My boiler is over-pressurizing consistently for the past week when heating. If I lower the pressure by letting some water out from the relief valve in back of the boiler, the water fills to about 12 psi just like it's supposed to but once it starts heating up it gets up to 30 at which point it starts dripping out through the pressure relief valve again.

Since it's happening while heating and the PRV seems to work properly (turned off the valve right after the prv to isolate) I looked at the expansion tank next.

The expansion tank is the old style - just a steel tank mounted horizontally above the boiler. The inlet to the expansion tank comes from the bottom of the air scoop. It's getting very little water in it, much less than a gallon and not hot water when I've checked it recently. Don't know how to proceed here. Water will come in to the expansion tank but apparently not enough.