CRASH Hydronic course
There are basically two types of expansion tanks used on hot water heating systems.
1- A tank filled with air and 1/3 water and has to be drained once the air is depleted
(absorbed into the system). Some do come with a sight glass
The other type is a bladder type sort of a balloon inside a steel tank.
The pressure settings are governed by the static head of your heating system.
Each system is different but the average is based on 12 -15 PSI setting
The static head is merely the cold fill of the water to the highest radiator and the gauge on the boiler will give the attitude reading .433 x the height
To prove this constant Lets assume your upper most radiator is 20 feet above the boiler the reading would be 8.66 on the altitude gauge marked with a red area along with the pressure settings. Now you have to add more pressure on the automatic feeder to make sure you have positive pressure on the system so one would add a total of 12 - 15 PSI
Now take the 8.66 PSI X 2.31 = 20 feet NOT enough pressure to keep the air out so we add a fudge factor of say 4 PSI more Here is how we get to the 12 PSI (simple huh)
We math figuring the pressure to have a system that is not air bound but yet low enough to not have the pressure relief valve opening every time the pressure approaches 30 PSI
The expansion tank takes up the expanding hot water as when water is heated it does expand and it a closed system the pressure does build.
People screw around constantly with the expansion tanks for no valid reason.
If it is not broken why try to fix it? Like the dummies who go around bleeding a system every year even when there is nothing air bound.
Al they are doing is killing their boiler as they are allowing fresh water in which contains oxygen which will in time rust out the boiler and other ferrous components
All one does is look at the Tri indicator (pressure, altitude, temperature)
The factory setting is set 12-15 PSI just like the automatic feeder.
If your getting water from a diaphragm type it is time to replace it as the bladder has given up the ghost
Just remember pressure temperature relationship