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g/plumbing • Posted by u/drain_dave •

Water hammer on a commercial building — where do you start diagnosis?

12-story office building, built in 1987. Tenants on floors 7-10 complaining about water hammer — loud banging when fast-closing valves actuate (solenoid valves on coffee machines and dishwashers mostly).

Building has a pressure-reducing valve (PRV) and an expansion tank. Both were replaced 2 years ago. Static pressure at the PRV is correct (65 PSI downstream).

Where do I start? Hammer arrestors at every offending fixture? Or is there something systemic I'm missing?

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u/pex_pioneer •

If systematic pressure reduction and expansion tank verification don't fix it, hammer arrestors are the right answer. Size per PDI WH-201 based on fixture flow rate. On a commercial install I'd put them at every solenoid valve anyway — they're cheap insurance.

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u/pipefitter_pat •

PRV and expansion tank are the right starting points but 65 PSI might still be too high for fast-closing solenoid valves. Drop the PRV to 55 PSI and see if the hammer improves before adding arrestors everywhere. Solenoid valves in commercial coffee equipment are sized assuming 40-60 PSI — at 65 PSI, the momentum of water stopping suddenly is significantly higher.

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u/backflow_ben •

Also check if the expansion tank is properly charged. If the pre-charge pressure doesn't match system pressure, the tank bladder is bottomed out and not absorbing any pressure wave. Should be charged to match your PRV setpoint. Easy to check with a tire gauge on the Schrader valve.

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