IM 178 / Page 43
External Time Clock
followed. When the circuit is closed, power is fed to DH1-1.
The MicroTech controller responds by placing the unit in the
occupied mode, overriding any set internal schedule.
For more information, please see the “Digital Inputs”
section of Bulletin No. IM 483, “MicroTech Applied Rooftop
Unit Controller.”
An external time clock can be used as an alternative to (or in
addition to) the MicroTech controller’s internal scheduling
function. The external timing mechanism is set up to open
and close the circuit between field terminals 101 and 102.
When the circuit is open, power is not supplied to digital in-
put D1 (terminal DH1-1) on the ADI board. This is the normal
condition in which the programmable internal schedule is
Smoke Detectors
The MicroTech controller responds by shutting the unit down.
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 (see Table 1).
The smoke detectors themselves must be manually reset
once they have tripped. Power must be cycled to the smoke
detector to reset.
Option smoke detectors can be located at the supply and
return openings. The wiring for these smoke detectors is
shown on any of the “Typical Main Control Circuit” schemat-
ics in the “Wiring Diagrams” section of this manual.
The sequence of operation for these detectors is as
follows: When the smoke is detected by either sensor, the
normally closed sensor contacts open. This removes power
from digital input D11 (terminal DH2-11) on the ADI board.
Freeze Protection
An optional freezestat is available on units with MicroTech
control that have hot 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 con-
troller will take different action, depending on whether the
fans are on or off. The freezestat is an auto reset type of
control; however, the controller alarm it causes is manual rest
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 controller will shut down the fans, close the
outdoor air dampers, open the heating valve, and set a 10-
minute timer. The MicroTech controller’s current alarm (menu
29) will be “Freeze Stat Fail.”
When the 10-minute timer expires, the controller begins
checking the freezestat again. If the freezestat is open, the
heating valve will close. If the freezestat closes again, the
heating valve will open, and the 10-minute timer will reset.
The unit will remain shut down until the “Freeze Stat Fail”
alarm is manually cleared. Refer to the operation manual
supplied with the unit for information clearing alarms (see
Table 1).
Fan off operation
If the freezestat detects a freezing condition while the fan is
off, the MicroTech controller will open the heating valve and
set a 10-minute timer. The MicroTech controller’s current
alarm (menu 29) will be “Freeze Stat Prob.”
When the 10-minute timer expires, the controller begins
checking the freezestat again. If the freezestat is open, the
heating valve will close. If the freezestat closes again, the
heating valve will open, and the 10-minute timer will reset.
When the freezestat opens again, the “Freeze Stat Prob”
alarm automatically clears. This feature protects the coil and
allows the system to start normally after a cold night.
Mixed Air Temperature Alarm
A mixed air temperature (MAT) sensor and an associated
“Low Airflow Alarm” are provided on VAV units with MicroTech
control and gas or electric heat. The MAT sensor is located
in the supply fan section of the unit at the supply air funnel.
Heat will be disabled whenever the airflow is detected to
be too low for safe heating operation. This condition is
indicated when the supply air temperature exceeds the
mixed air temperature by more than 60
°
F (16
°
C). In this case,
a “Low Airflow Alarm” is generated and heat will not be
reenabled until the alarm is manually cleared. Refer to the
operation manual supplied with the unit for information on
clearing alarms (see Table 1).
Duct High Pressure Limit
The duct high pressure limit control (DHL) is provided on all
VAV units, including the CAV-DTC unit that can be field con-
verted to VAV. The DHL protects the ductwork, the terminal
boxes, and the unit from overpressurization 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" w.c. (872 Pa). This setting
should be correct for most applications; however, it is adjust-
able. 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 D7 (terminal DH1-7)
on the ADI board will be de-energized. The MicroTech
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 (see Table 1).
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