88
McQuay IM 987
Unit Options
Duct High Pressure Limit
The duct high pressure limit control (DHL) is provided on all
VAV units. The DHL protects the duct work, the terminal
boxes, and the unit from over pressurization, which could be
caused by, for example, tripped fire dampers or control failure.
The DHL control is factory set to open when the discharge
plenum pressure rises to 3.5" wc (872 Pa). This setting should
be correct for most applications; however, it is adjustable.
Removing the front cover of the device reveals a scale
showing the current setting. Turning the adjustment screw
(located on the bottom of the device) adjusts the setting up or
down.
If the DHL switch opens, digital input MCB BI 14 on the Main
Control Board de-energizes. The MicroTech III controller then
shuts down the unit and enters the Off-Alarm state. The alarm
must be manually cleared before the unit can start again. Refer
to the operation manual supplied with your unit for more
information on clearing alarms (OM 920).
Variable Frequency Drive Operation
Refer to the vendor instructions supplied with the unit.
Convenience Receptacle/Section Lights
A Ground Fault Circuit Interrupter (GFCI) convenience
receptacle is provided in the main control box on all units. One
of the following is required:
1
Connect a separate field-supplied 115 V power wiring
circuit to the 115V field terminal block TB7, located in
the main control box.
2
Select the factory powered outlet option at time of
purchase.
Optional lights are available for certain sections in the unit.
Each light includes a switch and convenience receptacle and is
powered by the external 115V power supply connected to
TB7.
DesignFlow™ Outdoor Air Damper Option
DesignFlow™ airflow measurement stations are located inside
the louvered outdoor air intake doors between the intake
louver and outside air dampers. Essentially, they consist of a
vane that is repositioned by airflow, the amount of rotation
indicating the amount of airflow. They are calibrated precisely
at the factory and no further calibration is required. However, a
leveling adjustment is required in the field so that the
DesignFlow unit is in the same orientation as when it was
factory calibrated. See “DesignFlow Station Startup” below.
The rotational position of the DesignFlow unit vane is
translated into CFM by the microprocessor in the MicroTech
III control system. The position of the vane is determined by
two things—the force of the airflow impacting the vane and
the gravitational effect on the vane. Gravity is the only factor
at the lower CFM end of the range. On a correctly leveled unit,
this gravitational effect will be the same as when the unit was
calibrated in the factory.
Accurately leveling a station involves applying a precise
mechanical force against the vane. This force should cause the
vane to move to a specific position if the DesignFlow unit is
correctly leveled.
DesignFlow Station Startup
•
Before initial startup of the rooftop unit, carry out the
following procedure on both the right-hand (control panel
side) and left-hand (side opposite the control panel)
DesignFlow station vanes (see
Figure 98
).
Note:
This procedure is much easier to carry out with two
people—one making the mechanical adjustments and
the other viewing and recording readings on the
MicroTech III control panel.
1
Verify that power is supplied to the unit’s MicroTech III
control system. The DesignFlow startup procedure
cannot be completed without use of the MicroTech III
controls.
2
Unlock and open the louvered outdoor air intake door on
the side of the unit (see
Figure 98
).
3
The swinging vane on the measurement station is locked
in place for shipment. Unlock it by removing the two
shipping screws. One is located one inch from the top of
the vane and the other one inch from the bottom of the
vane. Both are about eight inches in from the outer edge
of the vane.
4
Examine the station for shipping damage. Manually
rotate the vane and verify that it does not rub against
anything.
Figure 98: DesignFlow Station
5
Manually hold the vane closed against the mechanical
stop at the top of the assembly. Then, read the current
vane leveling position on the MicroTech III keypad/
display.
Do this by viewing the
LH Lvl Pos=
or
RH Lvl Pos=
parameter in the DesignFlow setup menu. The
LH Lvl
Pos=
parameter indicates the current position of the vane
for the left-hand DesignFlow station (side opposite the
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