Honeywell T641A, T641C, T641B Installation Instructions Manual

T641A,B,C

Floating Control Thermostats

Installation instructions for the trained service technician.

Application

The T641A,B,C Floating Control Thermostats provide
spdt outputs to control floating actuators in applications
such as variable air volume (VAV) terminal units.

—T641A: standard spdt floating control thermostat.

—T641B: incorporates momentary system override switch

to provide external relay connection to be energized for
override in building management systems or other
applications.

—T641C: provides manual heat/cool changeover switch.

Operation

FLOATING CONTROL

The control provided by the T641 is conventionally
termed floating control. During floating control, a variation
of two-position control, the thermostat provides a three
output control, Advance motor, Reverse motor, and Hold.

On a change in temperature, the T641 drives the actuator
to an intermediate position and then opens the circuit to the
actuator. The actuator remains in this position until there is a
temperature change at the T641. The actuator is said to float
between the limits of the T641 to satisfy various load require­ments. See Fig. 1.

VAV SYSTEMS

VAV systems control the temperature within a space by
varying the volume of supply air. Air is delivered to the space
at a fixed temperature. The volume of supply air is controlled
by the space thermostat modulating the supply air damper.
When full heating and cooling flexibility is required in a zone,
it is handled by perimeter heating, or reheat capability in the
air terminal units. As individual zones shut down, the total air
flow in the system is regulated by a central duct static pressure
controller. The fan system is sized to handle an average peak
load, not the sum of the individual peaks. As each zone peaks
at a different time of day, extra air is borrowed from the off­peak zones. This transfer from low-load to high-load zones
occurs only in true VAV systems.

Pressure dependent systems do not incorporate an indi­vidual zone air flow sensor and depend on a stable system
pressure to maintain flow. These systems require slower
motors such as the seven minute ML6161 models that are
typically controlled by the T641 thermostats.

HEAT ANTICIPATION/COOL ANTICIPATION

Control of heating or cooling units with a thermostat does
not allow for the temperature to remain exactly at the thermo­stat set point, but varies within a certain temperature range.
Anticipation is added to the thermostat to reduce this range.

The anticipator is a small resistive heater in the thermo-

Fig. 1—T641 floating control.

R-Y ENERGIZED

DEADBAND

SET POINT

R-W ENERGIZED

FOR EXAMPLE, WITH SET POINT = 70°F
R-Y SWITCH WILL ENERGIZE AT 71.5°F
R-W SWITCH WILL ENERGIZE AT 68.5°F

71.5°F

SET POINT 70°F

68.5°F

stat which heats when the system is on or off. The heat
produced by the anticipator raises the internal bimetal tem­perature slightly faster than the surrounding room tempera­ture. The thermostat anticipates the need to shut off the
heating system sooner than it would if affected by room
temperature only. The T641 has fixed or voltage anticipation.

R-Y ENERGIZED

3° DEADBAND

R-W ENERGIZED

M2283

Installation

WHEN INSTALLING THIS PRODUCT…

1. Read these instructions carefully. Failure to follow

them could cause a hazardous condition.

2. Check the ratings given in the instructions and on the
product to make sure the product is suitable for your applica­tion.

3. Installer must be a trained experienced service techni­cian.

4. After installation is complete, check out product op­eration as provided in these instructions.

1 62-0101

J. H. • 9-92 • © Honeywell Inc. 1992 • Form Number 62-0101

CAUTION

1. Disconnect power supply to prevent electrical
shock or equipment damage.

2. To prevent interference with the thermostat link­age, keep wire length to a minimum and run
wires as close as possible to the subbase.

3. Do not overtighten thermostat captive mounting
screws, because damage to subbase threads can
result.

4. Do not short across coil terminals on relay. This
can burn out the thermostat heat anticipator.

IMPORTANT: An incorrectly leveled thermostat will cause

the temperature control to deviate from set point. It is
not a calibration problem.

LOCATION

Install the thermostat about 5 ft. [1.5 m] above the floor in

an area with good air circulation at average temperature.

Do not install the thermostat where it may be affected

by—

— drafts, or dead spots behind doors and in corners.
— hot or cold air from ducts.
— radiant heat from sun or appliances.
— concealed pipes and chimneys.
— unheated (uncooled) areas behind the thermostat,

such as an outside wall.

MOUNTING AND WIRING

IMPORTANT: The T641 is electrically compatible and

designed to operate with actuators that have seven
minute timings for a full stroke of 90° (angular). Using
with motors that have timings less than seven minutes
(90° full stroke) will result in unstable temperature
control.

The thermostat can be mounted on a horizontal outlet box
or directly on the wall. Choose the method that best fits your
installation.

In replacement applications, check the existing thermo­stat wires for cracked or frayed insulation. Replace any
wires in poor condition. All wiring must comply with local
codes and ordinances. The T641 utilizes a four-wire opera­tion, so an additional transformer common wire must be
present for correct wiring and operation. See Fig. 2 for
functional schematic of T641. Screw terminals are on the
back of the thermostat. See Fig. 3.

The part no. 221618 Cover Plate is available when replac­ing a competitive device that has left marks on the wall. See
Fig. 4.

1. Grasp the thermostat cover at the top and bottom with
one hand. Pull outward on bottom edge of the cover until it
snaps free of the thermostat base. Carefully remove and save
the packing material surrounding the mercury switches. See
Fig. 3.

2. Run wiring from the actuator (if necessary) to the
location. If the wiring is plastered into the wall, make a hole
next to the cable and loosen the wires so they can be pushed
back into the wall later. Thread wires through hole in packing
material saved in step 1. Connect the wires to the terminals on
the back of the thermostat. See Figs. 5 and 6.

3. Push the excess wire back through the hole and plug
any opening with packing material to prevent drafts that may
affect thermostat performance.

4. Through the thermostat two middle mounting holes,
use the screws provided to loosely secure the T641 to the wall
or outlet box.

5. Level the thermostat exactly using a spirit level or
plumb line. Tighten the two mounting screws at the middle
of the device. See Fig. 3.

Fig. 2—Functional equivalent anticipator circuit of T641.

SOLID STATE RELAY

HEAT

FALL

RISE

1

POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.

ONLY T641B HAS SYSTEM OVERRIDE SWITCH.

2

COOL

HEAT
ANTICIPATOR

COOL
ANTICIPATOR

N.O.

SOLID STATE RELAY

N.C.

ON

SYSTEM
OVERIDE

2

OFF

W

C

Y

2

X

R

DAMPER CONTROL

DRIVE
CLOSED
COIL

DRIVE
OPEN
COIL

SYSTEM OVERIDE RELAY

1

L1
(HOT)

L2

M6112