General Assist Technical Reference
Legacy Multi-Type Wall-Mounted Inverter Systems
System operation, control logic, protections and service diagnostics
Technical refresher for legacy multi-split inverter systems with up to four indoor units
This reference provides a concise overview of legacy multi-type wall-mounted inverter systems, including system architecture, compressor and fan control, EEV operation, defrost logic, protections, fault indication and common field checks.
Safety: Inverter equipment contains hazardous mains voltage and a high-voltage DC bus. Isolate the system and allow stored energy to discharge before handling inverter PCBs, capacitors or compressor wiring. Live measurements must only be performed by suitably qualified personnel using appropriate test equipment.
1. System overview
- Multi-split system with up to four indoor units (A-D) connected to one outdoor unit.
- Each indoor branch is controlled through an individual electronic expansion valve (EEV).
- The outdoor multi-controller combines indoor capacity requests and determines the required compressor operating frequency.
Typical indoor fan targets
| Fan setting | Approximate target |
|---|---|
| High | ~1330 rpm |
| Medium | ~1170-1260 rpm |
| Low | ~970-1190 rpm |
| Quiet / Super Low | ~500-1050 rpm |
Fan speed targets are approximate and vary by model and operating mode.
2. Inverter fundamentals
AC mains → Rectifier / smoothing circuit → DC bus → PWM inverter → Three-phase compressor
The inverter varies compressor frequency and voltage to control system capacity. Compared with a fixed-speed system, this allows faster pull-down, tighter room-temperature control and generally shorter defrost recovery periods.
3. Control and communications
| Signal direction | Typical information |
|---|---|
| Indoor units → Multi-controller → Inverter | Operating mode request, capacity code (F-code), timer status, Quiet and Power-save status. |
| Outdoor / Inverter → Indoor units | Operating state, defrost status, outdoor temperature information and EEV initialisation commands. |
- The first indoor unit requesting operation establishes the system operating mode.
- Typical priority is A > B > C > D.
- The multi-controller totals the indoor-unit capacity codes to determine compressor frequency.
4. Capacity and airflow control
| Compressor frequency | Follows the combined indoor F-code. Frequency may be reduced when Quiet or Power-save control is active. |
| Outdoor fan | Speed is scheduled according to ambient temperature and operating zone, with separate logic for cooling, heating and extreme conditions. |
| Indoor fan | Target speed varies by mode. During heating, cool-draft prevention can hold the indoor fan at very low speed until the indoor coil has warmed. |
5. Electronic expansion valve management
At power-up, each branch EEV is re-indexed to zero. The valve then opens to a preset pulse position according to the connected model, operating mode and whether the system is operating with a single room or multiple rooms. Heating corrections may be applied after a short stabilisation period.
6. Defrost and pre-heat operation
| Defrost | Initiated using combined time and temperature logic when the outdoor coil is sufficiently cold. The sequence controls EEV positions, changes the 4-way valve and operates the compressor at a defined frequency until coil temperature rises or the maximum defrost period is reached. |
| Crankcase / pre-heat | May energise after an extended OFF period at low ambient temperature to protect the compressor before operation. |
7. Protection functions
| Protection | Control response |
|---|---|
| Discharge temperature | Progressive compressor-frequency reduction followed by shutdown if temperature becomes excessive. Automatic restart may occur after cooling. |
| IPM protection | Protects against inverter over-current and over-temperature. Repeated trips can result in a lockout condition. |
| Current release | Limits outdoor input current. Thresholds vary with the operating zone and airflow conditions. |
| Other safeguards | Low-ambient cooling, heating overload, pressure switch A/B, CT error, active-filter protection and outdoor-fan error. |
8. Indoor control highlights
- Typical setpoint range: 18-30 °C cooling and 15-30 °C heating.
- Indoor capacity code is derived from the room-temperature / setpoint difference at regular intervals.
- Dry mode: uses a stepped fan strategy with moderate compressor frequency.
- Auto changeover: selects operating mode from room and outdoor conditions, with a sampling period at low indoor fan speed.
- Quiet mode: reduces indoor fan speed and applies tighter current limits.
9. Diagnostics and error indication
- Lamp codes on indoor and outdoor PCBs are used to classify thermistor, serial communication, pressure, discharge-temperature, current-trip, CT, active-filter and outdoor-fan faults.
- Thermistor resistance tables for room, pipe, outdoor coil, ambient and discharge sensors can be used for bench testing.
- Interpret both the major and minor indications where provided to help distinguish sensor, PCB and communication faults.
10. Field troubleshooting - quick guide
| Fault / symptom | Service approach |
|---|---|
| Communication not received at outdoor unit | Verify mains supply, then measure the serial communication rails. Use the service decision logic to isolate the indoor PCB, inverter board, interconnecting wiring or fusing. |
| Outdoor thermistor abnormal | Disconnect and measure the sensor. Compare the result with the applicable temperature-resistance table and allowable tolerance. Replace the sensor if outside specification. |
| Outdoor current trip / suspected IPM | Safely discharge the DC bus, verify DC bus voltage and compare phase outputs U-V, U-W and V-W. If the bus is correct but phase outputs are unbalanced, suspect the inverter board; otherwise continue compressor checks. |
| Active filter abnormal | Confirm supply voltage, connector seating and active-filter module integrity. Apply the specified bypass test where applicable to the model. |
11. Installer and service cautions
Allow for control delays. A system that does not restart immediately after a stop or mode change is not necessarily faulty. Check the operating sequence before condemning components.
- Observe the 3-minute restart lockout following stops or protection trips.
- During a heating/cooling mode change, the compressor stops before the 4-way valve changes position. Controlled delays are expected.
- Always de-energise the system and allow the DC bus to discharge before inverter module or capacitor work.
- Record commissioning baseline data such as compressor frequency, temperatures and operating current for future comparison.
Key takeaway
Legacy inverter multi-split systems combine the capacity requests from each indoor unit into a single compressor-frequency demand while independently controlling branch EEVs and system protections. Reliable fault diagnosis depends on accurate interpretation of serial signals, thermistor values, voltage/current measurements and lamp-code indications.
General Assist Technical Reference