Negative Head Shower Pump and Faulty Pressure Vessel

[It's me]

I am a little puzzled. My understanding of a negative head shower pump is that the pressure vessel detects a change in pressure when the shower tap is turned on. This pressure drop then switches on the pump.

The pressure vessel on our Stuart Turner pump is faulty and will not hold pressure at all, yet the pump still switches on. Can anyone suggest how this happens, apart from magic, smoke and mirrors?

James.

 

[Johntheo5]

If that's the case then it should just keep starting/stopping with no demand, if there is flow demand then the flowswitch will keep it running until the demand ceases, it should then return to continuous start/stop.

 

[It's me]

If that's the case then it should just keep starting/stopping with no demand, if there is flow demand then the flowswitch will keep it running until the demand ceases, it should then return to continuous start/stop.

It is odd. I noticed that there was a problem earlier this morning when the pump did not start properly for a shower and then, later, started to turn on and off randomly.

Even when there was no pressure at all because I had taken the valve out to replace it, the pump only started when I turned the shower control on and it turned off normally when I closed the shower valve. I do not understand how it turns on if there is no pressure in the vessel.

James.

 

[Johntheo5]

I might be wrong but I thought that the pressure switch also starts/stops the pump, if there is no/low pressure then the pump will start, normally, with the system/pump working properly, the pressure, say overnight might fall very slowly and the pump will start via the pressure switch and when the pressure rises it will stop, the small pressure vessel acts as a accumulator. The reason for a negative head pump is if the shower head is too high up and the head supplied by gravity isn't sufficient to provide the necessary > 1 or 2 LPM required to allow a flowswitch to start the pump.

 

[It's me]

Thank you for the replies which have given me a clearer understanding of how the pump works.

James.

 

[Johntheo5]

If there is no pressure when you press the pin on the schrader valve (air end of the pressure vessel) then it needs pumping up with a bicycle pump or whatever, you also need a pressure gauge, switch the pump off and slacken off the hot and cold flexible hoses to ensure no pressure at the water end then pump the pressure vessel to, I think, 0.9bar, 13 psi, reconnect hoses. even if you havn't got a pressure gauge, just pump it up a bit which should help until you can lay hands on one.

 

[It's me]

If there is no pressure when you press the pin on the schrader valve (air end of the pressure vessel) then it needs pumping up with a bicycle pump or whatever, you also need a pressure gauge, switch the pump off and slacken off the hot and cold flexible hoses to ensure no pressure at the water end then pump the pressure vessel to, I think, 0.9bar, 13 psi, reconnect hoses. even if you havn't got a pressure gauge, just pump it up a bit which should help until you can lay hands on one.

Thank you for the suggestion. Sadly, even with a new valve, the vessel does not maintain pressure for more than a day or so when pumped up. Water doesn't come out from the valve hole when the the insert is removed, so I assume that the actual vessel is leaking, rather than the membrane.

 

[Johntheo5]

I would think the air is leaking out somewhere around the air end valve, doesn't really matter anyway, can you fit a replacement pressure vessel or are they available/fttable?,
What sould work if you have room, is to install a small external expansion bessel, say 2 litres or so on the cold or hot discharge pipes. I don't know how one existing pressure vessel monitors the pressures, probably only one, you can test this by slackening off say the cold hose immediately the pump stops and see if this restarts the pump, if not try the hot. You can get nice mushroom shaped expansion vessels for around £50, make sure its the potable water type (normally white).

 

[It's me]

I would think the air is leaking out somewhere around the air end valve, doesn't really matter anyway, can you fit a replacement pressure vessel or are they available/fttable?,
What sould work if you have room, is to install a small external expansion bessel, say 2 litres or so on the cold or hot discharge pipes. I don't know how one existing pressure vessel monitors the pressures, probably only one, you can test this by slackening off say the cold hose immediately the pump stops and see if this restarts the pump, if not try the hot. You can get nice mushroom shaped expansion vessels for around £50, make sure its the potable water type (normally white).

Thank you for the suggestion. Stuart Turner, the manufacturer, have told me where to buy a replacement vessel.