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UK diy (uk.d-i-y) For the discussion of all topics related to diy (do-it-yourself) in the UK. All levels of experience and proficency are welcome to join in to ask questions or offer solutions.

negative head pump



 
 
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  #1  
Old February 15th 06, 08:26 AM posted to uk.d-i-y
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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

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  #2  
Old February 15th 06, 09:01 AM posted to uk.d-i-y
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Default negative head pump


"simon beer" wrote in message
...
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


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.


--
Bob Mannix
(anti-spam is as easy as 1-2-3 - not)


  #3  
Old February 15th 06, 11:59 AM posted to uk.d-i-y
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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.


  #4  
Old February 15th 06, 02:21 PM posted to uk.d-i-y
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Default negative head pump




"Bob Mannix" wrote in message
...

"simon beer" wrote in message
...
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


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.


--
Bob Mannix
(anti-spam is as easy as 1-2-3 - not)



Thank you for that.


  #5  
Old February 15th 06, 03:37 PM posted to uk.d-i-y
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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?
--
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Roger
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spam.


  #6  
Old February 15th 06, 04:06 PM posted to uk.d-i-y
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Default negative head pump

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.


  #7  
Old February 15th 06, 04:23 PM posted to uk.d-i-y
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Default negative head pump


"Christian McArdle" wrote in message
.. .
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.


Ah yes he's right, I realise. Damn Christians - always right ). Apologies
for any misleading from my first post.


--
Bob Mannix
(anti-spam is as easy as 1-2-3 - not)


  #8  
Old February 15th 06, 04:45 PM posted to uk.d-i-y
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Posts: n/a
Default negative head pump

Christian McArdle wrote:
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.


Why? Many pumps do exactly this.
  #9  
Old February 15th 06, 05:10 PM posted to uk.d-i-y
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Default negative head pump

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.


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.


 




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