Installation manual

IM 178 / Page 15
Installing Ductwork
For bottom discharge and return units not provided with a
McQuay roof curb, the installing contractor should make an
airtight connection by attaching field fabricated duct collars
to the bottom surface of either the roof curb’s duct flange or
the unit’s duct opening. Do not support the total weight of the
ductwork fromhe unit or these flanges. Refer to Figure 9.
Units with optional back return, side discharge, or front
discharge all have duct collars provided. For back return and
front discharge openings, the collars are exposed by remov-
ing unit shipping covers. For side discharge units, the duct
collar is exposed by removing the plenum section access
door and door gasketing.
Flexible connections should be used between the unit and
ductwork to avoid transmission of any potential vibration
from the unit to the structure.
Ductwork should be designed per ASHRAE and SMACNA
recommendations to minimize losses and sound transmission.
Where return air ducts are not required, it is recommended
that a sound absorbing T or L section be connected to the
unit return to reduce noise transmission to the occupied
space.
Ductwork exposed to outdoor conditions must be built in
accordance with ASHRAE and SMACNA recommendations
and local building codes.
NOTICE
On units with side discharge, access to plenum mounted
components becomes difficult once ductwork is in-
stalled.
Installer must provide access in the ductwork for plenum
mounted controls.
Figure 9. Installing ductwork
Installing Duct Static Pressure Sensor Taps
For all VAV units, duct static pressure taps must be field
installed and connected to the pressure sensors in the unit.
Sensor SPS1 is standard; additional sensor SPS2 is optional.
These sensors are located at the back of the unit’s discharge
plenum (see “Control Panel Locations” in the “Unit Descrip-
tion” section of this manual).
The duct static pressure sensing tap must be carefully
located and installed. Improper location or installation of the
sensing tap will cause unsatisfactory operation of the entire
variable air volume system. Following are pressure tap loca-
tion and installation recommendations. The installation must
comply with local code requirements.
Fragile sensor fittings. May damage pressure sensor.
If tubing must be removed from a pressure sensor fitting,
use care. Do not wrench the tubing back and forth to
remove or the fitting may break off.
1. Install a tee fitting with a leak-tight removable cap in each
tube near the sensor. This will facilitate connecting a
manometer or pressure gauge if testing is required.
2. Use different colored tubing for the duct pressure (HI) and
reference pressure (LO) taps, or tag the tubes.
3. Locate the duct pressure (HI) tap near the end of a long
duct to ensure that all terminal box takeoffs along the run
will have adequate static pressure.
4. Locate the duct tap in a nonturbulent flow area of the duct.
Keep it several duct diameters away from takeoff points,
bends, neckdowns, attenuators, vanes, or other irregu-
larities.
5. Use a static pressure top (Dwyer A302 or equivalent) or
the bare end of the plastic tubing for the duct tap. (If the
duct is lined inside, use a static pressure tip device.)
6. Install the duct tap so that it senses only static pressure
(not velocity pressure). If a bare tube end is used it must
be smooth, square (not cut an angle), and perpendicular
to the airstream (see Figure 10).
7. Locate the reference pressure (LO) tap somewhere near
the duct pressure tap within the building (see Figure 10).
If the reference tap is not connected to the sensor,
unsatisfactory operation will result.
8. Route the sensing tubes to the sensor(s) mounted on the
9.8
(249)
4.6
(117)
Unit Duct
Opening
Unit Base
Flexible
Connector
Roof Curb
Duct Flange
in Roof Curb
Ductwork
!
CAUTION