Honeywell Design and Application Technical Reference Manual page 190

Section 17 - Retrofit and Upgrades
COOLING
COMMERCIAL THERMOSTAT
Series 90 was widely used for fifty years as the primary form of electronic temperature control.
Due to the durability of these devices there are many of them still in use on rooftop air handlers
throughout the world. This is a typical configuration for a dry bulb economizer. The high limit is
typically set at 70 or 75°F (21 or 24°C). A relay is used to switch the mixed air control circuit as
the first stage of cooling to a minimum position when the outside air minimum position
potentiometer provides ventilation by preventing full closure of the outside air dampers.
63-8594-02 Honeywell Economizers
TRANSFORMER
L1
(HOT)
L2
T675 DRY BULB
HIGH LIMIT
CONTROL
OAT
F 120
110
130
100
140
90
15
80
16
70 170
60
180
R
B
RC
Y1
Y2
Before: Series 90 with Dry Bulb Changeover
MIXED AIR
DAMPER MOTOR
T1
T2
W R
W
1 0 0
8 0
R
1 2 0
6 0
4 0
1 4 0
° F
MINIMUM
POSITION
CONTROL
C
NC
C
NC
COOLING
CONTACTOR
184
B
W
120
F
110 130
100
140
90
15
R
80
16
70
170
60 180
B
T991 MIXED
AIR CONTROL
NO
RELAY
NO
T 1
L 1
T 2
L 2
M13847A