Vacuum in central heating system

[berkstom]

I had the 3 way valve replaced by a plumber, and every time the heating or water fires up the pipes vibrate, like water hammer (ie for about 1 second). The plumber came back, replaced the valve again, but it is still bad.

I have tried to bleed the system, in case there is an airlock. If I bleed the highest radiator with the heating OFF, the radiator sucks air into the system.

If I bleed a radiator with the heating ON, water comes out of the radiator.

Can anyone help?

 

[buck51]

It sounds like he was not a plumber why did he "replace the valve again"
Then try FAQs at top of forum

 

[berkstom]

Thanks Buck

He was sent by British Gas, so I hope he was qualified....

He said he would replace the valve in case the original replacement was opening too quickly.

I can't find any reference to air being sucked into the radiator when bleeding it, or am I missing something. It is a vented system.

 

[mogget]

It sounds like a blockage in the cold feed pipe from the header tank.

 

[berkstom]

Thanks Moggett

I have just tried loosening the drain valve by the boiler, and leting a litre or so of water out. The header tank water level fell at the same time, s I assume this means the feed pipe is clear.

 

[namsag]

Bleed it for longer and make sure the pump is completly off

 

[buck51]

Do you have a drain cock at a low point on the return pipework if so drain it down from there, untill it runs clear of any air if that dont clear it try the same on feed pipework, once you have a clear path to bottom of system then any remaining air will be push up and out through rads etc
PS and lock the valve in the open position with the lever before you start