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A
base
OPERATION
MODE SELECTION
The base can operate up to 4 distinct modes; heating, economizer, ventilation and cooling. There is a delay
between changing from one mode to another to prevent cycling of the equipment. Mode selection is
determined by the demand call and the ambient temperature. The base must have an ambient sensor
installed either within the appliance, or remotely located to measure the ambient temperature.
The first (left) seven segment LED describes the current operating mode, as:
1 = Ventilation
2 = Economizer
3 = Heating
4 = Cooling
The two seven segment LED on the right side show the current alarm reference number. Refer to the alarm
section for additional information.
OCCUPIED / UNOCCUPIED
During night, or unoccupied, operation, when the modulating room thermostat is calling for more than 60%
heating demand, the heater will operate at the maximum discharge of 120°F until the room thermostat
demand falls below 20%.
FAN CONTROL
VARIABLE AIR VOLUME
On a blower start signal, and before the blower is started, the VFD feedback signal is checked for a false
signal. If a false feedback signal is detected the unit will go into alarm and not start. If the signal remains
above the VFD minimum speed (%) for 30 seconds, the Air Proving fault alarm is set. After the blower is
started, the feedback signal must rise above the VFD minimum speed (%). If it does not, after 30 seconds the
Low Airflow alarm is set and the equipment shuts down.
VFD minimum speed should not be set to 0%. The design minimum is typically 35%, or 3.5Vdc. Confirm with
the unit function what the specific design minimum is. The feedback signal should not be scaled, and must
send a 3.5 to 10.0Vdc signal to the base. Confirm the feedback signal generates a full signal range to the
base. Adjust the output range to compensate if unable to meet 10.0Vdc minimum. The feedback output on
the ACH-580 drive is typically set to 10.6Vdc at maximum to ensure 10.0Vdc at the base. Confirm the
feedback display reads 100% on the computer or the keypad display.
AIR PROVING SWITCH
On a blower start signal the air proving switch must be in the open position. If a closed position is detected, the
fan is not allowed to start and a 30 second alarm (Shorted Air Proving) timer is initiated. The alarm will
automatically clear if the switch opens within 30 seconds.
If the flow proving switch is open, the blower can start normally. After the blower starts, the base needs to see
13
IOM74
November 2020 R105
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