Sizing Guide

4
Venting Guidelines for Category I Gas Appliances
Single-Appliance Vent - Tables 1 and 2 (continued)
3. Single-appliance venting configurations with zero (0) lateral lengths in
Tables 1 and 2 shall have no elbows in the venting system. For vent
configurations with lateral lengths, the venting tables include allowance
for two 90-degree turns. For each additional 90-degree turn, or
equivalent, the maximum capacity listed in the venting tables shall be
reduced by 10 percent (0.90 x maximum table cap
acity). Two or more
turns, the combined angles of which equal 90 degrees, shall be
considered equivalent to one 90-degree turn.
Note: Two 45-degree turns are equivalent to one 90-degree turn.
4. Zero (0) lateral (L) shall apply only to a straight vertical vent attached to
a top outlet draft hood or flue collar.
5. Sea-level input ratings shall be used when determining maximum
capacity for high-altitude installation. Actual input (derated for altitude)
shall be used for determining minimum capacity for high-altitude
installation.
6. For appliances with more than one input rate, the minimum vent
capacity (FAN Min) determined from the tables shall be less than the
lowest appliance input rating, and the maximum vent capacity
(FAN Max/NAT Max) determined from the tables shall be greater
than the highest appliance rating input.
7. Listed corrugated metallic chimney liner systems in masonry chimneys
shall be sized by using Table 1 or 2 for Type B vents, with the
maximum capacity reduced by 20 percent (0.80 maximum capacity)
and the minimum capacity as shown in Table 1 or 2. Corrugated
metallic liner systems installed with bends or offsets shall have their
maximum capacity further reduced in accordance with Paragraph 3.
8. If the vertical vent has a larger diameter than the vent connector, the
vertical vent diameter shall be used to determine the minimum vent
capacity, and the connector diameter shall be used to determine the
maximum vent capacity. The flow area of the vertical vent shall not
exceed seven times the flow area of the listed appliance categorized
vent area, flue collar area, or draft hood outlet area, unless designated
in accordance with approved engineering methods.
9. Tables 1 and 2 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.
Figure 1
Table 1 is used when sizing Type B
double-wall gas vent connected
directly to the appliance.
Note: The appliance type may be
either Category I draft
hood-equipped or fan-assisted.
Figure 2
Table 2 is used when sizing a single-
wall metal vent connector attached to a
Type B double-wall gas vent.
Note: The appliance type may be
either Category I draft
hood-equipped or fan-assisted.