56 McQuay IM 987
Sequences of Operation
Economizer Operation
When the outdoor air is suitable for free cooling, the switch in
enthalpy sensor OAE is in position “3” (line 245, Figure 79,
page 71) energizing analog input AIX5. When AIX5
energizes, the economizer is enabled. (Note: If selected from
the keypad, the enthalpy decision can be made based on
outdoor temperature. In that condition, if the outdoor air
temperature is less than or equal to the changeover set point,
the economizer is enabled.) If cooling is required, the
economizer dampers (ACT3) are modulated to maintain the
discharge air temperature setpoint. Analog input AIX5 drives
the outdoor air dampers toward the open and closed (line 256)
position. If the outdoor air dampers are wide open and more
cooling is required, the dampers hold their positions and
mechanical cooling is activated (see below).
When the outdoor air is not suitable for free cooling, the
switch in enthalpy sensor OAE is in position “1,”
de-energizing analog input AIX5. (Alternatively, the outdoor
air temperature is above the changeover setpoint plus the
economizer changeover differential). When the economizer is
disabled, the dampers are held at their minimum position.
Heating
Gas Furnace, Super Modulating Burner (20:1
Turndown)
Refer to the Super Mod Gas Furnace Control (1000 MBh)
schematic on page page 76 and the Standard Mod, Furnace
Control (1000 MBh) schematic on page page 78 for a
sequence of operation.
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