Had a similar problem and still occasionally do it seems to be caused by condensation in the flu. (my flu passes all the tests by the way and it pulls like a tornado)
If I have trouble reigniting it I tend to put the fire on for 5 to 10 minutes on high this heats the boiler up and seems to pull out a lot of the condensation that has run down the flu.
Then I will reset the boiler and 99% of the time it will ignite without the need to reset it again if it doesn't reignite after the first attempt, leaving the fire on I go and have a cup of tea and then I try again the whole time leaving the fire on.
My boiler is also about 2 years old, it did have a warranty job done on it and the pc board was replaced, I'm not convinced this was the problem as my gas installer was pretty certain it was just the postion of the sensor for the spark (name of it escapes me) was not quite right but after he took a look he said get it done as a warranty job just in case it is something else.
The first warranty bloke couldn't figure out how to get the fire off! The second one was great but he did insist it was the PC board, which he replaced and within a week the problem was back, called manufacturer again and a third bloke said it was because my chimney wasn't completely capped from top to bottom causing condensation. I'm not really sure how capping a chimney from the roof to just above the boiler is going to help condensation. In a victorian house it would cause the damp to get trapped in the chimney as it would only be able to go through the walls I would have had 20 feet of mildew to deal with inside 6 months! And the effect on condensation within the flu would have been close to negligable!
After a lengthy chat with my installer he said he was talking out of his back side agreed with me about the mould and I was further inlcined to agree with my installer because the third bloke even started touting for business to cap the flu for me at the top and bottom and then fill it with some bizarre extremely expensive aggregate.
Interestingly the boiler is fine now with the only blips occuring on very rainy days occasionally I think it just needed to settle in to the environment. The biggest difference I have noticed with old and new back boilers is that the new ones are like a computer whilst the old ones were virtually crank driven meaning the old timers would go on for decades! Not so sure about the length of service that you will get out of the pc boards to be honest I suspect just a few years at best!
Generally speaking I am very pleased with it though especially since it replaced an 18 year old glow worm with the obligatory monstrous wooden cased gas fire, the thought of it makes me cringe!
Alan, January 2010
