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!)
chas, January 2010
