I want to sanity check that my installation is wrong and to warn others of the possibility that theirs might be too.

The Legionella cycle that is run periodically on the water cylinder is supposed to heat the water up to 60C or 65C (you can choose). I don't know, but suspect that the system uses the ASHP to get most of the way to that temperature, but then uses the immersion heater to do that last 10C - 15C. The immersion system rightly has a safety cut off thermostat to avoid overheating the unvented cylinder (serious issue if that goes wrong).

Surprisingly, the Ecodan FTC6 doesn't throw and error if the Legionella temperature is not reached. Based on the immersion heater being involved, there are two simple failure modes that can cause this to happen:

1. RCBO/MCB off: the immersion heater has its own breaker in the consumer unit (fuse box). This could have been left turned off or could have tripped.

2. Cylinder's Immersion Thermostat Set wrong: The immersion heater has a companion bimetallic thermostat that is there for safety and has no software connection to go wrong (or that allows monitoring). This thermostat has a safety cut off which is understandably likely to be mandatory for an unvented cylinder. If this thermostat is set too low, it will stop the immersion heater heating the water to the required temperature. Also, the resettable safety may have tripped.

Below is an photo of my immersion heater's thermostat which looks to be set to (3) = 40C. Feels like this should be set to (5) = 70C [SAFETY WARNING: there are obvious issues here playing with these settings]. Can somebody please check their installation to see it they have the same thermostat and what it is set to.

Ecodan FTC6 Installation Manual:

COTHERM TSE thermostat:

My Thermostat:

My Ecodan's attempt at Legionella cycle:

Based on the setting on my immersion you are just set to low. Turn it to max setting or just below, switch immersion on and see how hot your cylinder gets, adjust down a little at time.