[paulbennett]

I'm a bit confused here; looking at the video and pics of your water heater/boiler, the knob the servo is turning looks like it sets the temperature the boiler heats the water to for the central heating loop. This is generally not the thermostat, and once set to a value you are happy with shouldn't need to be adjusted.

[krzyk]

Yes, it is not a thermostat, I have similar heater (also Vaillant) and it just sets the temperature of the water in the pipes (generally you set it higher when the outside temperature gets lower, and all the way up when it reaches the minimum at your location).