1. Purpose
The defrost control logic automatically manages outdoor coil frost removal to maintain heating performance and efficiency.
This document explains the conditions for defrost initiation and completion, enabling service technicians to assess normal operation or identify control faults.
2. Conditions for Starting Defrost Operation
The outdoor unit initiates a defrost cycle based on compressor runtime and outdoor heat exchanger temperature.
The control sequence varies depending on whether it is the first, subsequent, or integrating (continuous monitoring) defrost.
2.1 First Defrost after Start-Up
| Compressor Integrating Operation Time | Condition | Outdoor Heat Exchanger Temperature |
|---|---|---|
| Less than 22 minutes | Does not operate | — |
| 22–62 minutes | Defrost starts if below –9°C | |
| More than 62 minutes | Defrost starts if below –5°C |
Summary:
During initial operation, the unit waits until sufficient runtime and low outdoor coil temperature confirm frost accumulation.
2.2 Second and Later Defrost Cycles
| Compressor Integrating Operation Time | Condition | Outdoor Heat Exchanger Temperature |
|---|---|---|
| Less than 35 minutes | Does not operate | — |
| More than 35 minutes | Defrost starts if below –10°C |
Summary:
Subsequent defrosts require a longer compressor runtime and lower coil temperature, ensuring efficient operation without unnecessary defrosting.
2.3 Integrating Defrost (Continuous Monitoring Mode)
| Compressor Integrating Operation Time | Condition | Trigger |
|---|---|---|
| More than 240 minutes (continuous) | Coil temperature below –3°C | |
| Less than 10 minutes (intermittent) | Compressor OFF count = 40 times |
Notes:
If the compressor operates for less than 10 minutes repeatedly, the controller accumulates the OFF count.
When the count reaches 40, the defrost cycle is forced to start.
After any defrost operation, the OFF count is reset to zero.
3. Conditions for Defrost Termination
Defrost concludes automatically once either of the following release conditions is met:
| Release Condition | Description |
|---|---|
| Outdoor heat exchanger temperature ≥ 12°C | Coil temperature sufficiently high; frost melted. |
| Compressor operation time ≥ 15 minutes | Safety timeout to prevent over-defrosting. |
Summary:
The system ensures reliable defrost completion while preventing excessive downtime that could reduce heating capacity.
4. Field Service Notes
Normal Defrost Cycle Duration: Typically between 5–12 minutes, depending on ambient conditions.
Indicators: During defrost, the outdoor fan stops, and compressor continues operating — indoor unit may reduce air delivery.
Common Symptoms of Faulty Defrost Control:
Prolonged heating loss at low ambient temperature.
Outdoor coil remaining frozen.
Frequent or premature defrost cycles.
Diagnostic Tip:
If the unit fails to defrost, check:
Outdoor thermistor readings (resistance vs. temperature chart).
Compressor cumulative runtime and OFF count behavior.
PCB logic compliance with the above temperature/time thresholds.
5. Technician Summary Table
| Cycle Type | Compressor Runtime | Outdoor Coil Temperature Trigger | Release Condition |
|---|---|---|---|
| 1st Defrost | >22 min (–9°C) / >62 min (–5°C) | Below threshold | Coil ≥ 12°C or 15 min elapsed |
| 2nd+ Defrost | >35 min | Below –10°C | Coil ≥ 12°C or 15 min elapsed |
| Integrating | >240 min or 40 OFF counts | Below –3°C | Coil ≥ 12°C or 15 min elapsed |
6. Practical Service Guidance
Defrost intervals typically occur every 30–90 minutes depending on load and ambient.
When investigating abnormal defrost behavior, always verify:
Thermistor calibration (frost sensor at coil).
Compressor runtime logic (check via error history or control board diagnostics).
Outdoor fan relay operation (must stop during defrost).
