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Christian McArdle
 
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Default negative head pump

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.