Heat pump heating the cylinder much hotter than usual

What it usually means

Every heat pump manufacturer builds in a periodic high-temperature run of the hot water cylinder, and every manufacturer gives it a different name: Vaillant call it the anti-legionella function, Daikin call it disinfection, Mitsubishi call it the legionella prevention mode. The purpose is the same. Stored water at the forty-five to fifty degrees a heat pump prefers is an ideal temperature for legionella bacteria, so on a schedule the system drives the cylinder up past sixty degrees for long enough to kill them, then lets it drift back. Because a heat pump struggles to make water that hot efficiently, the cycle normally hands over to the electric immersion or back-up heater for the last few degrees, which is why the meter turns faster while it runs and why the hot water afterwards is noticeably hotter than usual.

The scheduling is where owners get caught out. Vaillant's default is four o'clock in the morning on a set day of the week; the Altherma 3 defaults to one in the morning on a day the installer chooses, taking the tank to seventy degrees and holding it for ten minutes; Mitsubishi's runs at a default three in the morning every fifteen days, to sixty-five degrees, held for half an hour. So the household experience is a cylinder unit that fires up at an hour when nothing should be running, an immersion heater that hums for an hour, a bathroom tap that is scalding at breakfast, and a controller that shows the water heating icon while the room thermostat is satisfied. None of that is a fault. It is arguably the one time a week the system is working hardest on purpose.

A genuine fault looks different in three ways. First, it produces a code: Daikin's AH means the disinfection cycle did not reach or hold its temperature, Mitsubishi's L4 means the tank went over its overheat limit, and Vaillant's F.1120 warns that the anti-legionella function cannot be guaranteed because the back-up heater has tripped. Second, it is not on a schedule: hot water that is very hot every day, or at random times, is an immersion running when it should not, and that is a control or sensor problem worth chasing because of what it does to the bill. Third, a real overheat trips something, a cylinder thermostat or a high-limit cut-out, and the hot water then stops rather than simply being hotter.

If the cycle itself is unwelcome, the answer is the schedule rather than the immersion switch. All three manufacturers let the day, time and target be changed from the controller, and Vaillant and Daikin allow it to be turned off, though they also say clearly that they do not recommend that, and the UK guidance on stored hot water agrees with them. What you should not do is disable the immersion heater at the isolator to stop the cycle, because on most systems that same heater is the frost protection and the back-up for the whole hot water function.