1-14. AIRFLOW SYSTEM.
General. The airflow system is divided into two separate air flows and is illustrated schematically in Figure 1-
6. Major items include the circulating air fan, primary and secondary heat exchanger, burner, combustion air fan.
Circulating Air Flow. In the circulating air flow, the air is either ventilation (ambient) or heated air.
Ventilation Mode. Ventilation air is drawn through the return air duct and/or the fresh air damper by the
circulating air fan. The ASH enclosure becomes pressurized, the air moves first around the secondary heat exchanger
then through the primary heat exchanger. From the primary heat exchanger the air is discharged through the supply air
Heating Mode. The airflow in the heating mode is identical to that described for the ventilation mode,
except the air passing over the heat exchanger is heated by the heat exchanger as required by either the remote
thermostat (Auto Heat Mode) or the discharge air thermostat (Manual Heat Mode).
Figure 1-6. Air Flow Schematic