When to defrost the outdoor unit of an air conditioner in winter and the principle of defrosting
When an air conditioner is in heating mode during winter, the outdoor unit will automatically detect and perform defrosting. Defrosting usually occurs after the outdoor unit's coil temperature is very low (e.g., around -5°C) and the unit has been running in heating mode for a period of time. The principle is mainly to briefly switch to "cooling" mode, directing the high-temperature refrigerant to the outdoor unit to melt the frost layer .
I. When to defrost
In heating mode, the outdoor unit's heat exchanger acts as an evaporator. When the coil temperature drops below 0°C, moisture in the air condenses into frost. Thick frost can clog the fins, impairing heat exchange and reducing heating capacity, prompting the system to automatically defrost. The mainstream approach uses a triple-factor judgment based on coil temperature, ambient temperature, and operating time (inverter units), while older fixed-frequency units often rely on timed defrosting.
Typical operating conditions: Outdoor Frost is most likely to form at temperatures between 0 and 5°C and high humidity , and it typically triggers every 40 to 90 minutes.
The temperature is very low but dry (e.g., below -5°C, very dry), frost forms slowly, and the defrosting interval is very long;
Example of trigger judgment: When the outdoor unit coil temperature drops to the set value and the temperature difference with the environment meets the conditions, and the heating has run for the minimum protection time, the motherboard enters the defrost state.
Termination condition: The coil temperature rises back to the set temperature, or the maximum defrost timeout period is reached (to prevent jamming).
Defrosting time per cycle: 3-10 minutes . The air conditioner panel will often display H1 or DF to indicate that defrosting is in progress.
Defrosting symptoms: The indoor unit fan stops and no hot air is emitted; the outdoor unit fan stops; the outdoor unit emits white mist (frost is rapidly vaporizing) and drips water. A hissing sound will be heard when the four-way valve switches. This is normal and you should not disconnect the power.
II. Defrosting Principle
Core component: Four-way reversing valve , used to reverse the flow of refrigerant.
1. Normal heating:
The compressor discharges high-temperature, high-pressure refrigerant → Indoor unit (condenser, releasing heat for heating) → Throttling → Outdoor unit (evaporator, absorbing heat, temperature below 0℃, frosting) → Back to the compressor.
2. Enter defrost mode:
When the four-way valve switches, the system temporarily switches to a refrigeration cycle ;
The outdoor unit fan stops, and the indoor unit fan stops (to prevent cold air from blowing out).
from the compressor is sent directly to the outdoor heat exchanger . At this time, the outdoor unit becomes a condenser, releasing heat inward and melting the frost on the fins.
3. Once the defrost has completely melted and the coil temperature reaches the target, the four-way valve switches back to heating mode, the indoor and outdoor fans resume operation, and heating continues.
In short: temporarily turn the outdoor unit into a heat exchanger, and use the high-temperature refrigerant discharged from the compressor to defrost it .
III. Other defrosting methods (high-end heat pumps/multi-split systems)
1. Hot gas bypass defrosting : Without switching the four-way valve or reversing the circulation, a small portion of the high-temperature exhaust gas from the compressor is directly introduced into the outdoor unit coil for defrosting, allowing the indoor unit to continue heating for better comfort; suitable for when the frost is relatively light.
2. Heat storage defrosting : Equipped with a heat storage tank, it uses the heat stored in the heat storage body during defrosting to reduce indoor temperature drop. It is mostly used in low-temperature air source heat pumps.
3. Electric heating-assisted defrosting : A few models have an electric heating element, which is generally used as an auxiliary function and not for primary defrosting.
IV. Normal Phenomena During Defrosting
If you see or hear the following, don't worry, it's normal for defrosting to be working:
Indoor unit stops blowing air : Displays "Defrost" or "Defrost", and hot air is not being emitted temporarily.
White smoke/water coming out of the outdoor unit : Water vapor or defrosting water produced by the melting of frost.
If you hear unusual noises , it could be the sound of ice melting and cracking or the sound of a four-way valve switching.
If you find that your air conditioner defrosts very frequently (for example, it stops blowing hot air after a short while), or defrosts continuously for several hours, this is usually not normal and may involve sensor malfunction or refrigerant shortage. It is recommended to contact after-sales service for inspection.
