How do you size the power venter to the appliance?
The best way to size the power venter to the appliance is by the BTU rating of the appliance. If you size the power venter by the flue pipe size you could get a power venter that would be too large for the appliance possibly even large enough to suck the fire out of the appliance or cause it to have an erratic fire, and making it very inefficient.
