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C7-3 DIAGRAM WITH D5 DIAGRAM
Honeywell W7400 Control System and T7400 Thermostat with Modulating Economizer
2-C7-3 Section with D5 Section
The Honeywell W7400 control / T7400 thermostat system, when applied to a GCS16 allows
fully programmable operation of the unit during occupied or unoccupied periods. Morning
warm-up capabilities are built-in to the control system. An external warm-up kit is not needed
and should not be used. This diagram shows a modulating economizer connected to a W7400
control system.
An economizer may be added to the system to allow outside air for cooling. A relay (K12 -
figure 95) must be added to interface the control to the economizer. In this sequence of opera
tion, the W7400 relay kit is specially wired to interface the control to an economizer using a
solid state enthalpy control. Relay kits for early electromechanical enthalpy controls (as used
in D2 wiring diagrams) cannot be used or control damage will result.All economizers for 2-5 ton
GCS16 units are equipped with electronic enthalpy controls and the updated relay kit must be
used. Unit jackplug J3 must be installed in place of relay kit for basic unit operation without
economizer (since it is improbable that a W7400 system be used without economizer, this wir
ing diagram only shows systems equipped with economizer).
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.
WARNING - CONNECT ONLY RELAY KITS DESIGNED FOR THIS CONTROL SYSTEM.
RELAY KITS FOR OTHER CONTROL SYSTEMS ARE NOT COMPATIBLE AND CON
TROL DAMAGE OR FAILURE WILL RESULT. FOR EXAMPLE, NEVER CONNECT A
W973 RELAY KIT TO A W7400 CONTROL SYSTEM.
CAUTION - DO NOT CONNECT A WARMUP KIT TO JACK J5 OF THE W7400 RELAY
KIT. THE WIRING OF THE WARMUP KIT IS NOT COMPATIBLE WITH THE WIRING OF
THE W7400 CONTROL SYSTEM AND DAMAGE TO COMPONENTS WILL RESULT IF
CONNECTED. THE W7400 SYSTEM HAS A WARMUP FEATURE BUILTIN. A WARM
UP KIT IS NOT NEEDED.
This control arrangement does not require field installed pigtails. The W7400 plugs in to the
GCS16 in the control mounting area of the blower compartment. Jack J17 connects to plug
P16. Plug P17 then connects to jack J16 (figure 94).
The W7400 relay kit mounts next to the W7400 control in the control mounting area of the
GCS16 blower compartment. No hard wiring is required. Jumper plug P3 is removed and dis
carded. Relay kit plug P5 connects directly to jack J3 in the blower compartment (figure 95).
The economizer plugs in to the relay kit. Economizer plug P4 connects directly to jack J5 of the
W7400 relay kit.
IMPORTANT - P14 MUST BE DISCONNECTED FROM J23 IN THIS APPLICATION
ONLY. IMPROPER UNIT OPERATION WILL RESULT IF P14 IS NOT DISCONNECTED.
IMPORTANT - DISCONNECT J26 FROM P26 AND CONNECT J28 TO P26 IN THIS AP
PLICATION ONLY. IMPROPER UNIT OPERATION WILL RESULT IF PROPER CON
NECTIONS ARE NOT MADE.
Operation Sequence:
1- Relay K11 (not furnished) is not used in this application and should not be installed.
2- Economizer minimum position and warm-up are controlled through terminals A and C on the
W7400 control. Outdoor air dampers are held closed during morning warmup and at minimum
position during all other unit operation. Dampers are held closed when the unit is not operating.
3- Heat demand W1 from thermostat T7400 is routed through the W7400 directly to the heat sec
tion of the GCS16.
4- Increased heat demand from the T7400 is not used.
5- Economizer outdoor air dampers are held closed any time heat demand is not present (relay
contacts K13-2) or blower is not operating (relay contacts K3-2).
Cooling:
I. Enthalpy Control in Low Position (outside air can be used for cooling).
First stage cool (all models):
6- Enthalpy control A6 has determined that outside air can be used for cooling and has switched
1K and 2K internally. Internal relay 1S is energized. Internal contacts 1S1 close to complete a
circuit through damper motor terminals T and T1.
7- Cooling demand Y1 is sent through N.C. K12-1 contacts 1-9. There is no affect. However,
blower demand allows outdoor dampers to modulate open.
8- 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.
Second stage cool (all models):
9- Economizer outdoor air dampers remain open.
10- 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).
11- Enthalpy control internal relays 1K and 2K switch. Internal relay 1S is deenergized and 1S1
opens. Outdoor air dampers close to minimum position.
12-Relay K12 is energized. K12-1 contacts switch to allow compressor to provide cooling and
K12-2 open to tell W7400 that outside air is no longer available for cooling.
13- Cooling demand Y1 is sent through K12-1 N.O. contacts 9-5 to GCS16 compressor circuit.
The compressor handles all cooling demand.
Night Setback (optional field installed)
14- Night setback and morning warmup functions are controlled internally in the W7400. Opera
tion sequence does not change.
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