Sizing Guide

15
Venting Guidelines for Category I Gas Appliances
Two or More Appliances - Tables 3 and 4 (continued)
15. Tables 3 and 4 shall be used for chimneys and vents not exposed to the outdoors below the roof line.
A Type B vent or listed chimney lining system passing through an unused masonry chimney flue shall
not be considered to be exposed to the outdoors.
16. Vent connectors shall not be upsized more than two sizes greater than the listed appliance
categorized vent diameter, flue collar diameter, or draft hood outlet diameter. Vent connectors for
draft hood–equipped appliances shall not be smaller than the draft hood outlet diameter. If a vent
connector size(s) determined from the tables for a fan-assisted appliance(s) is smaller than the flue
collar diameter, the smaller size(s) shall be permitted to be used provided the following conditions are
met.
a) Vent connectors for fan-assisted appliance flue collars 12 inches (300 mm) in diameter or smaller
are not reduced by more than one table size [e.g., 12 inches to 10 inches (300 mm to 250 mm)] is
a one-size reduction), and those larger than 12 inches (300 mm) in diameter are not reduced more
than two table sizes [e.g., 24 inches to 20 inches (610 mm to 510 mm) is a two-size reduction].
b) The fan-assisted appliance(s) is common vented with a draft hood–equipped appliance(s).
17. All combination of pipe sizes, single-wall, and double-wall metal pipe shall be allowed within any
connector run(s) or within the common vent, provided ALL of the appropriate tables permit ALL of the
desired sizes and types of pipe, as if they were used for the entire length of the subject connector or
vent. If single-wall and Type B double-wall metal pipes are used for vent connectors, the common
vent must be sized using Table 4 as appropriate. (See NFPA 54 for more information.)
18. Where a table permits more than one diameter of pipe to be used for a connector or vent, all the
permitted sizes shall be permitted to be used.
19. Interpolation shall be permitted in calculating capacities for vent dimensions that fall between table
entries. (See Example 3.)
20. Extrapolation beyond the table entries shall not be permitted.
21. For vent heights lower than 6 feet and higher than shown in the tables, engineering methods shall be
used to calculate vent capacities. For SI units, 1 inch = 25.4 mm; 1 inch
2
= 645 mm
2
; 1 foot, =
0.305 m; 1000 Btu per hour = 0.293 kW.