7 bar produces the M32 maintenance message; on a Mitsubishi L9 or a Daikin 7H, low pressure is a cause in its own right. Then do not top it up. Vaillant's instruction for this appliance is to call your installer rather than repressurise, and it is deliberate: many heat pump circuits contain glycol antifreeze, and adding plain water dilutes it, which can leave the system unprotected in exactly the weather it most needs to be. Look for the leak while you wait — stains or limescale bloom around radiator valves and under the hydraulic station, and a dripping outside discharge pipe that points at the pressure relief valve rather than a leak indoors. Pressure that needs topping up more than about once a year means a leak to find, not a level to correct.
• Isolators and breakers. There is usually a separate isolator for the outdoor unit, and it is easy for one to have been left off after other work. Check the consumer unit for a tripped breaker or RCD on either supply. Reset a tripped breaker once, and once only: a heat pump is a high-current appliance, and a breaker that trips again is protecting the circuit for a reason. Leave it off and call an electrician or your installer.
• The controller. Is the system showing as prohibited or stopped? Is a schedule or holiday mode active? Mitsubishi put both at the top of their own list and note that a system doing nothing because it has been told to looks identical to one that has failed. Fresh batteries in a wireless controller: a controller with dying batteries stays lit while it stops communicating.
• What changed. Any work on radiators, valves or the cylinder in the last few weeks introduces air, and air is a named cause of low-flow codes on both Mitsubishi and Daikin. A power cut, an electrician, fencing or digging near where the cable runs outside are the usual stories behind Daikin's U4 communication fault. Look at the outdoor unit's cable where it enters for damage or chew marks at ground level — look only; do not open the electrical enclosure on either unit.
• The immersion switch. Often a separate switch near the cylinder, used in a cold snap and forgotten. It is behind Mitsubishi's L4 cylinder overheat more often than the cylinder is, and it is the usual reason a bill has jumped.
• The filter. Ask your installer when the magnetic filter was last cleaned. If the answer is never, that is very likely a low-flow code's cause, and cleaning it means isolating and opening the primary circuit, which is an installer job. Ask; do not attempt.
• The outdoor unit's surroundings. Clear snow, leaves and debris from the grille and the space around it, and check nothing has been put against it. That is a genuine owner job on every make. Do not pick ice off the fins or pour anything on the unit, for the reasons in the defrost section of Heat pump defrost and sterilisation cycles.

Resetting, and why the manufacturers discourage it

Daikin's position is unambiguous: resetting an error code should be done by a licensed installer or your local dealer, and their published code table is an installer reference with a description column and no remedy column at all. Mitsubishi's controller offers "Reset error? No / Yes", and it is worth understanding what that does: clearing the message does not clear the condition, and a code that returns after a reset is telling you something. The controller stores up to sixteen previous check codes with the date each occurred, so there is no need to leave a fault showing to prove it happened, and if the error screen shows a telephone number it is your installer's, entered at commissioning. Vaillant keep the fault memory — the last ten faults — and the ability to clear it in the installer menu rather than the owner one, and for a fault that has genuinely stopped the system the reset is the installer's job.

What you can do first, before any reset, is establish whether you actually have heat. Vaillant's limp-home mode and Daikin's warning-versus-error distinction both mean a live code does not always mean a cold house, and the difference between "code showing, house warm" and "code showing, house cold" is the first thing the installer will ask.

Where the refrigerant line falls, exactly

Most current heat pumps are charged with R32, an HFC inside the UK's F-gas regime, and the retained F-Gas Regulation makes certification a legal requirement for anyone who recovers, charges or breaks into the circuit. Mitsubishi's U, F and A codes and Daikin's OU: codes are that circuit. Vaillant's aroTHERM plus is different in kind and not in consequence: it runs on R290, which is propane — a hydrocarbon outside the F-gas regime, and flammable. Vaillant's manual carries a risk-of-death warning for a leak in that circuit and defines a protective zone around the outdoor unit in which the owner may not add sockets, lights, vents, openings or structures, because propane is denser than air and collects at ground level. Nothing about either refrigerant is owner work, and nothing about either is work for a general electrician or a heating engineer without the certificate. When you book for an outdoor unit code, ask specifically whether the engineer holds F-Gas certification.

Who to call

• Your installer for the water side and the controls: pressure and leaks, low flow, air, filters, pumps and valves, communication faults between the units, schedules and settings, the sterilisation cycle's frequency, and any reset. Mention the exact code including any suffix; it changes what they bring.
• An F-Gas certified heat pump engineer for anything on the refrigerant side: Mitsubishi U, F and A codes, Daikin OU: faults, progressive icing that a defrost does not clear, any suspected loss of charge. Within the manufacturer's guarantee, that call goes through the manufacturer or the installer who registered it.
• An electrician for a breaker that trips again after one reset, or any question about the supply to either unit.

Right now

If water is escaping indoors, isolate the system at its own switch or breaker rather than waiting for a call. Do not try to release pressure, top anything up, or open the refrigerant pipework. If you can smell burning or see scorching at either unit, switch the circuit off at the consumer unit. And do not switch a heat pump off for the winter to save money while you are away — use holiday mode, because Vaillant publish that an isolated installation in freezing conditions must be drained by a competent person, and a frozen water side is a far larger bill than the one you were avoiding.

What it costs

The brackets are those the code pages on this site carry for the codes involved.

• An installer visit is typically £90 to £180, and on a communication fault or a flow problem it often resolves the matter. A visit to repressurise and check runs £90 to £150.
• Finding and fixing a leak, or an expansion vessel: £150 to £450 depending on where it is.
• A circulation pump or zone valve: £180 to £400 fitted on the Mitsubishi L9 page, £250 to £450 for a pump on the Daikin 7H page. Filter cleaning and air purging add little beyond the visit.
• An immersion heater or its control: £250 to £450; a cylinder thermistor is inexpensive.
• A control board (Daikin U4 where the cable and power check out): £300 to £600 fitted.
• An F-Gas certified diagnostic visit on an outdoor unit code: typically £120 to £250. Beyond that the range is very wide — a pressure switch or fan motor a few hundred pounds, a compressor replacement past £1,000 and often the point at which the age of the unit decides it.
• An investigation into a jumped bill: £90 to £250, and set against a back-up heater running unnoticed, which can add hundreds of pounds a quarter, usually the cheapest call you will make.

Sources

• Mitsubishi Electric UK: Ecodan controller and service documentation for the check-code letter rule, the error history, "Reset error?" and the four first checks; Ecodan cylinder unit installation manual for the electrical and defrost warnings.
• Daikin UK: Altherma installer reference guide (code table, suffixes, OU: prefix, the resetting statement); Altherma operation manual for warnings versus errors, 7H-07 pump deblocking and U4.
• Vaillant UK: aroTHERM operating and installation instructions (F, S and M code families, F.22 thresholds, limp-home mode, the freezing-conditions instruction); aroTHERM plus VWL A S2 installation and operation manual, doc 0020330792-03 (R290, the protective zone).
• The retained F-Gas Regulation (Regulation (EU) 517/2014 as it applies in Great Britain) and GOV.UK "Fluorinated gases (F gases)" guidance, on certification for installing, servicing, leak-checking and recovering refrigerant from HFC equipment, and on the regime's scope (fluorinated gases, which hydrocarbons such as propane are not).
• Cost brackets are those published on the code pages for Vaillant F.22, Mitsubishi L9, L4, Outdoor unit failure and No code, and Daikin 7H, U4 and Bill has jumped, each with its own sourcing.