Pressure Rise During Boiler Firing Up

My Utica hydronic boiler for all its years in operation has always maintained 12 lbs pressure either cold or fired to 180 degrees. Now, with a new expansion tank, same size and brand, the pressure rises from 12 lbs cold, to 20 lbs at 180 degrees. I realized the plumber had mounted the new expansion tank water chamber down. Extrol rep. indicated that there is a chance of air getting trapped in the water chamber with water chamber down. I called back the plumber, and had him redo the fitting to permit mounting the tank with air chamber down. While the tank was off during the redo, I rechecked the tank pressure, it was still at 12 lbs. Changing the tank position didn't help, the pressure is still rising to 20 lbs with the boiler fired up. I don't rely on the water pressure regulator to get to 12 lbs cold fill. I watch the gauge, and when it reaches 12 lbs, I turn off the water inlet valve to the boiler. I'm wondering if the fact that the boiler was kettling for the last couple of years has anything to do with this problem. After refilling the system, the kettling stopped. I thought that's a good thing, indicating the debris got flushed out. On the otherhand, what if the refill forced the debris into some of the heat exchanger channels, causing the problem. BTW system is heating just fine.