10
The installed length of the sidewall or vertical combustion air inlet
pipe, from the appliance to the inlet terminal outside of the
building,
must not exceed a maximum of 100 equivalent feet
(30.5M) in length
. The straight length that each elbow equates
to depending on the diameter and centerline radius, typically 7 to
19 feet per 90° elbow and roughly half this value for each 45°
elbow. The equivalency values must be verified with the
individual vent manufacturer, as they are not standard, and
depend on the precise dimensions, surface roughness, etc., that
a particular product is built with.
3.2.3
– Combined Combustion Air Supply
The air inlet pipes from multiple appliances can be combined in
a single common air inlet pipe if the cross-sectional area, and the
combustion air opening free area, are equal to or larger than the
total area of all air inlet pipes connected together. This exterior
opening for combustion air must connect directly to the outdoors.
Deduct the restriction in area provided by any screens, grills or
louvers installed in the common air inlet point, which can reduce
the free area by 25% to 75% based on the materials used.
Determine cross-sectional area using the equation below.
Eq. Ø =
√ [(d
1
)
2
+(d
2
)
2
+(d
3
)
2
+…+(d
n
)
2
]
Where,
Eq. Ø = Equivalent Total Diameter
d = Individual Pipe Diameters
Example:
Find the equivalent diameter of 3x pipes
at 8" (20.3 cm),
10” (25.4 cm) and 12” (30.5 cm).
Eq. Ø =
√ [(8)
2
+(10)
2
+(12)
2
] = √308 = 17.5”
Select 18” (82.8 cm) diameter pipe or larger
The total length of air inlet pipe for a water heater must be
calculated individually, and must not exceed a maximum 100
equivalent feet (30.5M) regardless of pipe diameter
. The total
equivalent length of the common piping must be added to
equivalent length of the branch for an individual water heater. For
instance, if the common piping is
50’ (15.25M) equivalent, and
the water
heater’s individual pipe branch is also 50’ (15.25M)
equivalent, this water heater is at its maximum length.
The total
equivalent length of the common air inlet pipe must not
exceed a maximum of 100 equivalent feet (30.5M) under any
circumstances.
3.2.3
– Room Combustion Air Supply
All Air From Inside the Building (US Only)
-
If combustion
air is drawn from inside the building (the mechanical
equipment room does not receive air from outside via louvers
or vent openings and the water heater is not equipped with
direct vent combustion) and the water heater is located in a
unconfined space, use the following guidelines:
o
The mechanical equipment room must be provided with
two permanent openings linked directly with additional
room(s) of sufficient volume so that the combined volume
of all spaces meet the criteria for an unconfined space.
Note: An "unconfined space" is defined as a space whose
volume is more than 50 cubic feet per 1,000 Btu per hour
of aggregate input rating of all appliances installed in that
space.
o
Each opening must have a minimum free area of one
square inch per 1,000 Btu per hour of the total input rating
of all gas utilizing equipment in the mechanical room.
o
One opening must terminate within twelve inches of the
top, and one opening must terminate within twelve inches
of the bottom of the room.
Figure 1
– All Air From Inside, Two Openings
All Air From Outdoors, Two-Opening
- If all combustion air
will be received from outside the building (the mechanical
room equipment is linked with the outdoors), the following
method can be used:
o
The mechanical equipment room must be provided with
two permanent openings, one terminating within twelve
inches from the top, and one opening terminating within
twelve inches of the bottom of the room.
o
The openings must be linked directly or by ducts with the
outdoors.
o
Each opening must have a minimum free area of one
square inch per 4,000 Btu per hour of total input rating of
all equipment in the room, when the opening is directly
linked to the outdoors or through vertical ducts.
o
The minimum free area required for horizontal ducts is
one square inch per 2,000 Btu per hour of total input rating
of all the equipment in the room. The ducts should not
exceed
100 equivalent feet (30.5M) in length
.
Figure 2
– All Air From Outside, Two Openings (Optional Duct)
All Air From Outdoors, One-Opening
- If all combustion air
will be received from outside the building (the mechanical
room equipment is linked with the outdoors), the following
method can be used:
o
One permanent opening, commencing within 12 inches of
the top of the enclosure, shall be provided.
