25 yr old warm air boiler was fine until flames started lapping up outside! dirty heat exchanger ??
since mains electric turned off (then on again) could not get boiler to fire up,even though pilot was on.
now i have turned off pilot cannot get that to stay on even when holding plunge down for ages!
it will still go out even while holding plunge switch in!!
be grateful for any advice.

mark, January 2008

Same thing happened to mine - a J25-32 Mk III. I took it apart to see what the problem was:

The heat exchanger fins were warped, buckled and cracked. All the air spaces were clogged with carbon and limescale. Basically, I need a new heat exchanger, as the old one leaks. I've replaced it before and remember paying about £50 for the replacement part. Now it's £170 -- if I've found the right part (no picture).

After dismantling for inspection, I cleared out the filth as best I could and it's working happily again, but I need to replace it soon, or it will recur.

If you're thinking of doing it yourself, the best thing is to drain your hot water system, dismantle and inspect. Don't forget to turn the gas off. Mine wasn't particularly difficult to dismantle, but it was a filthy job.

Dave, January 2008

Same thing happened to mine - a J25-32 Mk III. I took it apart to see what the problem was:

The heat exchanger fins were warped, buckled and cracked. All the air spaces were clogged with carbon and limescale. Basically, I need a new heat exchanger, as the old one leaks. I've replaced it before and remember paying about £50 for the replacement part. Now it's £170 -- if I've found the right part (no picture).

After dismantling for inspection, I cleared out the filth as best I could and it's working happily again, but I need to replace it soon, or it will recur.

If you're thinking of doing it yourself, the best thing is to drain your hot water system, dismantle and inspect. Don't forget to turn the gas off. Mine wasn't particularly difficult to dismantle, but it was a filthy job.

Dave, January 2008

DO NOT USE IT!!! get it serviced the heat exchanger is blocked. If you did get it to light you and your family will probably die from carbon-monoxide poisoning. consider yourself lucky it won't relight

Paul the gas man, January 2008