Horizontally Vented Furnaces (see Figure K5)
– Category III (Per NFGC and ANSI Z21.13 is
positive vent pressure and non-condensing)
Pressures in Category III venting systems are positive and
therefore, care must be taken to avoid flue products from
entering the heated space. Use only vent materials and
components that are UL Listed and approved for Cat-
egory III venting systems.
1. All vent pipe joints must be sealed to prevent leakage
into the heated space. Follow instruction provided
with approved venting materials used.
2. The total equivalent length of vent pipe must not ex-
ceed 50 feet [15.35 m]. Equivalent length is the total
length of straight sections, plus 5 feet [1.52 m] for
each 90 degree elbow and 2.5 feet [0.76 m] for each
45 degree elbow.
3. The vent system must also be installed to prevent
collection of condensate. Pitch horizontal pipe runs
downward ¼” per foot [21 mm/m] toward the outlet
to permit condensate drainage. Insulate vent pipe
exposed to cold air or routed through unheated
areas. Insulate vent pipe runs longer than 10 feet [3
m]. Insulation should be a minimum of ½” [12 mm]
thick foil faced material suitable for temperatures up
to 500°F [288°C]. Maintain 6” [152 mm] clearance
between vent pipe and combustible materials.
4. A vent cap approved for horizontal venting must
be provided. Vent cap inlet diameter must be the
same as the required vent pipe diameter. The vent
terminal must be at least 12” [305 mm] from the
exterior wall that it passes through to prevent deg-
radation of building material by flue gases. The vent
terminal must be located at least 1 foot [305 mm]
above grade, or in snow areas, at least 3 feet [1 m]
above snow line to prevent blockage. Additionally,
the vent terminal must be installed with a minimum
horizontal clearance of 4 feet [1.2 m] from electric
meters, gas meters, regulators or relief equipment. Pe-
riodically clean the screens in the vent terminal (where
applicable).
5. Through the wall vents shall not terminate over pub-
lic walkways, or over an area where condensate or
vapor could create a nuisance or hazard. Provide a
vent termination clearance to building or structure
features per Table I2.
Roof line
Exhaust vent
terminal
B
A
Air inlet
Tee fitting with drip
leg and cleanout cap
¼” [6 mm] per
1 foot [305 mm]
A = 10” [3.05 m] maximum horizontal run
(not to exceed 75% of vertical run)
B = 2 ft. [0.61 m]
Exhaust
Figure K4: Indoor vertical venting
A = 12” minimum
Exterior wall
A
Exhaust
Air inlet
Exhaust vent
terminal
Tee fitting
with drip leg
Pitch vent pipe
downward from
furnace ¼ inch
per foot
Figure K5: Indoor horizontal venting
IMPORTANT
Each furnace must have its own individual vent pipe and
terminal. Do not connect vent system from horizontally
vented units to other vent systems or a chimney.
WARNING
Do not use Type B vent within a building on horizontally
vented indoor furnaces.
Table K2: Vent Termination Clearances
Structure
Minimum Clearance
Door, window or gravity inlet
4 ft. [1.2 m] below
4 ft. [1.2 m] horizontally
1 ft. [305 mm] above
Forced air inlet within 10 ft. [3 m]
3 ft. [0.91 m] above
Adjoining building or parapet
6 ft. [1.8 m]
Adjacent public walkways
7 ft. [2.1 m] above grade
VCES-VHC-IOM-1F – VHC-36, -42 & -50
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