Grant Combi 90 Outdoor Central Heating Noise

[Paul in Angus]

Can anyone advise me on the potential causes of an annoying noise coming from the central heating?

I have an outdoor Grant Combi 90 that makes a single bang when the hot tap is opened and again when it’s closed. It only does this when the central heating is on and up to temp. If you turn the heating off when its up to temp (I assume max pressure in the system) then there is no noise when you operate the tap. If the heating is on but not been running for long then you get no noise either.

Up to now the engineer has changed the diverter valve twice, no difference. The system used to work ok, so I am thinking pump? Unfortunately the boiler cover I have ties me into one company, I am trying to get them to get grant out to look at the problem, but no look yet.

Thanks in advance.

Paul

 

[tipintim]

Diverter valve

 

[Puddle]

Was thinking diverter valve but if replaced twice must look elsewhere,pump pressure causing problem so not sure why that replaced,has it a internal by pass that may be blocked ?
Take it engineer has heard noise and not burner startup related

imho

 

[Paul in Angus]

the noise only starts when the central heating is on and up to temp so kind of rules the burner out. the engineer thinks it is pipe related, but as i said to him we have had no work done for years!

 

[tipintim]

open by pass a bit more. after water demend, the pump kicks in to disperse heat from boiler
Does it happen on all hot taps?
Turn water main down?
If it only happens at one tap i.e , its the jumper junping
Just shouting ideas

 

[kimbo]

without knowing which combi it is there are 3 things in the pipes that move when the hot water tap is turned (discounting the diverter valve assembly) on One is the flow switch Two is the check valve and Three is the flow restrictor. I would be looking at the Check valve on the incoming main inside the boiler they do break up and sometimes cause lessening of flow. The flow restrictor is a plastic insert in the incoming main which again sometimes breaks up it is usually green.

 

[kimbo]

I just went to a firebird combi today with the same problem, I changed the check valve and sorted out the problem.

 

[markproperjob]

oh yes.
check valve!

 

[dontknowitall]

Is the water pressure high? It could have been fine when installed but now much greater. If more than 3 bar then it would be sensible to fit a pressure reducing valve to protect the boiler.

Only a thought.