1-14. AIRFLOW SYSTEM. (Refer to Figure 1-8).
c. General. The airflow system is illustrated schematically on figure 1-7. Major items include motor B 1, air filters,
the heat exchanger, and the centrifugal fan. Related components include return air thermostat TS 1, discharge air
thermostat TS2, and temperature limit switch S2.
b. Ventilation Mode. Ventilation air is drawn through the air filters from the return air duct and/or the fresh air
damper. From the centrifugal fan, the air passes over surfaces of the heat exchanger. The air is then discharged from
the heater through the discharge air duct and/or fresh air damper. Operation of the discharge air thermostat TS2 is
described in paragraph 1-13.
c. Heating Mode. The heating mode airflow is described in the following paragraphs.
(1) Ventilation Airflow. Ventilation airflow in the heating mode is identical to that described for the ventilation
mode, except that the air passing over the heat exchanger is heated as required by demand of thermostats TS
1 or TS2. Operation of temperature limit switch S2 is also described in paragraph 1-12.
(2) Combustion Airflow. Combustion air is supplied by the centrifugal fan. It is high velocity air provided from the
top of the centrifugal fan scroll or housing. Combustion air is provided when motor B1 is operating. This air
passes from the centrifugal fan into the burner assembly, and after ignition of the fuel-air mixture, the air
passes through the exhaust tubes to the flue gas collector and exits the unit through the exhaust pipe.
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