How to Identify and Fix Mini-Split Refrigerant Issues

Ductless mini-splits provide a highly efficient heating and cooling solution for one room or a whole house. Mini-split systems are a perfect solution for a home with boiler heat and a need for air conditioning, older homes without ductwork, garages, sunrooms, and other areas without ductwork.

Like any mechanical system, a mini-split HVAC system will eventually have problems. One of those issues can be low refrigerant. In this article, we’ll discuss signs of low refrigerant and how to fix the issue. Spoiler alert: only a trained HVAC professional can detect and fix a refrigerant leak and safely perform a refrigerant recharge.

Mini-splits have low refrigerant for 3 basic reasons: wear and tear, improper installation, or a refrigerant leak. A refrigerant leak can result from both wear and tear and poor installation.

• Mini-split not cooling If your mini-split air conditioning system is struggling to cool a room or zone, it could be a sign of low refrigerant. Other issues could be to blame as well.
• Mini-split not heating If the mini-split struggles to heat your space, it could be a sign of low refrigerant. Other issues could be to blame.
• Whistling or hissing sound Mini-splits are known for their quiet operation. If you hear a whistling or hissing sound inside or outside near the mini-split heat pump, it is most likely due to a refrigerant leak. If you notice this sound, call an HVAC professional.
• Increased energy bills A ductless system that is low on refrigerant has to work harder to try and heat or cool your space. That decreased system efficiency can lead to increased energy bills. Other issues may be the cause.
• Frozen evaporator coil Frozen evaporator coils on one or more indoor units can be a sign of a refrigerant leak. If you have frozen coils, turn the affected indoor unit off and call for mini-split repair.

Some of these symptoms can be due to other issues, including needing to clean the mini-split filters. Before assuming your problem is low refrigerant, check and clean the mini-split air filters.

Here’s the thing. Your system shouldn’t be low refrigerant unless there’s a refrigerant leak or the system was improperly charged at installation.

If you got here by Googling how to add refrigerant to a mini-split, we’re not going to give you a tutorial on that. Adding refrigerant to a mini-split system is a job for trained HVAC professionals who know how to handle refrigerants.

And they shouldn’t just recharge (add) the refrigerant without troubleshooting why the refrigerant level is low. As mentioned above, the main reasons for a low refrigerant level are a refrigerant leak and improper charging when the system was installed.

How do I know if my mini-split is leaking refrigerant? You don’t know for sure until an HVAC technician troubleshoots the system and runs some tests. If your mini-split system is newly installed, call your mini-split system installer to come back and fix the problem. Many HVAC companies offer a one-year labor warranty at no extra cost. 

Pinpointing the location of a leak can take time. It can involve the use of an electronic leak detector, bubble testing, recovering the refrigerant, fixing the leak, pressure testing the system, and more. The technician will also examine the flare fittings on the refrigerant lines that connect the outdoor unit to the indoor unit(s). In some cases, simply tightening the fittings resolves a leak.

Last but not least, the technician will replace the recovered refrigerant and recharge the system according to the manufacturer’s specifications.

The answer to this depends on the age of the system. If you have a mini-split that’s been around since 2009, it may use R-22 (Freon®). If it was manufactured after 2010, it probably uses R-410A. Newly manufactured Trane/Mitsubishi mini-splits that use the newest, more sustainable refrigerant R-454B will be available in 2025.

Read our Complete Guide to HVAC Refrigerants to understand more about them.

Depending on the age of your system and the severity of the problem, you may want to consider replacing your mini-split system rather than pouring money into repairs. If your mini-split system uses a refrigerant that has been or is being phased out, you may want to consider replacing the system.

This is not a decision to be made lightly. Get more than one HVAC professional’s opinion. You can also refer to our Repair or Replace Guide for information to help you make your decision.

New mini-split heating and cooling systems are usually very reliable when made by a reputable company (such as Trane/Mitsubishi Electric) AND when properly installed.

Take care of your mini-split HVAC system to keep it running at peak performance. Clean the mini-split filters regularly to help prevent problems with your ductless mini-split system. Whether you have one mini-split AC unit or a multi-zone mini-split system that heats and cools your whole house, pay attention to the filters in each mini-split unit.

Dirty or clogged air filters can cause

• Poor or no cooling
• Poor or no heating
• Water leakage
• Mold and mildew issues
• Musty smell
• System damage

Other DIY maintenance includes keeping the mini-split heat pump outdoors free of debris or encroaching shrubbery. Keep the unit clear at least two feet around it.

Schedule professional ductless HVAC maintenance at least once a year for a mini-split air conditioner. Twice a year is better if you have a ductless mini-split heating and cooling system. Schedule with the mini-split installer you used if your system is still under warranty. Whatever you do, work with an HVAC company that has extensive experience with ductless heating and cooling systems.

During the maintenance visit, the HVAC technician should:

• Thoroughly inspect and clean the outdoor and indoor units.
• Ensure the ductless system operates efficiently.
• Check for refrigerant leaks.
• Check electrical connections.
• Make minor repairs.

Be sure to work with an HVAC company experienced with mini-split units. Consider signing up for a maintenance plan and the company will remind you when it’s time to schedule an appointment.

Anne Fonda, Content Writer at Trane Technologies

A Content Writer with Trane Technologies, Anne Fonda researches topics and writes for Trane® and associated residential HVAC brands. She works in collaboration with Trane Technologies subject matter experts, offering easy-to-understand, informative content on complex topics. Her goal is to help consumers make informed decisions on the products and services they need.

She has written for HVAC and other service provider websites for over 16 years. Before transitioning to web content writing, Anne had a 14-year stint as an award-winning journalist. She graduated cum laude from the University of Missouri-Columbia School of Journalism. When she’s not working, Anne enjoys playing word games, reading, gardening, spending time with family, and visiting gardens and museums.

Expert review by Kimberly Sexton, Senior Systems Engineer

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