Do I need a shower pump?

Discussion in 'Getting Started FAQ' started by Phil Jones, Apr 25, 2017.

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  1. Phil Jones

    Phil Jones New Member

    Hi all - new to this forum and in need of help!
    Previous home owners fitted an 10.8kw electric shower on a 6mm cable. Needless to say it burnt out and I've now isolated the thing.

    Problem is I now have no shower!
    So here's the conundrum - I have a gravity fed system. The cold tank and hot water cylinder are in the loft, above the bathroom, and the cold tap in the bath is fed from the mains.

    I was going to fit a positive shower pump on the tank in the loft, but I've been told I need a negative one as it is mixing gravity fed hot water with mains cold water. Is this true? Could I actually just get away with fitting a bath shower mixer tap and no pump?

    Thanks for any help received!
     
    Phil Jones, Apr 25, 2017
    #1
  2. Dr Bodgit

    Dr Bodgit Super Member

    No - the hot and cold supplies need to be the same pressure, which I think means both need to come from the cold water tank in the loft (the hot via the hot water tank - you have one?)

    You could fit a bath mixer tap, put in a new cold water feed to it from the cold water tank in the loft, then both that and the hot feed to the tap go via a twin impeller pump to boost the pressure.
     
    Dr Bodgit, Apr 25, 2017
    #2
    Phil Jones likes this.
  3. Phil Jones

    Phil Jones New Member

    Thanks - my mains pressure is 1.5 bar. As my cold tank is about 1.5 meters above the shower, would that not mean I have equal pressure, and a shower at 1.5bar?
     
    Phil Jones, Apr 25, 2017
    #3
  4. Dr Bodgit

    Dr Bodgit Super Member

    1 bar equates to 10 meters of height, so 1.5 bar is 15 meters - 10x the pressure as your cold water tank.
     
    Dr Bodgit, Apr 25, 2017
    #4
  5. Phil Jones

    Phil Jones New Member

    yes of course - sorry bad day. and thanks for such a quick response! So then what I don't get, is if my mains cold is 1.5 bar, and I fit a 1.5 bar pump to the hot cylinder in the loft, why would I need a negative pump?
     
    Phil Jones, Apr 25, 2017
    #5
  6. Dr Bodgit

    Dr Bodgit Super Member

    Whether you need a positive or negative head pump depends where the shower head is with respect to the level of the cold water tank.

    [​IMG]
     
    Dr Bodgit, Apr 25, 2017
    #6
    Phil Jones likes this.
  7. Phil Jones

    Phil Jones New Member

    exactly what I thought - but I called up Salamander, and they reckon that because the pump will meet with mains cold at the mixer, that makes it a negative system
     
    Phil Jones, Apr 25, 2017
    #7
  8. Dr Bodgit

    Dr Bodgit Super Member

    Ok that makes some sense - a negative head pump will continue pumping for a bit after the taps have been turned off in order to create a permanent positive pressure. And you'd need a positive pressure in order that the mains cold (at higher pressure) doesn't flow into the hot supply (I think) and create a back pressure on the hot. Seems like a bit of a bodge to me though.
     
    Dr Bodgit, Apr 25, 2017
    #8
    Phil Jones likes this.
  9. Phil Jones

    Phil Jones New Member

    ok - sounds like I'd be better running a separate cold feed from the storage tank, would you recommend a separate hot feed too? I'd rather not if I can get away with it
     
    Phil Jones, Apr 25, 2017
    #9
  10. Dr Bodgit

    Dr Bodgit Super Member

    Where does your hot feed come from - hw cylinder in an airing cupboard?
     
    Dr Bodgit, Apr 25, 2017
    #10
    Phil Jones likes this.
  11. Phil Jones

    Phil Jones New Member

    hot water cylinder in the loft. Cold water tank is directly above that. diagram here - sorry it's very basic but its representative of where stuff is
    tank system.jpg
     
    Phil Jones, Apr 25, 2017
    #11
  12. Dr Bodgit

    Dr Bodgit Super Member

    OK should be fairly straight forwards. You will need to 'T' of at the top of the hw tank to have a dedicated supply hot water supply to the pump, ideally you should have a Surrey or Warix flange (Warix is easier to fit, below) which provide two outlets - one for the vent and domestic hw water, the other is air free and goes to the pump. The pump can go in the loft at floor level (isolated from the floor to reduce noise).

    Vertical outlet goes to the pump; horizontal outlet goes to the vent and other hot water taps.

    [​IMG]
     
    Dr Bodgit, Apr 25, 2017
    #12
    Phil Jones likes this.
  13. Phil Jones

    Phil Jones New Member

    you don't think i'll need a negative pump then? Could have a problem fitting a flange like that, the cold tank almost sat on top of the cylinder :(
     
    Phil Jones, Apr 25, 2017
    #13
  14. Dr Bodgit

    Dr Bodgit Super Member

    Per the diagram above, if the shower head is below the level of the cold water tank then a positive head pump is what you need. Positive head pumps are triggered by detecting a flow of water (using reed valves), so you need a positive head for the water to start flowing.

    A Warix flange isn't absolutely necessary, but you risk getting air into the pump without one. My partner doesn't have one in her house, hasn't been a problem.
     
    Dr Bodgit, Apr 25, 2017
    #14
    Phil Jones likes this.
  15. Phil Jones

    Phil Jones New Member

    ok - sounds like I can go with my original plan, fit a bath shower mixer, a 1.5 (or 2.5bar) positive pump next to tank (which will boost other taps too) and see how it goes.
    Suppose as a backup I could put a non-return in the hot line at the mixer tap to make sure cold doesn't go back up the hot in the event of pump failure?
     
    Phil Jones, Apr 25, 2017
    #15
  16. Dr Bodgit

    Dr Bodgit Super Member

    So are you putting in a cold water feed from the loft tank?
     
    Dr Bodgit, Apr 25, 2017
    #16
  17. Phil Jones

    Phil Jones New Member

    not if I can avoid it. Single pump in loft for hot to mixer tap fed with mains cold. Check valve on pump outlet for safety. Sound ok? I know cold feed to twin pump would be better, but will be a lot more work
     
    Phil Jones, Apr 25, 2017
    #17
  18. Dr Bodgit

    Dr Bodgit Super Member

    In that case I'd go for a negative head pump, but that's not how I would want to plumb it.
     
    Dr Bodgit, Apr 25, 2017
    #18
  19. Phil Jones

    Phil Jones New Member

    what is your reason for going for a negative head pump though?
     
    Phil Jones, Apr 25, 2017
    #19
  20. Dr Bodgit

    Dr Bodgit Super Member

    Because the cold is at mains pressure, 10x the pressure of the unpumped hot.
     
    Dr Bodgit, Apr 25, 2017
    #20
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