I just drained the boiler of my hot-water heating system (so I could pull out some radiators - see previous post) and refilled it. All is well, I think, but I'm wondering about the expansion tank. How can you tell if the water is at the right level? The tank has a valve at the bottom with two screws on the side for controlling the valve. There's another valve in the pipe that leads to the boiler that says it must remain open unless draining the expansion tank.
I would guess that a pipe goes up into the tank (probably with the two-scrw valve at the bottom) and that if you drained from that valve, the water would be at the right level when water stopped coming out. But that's just a guess.