AIRSTAGE Technical Reference
Defrost Control Logic
Start conditions, termination criteria and field diagnostic guidance
For service technicians assessing heating performance and outdoor coil defrost operation
The defrost control automatically removes frost from the outdoor heat exchanger to maintain heating capacity and efficiency. Defrost initiation is based on compressor operating time and outdoor heat exchanger temperature, while completion is controlled by coil temperature or a maximum operating-time limit.
Technical note: The thresholds in this article should be assessed together with the applicable model service documentation. Confirm thermistor accuracy, compressor runtime and control-board behaviour before condemning components.
1. Conditions for starting defrost
The outdoor unit initiates a defrost cycle according to compressor integrating operation time and outdoor heat exchanger temperature. The sequence differs for the first defrost, later defrost cycles and integrating defrost control.
1.1 First defrost after start-up
| Compressor integrating operation time | Defrost condition |
|---|---|
| Less than 22 minutes | Defrost does not operate. |
| 22 to 62 minutes | Defrost starts when the outdoor heat exchanger temperature is below −9 °C. |
| More than 62 minutes | Defrost starts when the outdoor heat exchanger temperature is below −5 °C. |
What this means: During initial operation, the unit waits for sufficient compressor runtime and a low outdoor coil temperature before confirming that defrost is required.
1.2 Second and later defrost cycles
| Compressor integrating operation time | Defrost condition |
|---|---|
| Less than 35 minutes | Defrost does not operate. |
| More than 35 minutes | Defrost starts when the outdoor heat exchanger temperature is below −10 °C. |
What this means: Subsequent defrost cycles require at least 35 minutes of accumulated compressor operation and a lower outdoor coil temperature.
1.3 Integrating defrost
| Operating pattern | Trigger |
|---|---|
| More than 240 minutes of continuous compressor operation | Outdoor heat exchanger temperature below −3 °C. |
| Repeated compressor operation of less than 10 minutes | Defrost is forced when the accumulated compressor OFF count reaches 40. |
- The controller accumulates the OFF count when compressor operation repeatedly remains below 10 minutes.
- When the OFF count reaches 40, the defrost cycle is forced to start.
- The OFF count resets to zero after any defrost operation.
2. Conditions for terminating defrost
Defrost ends automatically when either release condition is met.
| Release condition | Purpose |
|---|---|
| Outdoor heat exchanger temperature reaches 12 °C or higher | Indicates that the coil temperature is sufficiently high and frost has melted. |
| Defrost compressor operation reaches 15 minutes | Provides a maximum-time release to prevent excessive defrost operation. |
Expected outcome: The system restores heating once the coil has warmed sufficiently or the maximum defrost time has elapsed.
3. Field service notes
| Typical duration | Approximately 5 to 12 minutes, depending on ambient and operating conditions. |
| Normal indicators | The outdoor fan stops while the compressor continues operating. Indoor airflow may reduce during the cycle. |
| Typical interval | Defrost may occur every 30 to 90 minutes, depending on load and ambient conditions. |
Symptoms that may indicate abnormal defrost control
- Prolonged loss of heating at low ambient temperature.
- Outdoor coil remains frozen.
- Defrost cycles occur too frequently or prematurely.
If the unit does not defrost, verify:
- Outdoor thermistor resistance against the applicable temperature chart.
- Compressor cumulative runtime and OFF-count behaviour.
- PCB control logic against the stated temperature and time thresholds.
4. Technician summary
| Cycle type | Runtime requirement | Temperature or count trigger | Release condition |
|---|---|---|---|
| First defrost | 22 to 62 minutes, or more than 62 minutes | Below −9 °C, or below −5 °C respectively | Coil at or above 12 °C, or 15 minutes elapsed |
| Second and later | More than 35 minutes | Below −10 °C | Coil at or above 12 °C, or 15 minutes elapsed |
| Integrating | More than 240 minutes continuous, or repeated runs below 10 minutes | Below −3 °C, or 40 OFF counts | Coil at or above 12 °C, or 15 minutes elapsed |
5. Practical service guidance
When investigating abnormal defrost behaviour, confirm each of the following before replacing parts:
- Thermistor calibration: Verify the outdoor coil frost sensor against the correct resistance-to-temperature data.
- Compressor runtime logic: Review cumulative runtime, short-cycle behaviour, error history and control-board diagnostics where available.
- Outdoor fan operation: Confirm that the fan stops during defrost as required.