Thank you Dana , I believe I might get what you are saying. at this time I have had my auto fill water valve shut off for about a month thinking that it might have been the issue but I truly feel its the expansion tank ..I have shut the unit down and have waited till the temperature has dropped down to 130 degrees.. When i checked the temperature / pressure gauge and the pressure was at around 18 to 20 pounds at that temperature of 130 degrees ..
I believe the pressure should be around 12-15 psi at a cold temperature on the boiler of 100 degrees ..Dana would i need to wait for the system to cool down at all ? before doing what you are saying ..The target idling pressure in your opinion would this be 12-15 psi ? Like i was thinking ? Thanks again for the info New to the boiler world ...
The target pressure is related to the vertical distance between the system's pressure gauge and the highest radiator on the system, but not to be below 10 psi (when cold). A column of standard temperature water exerts 0.433 psi per foot of depth, so if it's a boiler in the basement with the boiler's gauge 7' below the floor above, and it's a 2 story building with 10' per floor, and the top of the second floor radiator is 3' off the floor you're looking at 7' + 10' + 3' = 20' of elevation difference, so just to get the water to fill to the top of the top floor radiator takes 0.433 x 20' = 8.66 psi, call it 9 psi. It takes that much pressure to lift the water that even when the system is stone cold, no pumps running.
To keep the top of the system pressurized above room air pressure (so that it can't suck air into the system)when the pumps are running, add another 3 psi. So the target pressure when operating would 9psi +3psi= 12 psi. That would be a pretty reasonable target pressure for 95% of all homes out there. But if the vertical distance is greater than 20' it needs to bumped up accordingly. With some high head water-tube boilers or limed up cast iron boilers it might need another 2-3 psi to keep the boiler from experiencing flash-boil sizzle/pop/banging aka "kettling". But for most 1-2 story homes it will never need to be higher than 15 psi.
It's fine to set the system pressure while the system at 130F if that is at or below the normal low-limit operating temp. If it's a cold-start boiler DO note the pressure on a mild low to no heating day, when the boiler drops to <100F, just to be sure that it's still above 10 psi. Below 10 psi almost all cast iron boilers will experience some amount of kettling on a cold start- often with some alarming (but not particularly damaging) clanging & banging. Kettling doesn't damage the equipment, but it does reduce the heat transfer efficiency, and it's annoying as hell to listen to at 5AM when the setback thermostat calls for heat.