central heating pipe single bang and judder

Joined
3 Mar 2007
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Location
Manchester
Country
United Kingdom
I have a problem with the central heating. The boiler is a conventional one (Glowworm Ultimate), not a combi boiler.
When the central heating comes on or off the pipes will give a single bang and vibrate a lot as the water starts to flow. The vibration is only a single movement that causes the banging noise and is not as the pipes expand. It also does it as the central heating goes off.
There was a fault on the three port valve a few months ago but a new one was fitted and it had been fine since.
My first thought was that it was the pump 'shuddering' as it stopped or started but that doesn't seem to be the case.
Any ideas?
 
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Not entral heating but I used to get this with taps both hot and cold when turning them off.

It stopped by adjusting the rising mains tap under my kitchen sink.
 
could be the ball in a honeywell 3 port valve sticking on each side until the spring pulls it open or shut with a single bang.
 
It's not the three port valve as I had already replaced that and it made no difference.
Its just a single loud 'shudder' of the pipes when the central heating cuts out on the thermostat, almost as if someone has hit the pipe with a mallet.
:?: :?:
 
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Have you been able to get a rough idea of the area it is coming from?

Was the whole 3 port valve replaced or just the actuator?

Does it do it if just the hot water is on as well?
 
It only does it if the central heating is on. I can cause it to happen myself by opening the central heating switch manually and it will make the noise when it closes the valve.
I can't work out exactly where the noise originates though.
 
I take it the water is going the right way through the valve?
 
yes the water is flowing the right way. Can't think why a pipe would become unclipped by it's self though.
 
I originally thought that so I replaced the diverter valve for a second time but it still did it.
 
No trvs and I have tried the pump on it's slowest setting with no noticeable effect.
:(
 
Have you got a bypass or auto bypass valve between the pump and the 3 port valve? Is it set correctly if an auto one, about 3, or has it been shut if just a gate valve type? If gate valve open it about 1/2 to 1 turn.
 
There is no bypass valve installed and nothing between the pump and the 3 port valve.
There is a gate valve on the central heating pipe after it leaves that 3 port valve.
Should this be fully or only partially open?
 

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