1
62-0101
T641A,B,C
Floating Control Thermostats
Installation instructions for the trained service technician.
J. H. • 9-92 • © Honeywell Inc. 1992 • Form Number 62-0101
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 require-
ments. 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 off-
peak zones. This transfer from low-load to high-load zones
occurs only in true VAV systems.
Pressure dependent systems do not incorporate an indi-
vidual 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 thermo-
stat 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.
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 applica-
tion.
3. Installer must be a trained experienced service techni-
cian.
4. After installation is complete, check out product op-
eration as provided in these instructions.
R-Y ENERGIZED
R-W ENERGIZED
DEADBAND
SET POINT
R-Y ENERGIZED
R-W ENERGIZED
3° DEADBAND
SET POINT 70°F
M2283
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
68.5°F
stat which heats when the system is on or off. The heat
produced by the anticipator raises the internal bimetal tem-
perature slightly faster than the surrounding room tempera-
ture. 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.
