U.s. Gas Hot Water Heating Boiler
 

I live in the U.S. I have a very simple small old gas hot water heating boiler in my home. (BRYANT model: 234-3GW series C). It heats the house nicely. The boiler temperature stays between 135-140 degrees which is okay. No problem with that. THE BOILER IS NOT OVERHEATING. However, lately I've noticed problems with the pressure. Maybe this has been happening for years and I just have not noticed it. I don't go into the basement often. The problem is this. Whenever the weather temperature outside falls from about the 40's (or above) down in to the 20's (or below) the pressure gauge rises past the 30 pound limit such that it pops the relief valve. THE BOILER TEMPERATURE STAYS STEADY HOWEVER AND DOES NOT OVER HEAT (135-140 degrees or there-abouts). This happens a few times intermittently (spilling some water each time which is replaced by the automatic fill valve) and dropping the pressure each time to about 14-20 lbs. (which is good) until the system somehow stabilizes and STOPS doing this. This cycle will not happen again until the outside weather temperature rises high enough then falls low enough again. I'm stumped. I believe the expansion tank is working okay because when I knock on it it sounds very hollow and empty. But I don't know how expansion tanks work so I'm not sure. Please help.