Installation manual

IM-487 Page 61
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 a manual 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 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
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
(OM138 or 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
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 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 (OM138 or 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
(OM138 or Om137).
Variable Inlet Vanes
Variable inlet vanes are installed on the supply and return
fans of VAV units. They are also installed on the return fans
of constant volume units that have direct building static pres-
sure control capability.
The inlet vane assemblies consist of airfoil type inlet funnels
with integral sets of lever-actuated radial vanes. Supply fans
have one assembly on each side of the fan. Return fans have one
assembly on the inlet side of the fan only. When they open, the
inlet vanes direct air in the direction of wheel rotation.
The vanes are able to rotate 90 degrees from full closed to
full open. A stop limits the amount of travel open and must
not be changed. Driving the vanes past this stop will cause
the vane hub cam linkage to disengage. The vanes will turn
independently of each other, and the hub assembly will have
to be overhauled. The moving parts of the vanes are perma-
nently lubricated. All threaded fasteners must be periodically
checked for tightness.
All fans with inlet vanes are provided without motor actua-
tors for field control installation, or may be shipped from the
factory with complete linkage and actuator assemblies. Field
installed linkages and actuators would be connected at Point
B as shown in Figure 56 on page 63, Figure 57 on page 64
and Figure 58 on page 64.
Minimum Vane Position
Operation with the inlet vanes closed completely may result
in fan pulsation and excessive vibration. In units equipped
with MicroTech II controls, the vanes are automatically
opened approximately 5 degrees by the control system prior
to fan start-up. Preset field installed linkages and actuators to
close to no less than 5 degrees open.