My sealed hot water system has a manual fill-valve. The expansion tank is in the floor joists just above the boiler. The tank has no gauges or windows to check the level. Before draining, I noted the pressure on the (cold) boiler -- 20 psi. I refilled and bled the radiators until I was again at 20 psi with no air. But what about the tank? How does that fit into the equation? As I understand, it should have some air in it to allow for expansion of the water at operating temperature, but how do I know without a gauge? Should I just close the inlet valve to the tank and drain out all or some of the water, then reopen the inlet? Also, I'm new to this-old-house. How do I know what the pressure in the system really should be? The gas-fired bolier is rated at max 100 psi. The T&P valve is rated at 30 psi.

Thanks to all for help