General Assist Technical Reference
J Series Outdoor Fan Control
Cooling fan-speed control, intermittent operation and heating behaviour
Technical overview of J Series outdoor-unit fan control logic
During cooling operation, the outdoor fan speed is controlled primarily from the outdoor heat-exchanger middle temperature thermistor value THHMO. During heating operation, the outdoor fan operates at Hi continuously.
1. Cooling operation
Outdoor fan-speed selection during cooling operation is performed using the detected value of the outdoor heat-exchanger middle temperature thermistor, THHMO.
- When cooling starts from a stopped condition, the outdoor fan operates at the initial fan speed or at the fan speed stored in memory.
- When cooling restarts after thermostat OFF, the fan speed that was operating immediately before cooling stopped is memorised and operation restarts at that fan speed.
2. Cooling fan-speed table
| Step | Fan speed — normal | Fan speed — silent operation mode | Intermittent output |
|---|---|---|---|
| Step 1 | Hi | Lo | — |
| Step 2 | Lo | Lo | — |
| Step 3 | Intermittent 1 | 6.0 sec ON / 3.0 sec OFF | |
| Step 4 | Intermittent 2 | 6.0 sec ON / 6.0 sec OFF | |
| Step 5 | Intermittent 3 | 6.0 sec ON / 8.7 sec OFF | |
| Step 6 | Intermittent 4 | 3.5 sec ON / 7.4 sec OFF | |
| Step 7 | Intermittent 5 | 4.7 sec ON / 15.4 sec OFF | |
| Step 8 | OFF | — | |
Important: Intermittent fan operation is a normal control state. Depending on THHMO, the outdoor fan may cycle ON and OFF rather than run continuously.
3. Change of cooling-mode fan speed
The outdoor fan motor is controlled so that the heat-exchanger middle temperature thermistor THHMO remains within a target control range.
- If THHMO falls below THHMO-min, fan speed is reduced by one step from the current fan speed.
- If THHMO rises above THHMO-max, fan speed is increased by one step from the current fan speed.
- THHMO-max is fixed at 55°C.
- THHMO-min changes according to outdoor temperature THo.
| Outdoor temperature thermistor THo | THHMO-min | THHMO-max |
|---|---|---|
| THo < 20°C | 38°C | 55°C |
| 20°C ≤ THo < 40°C | 0.35 × THo + 31 | 55°C |
| 40°C ≤ THo | 45°C | 55°C |
How THHMO-min changes:
Below 20°C outdoor temperature, THHMO-min is fixed at 38°C. Between 20°C and 40°C, it rises linearly using 0.35 × THo + 31. At 40°C outdoor temperature and above, THHMO-min is fixed at 45°C. THHMO-max remains 55°C throughout.
Below 20°C outdoor temperature, THHMO-min is fixed at 38°C. Between 20°C and 40°C, it rises linearly using 0.35 × THo + 31. At 40°C outdoor temperature and above, THHMO-min is fixed at 45°C. THHMO-max remains 55°C throughout.
Example target values
| Outdoor temperature | THHMO-min | THHMO-max |
|---|---|---|
| 10°C | 38°C | 55°C |
| 20°C | 38°C | 55°C |
| 30°C | 41.5°C | 55°C |
| 40°C | 45°C | 55°C |
4. Heating operation
During heating operation, the outdoor fan operates at Hi all the time.
5. Diagnostic interpretation
| Observed behaviour | Control interpretation |
|---|---|
| Fan slows during cooling | This may be normal if THHMO has dropped below THHMO-min. |
| Fan speeds up during cooling | This may be normal if THHMO has risen above THHMO-max. |
| Fan repeatedly cycles ON and OFF | The controller may be operating in one of the five intermittent fan-speed steps. |
| Fan runs continuously at Hi in heating | Normal operation according to the supplied J Series fan-control information. |
Technical note: This article summarises the J Series outdoor fan-control information supplied. Exact control behaviour can vary by model and operating condition. Always refer to the applicable service manual when diagnosing a specific outdoor unit.