The only way to check pressure in the expansion tank is when there's no water pressure in the system. If the tank is shot, when the water heats, it can raise the pressure enough to cause the pressure relief valve to open. The only other reasons for the pressure to rise that I can think of are: there's a hole in the heat exchanger of the indirect allowing water to pressurized the boiler circuit, or, the autofill valve is defective and letting water in regardless. If the high temp limit is not operating, you could be getting some steam generated, but that would (or should) shut the whole thing down. Because steam expands radically, the pressure will rise, but it has to be quite hot with that pressure for that to happen, and isn't likely.