Loud momentary clunk when boiler turns off - any ideas?

[stevem999]

Hi, we have a Baxi back boiler, and noticed recently that whenever the boiler turns off you get a single clunking noise a second after the gas has turned off. No other noise whilst it operates other than the usual burner noise and occasional water noise, just this clunk each time the burners turn off. Any ideas if it's something I should worry about, or leave until next year's service?

Cheers

 

[Boilerman2]

Could be that the Gas valve is not shutting quite as quick as it once did, this can result in a noticeable "Thump" as the burner blows back as the gas pressure slowly reduces - have a look at the main burner flame as it switches off you may notice a blue flash from the combustion space when the bang occurs

 

[mickyg]

have you got a really old pump? there was one very old type pump, which i cant remember the name/model, that would often clunk as it stopped, and having a back boiler this is possibly located next to it somewhere in some boxing perhaps. This would explain the delay. As above though, gas valves can also clunk !

 

[stevem999]

Thanks for the pointers folks. I don't think it's the pump as that is located upstairs next to the hot water cylinder, and the boiler is downstairs so guess it would have to be a mighty clunk that it makes to come down the pipes so far.

I'll have a good look at the burners tonight and see if I can see this blue flash.

If it is the gas valve that is getting a bit slow, is this a cause for concern i.e. it's going to fail or is dangerous and letting gas escape? Please tell me it's not 'new boiler' territory!! Thanks

 

[bengasman]

The real danger with these boilers is not a gasleak, but spillage from poor maintenance.
It is imperative that they get a a full strip down service every year plus a flue and spillage test.
The noise could also be a steambubble from overheating due to poor circulation.

 

[stevem999]

Well it gets an annual service from a local gas company but not sure how much it is stripped down to be honest. It does get a flue test but no idea what a spillage test looks like so don't know if that happens.

Would the steambubble happen every time the boiler turns off?

 

[bengasman]

Well it gets an annual service from a local gas company but not sure how much it is stripped down to be honest. It does get a flue test ...

What do you mean by a flue test?
A real full service on these would take between one and two hours.
If you lift/take off the bottom cover and shine along the floor with a torch, it should be ( fairly ) clean. If you can see dust/debris against the chimney back, it probably was not done properly.

Would the steambubble happen every time the boiler turns off?

If the boiler turns off due to overheating: probably.

 

[stevem999]

Sorry, I've seen him use a smoke thing in the past to check that it'd drawing up the chimney ok, so assumed that was a flue test.

I'll go rooting underneath in a bit and see what it looks like. I suspect he's here for less than an hour when the service is done - he does bring a vacuum in though!

 

[bengasman]

Sounds ok so far.
It could be he does these a lot and is very quick with them. It also speeds things up if they are really serviced every year so you don't loose half an hour on trying to undo things that are stuck solid.