Ravenheat CS1 85 CH Turns itself on, then overheats

[mrnickyp]

Ravenheat CS1 85

I have a problem with the CH starting up on its own then overheating. The problem started with the DHW temperature fluctuating so I replaced the DHW thermistor sensor. This fixed the DHW problem. Last night I isolated the timer clock just leaving the room stat wired up to see if there was a problem with the timer. The CH still starts up on its own, randomly, and then overheats.

Here is a breakdown of the symptoms:-

No DHW in the morning. I need to fire up the CH for a minute then the DHW works.
When I return from work the red light is flashing and the CH rads are slightly warm.

What I've done:-

Reset boiler and switched to DHW only. CH still comes on, on its own.
Changed DHW thermistor to fix hot water. CH still comes on, on its own.
Isolated the timer. CH still comes on, on its own

The boiler has had a new PCB a few years ago and I have had to change the thermistors before.

Any help will be appreciated. Thank you.

 

[kevplumb]

sticky hot water switch by the sound of it

 

[mrnickyp]

sticky hot water switch by the sound of it

If the switch is at fault, would this turn on the CH if the timer is off and the room stat is at its lowest?

 

[kevplumb]

possibly as the boiler thinks there is a call for hw

 

[mrnickyp]

Sorry for the ignorance, but is the hot water switch the main DHW and CH reset switch located at the font of the boiler? Thanks.

 

[kevplumb]

no its inside the boiler

methinks time for a phone call

 

[mrnickyp]

Maybe so, Cheers.

 

[mrnickyp]

Is the "hot water switch" the microswitch that is part of the three way diverter valve. If so, could it be the valve pin that is sticking? Thanks.

 

[Agile]

Yes, thats what you should be checking.

Either pin not returning properly

OR

switch itself faulty ( not very likely but possible )


If you decide it may be the switch then disconnect a wire from it and see if it continues.

Can also be a high resistance or faulty CH thermistor making the boiler think its frosty!

I think they should be 25k at room temp but compare the two when the boiler is totally cold.

Tony