On 2008-02-11, Roger Mills wrote:
In an earlier contribution to this discussion,
Garry wrote:

Im struggling with the pump term "Pump Head". when used in central
heating application.

Is it the distance between the lowest point of the central heating to
the base of the pump.

Or

The distance between the highest point of the heating circuit to the
base of the pump.

Or something else.

Something else!

It's the pressure generated by the pump to overcome the friction losses in
the system, and thus to allow the water to circulate. If you look at a
pump's performance graph you will see that the head is highest at very low
flow rates, and lowest at high flow rates. [Think of the back pressure you
generate in the mains by putting your thumb over the cold tap spout].

The pressure is often expressed in terms of an equvalent static 'head' of
water (1 bar is approx 10 metres) but it has nothing to do with the physical
dimensions of your system.

Indeed. By contrast to the pump, a CH system's radiators and pipework
(including the boiler) will have a head (pressure) versus flow curve
where friction causes the head to *increase* as the flow increases.
Head is approximately proportional to the square of flow in pipework.

If you plot the two head versus flow curves on the same graph the
intersection shows the head and flow values where the combined CH/pump
system works.

Changing the pump setting will alter its head / flow curve and so change
the operating point.

--
John Phillips