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Fix a ductless mini-split that will not heat

We'll check the mode, thermostat, filter, reversing valve, and outdoor unit—or tell you when to call a pro.

Troubleshooting · Home appliances
15–45 min
What you'll need
  • Remote control (or wall control)
  • Mild soap and water for filter (or vacuum)
  • Replacement batteries for remote (if needed)
Step 1 of 5

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Steps

Goal: Confirm the symptom, rule out mode and thermostat, then isolate the heating fault.

  • Set the mode to Heat and the temperature above room temperature. Wait five minutes. Feel the air from the indoor unit.
  • Good: The fan runs but the air is cool—heating system has failed. Proceed to Check mode and thermostat.
  • Bad: The fan does not run—different problem (power, motor). Check power first.

Check mode and thermostat

Goal: Rule out incorrect settings before opening the unit.

  • Confirm the mode is Heat—not Cool, Dry, Fan, or Auto. Use the remote or wall control.
  • Confirm the set temperature is above the current room temperature. Raise it several degrees if needed.
  • Good: Mode is Heat and set temp is above room temp. Proceed to Filter and outdoor unit.
  • Bad: Wrong mode or temp—fix and retest.

Filter and outdoor unit

Goal: Clean the filter and confirm the outdoor unit can absorb heat.

  • Open the front panel of the indoor unit. Remove and clean the air filter. Reinstall.
  • Check the outdoor unit. It should run and not be heavily iced. If iced, the defrost cycle may be failing.
  • Good: Filter is clean and outdoor unit is running. Proceed to Reversing valve.
  • Bad: Filter was dirty or unit was iced—clean filter, wait for defrost, or call a pro.

Reversing valve

Goal: Confirm the reversing valve switches to heat mode.

  • Switch from Cool to Heat. Listen at the outdoor unit for a click or hiss from the reversing valve.
  • If you hear nothing, the valve may be stuck—call a technician.
  • If you hear the valve but it still blows cool air, low refrigerant or another fault—call a technician.

When to get help

Call an HVAC technician if:

  • The mode and thermostat are correct, the filter is clean, and it still blows cool air.
  • The outdoor unit is iced over and not defrosting.
  • You hear hissing (refrigerant leak).
  • The reversing valve appears stuck.

Refrigerant and valve work require certification.

Verification

  • The unit blows warm air within five minutes of starting Heat mode.
  • No ice buildup on the outdoor unit.
  • Filter is clean and airflow is strong.
  • No hissing or unusual noises.

Escalation ladder

Work from the device outward. Stop when the problem is fixed.

  1. Confirm symptom Verify the unit runs but blows cool air in Heat mode.
  2. Mode and thermostat Confirm Heat mode and set temperature above room temp.
  3. Filter and outdoor unit Clean the filter; check outdoor unit is not iced or blocked.
  4. Reversing valve Listen for valve actuation when switching to Heat.
  5. Call a pro Reversing valve stuck, refrigerant leak, or still no heat—call an HVAC technician.

What to capture if you need help

Before calling support or posting for help, have these ready. It speeds everything up.

  • Mode and thermostat setting
  • Filter condition
  • Outdoor unit condition (iced, blocked)
  • Whether reversing valve clicks when switching to Heat
  • Steps already tried

Does the unit run but blow cool air in Heat mode?

Set mode to Heat and temp above room temp. Wait five minutes. Feel the vent air.

Set the mode to Heat and the temperature above room temperature. Wait five minutes. Feel the air from the indoor unit. Good: fan runs but air is cool—heating fault. Bad: fan does not run—different problem (power, motor); check power first.

Yes No

You can change your answer later.

Is the mode Heat and set temp above room temp?

Mode must be Heat. Set temperature must be above room temp to call for heat.

Confirm the mode is Heat (not Cool, Dry, or Fan). Confirm the set temperature is above the current room temperature. Good: Heat mode, temp above room. Bad: wrong mode or temp—fix and retest.

Yes No

You can change your answer later.

Is the filter clean and outdoor unit not iced?

Dirty filter blocks airflow. Iced outdoor unit cannot absorb heat.

Clean the air filter. Check the outdoor unit—it should run and not be heavily iced. If iced, the defrost cycle may be failing. Good: filter clean, outdoor unit running. Bad: filter dirty or unit iced—clean filter, wait for defrost, or call a pro.

Yes No

You can change your answer later.

Do you hear the reversing valve when switching to Heat?

The reversing valve switches between cool and heat. You may hear a click or hiss.

Switch from Cool to Heat. Listen at the outdoor unit for a click or hiss from the reversing valve. No sound: valve may be stuck—call a pro. Sound but still cool: may be low refrigerant—call a pro. Sound and heats: fixed.
Question

Do you hear the reversing valve when switching to Heat?

Done

Call a technician

Call an HVAC technician if: mode and thermostat are correct, filter is clean, and it still blows cool air; the outdoor unit is iced and not defrosting; you hear hissing (refrigerant leak); or the reversing valve appears stuck. Refrigerant and valve work require certification.

Done

Reviewed by Blackbox Atlas

Frequently asked questions

Why would a ductless mini-split not heat?
Common causes: mode set to Cool or Fan, thermostat below room temp, dirty filter, faulty reversing valve, low refrigerant, or outdoor unit not defrosting. Check mode and thermostat first, then filter. Reversing valve and refrigerant need a technician.
Can I fix a ductless mini-split that will not heat myself?
Yes, for mode, thermostat, and filter. The reversing valve and refrigerant system require a technician. Do not attempt to add refrigerant or repair the valve yourself.
When should I call a technician for a mini-split that will not heat?
Call a technician if the mode and thermostat are correct, the filter is clean, and it still blows cool air; the outdoor unit is iced over and not defrosting; or you hear hissing (refrigerant leak).

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