Sizing Guide

19
Venting Guidelines for Category I Gas Appliances
Examples Using Common Venting Tables
Example 4: Common-Venting Two Draft Hood-Equipped Appliances
A 35,000-Btu/hour water heater is to be common-vented with a 150,000-Btu/hour furnace, using a
common vent with a total height of 30 feet. The connector rise is 2 feet for the water heater, with a
horizontal length of 4 feet. The connector rise for the furnace is 3 feet, with a horizontal length of 8 feet.
Assuming single-wall metal connectors will be used with Type B vent, what size connectors and combined
vent should be used in this installation? See Figure 9.
Solution
Table 4 should be used to size single-wall metal vent connectors attached to Type B vertical vents. In
the vent connector capacity portion of Table 4, find the row associated with a 30-foot vent height. For a
2-foot rise on the vent connector for the water heater, read the shaded columns for draft hood-equipped
appliances to find that a 3-inch diameter vent connector has a capacity of 37,000 Btu/hour. Therefore, a
3-inch single-wall metal vent connector can be used with the water heater. For a draft hood–equipped
furnace with a 3-foot rise, read across the appropriate row to find that a 5-inch diameter vent connector
has a maximum capacity of 120,000 Btu/hour (which is too small for the furnace), and a 6-inch diameter
vent connector has a maximum vent capacity of 172,000 Btu/hour. Therefore, a 6-inch diameter vent
connector should be used with the 150,000 Btu/hour furnace. Since both vent connector horizontal
lengths are less than the maximum lengths listed in Paragraph 2, Page 12, the table values can be used
without adjustments.
In the common vent capacity portion of Table 4, find the row associated with a 30-foot vent height, and
read over to the NAT +NAT portion of the 6-inch diameter column to find a maximum combined capacity
of 257,000 Btu/hour. Since the two appliances total only 185,000 Btu/hour, a 6-inch common vent can be
used.
Figure 9