Honeywell Design and Application - Page 190

Section 17 - Retrofit and Upgrades
63-8594-02 Honeywell Economizers 184
Before: Series 90 with Dry Bulb Changeover
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.
TRANSFORMER
T675 DRY BULB
HIGH LIMIT
CONTROL
MINIMUM
POSITION
CONTROL
MIXED AIR
DAMPER MOTOR
T991 MIXED
AIR CONTROL
COOLING
COMMERCIAL THERMOSTAT
COOLING
CONTACTOR
RELAY
RC
Y1
Y2
NC
C
NO
NC
C
NO
B
R
W
R
B
B
R
W
W R
T1
T2
L2
L1
(HOT)
M13847A
60
1
80
90
17
0
16
15
140
130
120
110
1
00
F
8
0
70
L1
L2
T2
T1
60
100
140
120
80
40
°F
60
1
80
90
170
16
15
140
130
12
0
110
1
00
F
8
0
70
OAT