Ravenheat combi boiler continually firing then shutting down?

My 3 year old Ravenheat CSI 85A combi boiler developed the following fault:
Firstly, the domestic hot water worked perfectly. However when the boiler fired up for the central heating the following happened: boiler calls for heat, pump and fan run and the ignition ignites. The burner stayed alight for about 3 seconds then went out; only for the whole start up process to begin again and again. I then discovered that if I turned on a hot tap the domestic hot water would run OK, but when the tap was switched off the boiler would continue to run until the roomstat turned it off. After that it would start the re cycling process over again.
I used the Ravenheat fault guide and concluded that the Dungs gas valve was faulty so replaced it at a cost of £160. The fault remained.
I managed to track down the problem and fixed it. The fault lies with the fan circuit board (the small printed circuit board to the left of the main circuit board), the permanent fix is to replace it. It would appear that the fan has 2 speeds on this model and one of the speeds was not coming on line, hence the fault. BUT to get you out of a 'cold' spot try this. The fan pcb has a 11 pin connector to it. Electrically isolate the boiler first then link out terminals 9 and 10 with a bent pin (if your board is not marked with terminal numbers the pin at one end that is not connected to a wire is pin 11).
Reassemble and the boiler should fire up and the fault solved. The boiler will run for ever like this, but loses a fan speed and a little efficiency. I bought the fan control PCB on E bay for £30.
I hope this helps, I know this fault has baffled a number of boiler engineers who all go for the replacement dungs gas valve - wrong.

Reg Hill
December 2010