Right next to the gage there should be a TPR valve. It normally is a 200 degree F or 30 psi TPR. If you see signs of water below it that would be a sign that their could me a problem… (water logged expansion tank)… Usually the problem is that someone has tightened the “Valve Stem Cap” on the top of the expansion tank. It should be loose so it can let out water. The cap is designed with cut-out slots on the sides so that the water doesn’t shoot out into the air and burn someone or other hazards.
Here in Michigan you have to be licensed to install a boiler system (No Homeowners Permit at all). They are best inspected by a licensed professional. (On that note, it is still good to know something about them. I installed an entire baseboard hot water heat system myself for a couple that I do handyman work for (their business) on their own home. We had a licensed Engineer to pull the permit, look over my work & be their for the inspection. Both felt I did the nicest, cleanest job installing the whole system they have seen in a long time. Made me feel proud that day.