Have you ever used a flashlight to look up into the part of a ductless minisplit where the air blows out? I should have done that a lot sooner than I did. It turns out these heat pumps can get nasty dirty inside. That photo above is from the inside of my wall-mounted ductless unit. Down below you’ll find a video I made about this issue, including how we cleaned it. But first, let’s talk about why this happens.

The causes

There are really four factors that lead to ductless minisplits getting so filthy. First, they don’t have good filtration. The air passes through a pretty thin filter that doesn’t fit tightly across the coil. So dirty air comes through and finds cold, wet surfaces to stick to.

Second, where the air comes out at the bottom of the unit is open to the room. That means humid, dirty air can reach the cold surfaces within. So if there weren’t enough dirt and moisture there already, now we’ve got more.

Third, the blower is downstream from the cooling coil. So it gets cold—really cold—and becomes an excellent condensing surface. Put all that together and you get some nasty dirty surfaces inside the unit.

The blower wheel takes the brunt of it because it’s the thing that’s moving the air. All those cold, wet blades are pushing the air and removing the particles. I guess it’s a kind of filtration, but when you look at that photo above, you’ll probably agree with me that it’s not the kind you want.

And fourth, humid climates provide a lot of airborne water to wet things down. You can still get a dirty minisplit in a dry climate, but it probably won’t get as bad or happen as quickly.

The fix

Well, there’s just one thing to do and that’s clean it. Or hire someone to clean it for you. In the video below, you can see how we cleaned my ductless minisplit. We bought two things: a small pressure washer made for cleaning coils and a plastic shroud with built-in funnel to keep the water where we wanted it. It worked really well, as you can see by all the dirt we got out of my minisplit and how clean it was afterward.

Unfortunately, most ductless minisplits aren’t made for ease of cleaning. Edward Louie, an engineer at Pacific Northwest National Lab, has been pushing hard for manufacturers to improve in this area. He and I both gave talks at a conference in Portland, Oregon, earlier this year, and I saw him disassemble and remove the blower wheel from a ductless minisplit in less than ten minutes. He also recently posted on LinkedIn about four manufacturers that make it easy to remove the blower wheel.

Now, if you’re brave enough, go get your flashlight and take a look at yours. You really shouldn’t let it go four years like I did.

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Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can follow Allison on LinkedIn and subscribe to Energy Vanguard’s weekly newsletter and YouTube channel.