Sizing Guide

www.hartandcooley.com Sizing Guide 13
Multiple-Appliance Vent Systems
How to Determine Each Vent Connector Size
(Table 4A or 5A, pages 14-16)
• Determine the MINIMUM TOTAL VENT HEIGHT for
the system from a sketch of the proposed system.
• Determine the CONNECTOR RISE for each
appliance.
• Enter the VENT CONNECTOR Table 4A or 5A at the
line showing VENT HEIGHT equal to or less than
that determined above. Continue horizontally on
that line for the first appliance CONNECTOR RISE
using the appliance nameplate BTUH rating (sea
level). Always use a Table entry that equals or
exceeds a draft-hood appliance input or that brackets
a fan-assisted
appliance input. Read the connector vent size for
that appliance at the top of the column.
• Using the same VENT HEIGHT, repeat the procedure
for each appliance, using its CONNECTOR RISE AND
BTUH rating.
CAUTION. NEVER use a connector size smaller than
the draft-hood outlet size. (Exception: Does not apply
to fan-assisted appliances.)
How to Determine Common Vent Size
(Table 4B or 5B, pages 14-16)
• Total all appliance BTUH input ratings that are to be
connected to this common vent.
• Enter the COMMON VENT TABLE 4B or 5B at the
same VENT HEIGHT used to determine the vent
connector sizes above.
• Move horizontally across from this VENT HEIGHT
figure using either the L line if the common vent has an
offset, or has a horizontal manifold (Figure 4D), or the
V line if the common vent is vertical with no offsets.
• Select the first value in the correct appliance
combination column that is equal to or greater than
the total of BTUH ratings.
• The size of the required COMMON VENT is found at
the top of this column.
CAUTION. Regardless of the COMMON VENT size
determined by the above procedure, the vent MUST be
at least as large as the largest connector. If more than
one connector is this same size, then use a COMMON
VENT one size larger.
Example
Connect a 45,000 BTU water heater with a draft hood
and 1-foot connector rise with a 100,000 BTU fan-
equipped furnace with a 2-foot connector rise to a
Common Vent with a Minimum Total Vent Height of
18 feet (Figure 6). All portions of the system are
Type B GAS VENT.
Water Heater Vent Connector Size
Use Vent Connector Table 4A under NAT. Read down
Minimum Total Vent Height column to 15 feet and read
across 1 foot connector rise line to BTU rating equal to
or higher than water heater input rating. This figure
shows 53,000 BTU and is in the column for 4-inch
connector. Since this is in excess of the water heater
input, it is not necessary to find the maximum input
for an 18-foot minimum total vent height. Use a 4-inch
connector (Figure 6A).
Furnace Vent Connector
Use Vent Connector Table 4A. Read down Total Vent
Height column to 15 feet and read across 2-foot
Connector Rise line to fan column. Note 4-inch vent
size shows 96,000 BTU per hour or less than furnace
input. However, with 20-foot Total Height, read across
2-foot connector rise line. Note 4-inch vent size shows
105,000 BTU per hour. Since 18-foot height is
3
/5 of
this:
3
/5(105,000 - 96,000) = 5400. 96,000 + 5400 =
101,400, which is the maximum input for 18-foot
Minimum Total Vent Height. Therefore a 4-inch
connector would be the correct size for furnace,
providing the furnace had a 4-inch or smaller draft
hood outlet (Figure 6B).
Common Vent Size
Total input to Common Vent is 145,000 BTU. Vent
goes straight through roof so use V line of Table 4B
under FAN + NAT column. Note that for 15-foot
Minimum Total Vent Height maximum BTU for 5-inch
vent is 164,000, which is greater than total input to the
common vent. Therefore the common vent can be
5-inch diameter (Figure 6C).
Figure 6
A.
B.
C.
ONE-FOOT
CONNECTOR
RISE
INTERCONNECTION
TEE
TWO-FEET
CONNECTOR RISE
145,000 TOTAL INPUT
COMMON VENT
5-INCH SIZE
18 FEET
MINIMUM
TOTAL VENT
HEIGHT
18 FEET
MINIMUM
TOTAL VENT
HEIGHT
18 FEET
MINIMUM
TOTAL VENT
HEIGHT
WATER
HEATER
45,000
WATER
HEATER
45,000
WATER
HEATER
45,000
4-INCH
SIZE
4-INCH
SIZE
FURNACE
100,000
FURNACE
100,000
FURNACE
100,000
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