Sizing Guide

2
Venting Guidelines for Category I Gas Appliances
Introduction
The vent sizing tables in this brochure are part of the National Fuel Gas Code. These tables
cover vent sizing for both fan-assisted combustion and draft hood-equipped appliances.
Vents for most draft hood-equipped appliances can still be sized based on the draft hood outlet
size, or they can be sized using the tables or other engineered methods. However, when
installing Category I fan-assisted combustion furnaces, the tables or other engineered
methods must be used.
In addition, the tables for fan-assisted combustion appliances have minimum as well as
maximum rates. These appliances generally have lower flue gas temperatures and less dilution
air, and therefore have an increased chance of continuous condensation if the vent is too large.
These vent sizing tables cannot be used for vented wall furnaces, decorative appliances, or room
heaters. The appliance manufacturer’s venting instructions should be checked to make sure
which sizing method is recommended.
Additional references for the development of the material in this Guide were taken with
permission from the following.
National Fuel Gas Code, NFPA Standard 54 ANSI Z223.1
The capacities given in the tables for Type B Gas Vent are consistent with those found in
publications by the appliance manufacturers and NFPA 54 National Fuel Gas Code.
For information on products to use with the systems designed by use of this Guide, refer to the
various specific product literature.
NOTE: This Guide should be used in addition to, not as a replacement for, American Metal
Products Group’s installation instructions. Always read and comply with the
manufacturer's installation instructions supplied with the appliance.