View Single Post
  #5   Report Post  
Posted to uk.d-i-y
Roger Mills \(aka Set Square\)
 
Posts: n/a
Default negative head pump

In an earlier contribution to this discussion,
Christian McArdle wrote:

Would someone be able to tell me please what a negative head pump
does over a so called ordinary pump or I suppose what is negative
head. Is it just when the pump is positioned lower than where you
are pumping to? many thanks


It is nothing to do with the position of the pump or hot water
cylinder. It is to do with the position of the highest outlet
compared to the position of the cold water tank.

Basically, a standard pump uses a flow switch to turn itself on. That
means that when you open a tap or turn on a shower, a small amount of
water flows. This is sensed by the switch which starts the pump up.
It relies on the fact that the cold tank is above the outlet.

If the outlet is close to the tank position (but below), you have a
low head situation. There might not be enough flow for the switch to
sense. If the outlet is above the tank position (common with loft
conversions) there will be no flow at all, so the pump doesn't start.

The solutions to get the pump started a

1. Blip the lower down bath tap to get the shower started (assuming
the bath tap is low enough for reliable gravity flow). The cheapest
solution.

2. Use a negative head switch, which is a pull switch that just kick
starts the pump. Fairly cheap solution.

3. Use pressure switches. These maintain pressure even after the pump
stops, so there is a brief flow of water even from negative head
outlets, enough to kick start the pump. The best, but most expensive
solution.

Once the pump has started, the standard flow switch will keep the pump
running, as the flow will continue.

Christian.


Doesn't negative head also imply that the pump needs to be able to suck
water into its inlet from a lower level, without cavitating?
--
Cheers,
Roger
______
Please reply to newsgroup.
Reply address IS valid, but is disposable in the event of excessive
spam.