On Jan 29, 1:49*pm, "Dave Liquorice"
wrote:
Domestic CH circulator type pumps. The graphs giving flow rate use
"head" as the other axis. What head is this? Highest to lowest point
in the loop? Pump to lowest, pump to highest? Something completely
differnt like flow resistance of the entire loop?

It is the hydraulic resistance of the system, expressed in metres head
of water.

If you have the pump running in a test rig, you'd measure the water
flow rate and the water pressure at the pump inlet and outlet. The
pressure difference is equal to density x g x h, where h is the
equivalent head of water.

A pipe circuit and radiators say, piped in 28mm tube would have one
resistance at a certain flow rate. If you pipe the same circuit in
22mm tube, it will have a higher resistance at the same flow rate
requiring a bigger pressure difference to shove the water around the
loop. You'd typically need a bigger pump or the same pump running at a
higher speed.