General Assist Technical Reference
Wall Hung Split Defrost Operation
Conditions for starting and completing automatic defrost
Technical overview of outdoor coil temperature and compressor runtime logic during heating operation
During heating operation, moisture can freeze on the outdoor heat exchanger. The system automatically monitors the outdoor coil temperature and compressor operating time, then starts a defrost cycle when the programmed conditions are met.
Normal operation: During defrost, the indoor fan may stop, heating output will pause and steam or water may be visible around the outdoor unit. This is expected and does not normally indicate a fault.
1. Conditions for starting defrost
Defrost begins when the outdoor heat exchanger temperature sensor, identified as Tn, detects a sufficiently low coil temperature after the required compressor operating time.
| Operation stage | Compressor integrating operation time | Condition to start defrost |
|---|---|---|
| First defrost after start-up | Less than 22 minutes | Defrost does not operate |
| 22 to 62 minutes | Tn ≤ -9°C | |
| More than 62 minutes | Tn ≤ -5°C | |
| Second and subsequent defrost operations | Less than 35 minutes | Defrost does not operate |
| More than 35 minutes | (Tn - Tn10) ≤ -5°C and (Tn - Tnb) ≤ -2°C, with Tn ≤ -6°C | |
| Integrating defrost / constant monitoring | More than 240 minutes of long continuous operation | Tn ≤ -3°C |
| More than 215 minutes of extended continuous operation | Tn ≤ -5°C | |
| Less than 10 minutes of intermittent operation | Defrost begins after 40 compressor OFF counts |
Intermittent operation note: When continuous compressor operation is less than 10 minutes, the control uses compressor OFF counts to determine when defrost is required. The OFF count resets after a defrost cycle occurs.
2. Conditions for completing defrost
The defrost cycle ends when either of the following release conditions is met:
| Defrost release condition |
|---|
| The outdoor heat exchanger temperature sensor value, Tn, rises above +13°C. |
| The compressor has operated in defrost for more than 15 minutes. |
Once a release condition is reached, the system exits defrost and automatically resumes normal heating operation. A short delay may occur while refrigerant pressures stabilise and the indoor coil warms.
3. Technical terms
| Term | Meaning |
|---|---|
| Tn | Current outdoor heat exchanger temperature sensor reading. |
| Tn10 | Outdoor heat exchanger temperature measured after 10 minutes of continuous operation. |
| Tnb | Average outdoor heat exchanger temperature over the previous five minutes. |
| Integrating operation time | Accumulated compressor operating time used by the controller when assessing frost formation. |
4. What customers may notice
- Heating temporarily stops while the outdoor coil is defrosted.
- The indoor fan may stop or operate at a very low speed to prevent cool air entering the room.
- The outdoor fan may stop during part of the cycle.
- Water may drain from the outdoor unit as frost melts.
- Steam or mist may appear around the outdoor unit, particularly in cold and humid weather.
- The complete interruption can last several minutes before heating resumes.
Further investigation may be required if the system remains in defrost for unusually long periods, repeatedly defrosts with very short heating intervals, does not return to heating, displays an error code, or the outdoor coil remains heavily iced after the cycle.
5. Summary
The defrost control continuously assesses outdoor coil temperature, temperature change and compressor operating time. Defrost is only initiated when the applicable temperature and runtime conditions are met. The cycle finishes when the outdoor coil warms above +13°C or when the 15-minute maximum compressor time is reached, after which the system returns to heating automatically.