IM 738-2
91
Unit Options:
Freeze Protection
Freeze Protection
An optional freezestat is available on units with MicroTech II
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 II 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 II controller shuts down the fans, closes the
outdoor air dampers, opens the heating valve, and sets a
10-minute timer. The MicroTech II 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 (OM 138 or
OM 137).
Fan Off Operation
If the freezestat detects a freezing condition while the fan is
off, the MicroTech II controller opens the heating valve and
sets a 10-minute timer. The MicroTech II 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.
Entering Fan Temperature Sensor
The entering fan temperature (EFT) sensor and an associated
“Lo Airflow Problem” alarm are provided on VAV units with
MicroTech II control and gas or electric heat. The EFT sensor
is located in the supply fan section of the unit at the supply air
funnel.
Heat is 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).
Note:
This value is not always 60°F. It depends on whether the
unit is gas or electric heat and on the burner/baffling
arrangement on gas heat units.
In this case, a “Lo Airflow Problem” alarm is generated and
heat is not enabled until the alarm is manually cleared. Refer to
the operation manual supplied with the unit for information
clearing alarms (OM 138 or OM 137).
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" 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 (OM 138 or
OM 137).
Summary of Contents for RDT-135c
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