It's very possible that the tank is the problem. You can open the drain valve of the tank and some water could come out, but sometimes they go into a vacuum lock. There could be a plug at the other end of the tank to break the vacuum and let most of the water come out.

If you don't like the tank, you could have a properly sized Extrol tank installed, which is what I would do.

Thanks for the reply. There wasn't a plug at the other end of the tank. Some water came out but not alot. Any idea whether replacing an expansion tank is a big deal money-wise and time-wise?

It's not a hard job if you have isolation valves nearby. Otherwise, you'll probably have to drain some of the system, which would be a good time to add isolation valves.

Sweat a T into the feed line and install the tank.

If you don't understand the system, get somebody knowledgable to do it.

Thanks--we'll definitely call in a professional to do this!

Hasn't been drained in over ten years? Don't you have the system checked out at least every other year by a service tech? If so, how come they are not draining the tank? Shut off the valve installed leading to the tank from the boiler. Open the drain valve and see what happens. Should be a metal tag on that valve saying must be left open during operation of the boiler so turn off/down the tstat when playing with the boiler.

As for the drip, it is normal when one tests a pressure relief valve. There is a seal to keep water in until the rated pressure is reached and debris can catch on the seal and drips happen. Use a coffee can to catch the water. No big deal.

The OP stated that it leaks upon heating, suggesting a possible expansion problem, but who knows from here.

Some ceiling tanks will never give a problem and some will fill with water frequently. Some installers prefer them; not me.

The ceiling tank at my parent's place has been there since at least 1964 and it has never caused any problems and hasn't been touched for many years.