The hidden costs of having a mini-split heating and air-conditioning system

Home Improvement

Air quality experts say units collect dust and mold and can be expensive to clean.

By Jim Morrison — Boston.com Correspondent

February 21, 2024

Mini-split heating and air-conditioning systems can boost your home’s energy efficiency, but they come with trade-offs, air quality experts say.

These small, electric systems are replacing traditional fossil fuel-burning boilers and furnaces in new buildings and as part of renovations, especially in the Northeast. The technology has been around for a long time, but it’s much improved over earlier heat pumps. A heat pump is essentially an air conditioner that also can work in reverse to heat a space. Mini-splits are small, ductless units consisting of a wall-mounted blower connected to an outdoor condenser. They can provide just cooling or heating and cooling.

Jeffrey C. May, an indoor air quality expert and mold tester with May Indoor Air in Tyngsborough, said that in order for the systems to work properly, they need to be professionally cleaned regularly, which is not easy or cheap to do.

“A traditional HVAC system has an air filter,” May said, “but mini-splits don’t. They just have that little insect screen, like the ones in a window air-conditioning unit, and those don’t filter very much.”

Since mini-splits are not designed to accommodate better filters, dust gets through the screen and eventually coats the fan blades, which significantly reduces their effectiveness. Dust also gathers on the cooling coils, which greatly hampers their efficiency. But worse, mold spores, which are ubiquitous, proliferate in dusty, damp places like the inside of a mini-split. It’s unclear how much mold can build up and whether it poses a health risk; some people are more sensitive to mold and may have respiratory issues.

How often a mini-split needs to be cleaned depends on many variables.

“It depends on the usage,” he said. “If you have a house with no carpets and not a lot of fleecy surfaces and maybe with two people and no pets, there’s not a lot of dust in the air, so maybe they could get away with cleaning their mini-split every few years. A family with carpets and pets and three kids might have to clean it every year. Or even more.”

Deborah Gordon, owner of Gordon Mycology Laboratory in Groton, is a mycologist and performs residential and commercial mold testing. She said virtually every mini-split she’s ever examined had mold on the inside and was in need of a good cleaning.

“The fan blades are plastic, and they collect dust,” Gordon said. “And then you have this humid environment, and it’s dark. The mold starts growing. Mini-splits are just notorious. It’s becoming a more common problem because mini-splits and heat pumps are becoming much more popular as people are getting away from fossil fuels.”

The fan blades are difficult to access and clean, she said. The unit needs to be partially disassembled to access it, involving skills beyond the typical homeowner’s. And spraying chemicals inside the unit risks damaging the interior components.

“Alcohol kills mold very effectively, but it also dries out the seals,” she said. “You can’t use bleach, because it can corrode things. Same thing with vinegar. There are many problems with using chemicals in these systems. Plus, they’re usually located up high in a wall, so you’d need a stepladder.”

Chris McDonough owns McDonough Environmental Services in North Attleborough and specializes in mold remediation. He said a mini-split system is so difficult to clean, it can take him half a day or so if it’s just dusty. If there’s mold growing in the unit, the time and expense go up. Way up.

“If it’s just dusty in there, we charge in the ballpark of $500 to $600 to clean it,” McDonough said. “If it’s moldy and you need a negative pressure machine or an air scrubber, that’s a lot more involved, and then it could be thousands of dollars. If you want to clean one of those systems really well, you’d have to pump down the return refrigeration, pull the coil from the actual unit. You know, bring it outside to power-wash if you want to get to every side, and at that point, you’re almost better off replacing the unit, depending on its age.”

He said it’s not a question of whether mini-splits will get moldy, it’s when.

So what can residents do to minimize dust and the mold that feeds on it in their mini-splits?

“Once a month or so, they can shut off the circuit breaker … open the front panel, and rinse out the screen,” he said. “Clean out as much as possible with Lysol wipes or dishwashing liquid and warm water. Or hire a professional to clean them.”

May suggested a relatively inexpensive, DIY project that will remove dust from the air and keep the interior of the mini-splits cleaner: building a Corsi-Rosenthal box using a box fan and high efficiency air filters. It won’t eliminate dust from a home, but it will slow the accumulation.

There are cleaning kits people can use to get the dust out of their mini-split as well. But Nelson Sandoval, owner of Better Breathing Environmental in Malden, said every mini-split is different, and these kits aren’t suited for every model. He also points out that disassembling and reassembling a mini-split properly can be complicated, but it’s required to clean the unit thoroughly.

“Mold needs three things to survive,” he said. “It needs humidity, the right temperature range, and food, which is dust. To eliminate mold, you need to eliminate at least one of those. It’s always the right temperature in a mini-split, and there’s always humidity in them, too. The best thing to do is keep them clean.”

He said traditional, whole-house air-conditioning systems have all of the same components and are subject to the same dust buildup and mold growth, even though they typically have better air filtration. It’s just as important to keep them clean as well.

“People say: “I never knew about these things. I never knew these things should be cleaned,’” Sandoval said. “Some of those systems are 40 years old, and people don’t know what needs to be done to maintain them.”

Jim Morrison can be reached at [email protected].