Sizing Guide

Metal Cap Terminations
Cap sizes 14" through 30" diameters are for gas vents
that extend at least 2 feet above the highest point
where they pass through a building and at least 2 feet
higher than any portion of the building within 10 feet.
If any adjacent structures are within 10 feet of the vent
and are higher, then the vent MUST terminate at least
2 feet above these structures. This recommendation
should be followed unless local code requirements state
otherwise.
Large offsets in the attic space are discouraged.
However, small offsets (laterals) may be used to
minimize the amount of vent pipe that must be
exposed above the roof in order to comply with the
above. NO gas vent should be terminated less than
5 feet in vertical height above the highest connected
appliance draft hood outlet.
General Termination Considerations
A cap or chimney housing offers protection against
the entrance of rain, snow and debris, as well as birds,
and will minimize the effect of wind on the vent. It will
protect the vent from downdrafts due to a wind that
impinges directly upon the vent. However, no vent
cap, cowl or top can overcome the adverse effect of a
region of high static pressure around the vent terminal
nor the effect of an interior region of low pressure.
Regions of high static pressure around the vent
terminal can be avoided by following the general rule
for the vent termination given above. Low or negative
interior pressures in the building may be caused by (1)
failure to provide for combustion air, (2) excessive use
of exhaust fans, and (3) tight construction resulting in
the lack of infiltration air. Vented clothes dryers and
fireplaces will also remove large amounts of air from
the interior, tending to produce a low interior pressure.
This also means that no B Vent installation shall
terminate by piercing a wall with a short vertical or
horizontal pipe and a cap.
Definitions
A SINGLE-APPLIANCE VENT is an independent vent
for one appliance (Figure 3).
TOTAL HEIGHT (H) is the vertical distance measured
between the appliance collar connection and the vent
termination (Figure 3).
TOTAL LATERAL LENGTH (L) is the actual horizontal
distance or length of offset between the appliance collar
and the main vertical portion of vent (Figure 3).
MULTIPLE-APPLIANCE VENT is a venting system
combining the connectors of two or more appliances at
one floor level to a common vertical vent.
Connector in
a combined vent system connects an individual appliance
flue collar to the common vent or manifold (Figure 4).
MINIMUM TOTAL VENT HEIGHT is the vertical
distance measured from the tallest appliance flue
collar outlet in the system to the termination of the
vent (see Figures 3 and 4). This minimum height is a
fixed dimension for any one vent system regardless of
the number or placement of appliances in the system.
CONNECTOR RISE for any appliance in a vent system
is the vertical distance from the flue collar outlet to the
point where the next connector joins the system (see
Figure 4).
COMMON VENT is that portion of the venting system
above the lowest interconnection. When the common
vent is entirely vertical, the system is called a
VERTICAL or V type. All others are called LATERAL
or L type (see Figure 4).
FAN-ASSISTED COMBUSTION SYSTEM is an
appliance equipped with a fan to either draw or force
products of combustion through the combustion
chamber and/or heat exchanger.
FAN MIN refers to the minimum input rating of a
Category 1 fan-assisted appliance attached to the vent.
www.hartandcooley.com Sizing Guide 7
Figure 3
TOTAL
HEIGHT
H
COMMON VENT
IS VERTICAL (V)
ALL APPLIANCES
DIRECTLY
CONNECTED
TOTAL
LATERAL
LENGTH
L
APPLIANCE
A.
B.
C.
H
RISE
OFFSET
ELBOW
OR TEE
RISE
D.
COMMON VENT WITH LATERAL (L)
MANIFOLD
1
Figure 4