START-UP
Operating Sequence
COOLING
3 to 6 Ton Units — When the outdoor-air temperature is above
the OAT setting and the room thermostat calls for cooling,
the compressor contactor is energized to start the compres-
sor and outdoor-fan motor. The indoor-fan motor (IFM) is
energized and the economizer damper moves to the mini-
mum position. After the thermostat is satisfied, the damper
moves to the fully closed position when indoor-fan motor is
deenergized.
When the outdoor-air temperature is below the OAT set-
ting 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 econo-
mizer. If the supply-air temperature is above 57 F, a switch
on the supply-air thermostat is closed between the T2 ter-
minal 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. The compressor and outdoor fan will be energized
and the position of the economizer damper will be deter-
mined by the supply-air temperature.
When the second stage of cooling is satisfied, the com-
pressor 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, and the IFM
is deenergized, the damper will move to fully closed
position.
Fig. 14 — Typical Economizer Supply-Air
Thermostat
Fig. 15 — Barometric Relief Damper
Characteristics — 3 to 6 Ton Units
Fig. 13 — Wiring Connections for Outdoor-Air
Thermostat
Fig. 16 — Barometric Relief Damper
Characteristics — 7
1
⁄
2
to 12
1
⁄
2
Ton Units
5
