Baxi Back Boiler Loud Banging When Shutting Off

[paul1978]

Hi,

I have a baxi back boiler, with a pump located near the boiler, and an immersion tank upstairs. Over the last year and half, whever the boiler shuts down (ie after it's run for a while and then shuts off, presumably due to thermostat saying it's hot enough now), I get a series of bangs. It started moderately quiet, and I was advised to run boiler silencer through the system. I've done this several times now, and the noise just keeps getting worse. At the moment the bangs have become startlingly loud, and actually rattles the fireplace.

The bangs are definitely coming from the boiler itself, not the tank or anything. They don't happen during operation, only once operation stops.

I've noticed the pump stops at the same time as the boiler, is that correct? If kettling is causing the bangs, I thought perhaps the pump should be continuing for a period after the boiler gas cuts out, so that the heat in the boiler can be dissipated rather than flash boiling a single section of water (I presume that flash boiling is the cause of the bangs, that's what every plumber I've spoken to has said is the cause).

I've drained and filled the system several times, and fully bled regularly.

Any thoughts anybody on what to do? I'm tempted to get a powerflush, but don't want to spend 400 quid and find it's just the same.... (as I say, I've run chemicals through so many times now it's not funny, can't believe there's much crap left in the system!).

Many thanks,

paul

 

[baxpoti]

Describe the noise.
What does it sound similar to.

 

[paul1978]

The noise typically does this:


Boiler and pump shuts off
First bang happens, usually moderate noise. Like a thumping bang, and it originates from the boiler, but can be heard throughout the rads.
Second and following few bangs are louder, and really make a bang. Loud enough to wake me in the morning now.

They're definitely bangs, and they're not from gas exploding or anything, I've watched the boiler closely when it happens, it's only after the gas turns off.

 

[gavinda]

what model backboiler is it?

some may have a pump overrun, possibly board controlled.

 

[paul1978]

not sure on model at the moment (at work) can post later... any thoughts in the meantime?

 

[namsag]

Your boiler stat has gone , when its turning off there is far too much heat left in the boiler and its banging through the pipes,
Bet your rads are red hot to the touch

 

[paul1978]

Good call - they're pretty hot - certainly you wouldn't want to leave your hand on them - isn't that normal though? I thought rads always got really hot? Is there a temperature range they should operate between to indicate correct flow temperature? Where should i measure this, and how?