Sizing Guide

20 Sizing Guide www.hartandcooley.com
CAUTION: It is important to keep the following
points in mind.
• Common vent height must always be computed as
the distance from the outlet of the connected
appliance to the lowest part of the opening from
the next interconnection above.
• If the connector rise is inadequate, increase
connector size, always making sure of maximum
available connector rise.
• Be sure that the air supply to each appliance is
adequate for proper operation. A separation of
appliance rooms from occupied areas and provision
for outside air supply is necessary.
• If an air shaft is used for installation of the common
vent, be sure that sufficient space is provided for
fittings, clearance to combustibles, and access for
proper assembly.
• These calculations apply ONLY when the entire
system is constructed of listed double-wall Type B
Vent materials.
Ratio of Connector Size to Common Vent Size
Whenever the area of the common vent becomes more
than 7 times the area of the vent connector entering it,
the connector rise must be increased one foot above the
allowable vent connector rise shown in the Tables. For
example, where appliance input is 90,000 BTU per hour
using a 5-inch (area 20 square inches) vent connector
in a system having a minimum vent height of 10 feet,
the vent connector rise must be 2 feet on the lower
floors where the common vent size is 12 inches (area
113 square inches) or less. However, as soon as a larger
common vent size is required, such as 14 inches (area
154 square inches), the vent connector rise must be
increased to 3 feet to avoid draft hood spillage.
This requirement does NOT apply when the
connector rise is originally over 5 feet and
consequently self-venting.
Offsets in Multistory Vents
A multistory common vertical vent may have a single
offset, provided:
A. The offset does not exceed 45°, and
B. The section of common vent that contains the
offset shall be reduced by 20%, and
C. The horizontal length of the offset does not exceed
1½ feet for each inch of common vent diameter.
Economy of Parallel Systems
It may frequently prove more economical to group
appliances to upper and lower common vent systems so
that smaller vent sizes can be used. Even though many
appliances may be connected to a single multiple-story
common vent, the increase in size caused by this may
prove uneconomical because of the space for access
required and the need for numerous fittings. An
alternate procedure is to use parallel common vents
with staggered connections at alternate floors, thereby
greatly increasing the minimum total vent height
available to each connected appliance.
Example of Multiple-Story Systems
To give an example of the method of using the tables
for multiple-story venting, consider Figure 14 as a four-
story apartment with each natural draft heater
arrangement as follows: 90,000 BTU/L input, 5" draft
hood, 1-foot connector rise, 10-foot vent height for
lower three floors, 6-foot vent height for top floor,
B-vent being used for complete system. The Common
Vent is vertical, so use V lines of Table 4B under
NAT+NAT column for figuring common vent size.
Table 6 shows the calculations for venting all four floors
into the common vent.
Figure 13
CONNECTOR: SIZE (1) DEPENDS ON
(2) DRAFT HOOD OR COLLAR SIZE
(3) APPLIANCE INPUT
(4) RISE
(5) TOTAL HEIGHT
COMMON VENT SIZE (6) DEPENDS ON
(5) TOTAL HEIGHT AND
(7) TOTAL INPUT OR
(8) LARGEST ENTERING
CONNECTOR OR COMMON VENT
(5) TOTAL VENT HEIGHT
EQUALS (RISE + DISTANCE
BETWEEN TEES)
MULTISTORY GAS VENT DESIGN PROCEDURE
FOR APPLIANCES AT INTERMEDIATE LEVELS,
USING COMBINED VENT TABLES
(2) DRAFT
HOOD SIZE
TEE
TEE
(8)
(6) COMMON VENT SIZE
(1) SIZE
(4) RISE
(3)
INPUT
(7) TOTAL INPUT
APPLIANCE
B VENT
CONNECTOR