vent size

Hello all,

Is there a rule of thumb for vent sizing. Say I have a three inch water heater vent going into a 6 inch. Is this acceptable or should I write it up as an “orphaned flue.” I come across this often and always call it out. I came across this info

Any one have an easy rule of thumb like a 40,000 btu unit can have a vent size from 3 to 6 inch…

Thank you

Call it out.

Too many tables to consider a “rule-of-thumb” .