I am removing a furnace from a gas vent.
What will be left is a residential water heater (typical 40MBH with 3" (I think) vent connecter) and 8" BVent up inside a house probably about 25 feet.
I am not clear about the rules. Is the 8" now too big for just the water heater?
But not too big if the vent connecter is 4"?
According to the 7x rule, you can leave an appliance with a 3" vent collar vented into a 7" but not 8" vent. However, if it is a 4" collar, you can vent that into a 10.5" vent. Doesn't make it a great idea and certainly doesn't guarantee performance but under the gas code, it is acceptable if all else is.
Now, venting a 40mbh WH into a cold exterior 8" vent even with 4" vent connector is a recipe for condensation and/ or backdrafting.
Consider replacing it with a power vented WH out the side and be done with it.
Try using a 4" "B" vent connector to the 8 " vertical vent
and look for any condensation at the draft hood if there is no condensing and the water heater is drafting (homeowner test with some smoke at the draft hood skirt and see if it is sucking the smoke in or blowing it out, sucking in is good) If you are a pro do you have a draft gauge or combustion analyzer?