1) Product scope

  • Split-type cassette indoor units (approx. 18k–24k BTU/h class).

  • Focus areas: functions, timers, fan control, drain/defrost logic, self-diagnosis, thermistors, and troubleshooting.

2) Key operating functions

Cold-draft prevention (heating): on heat start, the indoor fan runs super-low until the indoor coil warms, then steps up.
Three-minute delay: compressor is inhibited after power-up and following stops to protect the system.
Test run mode: bypasses thermostatic control for commissioning; never leave the unit in test for normal operation.
Weekly/ON/OFF timers: OFF timer stops at a set time; ON timer starts at a set time; Weekly timer supports two ON/OFF windows per day and a one-day “Day Off” override.

3) Fan control logic

AUTO (cooling): fan speed steps between LOW/MED/HIGH based on room–setpoint delta.
DRY: prioritises dehumidification; indoor fan runs LOW with coordinated starts/stops around compressor cycling.
HEATING: fan advances to the next speed step when the indoor coil rises to a defined temperature and backs off if it cools; two-minute minimum hold between step changes.
Manual (LOW/MED/HIGH): fixed speed selection.

4) Drain pump operation

  • Pump starts with the compressor.

  • Continues for ~3 minutes after compressor stops to clear the pan.

  • Float-switch trip stops compressor and fans; pump continues briefly.

  • If the float remains high for several minutes, the controller issues a drain fault indication.

5) Defrost (heating)

  • Defrost eligibility is based on cumulative heating run-time and low outdoor coil temperature.

  • During defrost, the outdoor fan stops and control follows a fixed sequence; completion requires coil warm-up and/or maximum defrost time expiry.

  • A short “mask” period prevents nuisance protection trips during transitions.

  • After defrost, the 4-way valve and compressor are sequenced back to heating, then normal control resumes.

6) Indoor coil anti-icing (cooling/dry)

  • If the indoor coil temperature drops near the ice threshold, outdoor fan is reduced; if the coil stays too cold, the compressor is stopped.

  • Recovery occurs once the coil warms above the reset temperature.

7) Self-diagnosis (typical wall remote)

  • Error display can be invoked from the controller; codes cover communication, room/coil/discharge/outdoor thermistors, outdoor pressure/temperature protections, indoor fan, model/configuration, and EEPROM/controller faults.

  • Always clear latched errors by isolating power for at least 3 minutes after rectification.

8) Thermistors — quick reference use

  • Room thermistor: ~10 kΩ at 25 °C (NTC).

  • Indoor coil thermistor: higher resistance at low temperature; used for cold-draft prevention and anti-ice control.

  • Outdoor coil & ambient thermistors: govern defrost; check against standard NTC tables.

  • Discharge thermistor: protects against high discharge temperature.
    Bench test: disconnect, stabilise at a known ambient, measure resistance, and compare to an NTC chart. Allow ±10% field tolerance.

9) Troubleshooting workflow

No operation

  1. Confirm supply, fuse, and low-voltage rails (5/12 V).

  2. Check reset circuit and microcontroller oscillator rails.

  3. Verify remote input path and LED/indicator drive.

Erratic display/controls

  • Inspect remote wiring and DIP/configuration; confirm key inputs switch correctly.

Indoor fan issues

  • If the fan runs when stopped: solid-state relay/drive short.

  • If the fan will not run or only LOW works: check drive outputs and relays; verify microcontroller outputs for each step.

Poor cooling/heating

  • Indoor/outdoor airflow (filters, coils, fans).

  • Refrigerant side (charge, restrictions); look for frosting patterns.

  • Confirm coil temperatures; watch for anti-ice/overheat interventions limiting capacity.

Drain faults

  • Verify pump runs with compressor and post-run.

  • Check float switch continuity, pan level, and hose gradient/obstructions.

  • If float remains high, isolate power, clear water, and test pump on bench.

Communication/IO faults

  • Measure DC levels on the indoor–outdoor communication rails; asymmetry or zero volts indicates wiring, device, or PCB IO faults.

  • For persistent comms codes, isolate to single indoor and test with correct addressing.

10) Commissioning & service checks (field list)

  • Addressing and DIP settings verified; controller in normal (not test) operation.

  • Supply voltage within tolerance; earth continuity confirmed.

  • Fan speeds: verify AUTO step changes and manual LOW/MED/HIGH.

  • Heating start: confirm cold-draft prevention behaviour.

  • Cooling: monitor coil temperature to ensure anti-ice logic is not engaged under normal load.

  • Drain: function test with added water to prove float and pump.

  • Defrost: observe at low ambient; confirm clean exit and stable return to heat.

  • Record baseline: room/coil/discharge temps, compressor behaviour, fan states, and any error history.

11) Reset & handover

  • After fixing faults or changing boards/sensors, power the system off for at least 3 minutes to clear latches, then retest.

  • Document final measurements, DIP/addressing, and timer settings for the site record.