CONTROLS
Control of the classroom unit is achieved through an external device,
which is generally termed “thermostat”. The complexity of the control
is a wide range from a simple mercury switch to a computerized
system of information monitoring room conditions to process its
control.
Internal Controls and Their Function
Temperature -
The economizer board monitors mixed air
temperature via a mixed air temperature sensor which controls the
damper. If the mixed air temperature drops below 59°F (15°C) or
50°F (10°C) during cool cycle, the damper will close to minimum
position (pot adjustable). This is a low mixed air ambient temperature
protection. When the mixed air temperature rises above 59°F (15°C)
or 50°F (10°C) during cool cycle the damper returns to normal
position (pot adjustable). This process may be repeated, ensuring
that your classroom unit will heat the room space in cases where
there is excessive building heat loss or extreme outdoor
temperatures while still providing maximum fresh air ventilation.
Outside air temperature is also monitored, see “Compressor
Protection”.
Fan Motors Thermal Overload Protection -
All fan motors have
internal thermal overload protection. The motor will automatically
restart when it has cooled down.
Fuses
- Inside the main electrical box there are line voltage fuses
protecting all internal components against breakdown, wiring shorts
etc,. These do not reset automatically, consult a service technician to
replace these fuses.
Electric Heat
- All electric resistance elements units have two levels
of protection. A high temperature thermal cut-out disc which opens at
150°F and resets automatically, and a line voltage fuse at the
element which opens at 219°F and will not automatically reset,
consult a service technician to replace these fuses.
Hydronic Coil -
There is one temperature disc sensing the air
temperature entering the hydronic coil. A temperature of 37°F will
open either circuit. A low entering temperature will close the damper
to all outside air, remove power to all fans and open the hot water
valve. The disc automatically resets when the temperature rises
above 59°F.
GENERAL CLASSROOM UNIT SEQUENCE OF OPERATIONS
The following is a suggested sequence of operations, which is
determined by the control thermostat. All classroom unit supplied by
Change'Air, arrive with a standard control terminal strip to interface
with a thermostat. Consultation with the design engineer will
determine the exact sequence required for your application, which
may vary from the example given.
Occupied Mode
Unit operation is energized by an occupied signal and provides fresh
air as per ASHRAE code 62-89. While occupied, the supply fan runs
continuously. Heat and Cool functions would cycle as needed with
the high speed supply fan and occupied set points from the
thermostat. Temperature protections move the fresh air damper to a
minimum position when the mixed air temperature falls below 50°F.
Unoccupied Mode
Fresh air damper is disabled. Unit will operate on Heat/Cool function,
cycling the high speed fan as required to meet unoccupied set points
determined by the thermostat.
ATTENTION:
Change’Air is not responsible for injury to untrained or unauthorized personnel who
attempt to do repairs based on the trouble shooting procedures outlined in this section.
Cool
When cooling is required, an economizer circuit will process the
signal. This is a multi-function board, controlling the cool signal to the
compressor (optional) and damper positioning. On models with
mechanical air conditioning, it will lock out the mechanical cool below
60°F outside temperature and introduce 80% fresh air to achieve
cooling. On models without mechanical air conditioning, free air-
cooling is the only function.
General Classroom Unit Trouble Shooting
ATTENTION: Change’Air is not responsible for injury to
untrained or unauthorized personnel who attempt to do repairs
based on the trouble shooting procedures outlined in this
section.
1. Problem - Unit Blows Fuses
a) Incorrect fuse; check fuse size.
b) Short circuit or incorrect unit wiring line voltage; check wiring (see
diagram)
c) Transformer short-circuited to ground.
d) Incorrect low voltage wiring; check wiring (see diagram)
2. Problem - Supply Fan Not Running
a) Check for 24VAC at terminal strip. Between G & X
b) Check for “supply VAC” at fan relay.
c) Check for “supply VAC” across fan motor.
d) Test fan capacitor.
3. Problem - Excessive Noise
a) Blower wheel rubbing- Check for proper positioning and
clearance of blower wheel. Adjust wheel as necessary.
b) Loosen blower wheel - Ensure wheel is properly tightened on
motor shaft.
c) Blower wheel out of balance - Replace blower
d) Motor mounting - Motor mounts may have come loose.
e) Motor hum:
i. Check capacitor in motor circuit.
ii.Check for obstruction of wheel.
iii. Check that motor speed control (if equipped) is functioning.
4. Problem - Damper Not Opening or Working Improperly
a) Does unit require “occupied signal” to enable actuator? Check
for any open “safety circuits”
b) Ensure damper circuitry is enabled; check for 24vac at actuator,
check for dc signal at actuator. If signals are present and
actuator not functioning replace actuator.
c) Determine if damper position is controlled from an Air Quality
device. If an Air Quality device is present test device output for
DC voltage and actuator for corresponding DC voltage “during
occupied state”
d) Without an Air Quality device an internal potentiometer located on
the economizer board (marked with an “N”) controls the damper
position. In an occupied state test the 0-10 VDC to the actuator,
adjusting the potentiometer as necessary.
Cooling
1. Problem - Temperature drop between mixed air and supply is
less than 18°F. If it is more than this, the unit is operating
properly.
14
Owner’s Manual
