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Electromechanical Controls -
Sequence Of Operation
These units are offered with electromechanical controls.
Note: The Condensate Overflow Switch (COF) (optional)
will shut the unit down if the float is raised and the
switch is closed.
Cooling without an Economizer
When the thermostat switch is set to the "Cool" position
and the zone temperature rises above the cooling setpoint,
the thermostat Y contacts close. The compressor contactor
(CC1) coil is energized provided the low pressure control
(LPC1), high pressure control (HPC1) and discharge line
thermostat (DLT 1) are closed. When the (CC1) contacts
close, compressor (CPR1) and both outdoor fan motors
(ODM1 and ODM2) start. If the first stage of cooling cannot
satisfy the cooling requirement, the thermostat closes Y2.
The compressor contactor (CC2) coil is energized provided
the low pressure control (LPC2), high pressure control
(HPC2) and discharge line thermostat (DLT 2) are closed.
When the (CC2) contacts close, compressor (CPR2) starts.
Evaporator Fan Operation (for Cooling
Only Units) - Standard Economizer
When the thermostat fan selection switch is set to the
"Auto" position, the thermostat energizes the indoor fan
relay coil (F) to start the indoor fan motor (IDM).The fan
relay (F) de-energizes after the cooling requirement has
been satisfied. When the heating cycle is terminated, the
indoor fan relay (F) coil is de-energized with heater
contactors.
When the thermostat fan selection switch is set to the "On"
position, the thermostat keeps the indoor fan relay coil (F)
energized for continuous fan motor operation.
Economizer Set-Up - Standard
Economizer
Adjusting the minimum position potentiometer located on
the unit economizer module (RTEM-EM) sets the required
amount of ventilation air.
Ambient temperature is controlling the economizing cycle
by sensing the outside air dry bulb temperature.
26
lists the selectable dry bulb values by potentiometer
setting.
Table 4. Potentiometer settings
Potentiometer
Setting
27 Btu/lb (63 kJ/kg)
A
25 Btu/lb (58 kJ/kg)
B
26
Table 4, p.
Dry Bulb
Enthalpy
73°F (22.8°C)
70°F (21.1°C)
Table 4. Potentiometer settings (continued)
Potentiometer
Setting
C
(a)
23 Btu/lb (53 kJ/kg)
22 Btu/lb (51 kJ/kg)
D
19 Btu/lb (44 kJ/kg)
E
(a)
Factory setting
Cooling with an Economizer - Standard
Economizer
The economizer is utilized to control the zone temperature
providing the outside air conditions are suitable. Outside air
is drawn into the unit through modulating dampers.
When cooling is required and economizing is possible, the
unit economizer module (RTEM-EM) opens the
economizer damper. The RTEM-EM continues to modulate
the economizer damper open/closed to keep the mixed air
temperature in the 50°F to 55°F range.
The thermostat will close the Y2 contacts to turn on
contactor (CC1) if mechanical cooling is required.
If economizing is not possible, the RTEM-EM drives the
damper to the minimum position setpoint when the indoor
fan relay (F) is energized and allows mechanical cooling
operation.
Heating Operation (Units with Electric
Heat) - Standard Economizer
When the system switch is set to the "Heat" position and
the zone temperature falls below the heating setpoint, the
thermostat closesW1 contacts the first stage electric heat
contactor (AH or AH and CH) is energized. If the first stage
of electric heat cannot satisfy the heating requirement, the
thermostat closes W2.
When the W2 contacts close, the second stage electric
heat contactor (BH or BH and DH) is energized, if
applicable. The thermostat cycles both the first and second
stages of heat "On" and "Off" as required to maintain the
zone temperature setpoint.
Drain Pan Condensate Overflow Switch
(Optional)
The Condensate Overflow Switch (COF) is utilized to
prevent water overflow from the drain pan. The float switch
is installed on the corner lip of the drain pan. When the
condensate level reaches the trip point, the COF relay
energizes and opens the 24 Vac control circuit which
disables the unit. Once the 24 Vac control circuit is opened,
a delay timer will prevent unit start-up for three minutes.
Dry Bulb
Enthalpy
67°F (19.4°C)
63°F (17.2°C)
55°F (12.8°C)
RT-SVX50M-EN
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