Why Does an Air Conditioner Need to Defrost?
Reverse cycle air conditioners don’t just cool your home in summer — they also provide efficient heating in winter. When set to heating mode, the outdoor unit of your system draws in heat from the outside air and transfers it indoors.
But in cold, damp conditions (like early mornings or rainy days in a Perth winter), moisture in the air can freeze on the outdoor coil. This frost build-up can reduce your system’s efficiency and even cause damage if left unchecked.
That’s where defrost mode comes in.
What Is Defrost Mode?
Defrost mode is a self-cleaning safety feature built into most modern reverse cycle air conditioners. When the system detects frost forming on the outdoor coil, it automatically switches into defrost mode to melt the ice.
This process involves temporarily reversing the refrigerant cycle, directing warm air to the outdoor coil to melt the frost.
While in defrost mode:
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The indoor unit will stop blowing warm air
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You might hear a change in sound from the outdoor unit
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It may blow cool air for a few minutes
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Some water may drip around the outdoor unit — this is normal
The entire cycle usually lasts 5–15 minutes, after which the system will switch back to heating mode.
What Should You Do When Your AC Is in Defrost Mode?
Here’s what we recommend:
✅ Be patient – This is a normal part of winter operation. Don’t try to switch modes or restart the unit.
✅ Avoid turning the system off – Let it complete the defrost cycle. Turning it off can interrupt the process and delay heating.
✅ Check airflow – Ensure the outdoor unit has clear airflow. Remove leaves or debris around it that might block ventilation.
If your system is going into defrost mode too often or not completing the cycle, it could be a sign of a fault or a maintenance issue. Contact a professional technician to inspect the system.
How Often Should Defrost Mode Happen?
In a typical Perth winter, defrost mode may only activate occasionally — usually on cold, humid mornings or rainy days when outdoor temperatures drop below 7°C.
Frequent or extended defrost cycles may be a sign of:
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Poor airflow
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Blocked filters or coils
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Low refrigerant levels
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A faulty sensor
If you’re experiencing this, it’s best to book a service with your local air conditioning specialist.
Tips to Help Reduce Defrost Frequency
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Keep your outdoor unit clean and unobstructed
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Ensure your filters are clean
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Book regular servicing, especially before winter
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Install your outdoor unit in a well-ventilated area (if you’re building or renovating)
Final Thoughts
Defrost mode might seem unusual the first time you see it, but it’s a perfectly normal and necessary part of how your reverse cycle air conditioner keeps you warm in winter. Understanding how it works can help you use your system with confidence and avoid unnecessary stress.
Need help with winter maintenance or system troubleshooting? Get in touch with Allfield Residential Air Conditioning Services — your trusted Perth air conditioning experts. We’ll make sure your system is running smoothly all season long.
