Hot water boiler pressure range

Sue2

I've got a gas-fired boiler for both heat and domestic hot water, in a 2-story house with old-fashioned cast iron radiators. I learned early on that I needed a larger than "normal" expansion tank to account for the large volume of water in the radiators (despite what the guys at the supply store said.) However, this year, something has changed and the pressure range in the system between cold and hot has increased to the point where, if it maintains 10-12 psi cold, it is running at 30+ psi hot, and spitting water out the relief valve. What causes the hot-to-cold range to increase? Is this a sign that my expansion tank has failed? Has too much air? Has not enough air?