Do mini-splits get moldy and can I do anything about it?

Mini-splits have surged in popularity over the past few years, bringing more efficient heating and cooling to homes and businesses around the world, without necessarily requiring a major interior HVAC-related rehab. 

Installing new heating and cooling (HVAC) systems in any space will usually raise new considerations for care and maintenance to maximize comfort, and mini-splits are no different. There is also increased awareness around indoor air quality issues, and their connection to physical and mental health. As mini-splits become more common, more people are asking questions about their impact on indoor air quality, and the potential for mold to form in their systems.

Mini-splits offer several advantages over other HVAC systems, including increased efficiency and easier installation. But since they recycle air within a given space and deal with moisture, especially in more humid climates, it’s natural to wonder if mold can become an issue. 

In this article, we’ll look at how new technologies from manufacturers are helping prevent mold from forming in mini-split systems, and different ways mini-split owners and installers can help keep their systems clean.

One of the best ways to prevent mold growth is to follow manufacturer recommendations for system sizing. This allows individual indoor units to run efficiently without either generating excess moisture, or running shortened cycles that can allow airborne particles to build up. 

It’s also recommended to set a desired temperature and let indoor units keep running, instead of turning them on and off. This allows mini-split systems to keep air moving and run other built-in operations that help keep mold at bay and improve overall air quality, while still operating more efficiently than other HVAC systems. 

In many cases, mini-split manufacturers are adding features and designing their systems to combat mold growth before homeowners or professional cleaners need to get involved. For example, Hitachi has introduced Mold Guard, a feature that will monitor conditions within the indoor unit of a mini-split system. When temperature and humidity are detected at levels that could lead to mold growth, Mold Guard heats the coils inside the unit to a temperature hot enough to kill any possible mold growth within a few minutes. 

Nearly all indoor units for mini-split systems contain a filter, and cleaning it regularly will help prevent mold and keep the unit running efficiently. If you live in a particularly humid climate, or if maximizing air quality is a priority, several manufacturers offer enhanced filters that provide added protection against mold.

Sharp offers Plasmacluster filters that it says capture more environmental particles using charged ions, which have been shown to eliminate airborne mold and suppress surface mold. Mitsubishi offers Dual Barrier Coating on its systems, which is applied to the heat exchanger, vanes, and blower wheel of indoor units to reduce particle buildup. According to the company, tests show that Dual Barrier Coating reduced accumulated dirt, dust, and mold by 90% compared to untreated systems.

Beyond the enhanced manufacturer filters and coatings, there are also common-sense steps homeowners can, and should, take to keep their systems clean and in optimal working condition.

Homes that use mini-split systems should ideally have their indoor units cleaned and serviced once a year by a professional. Homeowners should clean easily accessible components and filters from indoor units every 30 to 90 days. Not only will this help maintain high efficiency and optimal air quality in living spaces, but it can also help alert you to any buildup of mold, dust, or dirt within the system that could justify a deeper, professional cleaning.

Most systems make it relatively easy for homeowners to access the coils and fins of a mini-split indoor unit. These should be cleaned regularly: about once every three months, or more often if you notice any visible buildup or odor coming from the system. 

Manufacturers typically recommend against using a wet or damp cloth to clean the interior components of a mini-split indoor unit. Instead, use a dry cloth to gently wipe down vanes, coils, and interior panels that are within reach when the system is opened up. Using compressed air or a vacuum cleaner can be an easy way to clear debris from coils that are tightly spaced or otherwise difficult to fully access.

For a deeper, but still DIY clean, specialized “coil jet cleaners” or “bib kits” are available that use pressurized detergents capable of reaching deeper into the coils and blower wheel assembly than other cleaning techniques. If you don’t notice any buildup or odor within your indoor unit, these methods should be enough to keep mold out of your system, even in more humid climates.

Some indoor units are designed to allow homeowners to easily remove components like the blower wheel, where mold can accumulate. If this is the case, cleaning this component is one of the best ways to make sure your system is especially clean, and may not require the use of a “bib kit” to address difficult to reach parts of the system.

If you do notice signs of mold or other debris in your system, hiring a professional with expertise in deep-cleaning mini-splits could be a smart idea. Many service providers now have decades of experience installing, maintaining, and cleaning mini-split systems, and will be able to use specialized tools to clean deep within indoor units. Professional service providers will also be better prepared to deconstruct units with more complex designs to clean components that would be out of reach for homeowners. 

In addition to addressing any existing buildup, professional cleaning services may also be able to add a coating to components or give advice on how to use your mini-splits to reduce the chance of mold building up in the system. If your mini-splits are being serviced annually, professionals will be able to spot possible early signs of mold, allowing the problem to be addressed more quickly and with less intense cleaning, or by adjusting the homeowner's operating habits.

While mold in mini-splits can be an issue that impacts air quality and health, mold forming in mini-splits is far from inevitable. It’s also important to remember that if mold is found in indoor units, they can quickly and easily be cleaned and restored to good working order by homeowners, or in more advanced cases, professional cleaning services.

Proper sizing and usage go a long way towards preventing mold from appearing in the first place, and many manufacturers already include features in their systems that help prevent mold from forming. With a proper schedule for DIY and professional maintenance, mini-split owners can be confident that they’re reaping the benefits of clean air alongside the comfort and efficiency gains provided by mini-split systems.