Negative head pump

[V1k]

Hi, I have been told that I need to have a negative head pump installed to increase the preassure of the hot water system in my house.

I just wanted to know what the difference is between this type of pump and a positive head pump or a standard water pump and if I could use something else in it's place as I'm trying to keep costs down and these pumps are quite expensive.


Thanks.

 

[Richard C]

A negative head pump is required when there is insufficient head (water flow); the required water flow to trigger the flow switches in a positive head pump is 1 litre per minute, less than this & the pump is unlikely to work.

Who has advised you need a negative head pump & why? Do you want to increase just the hot water pressure? What are you attempting to achieve? What sort of water/heating system do you have?

 

[V1k]

I want to increase the preassure of just the hot water.

It's a standard hot water and central heating system with a big 50 gallon cold water tank in the loft and a hot water cylinder on the 1st floor of the house.

The bathroom taps (also on the 1st floor) literally dribble water and the water seems to come out fine in the downstairs WC and kitchen for a while but then splutters and just stops as well.

So I basically want to increase the preassure of just the hot water (which comes from the tank on the 1st floor) around the whole house if possible.

I called a company which sells pumps and the person on the phone told me that I needed a negative head pump - just didn't understand what this was and so wanted more advise and info on it.

 

[echoes]

The bathroom taps (also on the 1st floor) literally dribble water and the water seems to come out fine in the downstairs WC and kitchen for a while but then splutters and just stops as well.

Roughly how much water comes out before the flow splutters and stops?
1 - 2 litres by any chance?

 

[V1k]

The bathroom taps (also on the 1st floor) literally dribble water and the water seems to come out fine in the downstairs WC and kitchen for a while but then splutters and just stops as well.

Sounds like an airlock or a blockage, to which a pump is not the solution.

The plumber who came over thought the same thing to begin with but after testing and emptying out the cylinder, (to no avail) he advised the pump.

 

[Richard C]

Has it always been like that or has the situation just developed? I would also say that you have a blockage which adding a pump will not fix. It may not be in the H/W cylinder or pipe work from it causing the problem, it could be a blockage in the feed from the CW storage tank to the HW cylinder; you won’t get HW water out of the cylinder if C/W water can’t get in to replenish it.

 

[V1k]

The majority of the system is new to be honest, new pipes, joints etc, the only old part is the feed coming down from the upstairs to the downstairs supplying the kitchen and ground floor WC.

The ground floor supply pipe was working fine before the system was changed over too.

The only difference since the upgrade of system has been the addition of extra pipes (as the hot water cylinder was moved to a different part of the house).

 

[echoes]

The bathroom taps (also on the 1st floor) literally dribble water and the water seems to come out fine in the downstairs WC and kitchen for a while but then splutters and just stops as well.

Roughly how much water comes out before the flow splutters and stops?
1 - 2 litres by any chance?

 

[V1k]

The bathroom taps (also on the 1st floor) literally dribble water and the water seems to come out fine in the downstairs WC and kitchen for a while but then splutters and just stops as well.

Roughly how much water comes out before the flow splutters and stops?
1 - 2 litres by any chance?

Hard to say.... this is the ground floor WC hand wash basin tap. Fed by a 15mm copper pipe, it runs fully turned on for a few minutes, but would be difficult to say quantitatively how much water comes out?

 

[Richard C]

The ground floor supply pipe was working fine before the system was changed over too.

The only difference since the upgrade of system has been the addition of extra pipes (as the hot water cylinder was moved to a different part of the house).

So did the problem become immediately apparent after the HW cylinder was moved? Where has the cylinder been moved to? What is the vertical distance between the bottom of the CW storage tank & the inlet to the HW cylinder, how long are these extra (presumably horizontal) pipes & how are they routed?

 

[echoes]

Ok - I had a theory that if the cold feed to the tank was blocked or restricted, then turning on a tap would empty the contents of the vent pipe. As this happens, the decreasing presssure would reduce the flow to a trickle. Spluttering would indicate air either from an airlock, or by air being drawn in if the cylinder could not replenish from the CWS tank.

I had exactly the same problem as you last week, but accompanied by tell-tale debris coming out of the hot taps as well. I found some debris in the bottom of the CWS tank and removed it. But still had poor flow reducing to a trickle at the hot taps. If I turned off a hot tap for a minute and opened it again, the flow started fast(ish) but reduced back to a trickle within seconds.

Solution was to blast some cold mains back through my kitchen mixer tap, which cleared the problem.

