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combustion air are not available to the heater, poor combustion
and inefficient operation will result.
For outdoor installations, the heating unit cabinet combustion air
openings should be sized to provide 1 sq in. of free area per 4000
Btuh of input. Combustion air inlet and flue gas outlet must be lo-
cated in the same pressure zone to minimize effects of wind on
burner and heater performance.
For indoor installations, locate heating unit to ensure an adequate
supply of fresh air to replace air used in the combustion and venti-
lation process. Install air openings that provide a total free area in
accordance with the National Fuel Gas Code (ANSI Z223.1 or
NFPA 54) in the United States or CAN/CGA B-149 Installation
Code in Canada.
Do not install unit in a confined space (a space whose volume is
less than 50 cubic feet per 1000 Btuh of furnace input rating) with-
out providing wall openings leading to and from the space. Pro-
vide openings near floor and ceiling to provide air for combustion
and ventilation. Size openings for total Btuh of all appliances in
confined space and provide openings as follows:
Two-stage
Gas Valve
3/4" Npt Gas
Inlet
Connection
Draft Inducer
Assembly
Draft Inducer
Motor
Motor Support
Bracket
Wire Harness
To Jb/ignition
Control
Modulator Valve
(Modulating Systems Only)
Condensate Drain
Fitting
Fig. 144 — Gas-Fired Duct Furnace Components
• Air from inside building — openings 1 sq in. free area per
1000 Btuh, but never less than 100 sq in.
• Air from outside through a duct — openings 1 sq in. free
area per 2000 Btuh
• Air direct from outside — openings 1 sq in. free area per
4000 Btuh
VENTING
Safe operation of indirect-fired gas furnaces requires a properly
operating venting system that vents all the products of combustion
(flue gases) to the outside atmosphere.
Failure to provide proper venting affects furnace perfor-
mance and may result in a health hazard which could cause
serious personal injury or death.
Air Pressure
Switch
189
WARNING
Flame Rollout
Switch
Flame Sensor
Hi Limit Switch
Manifold
Pressure Tap
Spark Igniter
a62-577
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