Hot water is OK
Heating... it will click, pump starts, fan starts, gas is released at the higher pressure and is sparked/ignited.
3 seconds later the gas pressure is reduced.
It then shuts down (flame goes out).
and the cycle begins again.... give it long enough (say after 5 minutes or so) as the thing starts to get warmer... the cycle gets longer till it is running ok.

Eventually after a fer days of this behaviour it was reduced to just the start/stop cycle of 9 second long.

VideoVandal, February 2009

Change the two capacitors (grey block shaped things)
on the small electronic circuit board next to the main board. They are each 1uf (one microfarad) in value and rated at 400v.

Mike, March 2009

The pressure switch is there to detect the drop in air pressure and shut down the boiler if the fan fails.

So.. checked the pressure switch... cleaned the silicon tube.
Found the pressure switch casing to be cracked as it was mounted in such a way as to be pressed up against a pipe... so sealed the crack (super glue) and modified the mounting bracket (drilled a small hole in it) so the switch is not in contact with the pipe.

Oiled the fan motor spindle.

NOW the REASON for the cycle is...

When the gas pressure drops to the lower setting, the fan should also slow BUT NOT STOP.

On the fan control board (on the left hand side) is a 24V relay, a 10K resistor and two 1u 400V capacitors.

The fan is run at full speed on high gas pressure (direct 240VAC mains) but the relay switches the capacitors in circuit so the fan runs on decoupled mains (about 150VAC) for the lower pressure.

On my unit, the capacitors had degraded so I got 90VAC... so fan was tooooo slow, and the resulting pressure drop read by the pressure switch shut down the boiler.

The capacitors are Metalised Polyester 1uF 400V though 275V ones are ok to use... RS Components cost less than a fiver !

VideoVandal, February 2009