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C12 DIAGRAM WITH D5 DIAGRAM
Honeywell T7300 Thermostat with Modulating Economizer
8-C12 Section with D5 Section
The Honeywell T7300 programmable thermostat allows GCS16 units without economizer to
automatically setback or setup setpoints for unoccupied periods as well as control setpoints
more precisely than electromechanical thermostats. With the modulating economizer added,
the T7300 is capable of directly modulating the economizers and can directly control morning
warmup.
NOTE - In order to understand how these optional controls affect the operation of the GCS16,
you must first understand how all GCS16 components work. Refer to the operation se
quence for basic unit operation.
NOTE - These thermostats have a built-in clock for controlling setback. Optional time clock
CMC3-1, night thermostat and night relay kit are not needed and are not compatible.
Operation Sequence:
1- The GCS16 with T7300 is designed so that the fan switch in the thermostat should be left in the
ON mode at all times. This allows the blower to be controlled by terminal G in the thermostat.
The blower operates continuously during occupied periods and intermittently during unoccu
pied periods.
2- During a heating demand when the building is not occupied, the blower is activated only when
a heating demand passes through relay K25 in the GCS16. During a cooling demand when the
building is not occupied, the blower is activated through terminal G in the thermostat.
Heating:
3- Heating demand W1 from the T7300 energizes the heat section of the GCS16 directly. When
relay K13 is energized to start the combustion air blower, contacts K13-2 close. When con
tacts K13-2 close, 24VAC is passed to damper motor terminal TR. Outdoor dampers open to
minimum position.
I. Enthalpy Control in Low Position (outside air can be used for cooling).
First stage cool (all models):
4- Initial cooling demand Y1 is sent to enthalpy control A6 terminal 1.
5- Enthalpy control A6 has determined that outside air can be used for cooling and has switched
internal 1K and 2K internally.
6- Cooling demand is routed through enthalpy control to energize internal relay 1S. Internal con
tacts 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 dampers open. Supply air sensor R1 varies the voltage across T and T1
and the outdoor air dampers adjust accordingly. First stage cooling is provided by outdoor air.
Dampers are modulated by T7300 terminals A1 and A2 (RT2) and supply air sensor R1.
Second stage cool (all models):
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 compressor. The compressor provides all additional cooling.
II. Enthalpy Control in High Position (outside air cannot be used for cooling).
Cooling:
10- Enthalpy control internal relays 1K and 2K switch. Internal relay 1S is deenergized 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 compressor. The compressor han
dles all cooling.
Night Setback (optional field installed):
12- Night relay K11 (not furnished) is not used in this application and K11 contacts K11-1 and
K11-2 (if installed) should remain closed at all times.
13- Night setback and morning warm-up are controlled directly by the T7300. During night set
back, the T7300 changes to unoccupied setpoints. Operation sequence does not change.
Outdoor dampers are held closed by T7300 terminal A2.
14- During morning warmup (first heat demand of the day - after night setback) T7300 terminal A2
remains deenergized and the outdoor dampers remain closed.