Water hammer on start up

27 Feb 2007
Reaction score
United Kingdom
Hoping you can help.

In the last 10 days my central heating has started to water hammer at start up. It only happens once when the sysyem first kicks in after a period of being off for several hours. The heating is programmed to come on before the hot water. The water hammer is very much reduced when i initially turn down the pump speed to 1, then turn it up to 3. What also seams to help is if i manually move the three port valve towards the heating position.

Full pumped.
F & E tank.
Two floors-7 rads.
Potterton Flamingo 40 boiler-kettling. I drained down the system (2 or 3 times) 10 months ago after using sentinel cleanser. It was topped up with noise reducer and inhibiter.
Grundfos 15-50 pump-head replaced 10 months ago.
ACL lifestyle mid position valve.

Is it possible that the valve is not moving freely, and is not in its final position by the time the pump kicks in thereby causing a temporary block to the water flow?

Thanks for your help,

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Can anyone throw me a line on this one?

I've got a powerflush booked in from HSS for the weekend assuming that i've got a blockage somewhere. Lets hope i don't break anything or drown myself in the process :)

Could it be a worn valve that is blocking the flow?

Any help greatfully received,


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