single loud bang when dhw motorised valve shuts

leckybird · 8 Aug 2013

Any ideas please?
I have a Baxi solo boiler about 8 years old and the ch and dhw are controlled by 2-port motorised valves operated by the room and tank stats respectively. (fully pumped system)
Everything is fine with the ch but whenever the dhw valve closes there is an almighty single bang.
If the ch valve is open at the time then there is no problem and if I jam the valve open with the manual control lever there no problem when the tank stat shuts the boiler off.
I've bled the system more times than I can remember and drained and refilled the system twice but no success. Everything used to work fine so the 2 port dhw valve cannot be fitted backwards even though that would seem to be the obvious answer.
The pump is fitted to the return side of the boiler rather than the flow but once again it's been like that for years with no problems.

Agile · 8 Aug 2013

This is usually because the valve has been fitted the wrong way round and the pump pressure pushes it closed with a bang!

They sometimes have an arrow showing the correct direction of flow.

Tony

leckybird · 8 Aug 2013

Agile said:
This is usually because the valve has been fitted the wrong way round and the pump pressure pushes it closed with a bang!

They sometimes have an arrow showing the correct direction of flow.

Tony

Thanks for the speedy reply Tony.
The valve is fitted the correct way with the arrow pointing into the cylinder and it's been working fine for the past 7 years, it's only just started playing up.

picasso · 8 Aug 2013

The ball and arm can become worn with enough play to slam shut with the pump pressure, you can buy kits to service the valve but its a drain down so might be beter with a whole new valve.

Vic Spanner · 9 Aug 2013

By-pass. Closed, Blocked, Missing ????

leckybird · 9 Aug 2013

poxi said:
By-pass. Closed, Blocked, Missing ????

No bypass fitted which would cure the problem but as it's worked for the last 7 years I would like to know what's gone wrong?

leckybird · 9 Aug 2013

picasso said:
The ball and arm can become worn with enough play to slam shut with the pump pressure, you can buy kits to service the valve but its a drain down so might be beter with a whole new va
lve.

Sorry not to have replied earlier. I'll drain down tomorrow and check for wear.

Boilerman2 · 9 Aug 2013

Has some Twit fitted a new pump the wrong way round?? (well you never know!!)

leckybird · 9 Aug 2013

Boilerman2 said:
Has some Twit fitted a new pump the wrong way round?? (well you never know!!)

Afraid not, nothing has changed since the system was installed, it's just started playing up.

Onetap · 9 Aug 2013

leckybird said:
Any ideas please?
I have a Baxi solo boiler about 8 years old and the ch and dhw are controlled by 2-port motorised valves operated by the room and tank stats respectively. (fully pumped system)
Everything is fine with the ch but whenever the dhw valve closes there is an almighty single bang.

Water hammer. The pump should stop when as the valve closes, but the water will still be moving when the valve shuts rapidly under spring pressure and the water is brought to an abrupt stop.
The Y-plan (one 3-port) always has one side open.

Try stopping the pump first (if you can) and then closing the valve to confirm it is this.

I came across something similar with electrically operated fire dampers on a ventilation system. The high pressure pulse could blow the duct connections off the dampers.

Tom1981 · 9 Aug 2013

Same effect can be made if you remove the head and twist the spindle by hand, waters changing direction quickly causing the pipes to shudder. The head could be faulty, skipping teeth and slamming shut, should be a smooth return.

Onetap · 9 Aug 2013

Tom1981 said:
Same effect can be made if you remove the head and twist the spindle by hand, waters changing direction quickly causing the pipes to shudder.

Or shut a quarter turn tap rapidly.

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single loud bang when dhw motorised valve shuts

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