Honeywell T641A Installation Instructions Manual

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.
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-
Floating Control Thermostats
Installation instructions for the trained service technician.
Application
Operation
Fig. 1—T641 floating control.
SET POINT
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 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.
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.
J. H. • 9-92 • © Honeywell Inc. 1992 • Form Number 62-0101 1
T641A,B,C
R-Y ENERGIZED
DEADBAND
R-W ENERGIZED
R-Y ENERGIZED
3° DEADBAND
R-W ENERGIZED
Installation
M2283
62-0101