expansion tank connection pipe size

pardon_my_ignorance

what is the relationship between the connection pipe size and the tank size?

SWEI

Whatever the manufacturer decided it should be. Flow rates into and out of the tank should be quite low in the overwhelming majority of systems.

RobG

I just go with the size of the tanks inlet. It makes it pretty foolproof.

Solid_Fuel_Man

I would be very surprised if there was a system with less than 250 gallons of water capacity that would need any more than a 3/4 line to expansion.

Think about what expansion really is and heat input into the system. The flow is slow and steady upon a cold boiler startup and almost new nill when a system is running at operating conditions. Very large boiler fireing rates could be the only thing that drives fast flow to expansion.