Examples:

Header tank at 10 meters. Tap at 8 meters. Pressure = 1.0 - 0.8 = 0.2 = 2 meters of water.

Header tank at 20 meters. Tap at 18 meters. Pressure = 2.0 - 1.8 = 0.2 = 2 meters of water

Header tank at 1000 meters. Tap at 998 meters. Pressure = 100 - 99.8 = 0.2 = 2 meters of water.

Ok so far?

Note how the diameter of the pipe doesn't matter. You could put a large bulge in the pipe the size of a hot water cylinder anywhere you like. Makes no difference to the static pressure.

CWatters wrote on Jun 5^th, 2009, 10:31pm:


The head depends on the highest point, be it a tap/shower rose or the pipework.

If you lower the rose in a shower the pressure increases, by 0.1bar per/m.

I know and understand all of that and a lot more.  However your diagram still doesn't quite show how to calculate the pressure at the tap itself.  See if you agree with this one..

Edit: For some reason it's hard to read the text. Click on it to make it readable.

It's quite clearly explained here http://en.wikipedia.org/wiki/Hydraulic_head  

Wrong CW.

Lowering the outlet at the top of the cylinder will increase the head in this case, to the next highest point, the pipes in the loft.

You could get a secondary head, if the top of the cylinder to the outlet is greater than the storage tank to the highest point.

I would have to agree with.... cw ,  if the head is measured from the bottom of the cold water tank to the highest tap, it doesnt matter where the hot water cylinder is.

no expert here but you need the mass to be as high as possible otherwise your hydro electric dams would be 20 foot high

you need volume[mass] x height to give weight[ pressure]

a bit off pipe 200 ft tall above a tank will give you high pressure/flow for a very short time because the body off the water is low down

height is weight volume is weight so volume at height weighs more

Now do another one with the connection out the cylinder lowered to the floor level.

Or are you deliberately leaving  it stuck up in the air a 1mtr.

You ain't trying Bill, and it ain't theory.   The head is the distance to the highest part of the system, in this case the pipe from the cylinder is higher than the outlet, so by lowering the pipe you increase the head.

Head is not the higher the better, although it works out that way most of the time, but the greater the distance betwen the storage tank and the highest part, not necessarily the outlet.

Read this and see if it's any clearer, they are talking of fill head, which is slightly more than static head by the distance to the outlet.

Also notice the negative head, then under stand how the head increases as you lower the outlet.

http://www.stuart-turner.co.uk/media/5273-Installation-Monsoon-Universal-U1.5-2....

In the next pdf look at fig5 and you should be getting warm  

http://www.stuart-turner.co.uk/media/5199-Installation-Monsoon-Standard-1.5-2.0-...