Expansion Tank

New residential 40 gal. electric water heater with new Watts expansion tank. Due to WH location the expansion tank is installed with a T- fitting in the cold water line a couple of feet above. When filling the system with water I know opening a faucet will purge the air from the WH but what happens to the air that is in the pipe entering the Expansion tank ahead of the water since the expansion tank only has an inlet? Thanks

It eventually disappears, moves through the system, just like an air chamber installed with tees with short pieces of pipe.

The water eventually displaces the air.

The air in the expansion tank is contained solely by the bladder.

Did you set the bladder pressure to the working pressure of the main line?

You have to do that before even installing it.

air

When the tank is filled with air, the bladder fills the entire tank, so there is only a small amount of air on the water side, and if does not dissipate, then it just helps the air in the bladder control the pressure.