expansion tank question

[greystonedog]

I have a 40 gal gas HW tank, inlet pressure 55psig. I installed a 2 gal expansion tank on the cold line, set the pressure in it at 55psig. When my heater kicks on the line pressure in the house goes up to 75 lbs. Is this normal ? I thought the expansion tank would eliminate all pressure increase?

 

[Mr_David]

Make sure when you set the tank pressure, the system pressure is at 0.
Tank will not eliminate all thermal expansion.
It will be proportional to the amount of thermal expansion to the available air pocket in the tank bladder.
The larger the tank the more thermal expansion it can absorb.
As the air compresses in the tank by the TE that volume of air will reduce and it's pressure will increase as it's capacity
to absorb the TE diminishes.

 

[greystonedog]

I set the pressure in the tank before installing it, and I checked at 0 system pressure. I don't feel the 75 lbs max is a problem, but do you think I should install a second or larger tank?