Specifications

Page 37
C1 DIAGRAM WITH D5 DIAGRAM
Electromechanical Thermostat with Modulating Economizer
REMD24M
When a REMD24M economizer section is applied to the unit with electromechanical thermostat,
three stages of cooling are available dependent on the actions of the economizer enthalpy control.
By sensing outdoor temperature and relative humidity, the enthalpy control determines if outside
air can be used as a first stage of cooling. If so, 1st stage cooling is handled by outdoor air damp
ers and 2nd stage cooling is handled by the compressor. The enthalpy control continuously ad
justs the outdoor air dampers to maintain a balanced mixed air temperature. When outdoor air
conditions become unsatisfactory for cooling, the outdoor air dampers and the compressors han
dle all cooling demand.
NOTE-In order to understand how optional controls affect the operation of the GCS24, you must
first read and understand how all the GCS24 components work.
Factory jumper-plug P3 is removed from harness jack J3 and discarded. Economizer plug P4
replaces plug P3. These connections are made in the unit blower compartment.
Operation Sequence: C1 Diagram with D5 Diagram (economizer connected to the unit
with electromechanical thermostat)
NOTE-In this operation sequence the unit diagram has been omitted in order to concentrate on
the interaction between thermostat and economizer.
1- Economizer outdoor air dampers drive full closed anytime blower B3 is not operating (switched by
K3-2 in the unit).
2- Damper motor terminal TR is powered by unit contactor K3 when there is a blower demand or by
K25 when there is a heating demand. When 24VAC is applied to between terminals TR and TR1,
the damper motor is energized and the outdoor air dampers open to minimum position.
3- Blower B3 is energized (indirectly) by thermostat terminal G. On a cooling demand, thermostat
terminal G energizes contactor K3 which in turn energizes the blower (refer to operation sequence
on previous page for exact sequence). When K3 energizes, K3-1 closes to energize the blower
and K3-2 closes to energize the economizer (see step 2) and open the outdoor air dampers to
minimum position.
Enthalpy Low, 1st Stage Cool:
4- Initial cooling demand Y1 is sent to enthalpy control A6 and terminal 1.
5- Enthalpy control A6 has determined that outside air can be used for cooling and has switched
internal relays 1K and 2K.
6- Cooling demand is routed through enthalpy control to energize internal relay 1S. Internal contacts
1S1 close to complete a circuit through damper motor terminals T and T1.
7- When a voltage is applied across terminals T and T1 of damper motor, the damper motor ener
gizes and outdoor air dampers open. Supply air sensor R1 varies the voltage across T and T1
and the outdoor air dampers open. Supply air sensor R1 varies the voltage across T and T1 and
the outdoor air dampers adjust accordingly. 1st stage cooling is provided by outdoor air.
Enthalpy Low, 2nd Stage Cool:
8- Economizer outdoor air dampers remain open.
9- Additional cooling demand is routed from thermostat Y2 through enthalpy control terminals 3 and
5 to energize the 1st stage compressors. The 1st stage compressors provide all additional cool
ing.
Enthalpy High, 1st Stage Cool:
10- Enthalpy control internal relays 1K and 2K switch. Internal relay 1S is de-energized and 1S1
opens. Outdoor air dampers close to minimum position.
11- Cooling demand is sent from thermostat terminal Y1 through enthalpy control terminals 1 and 2
and through enthalpy control terminal 5 to energize the 1st stage compressors.
Enthalpy High, 2nd Stage Cool:
12- Additional cooling demand is sent from thermostat terminal Y2 through enthalpy control terminals
3 and 4 to energize the 2nd stage compressor.
Night Setback (optional field installed)
13- Optional field installed time-clock and night thermostat S12 must be connected for night setback
operation.
14- Blower B3 operates only during a heating demand when night thermostat is closed.
15- When clock contacts close, relay K11 energizes. Contacts K11-1 open to disable the day thermo
stat and contacts K11-2 open to drive the dampers full closed.
16- Night thermostat S12 is typically set with setpoints below thermostat S1. During unoccupied peri
ods, K11-1 opens while S1 is disabled. When S12 closes, power is applied to S1 and the unit
operates normally. When the setpoint is reached, S12 opens, S1 is disabled and unit operation
stops.
17- Shortly before the building is to be occupied, clock contacts open to de-energize relay K11. Con
tacts K11-1 close to restore power to thermostat S1 and contacts K11-2 close to restore power to
the minimum positioner. Outdoor air dampers open to minimum position during blower operation.