Boiler takes several minutes to light

[GeordieBri]

Boiler: GlowWorm Economy Plus fanned flue, model EP40F.

My boiler takes several minutes to light properly. During that time, the main burner gas supply appears to be flipped on and off every second. The sequence of events is as follows: -

-- Switch on boiler at timer.
-- Flue fan starts, and appears to be working normally.
-- Igniter starts, and pilot lights.
-- Main burner lights for around one second, then goes out.

The last event happens around once per second - I can hear the main burner solenoid valve (I guess) open, the main burner lights, then the solenoid valve closes. This happens once per second, until the main burner eventually stays alight, several minutes later, and the boiler operates normally.

No recent work has taken place on the system.

Thanks!

 

[kevplumb]

fan pressure switch would be my first point of call

 

[GeordieBri]

Cheers kevplumb.

Is there a quick way I can verify this? Could this be affected by the ambient temperature of the boiler? Thing is, it will light and stay alight after a while. Could this be because it's warmed up a bit, so the pressure's affected?