Heat pump defrost and sterilisation cycles: not faults

Two things a heat pump does that look like faults

A heat pump has two routines that an owner watching it for the first time reads as failures. One happens outside, in cold damp weather: the outdoor unit ices up, then steams. The other happens indoors, in the small hours: the hot water cylinder heats far beyond its normal temperature, the immersion or back-up heater runs, and the taps deliver water hot enough to hurt. Both are by design, both are documented by every manufacturer in this corpus, and neither has a fault code — which is why they are among the most searched heat pump questions and why nobody finds the answer on a code page.

The defrost

An air source heat pump pulls heat out of outdoor air by running its coil colder than the air. In cold, damp weather moisture freezes on the fins, and the unit periodically reverses to melt it. Vaillant describe what you see — a cloud of vapour rising off the outdoor unit — and their instruction is one sentence: you do not have to do anything. If your radiators went cool at the same time, that is the same event; the defrost takes its heat from the heating circuit and gives it back within a few minutes. Daikin's account is the same: a unit that frosts and then clears itself, with vapour coming off it as it does, is working exactly as designed, and the right response is to watch it for a full cycle before doing anything.

What you can do is keep the air path clear. Leaves, snow, garden debris, a bin, a fence panel, a stack of pots — anything against the grille or in the space around the unit starves it of the air it needs, and Mitsubishi list clearing it as the first owner action on any outdoor unit code. Check that meltwater can drain away from underneath; Daikin note that a blocked drain path turns one night of frost into a week of accumulating ice.

What you must not do is help. Daikin publish it as a safety instruction rather than a preference: do not use means to accelerate the defrosting process other than those recommended by the manufacturer, and do not pierce or burn refrigerant cycle parts. Mitsubishi's cylinder unit manual carries the same sentence. The fins are thin aluminium with refrigerant pipework directly behind them; most current units are charged with R32, which Daikin label mildly flammable, and Vaillant's aroTHERM plus runs on R290, which is propane. Puncturing a fin turns a service call into an incident, and the refrigerant is odourless, so a leak gives you no warning.

When to call: if the ice never clears, if it builds progressively from one cycle to the next, or if it is forming in the tray under the unit rather than on the fins. Vaillant and Daikin both name that pattern as a fault — a blocked condensate drain or a failed defrost — and it gets worse rather than better.

The sterilisation cycle

Legionella bacteria multiply in water between about 20°C and 45°C and are killed at 60°C and above, which is why HSE's guidance on hot water systems has stored water kept at 60°C; Vaillant's controller manual puts the same point under a risk-of-death heading. A heat pump ordinarily stores water cooler than a boiler would, because that is where it is efficient, so every manufacturer builds in a periodic heat-up that takes the cylinder above 60°C. Vaillant call it the anti-legionella function; Mitsubishi call it legionella prevention mode, LP mode, and show a Legionella icon on the controller while it runs; Daikin call it the anti-legionella or disinfection function. Owners call it the sterilisation cycle, and it produces three symptoms at once — a cylinder roaring in the night, the back-up or immersion heater running, and water at the tap hot enough to scald — each of which looks like a fault and none of which is.

What each manufacturer publishes:

• Vaillant (sensoCOMFORT and VRC 700 controls): the installer sets an anti-legionella day — one day of the week, daily, or off, which is the factory setting — and a time, with 04:00 as the default in Vaillant's system set-up guidance. At that time the cylinder is heated to a target of 70°C, the circulation pump runs, and the function ends when the cylinder sensor has read above 60°C for more than sixty minutes, or after 120 minutes regardless, so that it cannot get stuck if hot water is being drawn at the same time. On a heat pump system the control uses the back-up heater for it. If the absence or days-away function is active the cycle is skipped and run on the first day back. Vaillant's set-up guidance recommends a thermostatic mixing valve, because a cylinder at 70°C delivers scalding water without one.
• Mitsubishi Electric (Ecodan with the FTC controller): LP mode raises the stored water above 60°C. The defaults are a 65°C target, a heat-up every fifteen days, a start time of 03:00, a maximum of three hours for the heat-up, and thirty minutes held at the target once it is reached; the installer can set the frequency anywhere from daily to monthly and the target between 60 and 70°C. Mitsubishi are explicit about the cost: LP mode uses the electric heaters to supplement the heat pump, heating water for long periods is not efficient and will increase running costs, and the installer should weigh the need for it against wasting energy. Their manual also says the end user should understand the importance of the feature — which is this section.
• Daikin (Altherma): the disinfection function is scheduled once a week by factory default, adjustable by the installer at start-up, and it works even when the domestic hot water tank is disabled or the system is in holiday mode. If the tank is switched off during the disinfection the system reports error code AH, which is the one place this cycle does produce a code. The owner can also trigger it manually — Daikin give the example of a holiday home whose electricity has been off for a long time.

