Negative hot / Postive Cold?? Shower Pump

[biosbill]

Hello,

Hopefully a quick one;

Got to install a twin shower pump in the loft (to feed two showers in separate rooms). Can probably get enough positive pressure on the cold side, but hot will be pulled from the HW tank in the airing cupboard below (thus a negative head??).

Does this mean I need a "twin universal pump"?

I presumed it did, but then I looked in the loft at a friends installation: it has a positive feed (around 3ft) from loft cold water storage tank, but hot is pulled up from the HW tank in the bathroom a few rooms away, a considerable distance and height away.

Pump appears to be a Techflow T3 which from searching online is a standard positive twin pump. Does it only need the positive head on the cold side to start the pump running then can pull the hot through, but switching the hot alone probably wouldn't start the pump?

I'm confused!

Cheers

William

 

[bernardgreen]

Can probably get enough positive pressure on the cold side, but hot will be pulled from the HW tank in the airing cupboard below (thus a negative head??).

If the cold feed to the pump is from a cold water tank and the cold feed to the HW tank is from the same cold water tank then the hot feed to the pump will be almost the same pressure as the cold feed to the pump.

If the hot water tank is fed from a different cold water tank that is below the pump then the hot water be "negative head" to the pump.

 

[biosbill]

Hello,

Thank you. Ok, the HW tank is fed from the same tank as cold water (HW tank in cupboard, Cold water storage in loft).

Does this mean I can mount a positive twin pump in the loft with cold feed from the cold storage tank and hot feed from a flange on the tank below?

Kind Regards

William

 

[denso13]

Negative and positive are in relation to the outlet on the cold water cistern. If the shower head is above it it is negative, if below it is positive (generally).

 

[bernardgreen]

Does this mean I can mount a positive twin pump in the loft with cold feed from the cold storage tank and hot feed from a flange on the tank below?

Provided the pump is below the water level in the cold tank the answer is yes. Though be careful the pump doesn't pump water out of the tank faster than the feed to the tank can replace it.

Positive head pumps need water in their pump chamber when they start up. Negative head pumps ( in most cases ) can start dry and then create enough suction to pull water up to them. Many have a non return valve to keep some water in the pump chamber for the next time it has to start.

 

[flameport]

For shower pumps, positive head versions only start by sensing water flow - if the shower head is above the cold water storage there will be no flow and the pump will never start.
Negative or universal models have a small expansion vessel and non-return valve on the output of the pump so that the pipework is under pressure when the shower is off. Turning on the shower releases that pressure, closing a switch and starting the pump.

Shower pumps should be mounted as low as possible, usually on the floor next to the hot water cylinder.

 

[biosbill]

Hello,

Many thanks all, I think this clears up my question!

Understand the ideal location is next to the HW cylinder on the floor but in my case that does'nt really work so I will have to go for the loft.

Cheers

William

 

[denso13]

You still need some head to the pump from the cold water storage cistern outlet. Follow the manufacturer's instructions.