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"John Stumbles" wrote in message
...
wrote:
Our shower pump turns on momentarily whenever a hot tap is operated
in the kitchen or the bathroom. The pump runs for about half a second
just after the tap is turned off.
The pump/shower system is designed to switch on whenever the shower
mixer valve is opened, so I assume this malfunction is something to do
with a pressure drop in the water system. Can anyone suggest what I
need to do to correct this problem please?

I think what happens is that when the flow of water to the hot tap is
suddenly stopped the momentum of the flowing water causes a surge of
pressure in the pipework. If the water can go through the pump it
activates the pump's flow switch and turns on the pump.

If the shower valve is turned off water shouldn't be able to flow
through the pump - it has nowhere to go - but if there's air in the
pipework between the pump and the valve the high-pressure surge can push
enough water through, compressing the air as it does so, to trigger the
pump. Of course there's nowhere for the water to go to continously so
the pump only runs momentarily and then stops. (Although under certain
circumstances a repetitive pulsing of the pump can occur.)

If your pipework from the pump to the shower goes up and then down again
then you have the potential for air to get trapped at the high point of
the pipework. Fitting automatic air-bleed valves (e.g. BES part no
7704 BOTTLE AIR VENT 1/2" WITH CHECK VALVE £2.33 + VAT) at the high
points may cure the problem.

As noted above this is a problem if there is any air trapped in the hot
system.

One way to cure it is to disconnect/remove the switch that senses the hot
water flow (these are usually
reed switches attached to the outflow) leaving the pump actuated by the cold
flow only.

Only effect is that the pump doesn't turn on if valve is set to maximum
heat - which is unlikely and probably too hot for comfort anyway.

Chris K