Having lived only in houses with forced air heat, our new home's boiler is a different technology to learn. It's a Buderus; supposed to be very efficient. When it's not firing, the display shows a system pressure of 20 PSI; when heating, that number drops to 13 PSI. Is that normal, and what would be the reason for the pressure change?
Lacking major information regarding system type, gauge location with respect to expansion tank, pumps, boiler, etc... I'll take a stab at it.
First, is there a circ pump operating when the unit is firing? If so, this may be the reason for the lower pressure shown... or not...
The 20 PSI could be the total of the Atmospheric pressure (prx 14psi) that we all endure due to the weight of the atmosphere at see level, added to the weight of the water in the system... 2.31 ft of water equals 1 psi. If your system is a single story w/ base board or radiator, I'm guessing you have about 6' of standing water above said gauge... then again, maybe not. If atmospheric pressure is zeroed out your system would be 46.2 ft high... do you live in a 6-8 story home?
Check out Dan Hoolihans web site for "wet heads" You'll learn more about hydronics there than you'll ever get from newsgroups.
Somebody out there probably knows the site address, as it's not coming to mind right now. A Google search will find it quick.
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