Combi boilers.. Addional expansion tank?

[Plumber69]

Ones with built in expansion tanks. Do they usually require an additional one? I heard built in one's just handle expansion of just the unit.

 

[Dana]

Without a better description of the combi and your potabable water distribution system it's hard to say what (if anything) you need in terms of additional expansion capacity. There are no simple answers- you have to do the math.