Sizing Guide

11
Venting Guidelines for Category I Gas Appliances
Examples using Single-Appliance Venting Tables (continued)
Example 3: Interpolating Between Table Values
An installer has an 80,000-Btu/hour input appliance with a 4-inch diameter draft hood outlet that needs to
be vented into a 12-foot high Type B vent. The vent connector has a 5-foot lateral length and is also
Type B. Can this appliance be vented using a 4-inch diameter vent?
Solution
Table 1 is used in the case of an all Type B vent system. However, since there is no entry in Table 1 for
a height of 12 feet, interpolation must be used. Read down the 4-inch diameter NAT Max column to the
row associated with 10-foot height and 5-foot lateral to find the capacity value of 77,000 Btu/hour. Read
further down to the 15-foot height, 5-foot lateral row to find the capacity value of 87,000 Btu/hour. The
difference between the 15-foot height capacity value and the 10-foot height capacity value is 10,000
Btu/hour. The capacity for a vent system with a 12-foot height is equal to the capacity for a 10-foot height
plus 2/5 of the difference between the 10-foot and 15-foot height values, or 77,000 + 2/5 x 10,000 =
81,000 Btu/hour. Therefore, a 4-inch diameter vent can be used in the installation.
Figure 3 Figure 4