I am assuming that you have the older style expansion tank up between the joists, and not the newer rubber ballast type tanks that usually mount to the air scoop. If that is true, you shut off the expansion tank feed valve, and usually there is a bleeder valve hooked up to drain the tank. Sometimes it is a plug, sometimes a valve. There is pressure within, so don't do anything stupid.....hot or cold water comin' your way.
My pet peeve: You DO NOT have a furnace. You have a BOILER! Furnaces distribute warmed air, boiler distribute steam or heated water. I don't understand how the h..l folks get this confused, but many do. I'll stop ranting, all I really want to do is get you back operational.
Another thought is that you might have a pinhole leak in the HW coil. How high is the pressure going up to?
Obviously it isn't above 30 psi or the PRV would blow off.
If you can drain the pressure down to 15 psi, and have it stay there unless it rises when heated up, the problem sure seems like an expansion tank issue. Good luck, Mr BOILER man.