Expansion tank sizing

Whats the rule of thumb in sizing expansion tank? Currently I have a 70 btuh

gas fired lennox boiler with an ordinary steel expansion tank, I think there's no

bladder inside, thanks for any input you can give.

expansion

The amount of expansion to raise the pressure of a closed system is about a cup or two, so a 5 gallon expansion tank is adequate for all but the most antique systems.