Just got on this thread. there are two different types of expansion tanks, one for domestic water which is set for higher pressure and it is 3/4” connection. The second is for radiant, low pressure and 1/2” connection. Both types come in different sizes but most homes are fine with the 2.2 gal.
Any tank should be mounted with straps against the wall or something structural because when they do fail you don’t want the pipes to support it. Tanks should only be mounted vertically with the pipe connection either on the top or bottom, not horizontally because it puts uneven pressure on the bladder.
To test a tank, tap on the side closest to where the pipe connects. It should be a dead tone. Then tap on the other side, that should be full of air, it should be an open tone. If your not sure, test the schrader (sp?). If water comes out, it’s dead.
Sorry for the long answer, this is what I do, work on boilers/radiant.
