Product data

20
If the diversity within the space does not vary much, then
bypass can be installed as shown in the diagram below suc-
cessfully. Care must be taken to ensure that maximum by-
pass air at part load conditions does not exceed 25% of the
full load airflow.
The bypass junction must be installed with sufficient
duct length between it and the unit to allow complete mix-
ing of the bypass air an the return air. Without adequate
mixing, comfort control will be lost as the equipment will
be cycled off and equipment failures may result if left at the
extremes. When the HVAC equipment uses an econo-
mizer, a back-draft damper needs to be installed between
the connection of the bypass duct and the closet room re-
turn air grille.
Bypass controller considerations — Besides consider-
ing the minimum and maximum cfm’s required, as in the
system design guidelines, the following are a few system
concerns to consider when designing a bypass damper in a
VVT® system.
1. The maximum amount of air being bypassed should
match an equivalent amount of cfm of unconditioned
air based on the ability of the system to unload. For
example: If the equipment has two compressors and
one unloader, then the refrigeration portion of the
equipment is capable of unloading to about 25% of
unit capacity. Therefore the designer should make
sure the system will use 25% of the system cfm for
comfort conditioning and bypass 75% of the condi-
tioned air before energizing the compressors. This
will ensure that the system will have enough return air
mixing with the bypassed air to keep the compressor
running. Make sure that the compressor does not
cycle too frequently based on the leaving air tempera-
ture limits since this could lessen the reliability of the
compressors. The heating equipment will only be able
to unload 50%. The system may be designed to cycle
based on the leaving air temperature limits depending
on the rise designed in the heat exchanger and the
amount of outside air being brought in. Although this
is not efficient, the equipment reliability should not be
a problem.
2. Be sure to verify the pressure drop across the bypass
when in full bypass. The bypass should maintain the
system pressure without causing 100% of the air to
be bypassed. Tables are provided in the Comfort Sys-
tems Controls manual which will indicate the sound
and performance criteria of Carrier dampers. Calcu-
late bypass of 4500 cfm on a unit with a nominal air-
flow of 6000 cfm and external static pressure of
0.50 in. wg. The ZD-16 has two entries in the table
of 1475 and 1574. Three ZD-16 dampers in parallel
could be used for the job. The 16-in. damper will not
operate full open to produce these results, so it will be
necessary to balance the system and set the maximum
damper position to achieve these results in the field.
3. These airflows were tested at 0.50 in. wg and pro-
vided NC levels of 42 and 44 respectively. A noise
criteria of around 40 would be acceptable for a ducted
bypass with internally lined duct but probably not for
externally lined ducts since the noise could be trans-
mitted to the return registers. Duct design is impor-
tant to consider in bypass design since improper
noise levels can require the use of other dampers to
achieve the same bypassed cfm with reduced noise.
For the example above, two 35D, 16-in. dampers
can move the same air at a NC level of 29.5 Radi-
ated. These dampers can be ordered with sound
attenuators to lessen radiated noise also. Use the
damper test criteria to make the selection.
4. If designing a system with an economizer, be sure to
include power exhaust on the equipment to prevent
conditioned air from migrating out the return registers
into the space in a over pressure condition. This con-
dition can be identified since the doors will most likely
stand open. The same condition can occur if there is
a large exhaust system in the building for labs or
restrooms.
Controls (cont)
T
BC
ROOFTOP
UNIT
DO NOT INSTALL A SHORT-CIRCUIT
BYPASS HERE. IT WILL CAUSE
NUISANCE TRIPS WITH ZONE
CAPACITY INTERRUPTION
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
SIZE BYPASS DUCT TO HANDLE
100% OF BLOCK PEAK CFM AT
1000 FPM OR LESS
*
5-CONDUCTOR
SHIELDED
CABLE (24V)
STATIC PRESSURE
SENSOR AND BYPASS
DAMPER ACTUATOR
BYPASS
DAMPER
*
RETURN BACK DRAFT
DAMPER
LOCATE RETURN/BYPASS JUNCTION
A MINIMUM OF 15 FT. FROM
UNIT OUTLET
VVT BYPASS DESIGN
DUCTED RETURN LAYOUT FOR MANUFACTURING OFFICE