Okay it is me again. I have emptied the expansion tank, all was good for a short while. When the furnace is finished heating up and turns off, the pressure continues to increase to 30 psi, then of course the relief valve releases water, approx. 1/2 pail full.
If it's not the tank than it is the more than likely the pressure reducing valve (prv) is not regulating properly. This is located where the domestic water connects to the boiler for filling. There should be a gate valve or ball valve near the prv so you can turn the water off to verify the prv malfunction.