My ravenheat 85 a works fine when asking for HW however when the CH is demanded the burner will ignite, burn for 20 seconds or so, then extinguish.
After10 seconds,it will reignite and cycle like this maybe 8 times, then sometimes settles down (rarely now)and works normally until the room stat shuts it down.
Any ideas out there what may be the problem ?
I have measured the CH thermistor, i'm getting 9.4 k/ohms when the system is cold,so I tried a 9k/ohm resistor across the ch wires, just to prove it.The system still cycles even with the fixed resistance in place.
One other thing is if I run a hot water tap for a few seconds the burner stays lit and the CH works until the room stat shuts it down!!!!and the therm' drops down to approx 4.5 k/ohms when the piping is hot which I assume is good
Any ideas out there are welcome its doing my head in.

billthespark

bill the spark, November 2008

HI ALL
jUST A BIG THANKS TO WAYNE ,HE WAS SPOT ON WITH HIS DIAGNOSIS
REPLACED THE FAN PCB AND ALL IS WELL.
CHEERS

BILLTHESPARK, November 2008

I left you an answer earlier will leave my e-mail for if you have any other questions :)
It would be advised to also change the hot water sensor (captive} and central heating (thats the clip on 1 on the right of the combustion chamber) as these are a constant source of many failures and cost very little.I would replace these first and if no improvement replace that fan control board.
As always it is recomended that a compitent corgi registerd installer be used to carry out any repair to a gas appliance.( Thats a gas man covering his ass) LOL

wayneburgess@hotmail.co.uk, November 2008

The fan control board to the left of the pcb is nackerd bud.

Wayne, November 2008