ravenheat csi85at starting up on its own?

[pcage49]

my 8 year old ravenheat csi85at occasionally starts up on its own with no external signal.
There is no frost stat fitted and the timer is always set to manual on and the control is via the room thermostat input, even with this link removed the boiler tries to run.
This would not be a problem normally but the boiler is linked in with a solid fuel boiler and so has a zone valve, this is shut if gas boiler is not being told to run, therefor boiler tries to run agains a closed valve.
Is there a built in frost stat as the problem seems to have got worse these cold days, although other winters have been ok.
Anybody got an idea?
Thanks,
pcage49

 

[Slugbabydotcom]

One of the functions of the CH thermistor is to act as a frost stat

 

[pcage49]

thanks for reply, I changed the thermister about 2 years ago after it failed, but I did buy a second spare so could put this in.
I have made a temporary repair by putting my run signal through a relay that breaks the low water pressure switch circuit, this switches boiler off ok but not happy with it as a long term fix.

If it is thermister, I am miffed why it has never started up in other winters that were a lot colder than this year???

thanks,

P. Cage