Step 4 - Check Duct System; Duct Static Pressure Probe And Tubing  (Vav Only); Duct High-Static (Dhs) Limit Switch - Carrier Omnizone 50BV020-064 Installation, Start-Up, Service And Controls Operation And Troubleshooting

Step 4 — Check Duct System
All dual fan units are required to use a "pair of pants" configura-
tion as shown in Fig. 24. Refer to ASHRAE (American Society of
Heating, Refrigerating and Air-Conditioning Engineers) standards
for the duct connection to unit with 2 fans. An adequate straight
length of ducting from the unit should be allowed before elbows
are installed. If connecting an elbow directly to the fan outlet, a
minimum straight length of 3 fan diameters from the fan outlet is
recommended. Elbows should turn in the direction of fan rotation.
Abrupt turns will generate air turbulence, excessive noise, and
cause vibration, which can lead to component failure. Turning
vanes should be used in all short radius bends. Ensure that ducting
does not obstruct access to the unit for routine servicing.
A
Fig. 24 — Typical Fan Discharge Connections for
Multiple Fan Units
A supply air outlet collar and return air duct flange are provided
on all units to facilitate duct connections. Refer to dimensional
drawings (Fig. 7-23) for connection sizes and locations.
A flexible canvas duct connector is recommended on both supply
and return air sides of the units to be connected to the system
ductwork.
All metal ductwork should be adequately insulated to avoid heat
loss or gain and to prevent condensation from forming on the duct
walls. Uninsulated ductwork is not recommended, as the unit's
performance will be adversely affected.
Do not connect discharge ducts directly to the blower outlet(s).
The factory filter should be left in place on a free return system.
If the unit will be installed in a new installation, the duct system
should be designed in accordance with ASHRAE (American So-
ciety of Heating, Refrigeration and Air-Conditioning Engineers)
procedures for duct sizing. If the unit will be connected to an exist-
ing duct system, check that the existing duct system has the capac-
ity to handle the required airflow for the unit application at an
acceptable system static pressure. If the existing duct system is too
small, larger ductwork must be installed.
NOTE: A = 1-1/2 to 2-1/2 B
The duct system and diffusers should be sized to handle the design
airflow volumes quietly. To maximize sound attenuation of the
unit's blower(s), the supply and return air plenums should be insu-
lated for a length of at least 15 ft from the unit. Direct line of sight
from return air grilles into the unit's return should be avoided. If
return air is to be ducted to an equipment room, an elbow should
be installed within the equipment room. Running a return air drop
to near the floor of the room will aid in sound attenuation. Avoid
transmitting vibrations generated by the movement of air in the
ducting to the walls of the building. This is especially important
where ductwork penetrates walls. The maximum recommended
return air velocity is 1,000 fpm. Lower return air velocities will re-
sult in lower sound power levels. The use of round supply duct
plenums should be considered, as it will significantly reduce low
frequency sound at the equipment room. If rectangular supply ple-
nums are used, the aspect ratio of the duct should be kept as small
as possible (i.e., as close to square as possible). The large, flat sur-
face areas associated with large aspect ratio duct systems will
transmit sound to the space, and the potential for duct-generated
noise is increased. The maximum recommended supply air duct
velocity is 2,000 fpm.
DUCT STATIC PRESSURE PROBE AND TUBING 
(VAV ONLY)
On VAV systems, the duct static pressure sensor is factory-
supplied (ships inside control cabinet) and requires field
installation and field provided tubing and pressure pick-up port.
The pressure sensor low port should be left open to atmosphere.
The pressure sensor high port should be connected with tubing to
a pressure pick up port. The pressure pick up port should be in-
stalled as close to 2/3 of the way down the duct system in a
straight section of duct, away from any turning vanes, take offs, or
areas in the duct that could feature turbulence.
Install the duct static pressure probe with the tip facing into the air-
flow. See Fig. 25.
Airflow
Fig. 25 — Duct Static Pressure Probe
Use 1/4 in. OD approved polyethylene tubing for up to 50 ft
(3/8 in. OD for 50 to 100 ft) to connect the probe to the bulkhead
fitting mounted above the unit display panel. Carefully route the
tubing from the probe to this bulkhead fitting.
The static pressure control should be adjusted so that, at full air-
flow, all of the remote VAV terminal boxes receive the minimum
static pressure required plus any downstream resistance. Control
the system to the lowest static pressure set point that will satisfy
airflow requirements. Lower static pressure set points reduce total
required brake horsepower and reduce generated sound levels.
DUCT HIGH-STATIC (DHS) LIMIT SWITCH (VAV ONLY)
The duct high static limit switch is a field provided mechanical
safety that prevents duct over pressurization. The switch is option-
al and is field-provided.
27
Probe
Tubing