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
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I believe it means, in an open system, that the pump may be situated above the level of the water in the header tank. It sucks, in other words, rather than just circulating. The head referred to in "negative head" is that presented to the pump at the inlet, not what it develops on the outlet.
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 are:
Once the pump has started, the standard flow switch will keep the pump running, as the flow will continue.
Christian.
Thank you for that.
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?
No. Negative head pumps should be installed below the level of the cold tank, ideally adjacent to the hot water cylinder. The negative head applies to the outlet, not the pump location. It would be quite difficult to design a shower pump that could prime itself by sucking water up by a column of air, as the impellors required to pump water are quite different to those used to provide high pressure air suction. Also, the lubrication and cooling behaviour is very different.
It is also usually unnecessary and provides a strict limit on the pressure possible, in that it could only provide a maximum of 1 bar at zero head pump installation level, reducing with altitude.
Christian.
Ah yes he's right, I realise. Damn Christians - always right :o). Apologies for any misleading from my first post.
Why? Many pumps do exactly this.
I said it was difficult, not impossible. I don't doubt that some pumps are designed to do this, but it is a bit of a compromise for a shower pump. Even standard non-submerged well pumps, such as venturi jet designs usually need priming with water. I suspect you could run a standard shower pump above water level with a separate manual or automatic priming system and check valves to prevent priming loss.
BTW, for future reference do you know which manufacturers do a negative head shower pump suitable for suction priming?
Christian.
I suspect you may be referring to a negative head shower pump unit? If so this is capable of being used with a shower with the shower head higher than the water storage tank. Operation is achieved by the pump responding to a pressure switch at the outlet of the unit. When first connected the pump will run until the downstream pipework is up to a preset pressure when an inbuilt pressure switch will stop the motor. A non return valve built into the unit holds the pipework "full" and retains the pressure therein. Opening a valve (turning the shower on) releases this pressure so the pump starts up in response to the pressure switch and runs until the valve is closed when the pressure builds up aginst the shut valve and the pump stops again. Dripping taps cause pulsing of the motor which can be very annoying Ordinary shower pumps are used where a trickle of water will flow when a shower valve opens. This trickle operates a flow detector and the pump cuts in. Shutting the valve stops the flow and the motor stops.
HTH
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