86 McQuay IM 987
Unit Options
Factory installed smoke detectors have similar wiring and the
control sequence is as follows:
When smoke is detected by either sensor, the normally closed
sensor contacts open. This removes power from binary input
B18 on the main control board.
The Microtech III controller responds by shutting down the
unit. The controller is placed in the Alarm Off state and cannot
be restarted until the alarm is manually cleared. Refer to the
operation manual supplied with the unit for information on
clearing alarms.
The smoke detectors must be reset manually once they have
been tripped. Power must be cycled to the smoke detector to
reset.
Emergency Shutdown
The terminals 105 & 106 on TB2 can be used for any field
supplied component that requires a unit emergency shutdown.
When these terminals are used, the factory installed jumper
must be removed.
Freeze Protection
An optional freezestat is available on units with MicroTech III
control that have hot water, chilled water, or steam heating
coils. The sensing element is located on the downstream side
of the heating coil in the heating section of the unit. If the
freezestat detects a freezing condition and closes, the
MicroTech III controller takes different actions, depending on
whether the fans are on or off. The freezestat is an auto reset
type of control; however, the controller alarm that it causes is
manually reset if the fan is on and auto reset if the fan is off.
Fan On Operation
If the freezestat detects a freezing condition while the fan is
on, the MicroTech III controller shuts down the fans, closes the
outdoor air dampers, opens the heating valve, and sets a 10-
minute timer. The MicroTech III controller’s active alarm is
“Freeze Fault.”
When the 10-minute timer expires, the controller begins
checking the freezestat again. If the freezestat is open, the
heating valve closes. If the freezestat closes again, the heating
valve opens, and the 10-minute timer resets.
The unit remains shut down until the “Freeze Fail” alarm is
manually cleared. Refer to the operation manual supplied with
the unit for information on clearing alarms (OM138 or
OM137).
Fan Off Operation
If the freezestat detects a freezing condition while the fan is
off, the MicroTech III controller opens the heating valve and
sets a 10-minute timer. The MicroTech III controller’s active
alarm is “Freeze Problem.”
When the 10-minute timer expires, the controller begins
checking the freezestat again. If the freezestat is open, the
heating valve closes. If the freezestat closes again, the heating
valve opens, and the 10-minute timer resets.
When the freezestat opens again, the “Freeze Prob” alarm
automatically clears. This feature protects the coil and allows
the system to start normally after a cold night.
External Time Clock or Tenant Override
There are several methods of switching the rooftop unit
between occupied and unoccupied operation. It can be done by
the controller internal schedule, a network schedule, an
external time clock, or a tenant override switch.
If the internal schedule or a network schedule is used, field
wiring is not required.
An external time clock or a tenant override switch can be used
by installing a set of dry contacts across terminals 101 and 102
on the field terminal block (TB2). When these contacts close,
24 V (ac) is applied to binary input MCB-DI3, overriding any
internal or network schedule and placing the unit into occupied
operation (provided the unit is not manually disabled). When
the contacts open (24 V (ac) is removed from MCB-DI3) the
unit acts according to the controller internal time schedule or a
network schedule. Refer to the unit wiring diagrams for
specific wiring termination details.
Figure 96: External Time Clock or Tenant Schematic
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