Sizing Guide

20
Venting Guidelines for Category I Gas Appliances
Figure 10
Examples Using Common Venting Tables (continued)
Example 5: Common-Venting a Draft Hood-Equipped Water Heater with a Fan-Assisted Furnace
into a Type B Vent
In this case, a 35,000-Btu/hour input draft hood-equipped water heater with a 4-inch diameter draft hood
outlet, 2 feet of connector rise, and 4 feet of horizontal length are to be common-vented with a 100,000
Btu/hour fan-assisted furnace with a 4-inch diameter flue collar, 3 feet of connector rise, and 6 feet of
horizontal length. The common vent consists of a 30-foot height of Type B vent. What are the
recommended vent diameters for each connector and the common vent? The installer would like to use
a single-wall metal vent connector. See Figure 10.
Solution (Table 4)
Water Heater Vent Connector Diameter: Since the water heater vent connector horizontal length of 4 feet
is less than the maximum value listed in Paragraph 2, Page 12, the venting table values can be used
without adjustments. Using the Vent Connector Capacity portion of Table 4, read down the Total Vent
Height (H) column to 30 feet and read across the 2-foot Connector Rise (R) row to the first Btu/hour rating
in the NAT Max column that is equal to or greater than the water heater input rating. The table shows that
a 3-inch vent connector has a maximum input rating of 37,000 Btu/hour. Although this rating is greater
than the water heater input rating, a 3-inch vent connector is prohibited by 10.2.19. A 4-inch vent
connector has a maximum input rating of 67,000 Btu/hour and is equal to the draft hood outlet diameter.
A 4-inch vent connector is selected. Since the water heater is equipped with a draft hood, there are no
minimum input rating restrictions.
Furnace Vent Connector Diameter: Using the Vent Connector Capacity portion of Table 4, read down the
Total Vent Height (H) column to 30 feet and across the 3-foot Connector Rise (R) row. Since the furnace
has a fan-assisted combustion system, find the first FAN Max column with a Btu/hour rating greater than
the furnace input rating. The 4-inch vent connector has a maximum input rating of 119,000 Btu/hour and
a minimum input rating of 85,000 Btu/hour.
The 100,000-Btu/hour furnace in this example falls within this
range, so a 4-inch connector is adequate. Since the furnace
vent connector horizontal length of 6 feet is less than the
maximum value listed in Paragraph 2, Page 12, the venting
table values can be used without adjustment. If the furnace had
an input rating of 80,000 Btu/hour, then a Type B vent connector
(see Table 3) would be needed in order to meet the minimum
capacity limit.
Common Vent Diameter: The total input to the common vent is
135,000 Btu/hour. Using the Common Vent Capacity portion of
Table 4, read down the Total Vent Height (H) column to 30 feet
and across this row to find the smallest vent diameter in the FAN
+NAT column that has a Btu/hour rating equal to or greater than
135,000 Btu/hour. The 4-inch common vent has a capacity of
132,000 Btu/hour, and the 5-inch common vent has a capacity of
202,000 Btu/hour. Therefore, the 5-inch common vent should be
used in this example.