In my system i have an auto fill valve that initially sets the pressure in the system to 12 psi. I have my aquastat set at 190*F. When it is that temp its close to 28-29 psi. I'm thinking that i don't have enough expansion tank volume. I didn't install the system but it has 2 small expansion tanks. My boiler alone holds 135 gallon. Then i have a total of 560 ft of 1" pex and 150 ft of 1/2" pex. The expansion tanks are about 11" wide by 13" long. I'm not sure if that is sufficient. Kevin