DIY Doctor

Turning of Taps Triggers Shower Pump and Causes Water Hammer

Postby Tony_tea » Sun Mar 18, 2018 8:36 pm

I have very low hot water pressure so have a shower booster pump. The shower pump gets a lot of cavitation (air ends up in the feed to the second bathroom).

Come the winter (i.e. now!) once the central heating system is in use, frequently when a tap on a sink is turned off the pump gets a burst of pressure that triggers it momentarily, unless the tap is turned off very slowly. Often we get repeated triggering of the pump (sort of a slow motion water hammer). This usually happens only after someone has had a shower. If I run water in the second bathroom until the slug of air escapes through the bath tap (also fed by the booster pump) then the problem goes away, usually until the next time someone takes a shower.

I assume the air is effectively creating a spring which bounces when non boosted taps are turned off.

I've reduced the hot water temperature as low as is safe (in terms of legionaires...) to no effect. The pump flow is not adjustable (and in any case, on the previous pump it has no effect). In any case, I don't understand why the problem goes awat in the summer - the pump cavitates then as well but no hammer.

Any clues how to fix this? Stopping the cavitation itself seems problematic. Is there any mileage in fitting an air vent somewhere in the boosted supply pipe work so that the trapped air bleeds away once showering stops, or some sort of surge suppressor? Our sink taps are all ceramic disk, so turning them off slowly requires quite a conscious effort.

Any suggestions greatly appreciated

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