6 Signs that Your Ductless Mini-Split Air Conditioner Isn't Working (and How to Fix It)


With summer well underway, many homeowners are relying on their ductless mini split air conditioners to keep their spaces cool. However, like any appliance, they can sometimes run into issues – especially if your area is experiencing extreme temperatures (like a heat wave).

Here are the top 6 signs that your ductless mini-split air conditioner isn't working as well as it should, and the steps you can take to keep it running smoothly all summer long. 

1. No Cool Air

Sweaty couple near mini-split

The most obvious sign that your ductless mini split isn't working is if it's not producing cool air when in cooling mode. To test this, set your unit to a low temperature, the fan on high, and stand below it. If the air it produces is warm or room temperature, there could be a problem.

Potential Causes:

  • Refrigerant Leak: Refrigerant is a fluid that plays a crucial role in your ductless mini split AC’s ability to cool. When the system is set to cool, refrigerant absorbs and transfers heat from inside your home to outdoors by passing through a series of components – the evaporator coil (indoors), compressor, condenser coil (outdoors), and expansion valve. This cycle repeats continuously as long as the mini split system is running, ensuring consistent cooling. If there's a leak, there won’t be enough refrigerant to complete this process. Signs of a refrigerant leak include hissing sounds, decreased cooling performance, or ice buildup on the evaporator coil.
  • Compressor Issues: The compressor’s job is to pressurize the refrigerant and circulate it through the mini-split system, allowing heat exchange from inside to outside to occur. If the compressor is faulty, it won't be able to pressurize the refrigerant and cool the air effectively. If you suspect a compressor issue, it’s time to call a licensed HVAC technician. 

Additionally, it is wise to consider other home-related issues that might be affecting your ductless mini-split air conditioning’s performance. Learn more about some easy ways to increase your home’s energy efficiency and support your AC’s functionality during the hottest months of the year.

2. Weak Airflow

Man checking mini-split

If the air coming from your unit is weak or not blowing at all, there could be an obstruction or a mechanical issue impeding the flow of air. 

Potential Causes:

  • Blocked Vents: Ensure that there are no obstructions, like furniture, curtains, or other objects, that might block airflow. Then, turn off the unit and carefully inspect the vents for dust, dirt, or debris accumulation. Use a flashlight if necessary to get a clear view inside the vents.
  • Fan Problems: A broken or malfunctioning fan makes it impossible for a mini split to properly circulate air. Noises such as rattling, grinding, or buzzing could indicate a problem with the fan motor. You can also check fan movement by gently attempting to spin the fan blades manually – they should move without any resistance. Lastly, ensure that all electrical connections to the fan motor are secure, as loose or corroded connections can cause the fan to malfunction.
  • Dirty Coils: Dust and debris on the evaporator coils can impede airflow. To diagnose this issue, turn off the unit and open the front panel of the indoor mini split head to access the evaporator coils (they’re usually located behind the air filter). The coils should appear clean and shiny; if they are covered in dirt, it’s time to clean them. Use a soft brush or cloth to gently remove dust and debris from the coils, or use air conditioner coil cleaner spray. Don’t forget to inspect the condenser coils in the outdoor unit as well!
  • Clogged Filters: All air that flows through your ductless mini split will pass through a filter. If the filter is clogged with dust or debris, the airflow is restricted, which reduces cooling efficiency. If you notice that the air coming out of the unit is weaker than usual, it could be due to a blockage in the filter.

3. Foul Odors

If you notice unpleasant smells coming from your ductless mini split, this can indicate mold, mildew, or other issues that will reduce its ability to efficiently cool your space, lower your indoor air quality, or, in the case of components overheating, even create a dangerous situation for your family. 

