Pump head from feed cistern

[stevekyle]

I've just moved into my new flat and notice that the pump is in the airing cupboard, and the hot water cylinder and feed and expansion tank are in the cupboard also. the pump is a wilo, and has 5m head. i don't think the feed cistern water level is 1/3 of that above the pump! does this mean pumping over will occur? i have not had it on yet to see, but believe that 1/3 of the pump head should be met?? is this true?

thank you for your help

 

[D_Hailsham]

I've just moved into my new flat and notice that the pump is in the airing cupboard, and the hot water cylinder and feed and expansion tank are in the cupboard also. the pump is a wilo, and has 5m head. i don't think the feed cistern water level is 1/3 of that above the pump! does this mean pumping over will occur?

The "head" of a pump has nothing to do with the height of the F&E tank above the pump. "Head" is a measure of the pressure loss due to friction in the pipework of the central heating system. Two things have to be taken into account when selecting the correct size pump: the flow rate and the pressure loss. Most three and four bed houses will require a 5m head pump.

Pumping over can occur is the pump speed is set too high. If there is no sign of this, then I wouldn't worry any more.

 

[stevekyle]

Ok thanks for your repsonse.

The reason i ask is because in a plumbing book, by Treloar, which I have, it says the circulating pump should be the pump head (eg 5M) divided by 3 below the feed and expansion cistern's water level, to prevent undue movement in the cistern.

 

[Llan1observer]

the 1/3 is to determin the height of the open vent pipe
minimum =150mm + 40mm ( for every metre of system height)

 

[stevekyle]

no the vent pipe for the feed and expansion system should be a minimum of 450mm (dry head). for an open vent from a hot water cylinder it is 150mm PLUS 40mm per metre from the cistern to the bottom of the hot water cylinder.

 

[D_Hailsham]

The reason i ask is because in a plumbing book, by Treloar, which I have, it says the circulating pump should be the pump head (eg 5M) divided by 3 below the feed and expansion cistern's water level, to prevent undue movement in the cistern.

The pump has to have a minimum static head ( height to the water surface in the F&E tank); this is probably what the book is talking about. However the static head depends on the temperature of the water.

This is taken from the Grundfos Alpha2 data:

Water Temp 75°C; 0.5m head
Water Temp 90°C; 2.8m head
Water Temp 110°C; 10.8m head

As most systems are now designed for a max temperature of 75°C a 0.5-1m head is all that is required.