IO137 34
Common vent diameter - The total input to the common vent is 135,000 Btu / hr. Using Ta-
ble 4, common vent capacity, read down the total vent height “H” column to 30ft and across
this row to find the smallest vent diameter in the Fan + Nat column that has a Btu / hr rating
equal to or greater than 135,000 Btu / hr. The 4” common vent has a capacity of 132,000 Btu /
hr and the 5” common vent has a capacity of 202,000 Btu / hr. Therefore, the 5” common vent
should be used in this example.
Summary - In this example, the installer may use a 3” diameter single wall metal vent
connector for the water heater and a 4” single wall metal vent connector for the furnace. The
common vent should be a 5” diameter type B vent.
Common Venting into a Masonry Chimney - In this case, a 35,000 Btu / hr input 4” diame-
ter draft hood equipped water heater with 2ft of connector rise and 4ft of horizontal length is to
be common vented with a 100,000 Btu / hr fan assisted furnace with a 4” diameter flue collar,
3ft of connector rise and 6ft of horizontal length. The common vent is an 8” X 12” tile lined
chimney that is 30ft tall. What are the recommended vent diameters for each connector? Is
this an acceptable installation? Solution- Table 8 is used to size common venting installations
involving single wall connectors into masonry chimneys.
Water heater vent connector diameter - Using Table 8, vent connector capacity, read down
the total vent height “H” column to 30ft and read across the 2ft connector rise “R” row to the
first Btu / hr rating in the Nat Max column that is equal to or greater than the water heater in-
put rating. The table shows that a 3” vent connector has a maximum input of only 31,000 Btu /
hr while a 4” vent connector has a maximum input of 57,000 Btu / hr. A 4” vent connector
must therefore be used.
Furnace vent connector diameter - Using table 8 vent connector capacity, read down the
total vent height “H” column to 30ft and across the 3ft connector rise “R” row. Since the fur-
nace has a fan assisted combustion system, find the first Fan Max column with a Btu / hr rat-
ing greater than the furnace input rating. The 4” vent connector has a maximum input rating of
127,000 Btu / hr and a minimum input rating of 95,000 Btu / hr. The 100,000 Btu / hr furnace
in this example falls within this range, so a 4” connector is adequate.
Masonry Chimney - From Table 9, the equivalent area for a nominal liner size of 8” X 12” is
63.6 sq. in. Using Table 8, common venting capacity, read down the Fan + Nat column under
the minimum internal area of chimney value of 63 to the row for 30ft height to find a capacity
value of 739,000 Btu / hr. The combined input rating of the furnace and water heater of
135,000 Btu / hr, is less than the table value, so this is an acceptable installation.
Note 19
requires the common vent area to be no greater than 7 times the flow area of the
smallest appliance outlet area. Both appliances in this installation use 4” diameter outlets.
From Table 9, the equivalent area for an inside diameter of 4” is 12.2 sq. in. Seven times 12.2
is 85.4, which is greater than 63.6, so this configuration is acceptable.
Note 1
specifies that table values are for vents or chimneys that are not exposed to the out-
doors below the roof line. If the masonry chimney in this case were exposed below the roof-
line, then the appliance manufacturer, local gas utility, and/or authority having jurisdiction
must be consulted.
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