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About 12 months ago I fitted a new central heating system with a Vaillant Ecotec System Boiler, but standard Honeywell Y Plan Controls. Throughout the year a water hammer noise has become increasingly loud and frequent when the diverter valve closes one or other of the ports. I fitted an old Drayton valve which cured the problem but it 'let by' so I have now fitted a new Honeywell 4073 which has caused the return of the problem, although not as bad as it was. We've tried reducing the boiler bypass setting but it made no difference. I'm about to try fitting a shock arrestor but feel that this will mask the problem rather than cure it. Has anyone had a similar experience? (My Corgi registered gas fitter has not come across the problem before).

The by-pass is it an automatic one that's fitted out with the boiler?

[quote="The Heating Doctor"]The by-pass is it an automatic one that's fitted out with the boiler?[/quote]

Yes, the bypass is integral to the boiler.

That is the boiler by-pass and not the system by-pass, I would suggest that you fit an external automatic by-pass, approx £25. This should be fitted between the boiler flow & return within approx 1m of the boiler

Many thanks for that - I'll have a go at fitting a bypass next week. Fingers crossed!