The pressure relief valve can weep at 25 psig and release at 30 psig. If the relief valve is off calibration, replace it.
Make sure the pressure reducing valve at the fill line is not creating the excess pressure. If you have a 2-story building or less, 12 psig is enough to automatically fill with. If the pressure has crept up by failure of the pressure relief valve, then replacing the pressure reducing valve will help. You can do this by shutting the hand valve befor the fill valve, then draining the water till the pressure reaches about 12 psig. Then open the hand valve and watch the pressure. If the burner is not on and the pressure creeps up, the valve is at fault.
If the system did not blow the pressure relief valve in the previous season and the pressure reducing valve is OK, the expansion tank was large enough. This means that the expansion tank must be evacuated of as much water as possible. If the draining sequence (purge) does not get rin of enough water, the tank can be disconnected, drained and then reconnected to make sure there is as much air there as possible, When you pressurize the system again, the tank will become about 1/4 full as the 12 psig pressure forces the water into the tank. You can tell by rapping on the tank and hearing the change in sound.