Vaillant eco tec 938 comfort mode overheating

[Jg81]

Hi all

Having an issue where our boiler is overheating up to 80 degrees despite neither central heating or hot water demand being on.

The ‘C’ symbol is flashing at the time of overheating, even though comfort mode has been programmed off. The overheating is causing a type of vibration noise and the only way to ‘fix’ it is to turn the boiler on/off

Any ideas how to fix?

Thanks in advance

Jonathan

 

[Jg81]

 

[CBW]

I don’t think comfort/preheat would work unless it was turned on. Feel the flow pipe under the boiler, could be passing diverter. Failing that, blocked canoe filter/plate heat exchanger

 

[Jg81]

thanks for replying. Comfort is definitely programmed off but when the issue arises, the ‘C’ is flashing on the LCD panel. I’m assuming this means that comfort mode is on unless it means something else?

I’ve checked on the boiler and no other fault codes are being logged.

Boiler was only serviced a few weeks ago and no issues at service.

 

[CBW]

C isn’t a Fault code. Any motorised valves?

 

[Jg81]

Yes I think there are 2 valves

 

[CBW]

Yes I think there are 2 valves

Could be an end switch sticking?

 

[Jg81]

Would this also cause the comfort mode to activate even if programmed off? Is there any way to test if it’s a sticking valve? Thanks again

 

[CBW]

Would this also cause the comfort mode to activate even if programmed off? Is there any way to test if it’s a sticking valve? Thanks again

Not usually, but have seen some weird things happen. If one is sticking, usually found by testing for voltage.