Possible to transform PosHead pumped system to NegHead sys?

2 Jun 2003
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United Kingdom
I am currently renovating my bathroom, and due to tap choice (Hudson Reed's that uses narrow flexi connectors...even for the bath taps), I had to install a single impeller pump (ST Monsoon 3 br single) to boost my DHW, and connnect the taps to the cold mains. Also, the basin mixer tap (15cm below the bottom of my CW storage tank) in my loft conversion had always provided a feeble HW flow.

Now that I have installed the pump (on the GF beside the HW cylinder), it appears that when I try to use the basin tap on the top floor, the 15cm drop is insufficient to activate the pump reliably...sometimes it works ...at time not.

Is there a flow switch (air switch ???) that I could install that will allow the top floor tap to reliably activate pump supply ?

My other options is to replace with a larger tank and swap the location with the CH tank (which is currently above the CW tank), hopefully that would add about 50 cm to the height difference...(BUT MORE GRAFTING !!!!!!!!!)

many thanks in advance
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As you say you have barely sufficient flow to trigger the flow switch that is built into the pump. Even raising the water level in the cold feed tank by a very small margin (say 20mm) might just make the difference. You could possibly do this by adjusting the float valve, or sliding a sheet of MDF or ply under the base of the tank.

If that doesn't work, it might be possible to link a manual switch into the pump control circuit, but you'd have to check that with the pump manufacturer. Also negative head pumps are available which would operate in your situation. It may even be possible to convert your existing pump to negative head operation.

Another approach would be to change the basin taps to ones that allow more flow, or even to change the tap tails to slightly less restrictive ones.

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