Sizing Guide

10
Venting Guidelines for Category I Gas Appliances
Examples using Single-Appliance Venting Tables
Example 1: Single Draft Hood-Equipped Appliance
An installer has a 120,000-Btu/hour input appliance with a 5-inch diameter draft hood outlet that needs to
be vented into a 10-foot high Type B vent system. What size vent should be used, assuming (1) a 5-foot
lateral single-wall metal vent connector is used with two 90-degree elbows, or (2) a 5-foot lateral single-
wall metal vent connector is used with three 90-degree elbows in the vent system? See Figure 3.
Solution
Table 2 should be used to solve this problem, because single-wall metal vent connectors are being used
with a Type B vent.
(a) Read down the first column in Table 2 until the row associated with a 10-foot height and 5-foot lateral
is found. Read across this row until a vent capacity greater than 120,000 Btu/hour is located in the
shaded columns labeled NAT Max for draft hood-equipped appliances. In this case, a 5-inch
diameter vent has a capacity of 122,000 Btu/hour and can be used for this application.
(b) If three 90-degree elbows are used in the vent system, then the maximum vent capacity listed in the
tables must be reduced by 10 percent (see Paragraph 3). This implies that the 5-inch diameter vent
has an adjusted capacity of only 110,000 Btu/hour. In this case, the vent system must be increased
to 6 inches in diameter. See the following calculations.
122,000 x 0.90 = 110,000 for 5-inch vent
From Table 2, select 6-inch vent.
186,000 x 0.90 = 167,000; This figure is greater than the required 120,000.
Therefore, use a 6-inch vent and connector where three elbows are used.
Example 2: Single Fan-Assisted Appliance
An installer has an 80,000-Btu/hour input fan-assisted appliance that must be installed using 10 feet of
lateral connector attached to a 30-foot high Type B vent. Two 90-degree elbows are needed for the
installation. Can a single-wall metal vent connector be used for this application? See Figure 4.
Solution
Table 2 refers to the use of single-wall metal vent connectors with Type B vent. In the first column, find
the row associated with a 30-foot height and a 10-foot lateral. Read across this row, looking at the
FAN Min and FAN Max columns, to find that a 3-inch diameter single-wall metal vent connector is not
recommended. Moving to the next larger size single-wall connector (4 inches), we find that a 4-inch
diameter single-wall metal connector has a recommended minimum vent capacity of 91,000 Btu/hour and
a recommended maximum vent capacity of 144,000 Btu/hour. The 80,000 Btu/hour fan-assisted
appliance is outside this range, so the conclusion is that a single-wall metal vent connector cannot be
used to vent this appliance using 10 feet of lateral for the connector. However, if the 80,000 Btu/hour
input appliance could be moved to within 5 feet of the vertical vent, then a 4-inch single-wall metal
connector could be used to vent the appliance. Table 2 shows the acceptable range of vent capacities for
a 4-inch vent with 5 feet of lateral to be between 72,000 Btu/hour and 157,000 Btu/hour. If the appliance
cannot be moved closer to the vertical vent, then Type B vent could be used as the connector material.
In this case, Table 1 shows that, for a 30-foot high vent with 10 feet of lateral, the acceptable range of
vent capacities for a 4-inch diameter vent attached to a fan-assisted appliance is between 37,000
Btu/hour and 150,000 Btu/hour.