Does the pressure, within a range, matter in a FHW system?

[MrEd]

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?

 

[pybyr]

Water carries the same BTU regardless of pressure. Some pressure is needed with a multi-floor system unless you are going to do "drainback"- the weight of the water in the vertical piping above the boiler will add approximately 0.4 psi peer foot. The pumps don't care, as the pressure within the system is the same on both inlet and outlet of each pump.

 

[benjamin]

The pressure makes a big difference in the temperature your system will function at. Any pump is much more likely to "cavitate" or air lock at a lower pressure and higher temperature. I think some pumps may publish charts for pressure/temp recommendations.

Of course if you have a closed system (with the minimum expansion tank size) that is 5psi cold then by the time you're pushing the temperature limits, the pressure will have risen quite a bit as well.