my friends boiler wont turn off when you turn the thermostat on the boiler to the off position, he has been told that some potterton boilers do this due to stop legionnaires disease, he has a combi potterton boiler, have you ever heard of this?

Paul, February 2010

The only thing concerning legionnaires disese that i know of applies to stored hot water (hot water tanks), where the temperature should be above 50 deg C or below 20 deg C in order to prevent legionella from forming. this should ideally be controlled by a cylinder thermostat rather than the boiler thermostat, as that way you can have different temperatures for the radiators and hot water without being scalded by the water.
This would not apply to a combi boiler as it is fed from the mains water supply.
It may be that the boiler has a pre-heat function for the hot water, or the flow switch may be stuck on.

Ian, February 2010