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FIGURE 23 - DOWNFLOW/HORIZONTAL
VENT PIPE CONNECTION
COMBUSTION AIR
All installations must comply with Section 5.3, Air for Combus-
tion and Ventilation of the National Fuel Gas Code, ANSI
Z223.1 or Sections 7.2, 7.3 or 7.4 of CAN/CGA B149.1 or .2
Installation Code - latest editions.
An unconfined space is not less than 50 cubic feet per 1000
Btu/hr input rating for all appliances installed in that area.
Rooms communicating directly with the space containing the
appliances are considered part of the unconfined space, if
openings are not furnished with doors.
A confined space is an area with less than 50 cubic feet per
1000 Btu/hr input rating for all appliances installed in that area.
The following must be considered to obtain proper air for
combustion and ventilation in confined spaces.
Air Source from Inside the Building -
Two permanent openings, one within 12 inches of the top of the
confined space and one within 12 inches of the bottom, shall
each have a free area of not less than one square inch per
1,000 Btuh of total input rating of all appliances located in the
space. The openings shall communicate freely with interior
areas having adequate infiltration from the outside.
NOTE: At least 100 square inches free area shall be used for
each opening.
Air Source from Outdoors -
1. Two permanent openings, one within 12 inches of the top
of the confined space and one within 12 inches of the
bottom, shall communicate directly, or by means of ducts,
with the outdoors or to such crawl or attic spaces that freely
communicate with the outdoors.
a. Vertical Ducts - Each opening must have a free area of
not less than one square inch per 4,000 Btuh of total
input of all appliances located in the space.
Unitary Products Group
EXAMPLE:
Total Input of All Appliances
4000
b. Horizontal Ducts - Each opening must have a free area
of not less than one square inch per 2,000 Btuh of total
input of all appliances located in the space.
NOTE: Ducts must have the same cross-sectional area as the free
area in the opening to which they are connected. The minimum
dimension of rectangular ducts shall be three inches.
2. One permanent opening, commencing within 12 inches of
the top of the enclosure shall be permitted where the
equipment has clearances of at least 1 inch from the sides
and back and 6 inches from the front of the appliance. The
opening shall communicate through a vertical or horizontal
duct to the outdoors, or spaces (crawl or attic) that freely
communicate with the outdoors and shall have a minimum
free area of:
a. 1 sq. in. per 3000 Btu per hr of the total input rating of
all equipment located in the enclosure.
b. Not less than the sum of the areas of all vent connectors
in the confined space.
3. Louvers, Grilles and Screens
a. In calculating free area, consideration must be given to
the blocking effects of louvers, grilles and screens.
b. If the free area of a specific louver or grille is not known,
refer to Table 6 to estimate free area.
TABLE 6 - ESTIMATED FREE AREA
ESTIMATED FREE AREA
Wood or Metal
Louvers or Grilles
Screens**
* Do not use less than 1/4" mesh
** Free area or louvers and grilles varies widely. Installer
should follow louver or grillle manufacturer's instructions.
NOTE: If mechanically operated louvers are used, a means to
prevent main burner ignition and operation must be pro-
vided should louvers close during startup or operation.
Special Combustion and Ventilation Considerations
Operation of a mechanical exhaust, such as an exhaust fan,
kitchen ventilation system, clothes dryer or fireplace may cre-
ate conditions requiring special attention to avoid unsatisfac-
tory operation of gas appliances.
Specially Engineered Installations
The above requirements shall be permitted to be waived where
special engineering, approved by the authority having jurisdic-
tion, provides an adequate supply of air for combustion, venti-
lation and dilution of flue gases.
650.75-N4U
= Square Inches Free Area
Wood 20 - 25%*
Metal 60 - 70%*
1/4" mesh or larger 100%
15
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