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C. Unit Controls
All compressors have the following internal-protection
controls:
1. High-Pressure Relief Valve — This valve (internal to the
compressor) opens when the pressure differential be-
tween the low and high sides becomes excessive and will
automatically reset when pressure returns to normal.
2. Compressor Overload — This overload interrupts power
to the compressor when either the current or internal
temperature becomes excessive, and automatically re-
sets when the internal temperature drops to a safe level.
This overload may require up to 60 minutes (or longer)
to reset; therefore, if the internal overload is suspected
of being open, disconnect the electrical power to the unit
and check the circuit through the overload with an ohm-
meter or continuity tester.
D. Cooling Sequence of Operation
Without Economizer
Room thermostat calls for cooling. Circuit closes between
24-v control circuit terminals R and Y1 and terminals R and
G. Power to terminal R is supplied through the 24-v trans-
former (transformer is internally protected against over-
load). Terminal G energizes the indoor (evaporator) fan con-
tactor (IFC) through normally closed contacts T and B of the
time-delay relay (TDR) and the evaporator fan starts.
The 24-v power through terminal Y1 energizes the compres-
sor contactor (C), starting the compressor and condenser fan.
When the thermostat is satisfied, C1 is deenergized and the
compressor and OFM shut off on 072 units, the IFM (indoor
fan motor) shuts off immediately. After a 30-second delay on
036-060 units, the IFM shuts off. If the thermostat fan selec-
tor switch is in the ON position, the evaporator motor will
run continuously.
With Varislide™ Economizer
When the outdoor-air temperature is above the (OAT) set-
ting and the room thermostat calls for cooling, compressor
contactor is energized to start the compressor and outdoor-
fan motor. The evaporator (indoor) fan motor (IFM) is ener-
gized and the economizer damper moves to the minimum
position. After the thermostat is satisfied, the damper moves
to the fully-closed position when evaporator-fan motor is
deenergized.
When the outdoor-air temperature is below the OAT setting
and the thermostat calls for Y1 and G (cooling), the econo-
mizer damper moves to the minimum position when the
evaporator-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 tem-
perature 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 termi-
nals. This causes the economizer damper to remain in an in-
termediate 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 modu-
late 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 inte-
grated with mechanical cooling, providing second stage cool-
ing. The compressor and condenser fan will be energized and
the position of the economizer damper will be determined by
the supply-air temperature.
When the second stage of cooling is satisfied, the compressor
and condenser-fan motor will be deenergized. The damper po-
sition will be determined by the supply-air temperature.
When the first stage of cooling is satisfied, there is a
30-second delay before evaporator fan shuts off (036-060). There
is no delay on 072 units. The damper will move to the fully-
closed position.
With PARABLADE Economizer
When the outdoor air is above the enthalpy control setting,
and the room thermostat calls for cooling, the compressor con-
tactor is energized to start the compressor and the outdoor
fan motor. The indoor fan motor is energized and the econo-
mizer damper moves to the minimum position. After the room
thermostat is satisfied the damper will spring return to the
fully closed position.
When the outdoor air is below the enthalpy control setting
and the thermostat calls for cooling, the economizer outdoor
air damper is opened proportionally to maintain between
50 and 56 F at the mixed air sensor. If outside air alone can-
not satisfy the cooling requirements, economizer cooling is in-
tegrated with mechanical cooling. When the room thermostat
is satisfied, the damper will spring return to the fully closed
position.
Time Guard II Device
If the unit is equipped with accessory Time Guard II recycle
timer, the unit will delay 5 minutes between compressor starts.
Loss-of-Charge/Low-Pressure Switch (LPS)
When the liquid line pressure drops below 7 psig, the LPS
opens 24-v power to the compressor contactor and stops the
compressor. When the pressure reaches 22 psig, the switch
resets and the compressor is allowed to come back on.
High-Pressure Switch (HPS)
When the refrigerant high-side pressure reaches 428 psig, the
HPS opens 24-v power to the compressor contactor and stops
the compressor. When the pressure drops to 320 psig, the switch
resets and the compressor is allowed to restart.
Freeze-Protection Thermostat (FPT)
When the evaporator-coil leaving refrigerant temperature drops
below 30 F, the FPT opens 24-v power to the compressor con-
tactor and stops the compressor. When the leaving refriger-
ant temperature warms to 45 F, the switch resets and the
compressor is allowed to restart.
—19—
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