Sizing Guide

3
Venting Guidelines for Category I Gas Appliances
Definitions
1. Appliance Categorized Vent Diameter/Area — The minimum vent area/diameter permissible for
Category I appliances to maintain a nonpositive vent static pressure when tested in accordance with
nationally recognized standards.
2. Fan-Assisted Combustion System — An appliance equipped with an integral mechanical means to
either draw or force products of combustion through the combustion chamber or heat exchanger.
3. FAN Max — The maximum input rating of a Category I, fan-assisted appliance attached to a vent or
connector.
4. FAN Min — The minimum input rating of a Category I, fan-assisted appliance attached to a vent or
connector.
5. FAN+FAN — The maximum combined appliance input rating of two or more Category I, fan-assisted
appliances attached to the common vent.
6. FAN+NAT — The maximum combined appliance input rating of one or more Category I, fan-assisted
appliances and one or more Category I, draft hood-equipped appliances attached to the common vent.
7. NA — Vent configuration is not permitted due to potential for condensate formation or pressurization of
the venting system, or not applicable due to physical or geometric restraints.
8. NAT Max — The maximum input rating of a Category I, draft hood-equipped appliance attached to a
vent or connector.
9. NAT+NAT — The maximum combined appliance input rating of two or more Category I draft hood-
equipped appliances attached to the common vent.
Single-Appliance Vent - Tables 1 and 2
Notes for Single-Appliance Vent
1. These venting tables shall not be used where heat reclaimers, draft regulators and vent dampers are
installed in the venting system.
a) The maximum capacity of the vent system shall be determined using the “NAT Max” column.
b) The minimum capacity shall be determined as if the appliance were a fan-assisted appliance,
using the “FAN Min” column to determine the minimum capacity of the vent system. Where the
corresponding “Fan Min” is “NA,” the vent configuration shall not be permitted, and an alternative
venting configuration shall be utilized.
2 . If the vent size determined from the tables is smaller than the appliance draft hood outlet or flue collar,
the smaller size shall be permitted to be used, provided the following requirements are met.
a) The total vent height (H) is at least 10 feet (3 m).
b) Vents for appliance draft hood outlets or flue collars 12 inches (300 mm) in diameter or smaller are
not reduced more than one table size.
c) Vents for appliance draft hood outlets or flue collars larger than 12 inches (300 mm) in diameter
are not reduced more than two table sizes.
d) The maximum capacity listed in the tables for a fan-assisted appliance is reduced by 10 percent
(0.90 maximum table capacity).
e) The draft hood outlet is greater than 4 inches (100 mm) in diameter. Do not connect a 3-inch
(80 mm) diameter vent to a 4-inch (100 mm) diameter draft hood outlet. This provision shall not
apply to fan-assisted appliances.