If your reverse cycle air conditioner isn’t heating, first check the remote is on heat (the sun symbol) and the set temperature is above the room temperature. Then check whether the unit is simply defrosting. On cold mornings the outdoor unit ices up, and the system stops heating for a few minutes to melt it. The indoor fan stops, a lamp may flash, and steam may rise from the outdoor unit. That is normal. If the unit blows cold air for longer than 15 minutes, or never warms at all, it needs a technician.
Checks first
- Mode: make sure the remote shows heat, not auto, cool or fan.
- Set temperature: set it a few degrees above the room temperature. 20 to 22 degrees is comfortable in winter.
- Warm-up delay: many units wait a few minutes before the indoor fan starts in heat mode, so they don’t blow cold air. Give it five minutes.
- Filters: blocked filters reduce heating as much as cooling.
- Outdoor unit: clear leaves and make sure water can drain away from underneath it during defrost.
Defrost mode explained
In heat mode the outdoor coil gets colder than the outside air, so on cold, damp Melbourne mornings frost builds up on it. The system briefly reverses to melt the frost. During defrost the indoor fan stops or slows, and you may hear a whoosh and see steam outside. It usually lasts a few minutes, then heating resumes. Frequent defrosts are normal in cold, wet weather.
Causes and what each one needs
| Cause | What you’ll notice | DIY or technician |
|---|---|---|
| Wrong mode or set temperature | Fan runs, air is not warm | DIY |
| Normal defrost cycle | Heating pauses for a few minutes; steam outdoors | Nothing to fix |
| Dirty filters | Weak, lukewarm airflow | DIY |
| Outdoor unit iced solid and not clearing | Thick ice that stays; poor heating all day | Technician: defrost sensor, fan or drainage |
| Reversing valve fault | Cools when set to heat, or won’t change modes | Technician |
| Low refrigerant (leak) | Heating weaker every week | Technician (ARCtick-licensed) |
| Sensor or control board fault | Error code; erratic operation | Technician |
Before winter
Run the unit in heat mode for 15 minutes in autumn, before the first cold night, so any fault shows up while technicians have time. An autumn service is the best time to catch heating problems; see how often to service your air conditioner.
If it still won’t heat, book a technician, or start from our troubleshooting hub.
For general advice on staying safe around electrical appliances at home, see Energy Safe Victoria.
Reviewed by AC repair and service specialist on 22 October 2026. How we write our articles