7
1
⁄
2
to 12
1
⁄
2
Ton Units — When the outdoor-air temperature
is above the OAT setting and the room thermostat calls for
cooling, the compressor contactor no. 1 is energized to start
compressor no. 1 and outdoor-fan motor. The indoor-fan mo-
tor (IFM) is energized and the economizer damper moves to
the minimum position. Upon a further call for cooling, com-
pressor contactor no. 2 will be energized, starting compres-
sor no. 2. After the thermostat is satisifed and the IFM is
deenergized, the damper moves to the fully closed position.
When the outdoor-air temperature is below the OAT setting
and the thermostat calls for Y1 and G, the economizer damper
moves to the minimum position when the indoor fan starts.
The first stage of cooling is provided by the economizer. If
the supply-air temperature is above 57 F, a switch on the
supply-air thermostat is closed between the T2 terminal and
the 24 vac terminal. This causes the damper to continue to
modulate open until the supply-air temperature falls below
55 F or the damper reaches the fully open position.
When the supply-air temperature is between 55 F and
52 F, the supply-air thermostat has open switches between
the T2 and 24 vac terminals and between the T1 and 24 vac
terminals. This causes the economizer damper to remain in
an intermediate open position.
If the supply-air temperature falls below 52 F, a switch on
the supply-air thermostat is closed between the T1 terminal
and the 24 vac terminal. This causes the damper to modulate
closed until the supply-air temperature rises above 55 F or
the damper reaches the minimum position.
When the supply-air temperature is between 55 F and
57 F, the supply-air thermostat has open switches between
the T2 and 24 vac terminals. This causes the economizer damper
to remain in an intermediate open position.
If the outdoor air alone cannot satisfy the cooling require-
ments of the conditioned space, economizer cooling is in-
tegrated with mechanical cooling, providing second stage
cooling. Compressor no. 1 and outdoor fan will be ener-
gized, and the position of the economizer damper will be
determined by the supply-air temperature. Compressor no. 2
is locked out.
When the second stage of cooling is satisfied, the compres-
sor and outdoor-fan motor will be deenergized. The damper
position will be determined by the supply-air temperature.
When the first stage of cooling is satisfied, the damper will
move to fully closed position.
HEATING — When the room thermostat calls for heat, the
heating controls are energized as described in the base unit
Installation, Start-Up and Service Instructions. The indoor-
fan motor (IFM) is energized and the economizer damper
moves to the minimum position. When the thermostat is sat-
isfied, the damper moves to the fully closed position.
VENTILATION (CONTINUOUS FAN) — When the room
thermostat is set for continuous fan, the economizer damper
remains at minimum position for as long as the indoor fan
is energized. When the indoor fan cycles off, the damper moves
to the fully closed position.
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