CW tank in airing cupboard - neg or pos shower pump?

[pdhan]

Dilemma explained in heading really, some more details:

Water tank sits on top of HW cylinder in airing cupboard, about 120-140cm from floor. Shower outlet in bathroom will probably be below this, but shower head (when raised) won't be.

1) I read in a separate post that I a positive pump will start if I lower the head enough below bottom of CWS level. However pump manufacturer recommends min 2m supply head (which I don't have as i assume this is calculated from bottom of the CSW) so I guess pressure will be a problem?

2) somebody told me that it's the position of the pump relative to the tank that decides if it should be positive or negative, but after looking at various diagrams I don't think this could be correct. Should/can a negative pump still be placed below tank level though?

Many thanks for any clarifications and recommendations!

 

[swbjackson]

It's the position of the outlet in relation to the head of water that matters. You need a negative head pump and yes it can go below the cylinder.

 

[pdhan]

Thanks for your reply. I was actually recommended a positive pump elsewhere and it looked like that would work according to Grundfos installation doc as well.

In this they said that the outlet had to be >250mm below the water level in the tank. Confusingly in other installation instructions I've seen they talk about the outlet in relation to the bottom of the tank (which seems to be your advice too) - in my case this would change everything!

In the end I had a positive pump installed with good results. I assume a negative would have worked too as I seem to be in some sort of grey area between the two.

In case it helps anyone else this is my setup:

• water tank c. 1m from the floor
  water level = 2m from the floor
  bath mixer (=shower outlet) sits on top of bath at 60cm from the floor

As a side note one annoying things was that there was no way a surrey flange would fit between my HWC and tank so had to do without it - reasoned that most air will have dissipated through the night when the water heats up anyway.[/list]