IM-738 Page 65
External Time Clock
An external time clock can be used as an alternative to (or in
addition to) the MicroTech II 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 input DI
(terminal DH1-1)on the ADI board. This is the normal condi-
tion in which the programmable internal schedule is followed.
When the circuit is closed, power is fed to DH1-1. The Micro-
Tech II 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 696, "MicroTech II Applied Rooftop Unit
Controller."
Smoke Detectors
Optional 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" wiring
schematics within the section “Wiring Diagrams” on page 54 .
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 digi-
tal input MCB BI 18 on the Main Control Board.
The MicroTech II 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 informa-
tion on clearing alarms (page 55, OM138 or page 58,
Om137).
The smoke detectors themselves must be manually reset once
they have tripped. Power must be cycled to the smoke detec-
tor to reset.
Freeze Protection
An optional freezestat is available on units with MicroTech II
control that have hot water or steam heating coils. The sens-
ing 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 II controller
will take different action, depending on whether the fans are
on or off. The freezestat is an auto reset type of control; how-
ever, 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 II controller will shut down the fans, close the
outdoor air dampers, open the heating valve, and set a 10-
minute timer. The MicroTech II controller's active alarm will
be "Freeze Stat Fault."
When the 10-minute timer expires, the controller begins
checking the freezestat again, If the freezestat is open, the heat-
ing 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 (page
55, OM138 or page 58, Om137).
Fan Off Operation
If the freezestat detects a freezing condition while the fan is
off, the MicroTech II controller will open the heating valve and
set a 10-minute timer. The MicroTech II controller's active
alarm will be "Freeze Problem."
When the 10-minute timer expires, the controller begins
checking the freezestat again. If the freezestat is open, the heat-
ing 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 II control and gas or electric heat. The MAT sen-
sor is located in the supply fan section of the unit at the sup-
ply 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 tem-
perature by more than 60°F (16°C). In this case, a "Low Air-
flow Alarm" is generated and heat will not be re-enabled
until the alarm is manually cleared. Refer to the operation
manual supplied with the unit for information on clearing
alarms (page 55, OM138 or page 58, Om137).
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 converted to VAV. The DHL protects the ductwork, 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" w.c. (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 will be de-energized. The MicroTech II
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
(page 55, OM138 or page 58, Om137).
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