Diverse couple smelling something bad

Potential Causes:

  • Mold or Mildew: Moisture buildup inside the unit can lead to mold growth. Inspect the indoor and outdoor units for any obvious signs of mold growth. Then, find the drain line, which usually runs from the indoor unit to the outside. It may not be properly connected or could be clogged, preventing drainage and causing water to stagnate and smell. To test the drain line, pour a small amount of water into the drain pan of the indoor unit to see if it flows freely through the drain line. Improper drainage creates a moist, humid environment that mold and mildew love. 
  • Burning Smell: If you sense a burning smell when your ductless mini split AC is running, this could indicate electrical issues or overheating components. In this case, turn the unit off immediately and call a licensed HVAC technician. 
  • Overdue Air Filter Cleaning or Change: Warmer weather can often lead to more airborne dust and dirt, as well as other particulates like pollen and pet hair/dander. If not regularly checked and cleaned or replaced, your ductless mini split air conditioner’s air filter can be clogged by these substances and cause a stale odor, as well as potentially serious respiratory issues related to air quality.

4. Unit Won't Turn On

Frustrated woman with AC remote

If your mini split system simply won't turn on at all, there might be an electrical issue with the unit, or a problem with the remote or smart thermostat you use to control it. 

Potential Causes:

  • Power Supply: The first step is to check if the circuit breaker that powers the unit is on. Breakers can trip for many reasons, and thankfully, this is a super easy fix.
  • Remote Control: Most ductless mini-split air conditioners come with a remote to control its settings. As with all remotes, sometimes the batteries die. Change the batteries and see if your unit will turn on. If you use a smart controller for your mini split air conditioner, ensure that it’s plugged in and still paired.
  • Internal Fuse: There might be an internal fuse that needs to be replaced. Before attempting to address this on your own, be sure to consult the user manual that came with your ductless mini split air conditioner for specific instructions. And as with all things electrical, don’t begin any work before you can confirm that the power supply is disconnected. If you are not comfortable working with electrical components, it’s best to call a professional HVAC technician to replace the fuse.

5. Leaking Water

Water leaking

Water leaking from your mini split is a sign of a problem that needs immediate attention.

Potential Causes:

  • Clogged Drain Line: A blockage in the drain line can cause water to back up and leak. To test the drain line, pour a small amount of water into the drain pan of the indoor unit to see if it flows freely through the drain line. 
  • Condensation Issues: Excessive condensation can occur due to improper temperature settings or humidity levels. Measure the humidity levels in the room where the ductless mini split is installed using a hygrometer. High humidity levels can be a sign of improper moisture removal by the unit. If the humidity in the room is significantly higher than in other areas of the house, it might indicate that the AC is not effectively removing moisture.
  • Improper Installation: If the unit wasn't installed correctly, it could cause water to leak. If you’ve checked the drain line and confirmed that there is no excessive condensation, call the technician that installed your ductless mini split air conditioner and ask them to double check everything was installed correctly. 

Remember, a good smart controller for your ductless mini split should come equipped with humidity sensors that help you monitor moisture levels in your home, allowing you to make the temperature adjustments needed to prevent costly damage, and keep your family comfortable.

6. High Energy Bills

Older couple worried holding bill

An unexpected increase in your energy bills can indicate that your ductless mini split is no longer working as it should, and consuming more electricity than necessary to cool your space. 

Potential Causes:

  • Old Unit: Older ductless mini split air conditioner units may become less efficient over time and require more power to operate. The average lifespan of a ductless mini split air conditioner is about 12-15 years. If your unit is approaching or has surpassed this age range, it might be time to consider a replacement.
  • Dirty Filters: Air filters that are clogged with dust and debris will reduce airflow through the air conditioner, forcing it to work harder and consume more energy to reach your desired set points. It's a good practice to check the air filter every month, especially during periods of heavy use, such as summer and winter. To clean the filter, vacuum it to remove dust or wash it with mild soap and water.  Make sure it is completely dry before reinserting it. Replacement filters are relatively inexpensive and easy to locate, so we recommend keeping a few on-hand. 
  • Malfunctioning Components: Faulty parts can cause the system to use more energy. If your unit is new and the filter is in good condition, it may be time to call a licensed HVAC technician to inspect your system. 

By staying vigilant and addressing problems early, you can keep your ductless mini split air conditioner running smoothly and enjoy a comfortable indoor environment year-round.

Ready for the convenience, control, comfort, and cost savings that come from choosing smart control for your home’s AC?

Learn more about Mysa Smart Thermostat for Ductless Mini-Split AC now.

by LastSale
by LastSale