Sizing Guide

Venting Guidelines for Category I Gas Appliances

Two or More Appliances - Tables 3 and 4 (continued)

4. If the vent connectors are combined prior to entering the common vent, the maximum common vent

capacity listed in the common venting tables shall be reduced by 10 percent (0.90 x maximum

common vent capacity). The length of the common vent connector manifold (LM) shall not exceed

18 inches/inch (180 mm/mm) of common vent connector manifold diameter (D). (See Figure 7.)

5. If the common vertical vent is offset as shown in Figure 8, the maximum common vent capacity listed

in the common venting tables shall be reduced by 20 percent (0.80 x maximum common vent

capacity), the equivalent of two 90-degree turns. The horizontal length of the common vent offset

(LM) shall not exceed 18 inches/inch (180 mm/mm) of common vent diameter (D).

6. Excluding elbows counted in

Paragraph 5, for each additional 90-degree turn in excess of two, the

maximum capacity of that portion of the venting system shall be reduced by 10 percent (0.90 x

maximum common vent capacity). Two or more turns, the combined angles of which equal 90

degrees, shall be considered equivalent to one 90-degree turn. (See NFPA 54 for more information.)

7. The cross-sectional area of the common vent shall be equal to or greater than the cross-sectional

area of the largest connector.

8. Interconnection fittings shall be the same size as the common vent.

9. 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.

Figure 5

Table 3 is used when sizing Type B double-wall vent

connectors attached to a Type B double-wall common vent.

Note: Each appliance type may be either Category I draft

hood-equipped or fan-assisted.

Figure 6

Table 4 is used when sizing single-wall vent connectors

attached to a Type B double-wall common vent.

Note: Each appliance type may be either Category I draft

hood-equipped or fan-assisted.