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T921A,B,E-G Proportional
T921 Thermostats provide 3-wire, low voltage
proportional control for Series 90 (135 ohm) valve
motors, damper motors, and balancing relays in
heating or cooling system applications.
T921A
Accessory faceplates available for horizontal mount-
ing of T921A,B,E models.
COOL-HEAT changeover switch in the T921B.
Night setback mechanism and switch in the T921E.
A spdt (single pole double throw) Class 2 auxiliary
switch in the T921F,G, which operates at the low
(T921F) and high (T921G) end of the throttling
range.
Control Thermostats
T921B
J. D. • Rev. 1-94 • ©Honeywell Inc. 1994 • Form Number 60-2242-2
T921E
A bellows element adjusts the potentiometer slider
to regulate motor operation.
Various temperature ranges are available.
All models are available with a thermometer and a
locking cover.
CONTENTS
Specifications ................................................. 2
Ordering Information ..................................... 2
Installation ..................................................... 4
Setting and Adjusting ..................................... 7
Operation ....................................................... 7
1
T921F
T921G
60-2242-2
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Summary of Contents for Honeywell T921A
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Page 1: Table Of Contents
(T921F) and high (T921G) end of the throttling range. CONTENTS Specifications ..........2 Ordering Information ........2 Installation ............. 4 Setting and Adjusting ........7 Operation ............7 60-2242—2 J. D. • Rev. 1-94 • ©Honeywell Inc. 1994 • Form Number 60-2242—2... -
Page 2: Specifications
If you have additional questions, need further information, or want to comment on our products or services, please write or phone: 1. You local Honeywell Home and Building Control Sales Office (check the white pages of the phone directory). 2. Home And Building Control Customer Satisfaction Honeywell Inc., 1885 Douglas Drive North... - Page 3 [32] REPLACEMENT PART: 130224 Locking Cover Knob. OPTIONAL SPECIFICATION (USING THE T991B): Alternate proportional control similar to the T921A, but uses an outdoor temperature, automatic control point reset. T991B with 3-1/2: 1 reset and 50 to 120 F [10 to OUTDOOR BULB SIZE VARIES INVERSELY WITH THE RESET RATIO.
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Page 4: Installation
MOUNTING (T921A,B,E ONLY) T921 models are shipped with a faceplate for vertical Disconnect power supply before installation to mounting. When mounting the T921A,B,E horizontally, prevent electrical shock and equipment damage. see Table 2 to order the faceplate with the appropriate scale. - Page 5 INSTALLATION Fig. 4—Vertical mounting of T921A,B,E-G Fig. 5—Terminals on back of thermostats. Thermostats on horizontal conduit box. T921A,B,E Thermostats can also be mounted horizontally on vertical conduit box using add-on faceplate. (Do not mount T921F,G horizontally.) NOTE: ACCESSORY FACEPLATE FOR USE WHEN THE THERMOSTAT IS MOUNTED HORIZONTALLY.
- Page 6 T921A,B,E-G INSTALLATION • SETTING AND ADJUSTING Fig. 7—Typical wiring diagram for T921B. Fig. 9—Typical wiring diagram for T921F. SERIES 90 MODULATING T921F MOTOR SERIES 90 MODULATING MOTOR T921B (HOT) TRANSFORMER (HOT) TRANSFORMER 2-POSITION AUXILIARY ACTUATOR SWITCH POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.
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Page 7: Setting And Adjusting
T921A,B,E-G SETTING AND ADJUSTING • OPERATION This switch energizes a heater that offsets the ther- Fig. 11—Internal view of vertically mounted mostat approximately 10 F [6 C], lowering the T921 (horizontally mounted model is similar). temperature for night operation. Leave this switch in the OFF position for normal operation;... - Page 8 OPERATION T921B Fig. 13—T921G auxiliary switch operation. 1. Place the changeover switch (see Fig. 11) in the HEAT position and follow steps 1 through 3 of the T921A TEMPERATURE procedure. 2. Move the switch to the COOL position and repeat...