Banging Apollo boiler!!

[claude2]

My vintage Myson Apollo 15/30 boiler has started to make some loud banging noises. This happens on the central heating cycle after about 40mins, when everything is at full temperature. It bangs loudly about 5 times.
I have bled all valves, and the water in the header tank appears ok, but the problem persists.
All help gratefully received!

 

[compheat]

If it only starts after everthing is mad hot i'd say it was your high/low stat causing the problem, but having said that I would expect your overheat stat to turn boiler off.

 

[claude2]

Thanks for reply. It only starts when everything is hot. I only use the boiler on the 'low' setting, so it could be the Low stat. Is there a chance it could be the pump over run stat? I am not sure if the over run stat could cause this.

 

[DeltaT2]

The Apollo is a low water content boiler that needs a large flow rate, so the banging could be a result of circulation pump, by-pass, Hi/Lo stat, pump over-run stat or most likely crud in your HE.

Time for a new boiler me thinks.

 

[claude2]

Fair comment, but is it worth trying to replace a stat, before I replace the boiler? It does not bang when heating the water, so does that rule anything in or out?

 

[Agile]

The water heating circuit is low resistance!

Do you have TRVs on all rads with no auto bypass valve?

These boilers are almost 30 years old now and most never serviced or having the stat temps checked.

The lowstat is marked at 68 C

Tony

 

[DeltaT2]

The spares are cheap & available, but I'd check the hydrolics first - as Tony has said. But my money is on crud in the HE.

 

[namsag]

Firstly i doubt its poor circulation as in blockage as has been suggested otherwise it would be banging continously .
Run it on the hi stat and see if its still the same if its not you have a faulty lo stat.
If it is then its possible both stats are faulty.
But as you have already suggested it could be the pump over run stat i have experienced the exact exact same scenario with them during operation they break contact during operation and cut power to the pump momentarily thus no circulation and the banging then it makes contact again and runs perfectly ok.

 

[DeltaT2]

Firstly i doubt its poor circulation as in blockage as has been suggested otherwise it would be banging continously .
Run it on the hi stat and see if its still the same if its not you have a faulty lo stat.
If it is then its possible both stats are faulty.
But as you have already suggested it could be the pump over run stat i have experienced the exact exact same scenario with them during operation they break contact during operation and cut power to the pump momentarily thus no circulation and the banging then it makes contact again and runs perfectly ok.

That's a bit of a contradiction son, but hey as I said; it could be any of the things I listed above............