Honeywell VVT, VVT-HC User Manual

VVT, VVT-HC (T641A,C)

Floating Control Thermostats

Installation Instructions for the Trained Service Technician.

Application

The VVT and VVT-HC Thermostats provide spdt outputs
to control floating actuators in applications such as variable
air volume (VAV) terminal units.

—VVT (T641A): standard spdt floating control

thermostat.

—VVT-HC (T641C): provides manual heat/cool

changeover switch.

Operation

FLOATING CONTROL

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

On a change in temperature, the thermostat 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 thermostat. The actuator
is said to float between the limits of the thermostat to satisfy
various load requirements.

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. Because 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 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 dampers, which are typically
controlled by the VVT (T641A) or VVT-HC (T641C) 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 thermostat
that 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 VVT
(T641A) or VVT-HC (T641C) have fixed or voltage antici­pation.

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.

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.

J. H. • 7-92 • © Honeywell Inc. 1992 • Form Number 69-0726

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 VVT (T641A) and VVT-HC (T641C)

are 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 thermostat utilizes a four-wire opera­tion, so an additional transformer common wire must be
present for correct wiring and operation. See Fig. 1 for
functional schematic of thermostat. Screw terminals are
located on the back of the thermostat. See Fig. 2.

The Honeywell part no. 221618 Cover Plate is available
when replacing a competitive device that has left marks on
the wall. See Fig. 3.

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 Fig. 4.

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 thermostat to
the wall or outlet box.

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

IMPORTANT: An incorrectly leveled thermostat will cause

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

6. Install two screws in top mounting holes and tighten.

7. Replace the thermostat cover.

WARNING

!

Because the cooling anticipator operates con­tinuously in the floating band, the VVT (T641A)
and VVT-HC (T641C) Thermostats incorporate a
factory offset that calibrates the thermostat. The
offset causes the thermostat to appear to be switch­ing 10.5°F lower when not powered compared to
actual operation. The cooling anticipator is needed
to provide proper thermostat cycling by matching
the current draw of the damper actuator. To
ensure proper thermostat calibration and operation,
the C terminal must be used.

Fig. 1—Functional equivalent anticipator circuit of thermostat.

SOLID STATE RELAY

HEAT

FALL

RISE

1

HEAT
ANTICIPATOR

COOL
ANTICIPATOR

COOL

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

N.O.

SOLID STATE RELAY

N.C.

2

W

C

Y

R

DAMPER CONTROL

DRIVE
CLOSED
COIL

DRIVE
OPEN
COIL

1

L1
(HOT)

L2

M6112