Shock arrestor on cold water pressurising hot water?

[jimbo84]

Hi all, new here. Spark, not a plumber.

I have a question about something at my house that's puzzling me.

Combi boiler system in a terraced house. I've had water hammer/water shock whenever something is turned off quickly, mainly the washer. I fitted a PRV just after the stopcock and reduced the incoming pressure to around 2 bars which stopped the problem but the shower (mixer shower running from the boiler) was terrible at this pressure so upped it 3 bar. Hammer/shock returns but not as bad.

Bought a shock arrestor (Altechnic diaphragm type) and fitted it just before the washer. Happy days it's cured the hammer when the washer/toilet valves shut off. Problem is when a cold water valve or tap is opened anywhere in the house there's hammer/shock coming from the hot tap side now this is when something is opened on the cold side, not shut off. Puzzled me how this is possible? Is the shock arrestor pressurising the hot water by back feeding through the boiler!?

Any ideas or help would be much appreciated.

For info the incoming pressure is 3.5 bars. It's been reduced to 3 bars now by the PRV valve. The arrestor is pre-charged at 3.5 bars.

 

[Last Plumber]

When you say hammer is this an on going banging or just one thump when you open something

 

[ShaunCorbs]

Arrestor needs to be fitted before boiler / cold feed to the boiler should sort you out

 

[jimbo84]

When you say hammer is this an on going banging or just one thump when you open something

It's just a one off thud/shock, it's not the multiple hammering you sometimes get with older houses.

Arrestor needs to be fitted before boiler / cold feed to the boiler should sort you out

That would make sense I'll try putting it before the boiler.

Thanks for the replies.

 

[Scott_D]

A loose pipe somewhere kocking as you shut the water off

 

[Last Plumber]

Could be a loose Jumper in a stop tap

 

[jimbo84]

Update: Removed the shock arrestor from the washing machine pipe and fitted it to the cold water feed to the boiler. This stopped the symptoms mentioned in the original post but started the Thud/shock when the washer/toilet was shutting off again.

I've completely removed the shock arrestor as it seemed to be causing more issues than it was solving. I've turned the washer and toilet isolators to half way and this has solved the issue of the hammer/thud/shock in the pipes.

Thanks for all the suggestions and replies.