Water heater flue

New HI furnace installed. Water heater connected to the old furnace flue. What are your thoughts about this connection? I know this has been discuss here before, just too lazy to look it up.


It should draw okay. I really see no problem. Having a bigger than needed flue is not an issue.

Thanks, James, it is MILLER time. It has been a long day.

Actually, the larger flue from the furnace could affect the draw from the WH. Going from a 3" flue to a 6" flue can reduce the draw. when the furnace was installed it would help the DWH draw. The only way to tell is to check for spillage that doesn’t stop within 60 seconds. BTW, I might be wrong, but this is what I learned trying to get my BPI cert. Any HVAC guys out there to correct one of us?

Additional Guidelines for Single-Appliance
Vent Systems
The flow area of the vertical vent shall not exceed
seven times the flow area of the appliance flue collar
area or the draft hood outlet area. For instance, if:
The flue collar diameter is: 3" 4" 5" 6" 7" 8"
The maximum common
vent diameter allowed is: 8" 10" 12" 14" 18" 20"