I have been wondering what the effect of the pressure differences in a closed FHW system? I seem to be able to run fine within a pretty big range, basically anything from about 5 psi upto about 25-30 psi in my system. But does it matter? i.e. If I consistently ran at 5 psi, versus consistently running at 25, are the heat-transfer/carrying properties any more or less efficient? Do the pumps have to work harder or easier at different pressures?