Vaillant [Sterilisation cycle]what it means

Once a week or so, at an hour when nobody is using hot water, a Vaillant heat pump system does something that looks wrong. The immersion heater status appears on the indoor unit, the cylinder climbs well past its normal setpoint, the electricity meter runs faster than it has all day, and the water at the tap the next morning is hotter than usual. Vaillant call this the anti-legionella function and they publish exactly how it behaves. On the sensoCOMFORT controller it has two settings: the day, which defines the days on which the function runs, and the time, which has a factory setting of 04:00.

On those days the water temperature is increased to above 60 °C, the circulation pump is activated, and the function ends after 120 minutes at the latest. If the Absence function is active the cycle is skipped and runs as soon as the absence ends. And Vaillant state plainly why the immersion heater is involved: heating installations with heat pumps use the back-up heater for legionella protection, because the heat pump itself is run at a cylinder temperature of 50 to 55 °C for efficiency and the last few degrees are not its job.

The reason it exists is not cosmetic. Vaillant’s indoor unit manual carries a danger warning headed risk of death from legionella: the bacteria multiply at temperatures below 60 °C, and the owner is told not to set any water temperature below 60 °C without consulting the competent person first. So the cycle is the system making up for the low everyday setpoint. The status codes it produces are all S codes — S.135 domestic hot water: immersion heater active, and S.134 while the compressor does the first part — and none of them is a fault. What is a fault is when the cycle goes wrong: F.1100 if the back-up heater’s safety cut-out opens, F.1120 if the heater loses a phase, F.821 if its flow sensor fails, or hot water that is scalding every day rather than once a week, which is a cylinder sensor or control problem and not the cycle at all.

1. 1

   Check the time and day it happened against the controller’s anti-legionella settings. Vaillant’s factory time is 04:00. If it matches, nothing is wrong.

2. 2

   Run hot taps with care the following morning. Vaillant’s cycle takes the cylinder above 60 °C, which is hotter than the everyday 50 to 55 °C the heat pump holds, and a mixer set for the usual temperature will deliver hotter water.

3. 3

   Do not switch the function off to save electricity. Vaillant’s danger warning is that legionella multiply below 60 °C, and the everyday setpoint sits below that on purpose. If you think it runs too often, ask your installer to review the day setting rather than disabling it.

   Vaillant tell owners not to set any water temperature below 60 °C without consulting the competent person first. The anti-legionella cycle is what makes the lower everyday setpoint safe.
4. 4

   If the water is too hot every day, or if F.1100, F.1120 or F.821 appears, that is a fault in the back-up heater or its controls rather than the cycle. Book the installer and mention the code.