T&P valve draining - expansion tank issues?
The tank should be charged to the pressure at the attachment point.  If the tank is attached to a point in the system where the pressure is supposed to be 25 psi, the tank should be pre-charged to 25 psi.  If the tank is attached to a point in the system where the pressure is supposed to be 55 psi, the tank should be charged to 55 psi.

When the water pressure raises above the air pressure in the expansion tank, the water will begin to fill the tank and compress the air further.  As the water pressure decreases, the air pressure in the tank forces the water out of the tank. If the tank is charged too high, water will not be able to compress the air and fill the tank.



Note: Measuring the pressure in the tank while it's connected to a pressurized water system, will show the pressure in the water system not the pre-charged air pressure of the tank. To measure the air pressure of the expansion tank, you must isolate it from the system.