Andrew Gabriel wrote:
.... snipped


The GlowWorm book suggests a minimum pressure of 1 bar, is this just to
stop local boiling in the heat exchanger?


1 bar will ensure pipework up to 30 feet above the pressure
guage is pressurised, which is enough for most houses. In a
3 or more storey house with the pressure guage in the basement,
you will need higher pressure if the pipework extends more than
30 feet in height above the pressure guage. (The pressure at 33
feet above the pressure guage will be 1 bar less than the guage
reads.)

.... snipped

Sorry, I don't understand that explanation. The system is sealed,
therefore it's all at the same pressure; how does atmospheric pressure
(32 ft of water) affect anything?

Dave