Product data

19
VVT® application guidelines (50HJQ014,016)
General — The VVT system is a zoning system that
allows a single-zone heating/cooling system to operate as
a multiple-zone system. For this system to function correct-
ly and not adversely affect the reliability of the heating/
cooling system, the information provided here must be
followed.
There are two parts of the VVT system installation that
are important for successful operation: the bypass damper
and the HVAC (Heating, Ventilation, and Air-Conditioning)
equipment configuration settings. The following informa-
tion will provide important details for these two items as
well as specific instructions for the 50HJQ014,016 roof-
top units. This information is intended as a supplement to
the Installation Instructions shipped with the equipment.
Bypass damper — The bypass damper and controller
serve to accomplish two functions in the VVT system.
First, the bypass damper keeps supply duct pressure and
velocity down at partial load. This permits the VVT system
to utilize standard low-velocity, low-pressure supply duck-
weeds, duct insulation, and duct sealing methods. Without
the bypass damper, the restriction of the zone dampers
that are closing at partial loads would cause duct static
pressure to climb, adding stress to all supply duct materials.
Accompanying velocity and noise would create problems
in the conditioned space.
Second, when the bypass damper is set up correctly, it
ensures that the airflow through the HVAC equipment re-
mains above the minimum required levels. (See capacity
ratings for minimum heating cfm required for proper opera-
tion of the rooftop heating system. These minimum heating
cfm levels must be maintained during all phases of heating
operation. The heating cfm values take precedence over
listed minimum cooling cfm values.) This permits the use of
constant volume rooftop units on VVT applications. With-
out the bypass damper, the restriction of the zone dampers
that are closing at partial loads will reduce the airflow
through the unit to below the minimum levels.
The installer is responsible for ensuring that the bypass
controller is configured and the system is balanced so that
the minimum airflow is higher than 300 cfm/ton (40 L/s
per kW). To accomplish proper configuration set the Sys-
tem Setup Enable Function (Category 5, option 7) to ON.
This will close the bypass damper and force all of the zone
dampers to open. The air balance should be completed
after the zones minimum airflow has been set, verifying the
total airflow through the equipment is above the minimum
airflow level.
VVT bypass design layout — Particular attention must
be given to bypass duct layout. It has been a source of VVT
system operating problems when neglected. Proper layout
consists of the five factors:
1. Proper location of the bypass
2. Adequately sizing the bypass
3. Avoiding short-circuiting of supply and return air
4. Proper location of the static pickup
5. Facilitating the mixing of bypass air with return air
The recommended location of the bypass damper is for
it to duct the bypass air either into the ceiling plenum or
into unoccupied spaces. See figure below for a typical in-
stallation. This method of installation prevents interference
with intended fresh air intake through the unit and econo-
mizer operation.
It is highly recommended that the bypass damper is not
ducted between the rooftop unit’s supply duct and the re-
turn duct as shown in the diagram below. This type of in-
stallation may result in the return duct becoming pressur-
ized causing many problems such as:
• Preventing intended fresh intake for ventilation from
entering the unit.
• Preventing proper operation of the economizer and
causing mechanical cooling to energize needlessly.
• Preventing proper mixing of bypass air and return air
resulting in low return temperatures in cooling, and
high return temperature in heating.
ROOFTOP
UNIT
LOCATE PICKUP IN AREA OF LEAST TURBULENCE,
5 DUCT DIAMETERS DOWNSTREAM OF BYPASS
TAKEOFF BUT UPSTREAM OF FIRST ZONE TAKEOFF
OR MAJOR JUNCTION
STATIC
PRESSURE
TUBING
STATIC
PRESSURE
PICKUP
5-CONDUCTOR
SHIELDED
CABLE (24V)
T
BC
STATIC PRESSURE SENSOR
AND BYPASS DAMPER ACTUATOR
BYPASS DAMPER WITH SHORT
DUCT - SUPPORT PROPERLY
AIM BYPASS DAMPER DISCHARGE
AWAY FROM RETURN INLET
AVOID RETURN GRILLES IN
CLOSE PROXIMITY TO BYPASS
DISCHARGE AIR PATH
MONITOR
THERMOSTAT
BYPASS
CONTROLLER
MOUNT BYPASS CONTROLLER
ON THE WALL NEXT TO THE
MONITOR THERMOSTAT
VVT BYPASS DESIGN
CEILING PLENUM LAYOUT FOR MANUFACTURING OFFICE