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C7-3 DIAGRAM WITH D8 DIAGRAM
Honeywell W7400 Control System and T7400 Thermostat with ThreePosition Economizer
D-ELECTRONIC CONTROL SYSTEMS
1-C7-3 Section with D8 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 builtin to the control system. An external warmup kit is not needed
and should not be used. This diagram shows a threeposition 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 units blower compartment. No hard wiring is required. Jumper plug P3 is removed and
discarded. 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 shown) is not used in this application and should not be installed.
2- Economizer minimum position and warmup are controlled through terminals A and C on the
W7400 control. Outdoor air dampers are held closed during morning warmup and at mid
(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 anytime 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
internal 1K and 2K internally. Since J26 is disconnected from P26 there is no affect.
7- Cooling demand Y1 is routed through N.C. K12-1 contacts 1-9 and through discharge air ther
mostat S13 to enthalpy control terminal D and damper motor terminal D.
8- When 24VAC is applied across terminals D and T of damper motor, the damper motor ener
gizes and outdoor dampers open fully. 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 demand.
II. Enthalpy Control in High Position (outside air cannot be used for cooling).
11- Enthalpy control A6 has determined that outside air cannot be used for cooling and has
switched 1K and 2K internally. Relay K12 is energized. K12-1 contacts switch to allow com
pressor to provide cooling and K12-2 open to tell W7400 that outside air is no longer available
for cooling.
12- Cooling demand Y1 is sent from through K12-1 N.O. contacts 9-5 to GCS16 compressor cir
cuit. The compressor handles all cooling demand.
Night Setback
13- Night setback and morning warmup functions are controlled internally in the W7400. Opera
tion sequence does not change.
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