When the boiler is on, the top of the expansion tank (the circular end that attaches to the rest of the system) gets so hot, one cannot leave one's hand on it for more than a moment. Is this a problem?
The high end temp for most boilers is 180 - 200 deg. You do not want to put you hand on the pipe at that temp. You should have a gage on the boiler giving you the water temp.as well most closed radiant systems have a temp gage in the primary loop.