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

 

[stevetheplumber]

not familiar with these boilers but obvious things to check woulkd be frost stat either internal or external is there a preheat or warm start setting on these boilers thats gotten switched on?
id next remove the xternal controls and put a link in to decide if its external controls calling for heat or a boiler fault
if none of above your going to need someone with a multi meter to check where power is coming from

 

[mattyp]

How hot are the rads getting when this occurs ? The boiler will fire periodicaly for the hot water store [providing the h/w is switched on at the programmer ]. It is possible the diverter valve is letting by a little .

 

[robovanbasten]

Thanks for that. Think I'll check it with a multimeter to see what's calling for hot water. What's the easiest way of doing this?

 

[robovanbasten]

Hi Matt, yeah rads do get hot. Just had a new dv and water-to-water heat exchanger.... both fitted by worcester engineer.
I think it might be one of the relays on the PCB, and I don't think the relay can be changed with changing the whole PCB. But they are £80 which is a lot if I'm not 100% sure that's what's wrong.

 

[mattyp]

Do you have any motorised valves on the ch system, unlikely but a possibility if you have any underfloor heating ?

 

[robovanbasten]

Do I don't have any motorised valves.

Am I right in thinking that the boiler should only produce CH if the programmer is ON and room stat is ON?

 

[mattyp]

yes, the c/h should only come on when the programmer is on and with the roomstat calling for heat [turned up]

 

[robovanbasten]

See that's the thing, at times it does work like that. Turn the room stat up and down causes the boiler to engage/disengage. But sometimes turning the room stat down the boiler stays engaged (as though there's a relay on the PCB that's stuck!?).

 

[mattyp]

Hard to say without looking, you need to establish if you are getting 240 volts on the c/h switch live to the boiler, by rights when the controls are off the switch live should not be live ! if you dont have 240 on the switch live and the c/h is still on- its only going to be a fault on the boiler ! Hope that makes some sort of sense ! You will need a heating engineer to check this out for you .