Sizing Guide

www.hartandcooley.com Sizing Guide 17
Additional Guidelines
for Multiple-Appliance Venting
When common-venting, connector Tables 4A and 5A
allow for connector lateral lengths of 1½ feet
(18 inches) for each inch of connector diameter as
follows:
Connector Lateral Length Allowance
Diameter 3" 4" 5" 6" 7" 8" 10" 12" 16" 24"
Length, Ft. 4½ 6 7½ 9 10½ 12 15 18 24 36
Table 3
It is permissible to double the length shown by reducing
the maximum connector capacity by 10%. Next,
determine FAN MIN by using the corresponding single-
appliance table treating each appliance and connector
along with the common vent as a single-appliance vent
system. If the input is still above FAN MIN, go ahead
and double the connector length.
If the vent connectors are combined prior to entering
the common vent (Figure 4D, page 7), the maximum
common vent capacity shall be reduced by 10%. The
horizontal length allowance shall not exceed 1½ feet
(18 inches) for each inch of common vent manifold
diameter. This length limitation also includes the
common vent offset in the attic added together.
When manifolding a fan-assisted appliance with a draft-
hood-equipped appliance, the fan-assisted appliance
should be positioned closer to the common vertical
vent.
If the common vent has a horizontal offset (Figure 4C,
page 7), the maximum common vent capacity shall be
reduced by 20%, the equivalent of two 90° elbows. This
20% reduction is listed in the L line in Tables 4B and
5B. The horizontal length of the common vent offset
shall not exceed 1½ feet for each inch of common vent
diameter. A 10% reduction in common vent capacity
must be taken for each additional 90° fitting.
The common vent diameter shall be at least as large as
the largest vent connector diameter.
Interpolation is permitted between table entries.
Extrapolation beyond table entries is not permitted.
The “7 times rule” (page 9) also limits the size of the
common vent to no more than 7 times the cross-
sectional area of the smallest connected appliance vent
connector or flue collar area; do not use the connector
pipe area.
Additional Guidelines
for Multiple-Appliance Venting
Use available headroom for maximum connector
rise
Always use available headroom for maximum
connector rise after allowing for the listed clearance
to combustibles. Obtain maximum connector rise by
such methods as extending the connectors between the
floor joists. Increased venting power and efficiency of
the system permits reduction of vent and connector
sizes.
Alternate ways for increasing vent height
and connector rise
If a combined vent cannot be used because of
limitations in connector rise or total vent height,
alternatives such as those illustrated (Figure 7) may be
used to secure greater connector rise or greater total
vent height. Type B GAS VENT CONNECTOR must be
used for Figure 7B.
Figure 7
IF THIS
CAN’T BE
USED
IF THIS
CAN’T BE
USED
A.
B.
1
RELOCATE TOP
NEAR PEAK FOR
ADDITIONAL
HEIGHT
RELOCATE TEE
IN ATTIC FOR
ADDED
CONNECTOR
RISE*
*Use B-vent connector
to go into attic space.