What this means in practice. If your immersion heater runs for an hour or two in the small hours on a schedule — weekly, fortnightly, or on the day your installer chose — that is the cycle, not a fault, and it is the most expensive hour of the week by design. If the water at the tap is briefly much hotter than usual the next morning, that is the same cycle, and if there is no mixing valve on the outlet, ask your installer about one. If you want the frequency, time or target changed, that is an installer-level setting on all three makes; Mitsubishi and Vaillant both note that local guidance governs how often it should run, so do not simply switch it off. And if your electricity use has stepped up permanently rather than for an hour a week, that is a different question, answered in the next section.

Running all the time, and the bill

A heat pump is meant to run long and low. Where a boiler heats hard and stops, a heat pump holds a low flow temperature for hours, and a unit that seems never to switch off is usually the system working. Vaillant and Daikin both publish the corollary: turning the setpoint up will not heat the house faster, and on most systems it costs more, because a higher flow temperature is less efficient.

The signal worth acting on is the bill. Compare your electricity use against the same period last year, not against last month — heating varies with weather — and if it has stepped up with no change in habits, the likeliest cause is the back-up or immersion heater doing work the heat pump should be doing. Daikin treat this as the most important heat pump fault with no code. Check whether anyone has switched the immersion on manually — there is often a separate switch near the cylinder, used during a cold snap and forgotten — and look for any warning or maintenance message on the controller, including ones already cleared. Then ask your installer the specific question: is the back-up heater being called, and by what? Give them the consumption figures; that is the evidence. Mitsubishi's L4, a cylinder overheat, is often the same thing seen from the other side — the immersion running when it should not.

Other messages that are not faults

• Vaillant S codes are status codes describing normal operation and M codes are maintenance messages where the unit keeps working; only F codes are faults. Comfort protection mode means the system has reduced what it does rather than stopped, and Vaillant's limp-home behaviour means a live fault code does not always mean a cold house — check whether you have heat before treating it as an emergency.
• Daikin warnings are distinguished from errors on the home screen; a warning means the unit is still running. 7H-07 is pump deblocking, an anti-seize routine that runs the pump deliberately during idle periods, and nothing is wrong.
• Mitsubishi's controller stores up to sixteen previous check codes with dates, so a code you cleared is still on record for the engineer; there is no need to leave a fault showing to prove it happened.
• Nothing happening and no code is, four times in five on Mitsubishi's own account, one of four things: the controller showing the system as prohibited or stopped, a schedule or holiday mode, dead batteries in the wireless controller, or a tripped breaker or an isolator left off. Reset a breaker once only.
• Water at the tap too cold is usually the hot water schedule reheating overnight and a cylinder emptied by the evening, or a low setpoint. Daikin's powerful operation is for one-off demand and uses the immersion, so it is not a permanent fix.

One rule for winter: if you go away, use holiday mode rather than switching the system off. Vaillant publish the reason — the installation must remain in operation during freezing conditions with all rooms sufficiently heated, or be drained by a competent person — and isolating a heat pump in January risks freezing the water side. If a code is genuinely showing, Heat pump faults: what you can check, and who can touch it draws the line between the water circuit and the refrigerant circuit.

Sources

• Vaillant: aroTHERM operating instructions (vapour and defrost, "you do not have to do anything"; freezing conditions); VRC 700 installation instructions §7.9.3–7.9.8 and sensoCOMFORT VRC 720 installation instructions (anti-legionella day and time, 70°C target with 5 K hysteresis, >60°C for 60 minutes or 120 minutes maximum, back-up heater, absence function); aroTHERM plus system schematics and set-up guide (04:00 default, TMV recommendation). Fetched 3 September 2026.
• Mitsubishi Electric: Ecodan cylinder unit installation manual (EHPT20X series), Legionella Prevention Mode settings table (65°C, 15 days, 03:00, 3 hours, 30 minutes; electric heaters; running costs; local guidance), the defrost-acceleration warning, and the four first checks from Mitsubishi's UK support.
• Daikin: "How does the anti-legionella function work?" FAQ (weekly default, holiday mode, error AH, manual trigger); Altherma operation manual (defrost acceleration and piercing warning, R32 labelling, powerful operation, setpoint advice).
• HSE, HSG274 Part 2, "The control of legionella bacteria in hot and cold water systems", on 60°C storage and the 20–45°C growth range.
• The published meaning of each code here is on its own page, with the source it was taken from.