Product data

To retrofit an application with Carrier Comfort System
controls, the unit controller is replaced with a CHR-06 or
CHR-06W relay pack. The control relay pack is wired to
the terminal strip of the unit and provides cooling, heating,
fan, reversing valve,or economizerrelay controlfor theunit.
A monitor thermostat must be wired to each relay pack.
For retrofitand plannedrenovation applications using ex-
isting zoning control damper actuators, or adding new zon-
ing control actuatorsand dampers,the properactuator must
be selected. For each damper actuator upgraded or added
to the system, a corresponding zone controller must be in-
stalled to control the actuator.
Replacement actuators are available in 3 configurations:
ZD/RD, MA, and HTA. The ZD/RD actuators come fac-
tory installed on round (6, 8, 10, 12, 14, 16-in.) or rec-
tangular (8-in.by 10, 14,18, or 24-in.) dampers. Complete
ZD/RD dampers withactuators aretypically usedin planned
renovations where ducting and new zoning capabilities are
being added. The MA and HTA actuators are used on ex-
isting dampers to upgrade the actuator.
Each actuator is designed to accept a
1
⁄ 2 -in. diameter
damper shaft. The zone controller can be configured to op-
erate the damper in either clockwise or counterclockwise
directions, with the specific degree of travel.
DAMPER ACTUATOR CHARACTERISTICS
ACTUATOR
TORQUE RATING
(in. lb)
DEGREE OF
ROTATION
ZD/RD 18 45
MA-08 18 60
HTA-02 80 60
HTA-03 45 90
Typicalwiring requirementsare: 7-conductorwire fromunit
to unit controller relay pack; 3-conductor wire with shield
between thermostats and 5-conductor shielded wire from
the monitor thermostat or zone controller to the damper.
Refer to the Accessories section for information on the
sensors available to maximize the operation of the comfort
system.
In addition to sensors, the following can be added to up-
grade an application: full capability for communication to
provide both local and remote access; pressure indepen-
dent operation for specific zone airflow requirements; and
zone supplemental heat during periods of central unit cool-
ing. These topics are covered in detail in other parts of the
Application data section.
Selecting a bypass system for VVTt Comfort
Systems
The purpose of the bypass is to account for fluctuations in
the supply air pressure caused by the zone dampers modu-
lating to satisfy individual set points. The bypass system al-
lows a constant volume HVAC unit to supply variable vol-
umes of air to the system. The system bypasses air from
the supply side of the unit to the return side of the unit us-
ing either a plenum or ducted return.
The components needed to build a bypass for a VVT sys-
tem are: a static pressure pickup, a static pressure sensor, a
bypass damper (with actuator), and a bypass controller.
The static pressure pickup delivers the system static pres-
sure to the static pressure sensor. The damper modulates
open or closed based on maintaining a pre-configured static
pressure set point. The configuration of the bypass system
parameters is done at the bypass controller. During system
mode changeover, the bypass damper will remain open un-
til the configured supply-air temperature isreached, thereby
averting the sending of undesirable conditioned air to call-
ing zones.
The typical environment of the bypass system is 1 in. or
less of external static pressure. For typical rooftop systems,
the standard VVT damper (ZD/RD) should be used. For
conditions which require a larger static pressure, a 35D VAV
box and a high torque actuator (HTA) should be selected.
Determining the proper size for the bypass damper is
critical for the operation of the VVT system. If the damp-
er selected is too large, it may have to modulate more
than necessary to react to system pressure changes. The
ability of the system to stay within a pressure range is
compromised.
When the damper is undersized, the capability of the
damper to control the pressure may be compromised due
to the inability to bypass enough air volume. An undersized
damper also creates higher airflow velocities which add to
the noise generated by the system.
Care should be taken in the layout of the system to pre-
vent undesirable ‘‘feedback’’ into the zones. Return grilles
in ceilings should be located to minimize the possible back-
flow from a plenum type bypass system. The ceiling return
should not be located too close to the plenum bypass ducts
to prevent hot or cold bypass air from entering the occu-
pant space through the plenum return.
On jobs using a ducted return air system, the use of an
economizer could inadvertently cause the unit bypass air to
be sent to the zones, since the return dampers of the HVAC
unit may be closed. If the return dampers are closed, the
airflow can go backwards through the return duct and into
the conditioned spaces through the return grilles. The re-
turn duct should be sized properly to handle bypassed air
from the unit. A backdraft damper may need to be located
between the connection of the bypass duct and the closest
roomreturn ifbypassedair isentering the conditionedspace.
Care must be taken when sizing the system to prevent
undesirable impact on unit temperature limits. In situations
where most of the unit discharge air is being bypassed into
the return system (when most of the zone dampers would
be closed), the temperature of the system return air is go-
ing to be close to the temperature discharge air. When the
return air is drawn back through the unit to be heated or
cooled, the resulting discharge air temperature can be close
to or exceed the safe temperature limits of the unit. The
safety limit switch will shut down the unit while zone space
temperatures remain unsatisfied. Size the system so cycling
of the return air does not result in unsafe temperatures.
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