Short answer: Settings, restricted airflow and a defrost cycle are starting points to check when a mini split is not heating. Check the mode and temperature, clean the filter, and look at the outdoor unit. If defrost lasts longer than 15 minutes, or heat never returns, a technician should look at it.
Is it just defrosting?
Cool air in winter can be normal. In heat mode the outdoor coil can ice up, and the system briefly sends heat toward that coil to melt it. Lennox says frost can begin forming when the outdoor temperature falls below about 47°F (8°C), that defrost generally takes 3 to 15 minutes, and that you may see steam around the outdoor equipment.
Lennox gives two limits for when to book service: a defrost cycle that lasts longer than 15 minutes, or one that returns more than once within an hour.
If you saw "DF" on the display, that is defrost. Read what DF means on a mini split.
Let's go through it in order
Grab your remote and a flashlight. Do these in order, and stop as soon as the heat comes back.
1. Check the power. Look at the indoor unit. Is the display lit? Lennox says to confirm the breaker and the outdoor disconnect are on, and to reset a tripped breaker once, not repeatedly.
2. Check the remote and settings. On the remote, select HEAT (usually a sun symbol), cancel any timer, and set the temperature 3 to 5 degrees above the current room temperature. If the unit ignores the buttons, replace the remote's batteries. Trane suggests turning the unit off for several minutes and back on to clear a glitch. You should hear the beep and see the display light up. Warm air can take a few minutes to arrive, so give it time.
3. Write down any error code first. If the display shows a code or the lights are blinking, take a photo of it with your phone. Lennox advises recording any code or flashing-light pattern before you reset anything. If you cut power, wait at least 30 seconds before restoring it. If the same error comes back, the fault is still there.
4. Clean the filter. Turn the unit off first. Lennox and Trane both say clogged filters cut airflow, and a heavy restriction can contribute to a frozen coil or a protective shutdown. Wash a reusable filter in warm water, rinse it, and let it dry completely. See how to clean a mini split.
5. Step outside and look at the outdoor unit. Trane says to clear snow, ice and debris from the top of the unit, shovel away drifts, and trim shrubs back two feet. Lennox recommends 24 inches of open space on all sides. Turn the unit off before you remove loose snow by hand. Lennox is clear on two things: do not chip ice off the coil, and do not spray water into the cabinet.
Possible explanations for the symptoms
| What you notice | Likely cause | What to do |
|---|---|---|
| Cool air for a few minutes, then warm air returns | Normal defrost | Wait |
| Steam from the outdoor unit | Frost melting | Normal |
| Nothing happens when you press buttons | Remote batteries or no power | Replace batteries, check the breaker once |
| Weak warmth, dirty filter | Restricted airflow | Clean the filter |
| Outdoor unit packed with snow or leaves | Poor airflow outside | Switch off, clear by hand |
| Cools fine but never heats | Possible reversing valve problem | Call a technician |
| Defrost over 15 minutes, or repeating within an hour | Possible fault | Call a technician |
When to call a technician
Lennox says refrigerant leaks, electrical failures, faulty sensors and reversing valve problems need specialized testing and repair. A mini split that cools normally but cannot heat is one possible sign of a reversing valve issue. Trane adds that low refrigerant comes from a leak, installation problems or an improper charge, and that adding refrigerant without finding the leak does not fix it.
Call a licensed HVAC technician if:
- Basic checks do not bring the heat back
- The breaker trips again after one reset
- An error code returns after a restart
- The unit behaves unpredictably
- Ice keeps building on the indoor or outdoor unit
If your system is newly installed, call the installer first. Trane notes that many installers include a one-year labor warranty.
Keeping it heating all winter
Lennox suggests switching the system to HEAT before cold weather arrives to confirm it responds, listening for unfamiliar sounds, and clearing leaves and plants from the outdoor unit. Book a yearly professional check, and never stack things against the outdoor unit or let snow pile on top of it. Trane recommends service twice a year for a heating and cooling system, before each season.
Related reading
- If a Carrier unit shows a code while it is not heating, read the Carrier error code guide.
- Settings matter too, so see the guide on winter temperature settings.
Sources
This is general information, not a repair manual for your exact model. Follow your unit's own manual.
