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Positive head v negative head

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  • 28-07-2014 11:01am
    #1
    yeppydeppy
    Registered Users Posts: 1,163 ✭✭✭


    I want to get a shower pump but don't know if it should be positive head or negative head. The shower is weak at the moment as the hot water tank is in the hotpress beside the bathroom. There is a cold water tank in the attic but the pressure is very low.
    Can anyone give some advice?


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    #2
    DGOBS
    Hosted Moderators Posts: 3,496 ✭✭✭DGOBS


    Positive head means simply the water storage tank is above the height of the shower, a negative head pump is required where the shower level as equal to or lower than the shower (as in a shower in an upstairs dimer bungalow where the storage tank is in the attic on the same level)


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    #3
    John T Carroll
    Closed Accounts Posts: 757 ✭✭✭John T Carroll


    yeppydeppy wrote: »
    I want to get a shower pump but don't know if it should be positive head or negative head. The shower is weak at the moment as the hot water tank is in the hotpress beside the bathroom. There is a cold water tank in the attic but the pressure is very low.
    Can anyone give some advice?

    The positive head pump is used in situations where the draw off ( showerhead, tap, etc) is located below the level of the cold water supply header tank, the pump uses a flow switch to start/stop the pump, when the shower (or tap) is switched on, a small quantity will flow which operates the flow switch and starts the pump.
    The negative head pump is used in situations where the draw off (showerhead, tap etc ) is located above the level of the cold water supply header tank, the pump has an accumulator and a pressure switch, once the shower is switched on, the pressure drops and starts the pump, the pump would not start if it was positive head as there is no gravity to operate the flow switch. A negative (universal) head pump can be used in both cases but is more expensive than the positive head pump, I know of people who have retrofitted a positive head pump instead of an negative head pump (for cost reasons) but the only way they can get the pump to start is to remove the shower head and throw it on the ground to try and get a drop of water to flow from it and start the pump.

    Re your required pressure, I think you can get a pumps with discharge pressures between 1.5 Bar and 3 Bar pressure. If you are only using one shower at the time then 1.5 bar will probably suffice but all pumps will (or should) specify a specific flowrate at any given design pressure. For example if you want something equivalent to an electric shower then you probably need something like 20 LPM at 1.5 Bar pressure, you only need 5 to 10 LPM for the shower but other people in the house may want some water at the same time and you still want to maintain the 1.5 Bar pressure at the shower head.

    Apoligies for being a bit longwinded but that's the gist of the differences, as I see it, at any rate.

    Should have also said that DGOBS explained it quite well. In your case a positive head pump should do the job.


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