Johnson Controls T25 Catalog Page

RODUCT BULLETIN

P

T25 Two-Stage Room Thermostat

T25 thermostats are for line- or low-voltage service
requiring accurate control of of the following operating
functions:

• control of two stages of heating, such as two-rate

unit heaters or duct furnaces, commercial heat
pumps, and other units.

•

control of two stages of cooling, such as two-stage
compressors, dual compressor units, and other
units.

• automatic changeover control of heating and

cooling on three- and four-pipe fan coil
installations and similar applications; automatic
changeover from heating to cooling for unit
operation.

Product/Technical T25

Issue Date 01/25/02

Typical uses are for valves, relays, fan coils,
compressor controls, and other applications where low
differentials and accurate sensing are required. Two
Single-Pole, Double-Throw (SPDT) switches permit
independent control circuits.

Each switch may be wired to make or break the control
circuit on a rise in temperature. A removable jumper
across the “common” terminals is supplied as
standard.

Features and Benefits

Dependable, Field-proven

❑

Pennswitches

Sensitive, Liquid-charged

❑

Temperature Element with an
Efficient Lever Mechanism

Standard Bimetal Thermometer Requires no calibration

❑

Semi-concealed, Field-settable

❑

High-end Temperature Stop

Provides accurate measurement and long-term stability

Achieves maximum sensitivity to ambient air
temperature changes without using anticipators; no
leveling required

Deters unauthorized temperature settings above a
pre-determined maximum

Figure 1: T25 Two-Stage Room Thermostat

Automatic Changeover Eliminates the need for manual change between cooling

❑

and heating

© 2002 Johnson Controls, Inc.

Code No. LIT-125640 www.johnsoncontrols.com

1

Product Overview

T25 thermostats are provided with a dependable,
easy-reading, bimetal, pointer-type thermometer.
The liquid-charged sensing element provides
maximum sensitivity to surrounding air temperature
changes. Coupled with a highly efficient diaphragm
and lever mechanism, the element actuates the
enclosed Pennswitches. This provides a low
operating differential and dependable switching
action without the necessity of either heating or
cooling anticipators.

Eliminating anticipators makes every thermostat a
stock control. Thermostats may be used at voltages
up to 277 VAC, for single-stage or two-stage heating
and/or cooling with a wide range of current loads.

T25 Thermostats are available with concealed
adjustments. (See Figure 3.) All thermostats have
Allen-head cover screws to discourage unauthorized
tampering. The concealed high-temperature stop
allows adjustments in 2F° (1.1C°) increments from
68 to 80°F (20 to 27°C). This feature deters
unauthorized temperature settings above a
pre-determined maximum. (See Figure 2.)

Set Point

Knob

High

Temperature

Stop

0

2

1

5

5

2

1
0

0

3

5

C

Figure 2: Interior of T25

Mounting

Plate

Studs

IMPORTANT: The T25 Series Thermostats are
intended to control equipment under normal operating
conditions. Where failure or malfunction of the
T25 Thermostats could lead to an abnormal operating
condition that could cause personal injury or damage
to the equipment or other property, other devices (limit
or safety controls) or systems (alarm or supervisory
systems) intended to warn of, or protect against,
failure or malfunction of the T25 Thermostats must be
incorporated into and maintained as part of the control
system.

Operating Temperature Differential

The operating temperature differential of any room
thermostat depends on:

• current flow through the thermostat (amperage load)

• air velocity over the thermostat

• rate of temperature change to which the thermostat

is subjected

• whether the thermostat is operating heating or
cooling equipment

Graphs (Figure 4 and Figure 5) show the operating
temperature differentials of these thermostats under
various electrical load conditions.

The air velocity was 25 feet per minute, (.1 m/sec) and
the rate of temperature change was 6F° (3.3C°) per

hour. Higher air velocity and/or a lower rate of
temperature change result in a lower operating
differential.

C° F°

6

3

l

5

a

i

t
n
e

4

r
e

f

f

2

i
D

3

g
n

i

t
a

2

r
e

1

p
O

1

Figure 3: T25 With Concealed Adjustment

2 T25 Two-Stage Room Thermostat Product Bulletin

0246

Amperage Load (units in amperes)

Figure 4: T25 Heating operating differential