Yesterday our expansion tank overflowed. We discovered that the overflow was blocked by scale, which we cleared quickly, and the ballcock valve was leaking. It is a diaphragm ballcock and I assume that the seal has worn. Today I have tried to unscrew the knurled nut, with no success, it is in a small tank which is difficult to get to. Tried using a Stilson, but the whole unit turned. I am very wary of taking apart at the pipe, for fear of more water coming out.
HELP!

Cen can you give me a picture? Where is the overflow located on your tank? This is a typical installation. Does yours look like this? Back to you, Tom

Are you in the USA?