Boiler CH keeps cutting in

[robovanbasten]

Hi all,
I have a Worcester Bosch Highflow 400 OF that has an intermittent fault.
The central heating will cut in (i.e. gas ignites & pump runs) even if the attached programmer is set to OFF and the room stat is also set to OFF!!?

As I understand it the boiler should only produce CH if the programmer is either [set to ON (constant) or is within it's timed ON period] & the room stat is set above room temperature.

As I said the fault is intermittent, sometimes it behaves - but mostly not.

I've checked the room stat in conjunction with the programmer and when it's behaving it all seems ok, i.e.
If the programmer is set to ON (constant) or within a timed ON period, flicking the room stat above room temp (audible click) then the boiler ignites & pump runs - correct!

Anyone have any idea what's going on!?

Regards,
Chris

 

[Foobix]

The boiler has a heat bank for hot water.Could it not be the boiler is firing periodically to maintain the temperature in the heat bank.
Do the radiators heat up when the boiler is misbehaving?

 

[robovanbasten]

Hi Foobix,
I've turned the hot water off when doing my tests. So it's not that heat bank.