Sizing Guide

12
Venting Guidelines for Category I Gas Appliances
Notes for Multiple-Appliance Vent
1. These venting tables shall not be used where heat reclaimers, draft regulators and vent dampers are
installed in the venting system.
2. The maximum vent connector horizontal length shall be 18 inches/inch (180 mm/mm) of connector
diameter as follows.
Two or More Appliances - Tables 3 and 4
Connector Diameter Maximum
(inches)
Connector Horizontal Length
(feet)
3 4½
4 6
5 7½
6 9
7 10½
8 12
9 13½
10 15
12 18
14 21
16 24
18 27
20 30
22 33
24 36
3. The vent connector shall be routed to the vent utilizing the shortest possible route. Connectors with
longer horizontal lengths than those listed above are permitted under the following conditions.
a) The maximum capacity (FAN Max or NAT Max) of the vent connector shall be reduced 10 percent
for each additional multiple of the length listed above. For example, the maximum length listed for
a 4-inch (100 mm) connector is 6 feet (1.8 m). With a connector length greater than 6 feet (1.8 m)
but not exceeding 12 feet (3.7 m), the maximum capacity must be reduced by 10 percent (0.90 x
maximum vent connector capacity). With a connector length greater than 12 feet (3.7 m) but not
exceeding 18 feet (5.5 m), the maximum capacity must be reduced by 20 percent (0.80 x
maximum vent capacity).
b) For a connector serving a fan-assisted appliance, the minimum capacity (FAN Min) of the
connector shall be determined by referring to the corresponding single-appliance table. For
Type B double-wall connectors, Table 1 shall be used. For single-wall connectors, Table 2 shall
be used. The height (H) and lateral (L) shall be measured according to the procedures for a
single-appliance vent, as if the other appliances were not present.