 

[V1k]

Ok - I had a theory that if the cold feed to the tank was blocked or restricted, then turning on a tap would empty the contents of the vent pipe. As this happens, the decreasing presssure would reduce the flow to a trickle. Spluttering would indicate air either from an airlock, or by air being drawn in if the cylinder could not replenish from the CWS tank.

I had exactly the same problem as you last week, but accompanied by tell-tale debris coming out of the hot taps as well. I found some debris in the bottom of the CWS tank and removed it. But still had poor flow reducing to a trickle at the hot taps. If I turned off a hot tap for a minute and opened it again, the flow started fast(ish) but reduced back to a trickle within seconds.

Solution was to blast some cold mains back through my kitchen mixer tap, which cleared the problem.

Hi, this is what the plumber already tried but it didn't help. The system does look as if it is sucking in air from somewhere but god only knows how.


With regards to:

V1k wrote:

The ground floor supply pipe was working fine before the system was changed over too.

The only difference since the upgrade of system has been the addition of extra pipes (as the hot water cylinder was moved to a different part of the house).

So did the problem become immediately apparent after the HW cylinder was moved? Where has the cylinder been moved to? What is the vertical distance between the bottom of the CW storage tank & the inlet to the HW cylinder, how long are these extra (presumably horizontal) pipes & how are they routed?

To be honest, I can't really remember how the system worked before the cylinder was moved as I just bought the house then and hadn't tried using the hot water at that point.

The distance between the CW tank in the loft and the entry point on the cylinder is over 2 meters. The cylinder and cold water tank have been moved horizontally accross the house - from one side of the house to the other - approximately 20 feet. This would be the length of the horizontal pipes between the cylinder and the taps in question (on the 1st floor
). The ones on the ground floor are the same with the addition of vertical pipes - approximately 2 metes in height.

 

[V1k]

The question that I had was what is so specific about a negative head pump in comparison to other pumps?

Other advise is also welcome.

 

[echoes]

The question that I had was what is so specific about a negative head pump in comparison to other pumps?

A negative head pump is required when there is insufficient head (water flow); the required water flow to trigger the flow switches in a positive head pump is 1 litre per minute, less than this & the pump is unlikely to work.

Basically, a pump needs a certain flow through it to start the motor. A positive head pump is used when there is sufficient head, or pressure to cause at least 1l/min flow rate through the pump, at which point the switch activates and the motor starts.
A negative head pump is used where this flow is not achieveable (there is insufficient head/pressure between the outlet and the tank to start flow through a positive head pump). It has some pressurised vessels to 'kick-start' the flow.

Other advise is also welcome.

Unless the force of gravity is peculiarly different in your house, I dare to suggest that your plumber suggesting that you install a pump is not giving you good advice.
Between your loft tank and your downstairs taps is at least 3m of head (=0.3 bar pressure). The laws of physics make it so.
I bet 50p there is a stubborn airlock or blockage. And a further 10p that it is where Richard C suggested!

 

[V1k]

The question that I had was what is so specific about a negative head pump in comparison to other pumps?

A negative head pump is required when there is insufficient head (water flow); the required water flow to trigger the flow switches in a positive head pump is 1 litre per minute, less than this & the pump is unlikely to work.

Basically, a pump needs a certain flow through it to start the motor. A positive head pump is used when there is sufficient head, or pressure to cause at least 1l/min flow rate through the pump, at which point the switch activates and the motor starts.
A negative head pump is used where this flow is not achieveable (there is insufficient head/pressure between the outlet and the tank to start flow through a positive head pump). It has some pressurised vessels to 'kick-start' the flow.

Other advise is also welcome.

Unless the force of gravity is peculiarly different in your house, I dare to suggest that your plumber suggesting that you install a pump is not giving you good advice.
Between your loft tank and your downstairs taps is at least 3m of head (=0.3 bar pressure). The laws of physics make it so.
I bet 50p there is a stubborn airlock or blockage. And a further 10p that it is where Richard C suggested!

Ahh, just read the post from Richard C, hadn't seen it before - that pipe was removed and checked (the pipe between the CW tank in the loft and the entry to the cylinder and then put back again - it's a 28mm copper pipe.

And I'm guessing that there is enough pressure coming out of the hot water cylinder to output more than 1l/min so as not to need a negative head pump or is this wrong?

(the plumber suggested a pump, I looked online and called a shop who suggested a negative head pump).