NOTE: Slope combustion-air and vent pipes downwared toward
furnace a minimum of 1/4 in. per linear ft with no sags between
hangers.
When combustion-air pipe is installed above a suspended
ceiling, pipe must be insulated with 3/8-in. thick Armaflex-
type insulation. Combustion-air pipe should also be insulated
when it passes through warm, humid space.
An abandoned masonry chimney may be used as a raceway for
properly insulated and supported combustion-air and vent pipes.
Each furnace must have its own set of combustion-air and vent
pipes and be terminated individually, as shown in Fig. 42.
Other gas appliances with their own venting system may also use
the abandoned chimney as a raceway providing it is permitted by
local code, the NFGC or NSCNGPIC, and the vent or liner
manufacturer’s installation instructions. Care must be taken to
prevent the exhaust gases from one appliance from contaminating
the combustion air of other gas appliances.
When vent pipe is exposed to temperatures below freezing,
such as when it passes through an unheated space or when a
chimney is used as a raceway, pipe must be insulated as
shown in Table 8 with Armaflex-type insulation. Failure to
follow this caution will result in intermittent unit operation.
Combustion air must not be taken from inside structure
because inside air is frequently contaminated by halogens,
which include fluorides, chlorides, bromides, and iodides.
These elements are found in aerosols, detergents, bleaches,
cleaning solvents, salts, air fresheners, adhesives, paint, and
other household products. Locate combustion-air inlet as far
as possible from swimming pool and swimming pool pump
house.
Excessive exposure to contaminated combustion air will
result in safety and performance related problems.
Solvent cements are combustible. Keep away from heat,
sparks, and open flame. Use only in well-ventilated areas.
Avoid breathing in vapor or allowing contact with skin or
eyes. Failure to follow this warning could result in fire,
property damage, personal injury, or death.
All combustion-air and vent pipes must be airtight and
watertight. Pipes must also terminate exactly as shown in Fig.
36, 37, 38, 39, or 40. Failure to follow this warning could
result in property damage, personal injury, or death.
NOTE: The minimum combustion-air and vent pipe length (each)
for these furnaces is 5 ft. Short pipe lengths (5-8 ft) may discharge
water droplets. These droplets may be undesirable, and a 12-in.
minimum offset pipe section is recommended, as shown in Fig. 34,
to reduce excessive droplets from exiting vent pipe outlet.
Combustion-Air and Vent Pipe Diameter
Determine combustion-air and vent pipe diameter.
1. Using Table 7, individually determine the smallest
combustion-air and vent pipe diameters permitted for each
pipe. Pick the larger of these 2 pipe diameters and use this
diameter for both combustion-air and vent pipes.
2. When installing vent systems of short pipe length, use the
smallest allowable pipe diameter. Do not use pipe size greater
than required becuase incomplete combustion, flame distur-
bance, or flame sense lockout may occur.
NOTE: Do not count elbows or pipe sections in terminations or
within furnace. See shaded areas in Fig. 36, 37, 38, 39, and 40.
Fig. 33—Combustion-Air and Vent Pipe
Connections
A96187
COMBUSTION-
AIR
COMBUSTION-
AIR
AIR
FLOW
VENT
VENT
VENT
AIR
FLOW
AIR
FLOW
AIR
FLOW
UPFLOW DOWNFLOW
HORIZONTAL-LEFT DISCHARGE HORIZONTAL-RIGHT DISCHARGE
Select 1 vent pipe connection and
1 combustion-air pipe connection.
COMBUSTION-
AIR
COMBUSTION-
AIR
COMBUSTION-
AIR
COMBUSTION-
AIR
VENT
VENT
VENT
NOTE: Select 1 vent pipe connection and
1 combustion-air pipe connection.
NOTE:
25
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