All the time I have had this boiler, through a couple of circuit board replacements and gas valve itself, it has still had an intermittent problem ( usually when it fires up first thing, but now randomly throughout the day), when it starts to oscillate loudly between on and off, like a loud buzzer, then will finally click on and stay on. WHat could it be? I have a spare circuit board taken out of a decommissioned boiler of a friend if i need to change it again, but I dont think that is the problem? Any ideas?

David Nicholas, December 2010

If you have a fan that starts up before the boiler ignites , then there is also an AIR PRESSURE switch.

The problem may be due to a partially blocked plastic pipe between the switch & fan.

It goes via an inline metal restrictor ( inside the plastic tube) that can block up.

Remove the tube and suck/blow so that you can hear the Air Pressure Switch clicking...then replace the tube.

A very common Netaheat fault....on these boilers with the fan + pressure switch !

Beware scrapping the boiler as most parts are still available (Ebay etc) so don`t jump to any expensive conclusions.
(and don`t believe what the CH engineers or Gas Board tell you. These old boilers are built to last....more than 28 years if necessary!)

joethebolierman, December 2010

sounds like the solenoid on the gas valve check connections etc

brian, December 2010