Intermittent problem with a boiler failing to light on demand

[martinw]

I have a Glow Worm 30Sxi condensing boiler which fails to respon

(sometimes) to a demand for heat from either the central heating or ho water.

I works fine when it is cold (or has been off for about an hour) bu when it is cycling, working normally to maintain the house temperature it often takes 2 or 3 attempts to light (It is all automatic and wil try only three times).

Unfortunately it does not always light by the third attempt so th systems fails and gives a ?F4 Ignition fault (lockout) went out whe lit?

When this is noticed and sufficient time has passed a simply off/o works fine and the boiler relights.

There is an expansion vessel fitted (since it is a large house) whic maintains the system pressure (cold 1.6 bar, hot, 81 deg C, 2.2 bar and the flue is the simple horizontal through the wall type (at slight angle).

I?ve had my plumber and electrician round who cannot find any problem with the boiler, but are willing to carry on looking, providing I carr on paying them.

I therefore need any good practical advice please.

The manual states possible causes as:

No gas. Possibly not:-).

Insufficient gas. Again no, it heats up over 15 radiators.

Incorrect gas valve adjustment. I do not know?

Electronic ignition lead defective. It works fine when the system i not hot?

Check air inlet duct. The flue is only a meter and on a slight slope.

Check connections to igniter unit. I was told this was fine, but?

Thanks

Marti

-- martinw

[Ed Sirett]

The manual is listing out reasons for the boiler not igniting. Some of these are more plausible (in your circumstances) than others.

Intermittent faults are really tricky to track down. It would greatly help if you could make note of when the lockouts occur, which you have done.

The burner unit looks very very like the one used in the Vaillant Ecomax boilers.

I would check of get checked the spark gap size. I would check of get checked the flue gases, the manual has a lot of info about this. When the boiler lights does it rumble like thunder for a second or two this might indicate a weak mixture?