Ductless Mini-Splits Can Get Nasty Inside
Have you ever used a flashlight to look up into the part of a ductless mini-split 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 mini-splits 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 mini-split 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 mini-split. 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 mini-split and how clean it was afterward.
Unfortunately, most ductless mini-splits 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 spoke at a conference in Portland, Oregon earlier this year, and I saw him disassemble and remove the blower wheel from a ductless mini-split 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. Don’t be like me.
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 subscribe to Energy Vanguard’s weekly newsletter and follow him on LinkedIn.
Related Articles
How to Clean Out Your Air Conditioner’s Condensate Line
4 Ways a Bad Duct System Can Lead to Poor Indoor Air Quality
* This is a TruTech Tools affiliate link. You pay the same price you would pay normally, but Energy Vanguard may make a small commission if you buy after using the link.
Comments are welcome and moderated. Your comment will appear below after approval.
Each of those dark brown spots at the bottom of the picture is a mold colony that grew from an individual spore. Using a quaternary ammonium compound in the water you are cleaning the unit with will kill the mold. It will grow back, but regular cleaning will keep it to a minimum. Here is a cheap option for making your own disinfecting solution: https://bit.ly/3Xvacoa
It’s food grade.
And, the dust and dirt built up can also lead to a clogged condensate drain and a leaky unit.
I have experienced this first hand. Not good.
More than “gunk”, that is more than likely mold colonization.. dust plus moisture= mold food.
How many PSI is that pressure washer? What is the max that can be safely used on a minisplit?
Thanks for the tip on cleaning the mini split; wish you had mentioned how you rigged up the plastic funnel, but, a $225 small washer seems like overkill to me. I think a quart spray bottle- the type with the large trigger handle would work nicely on spraying that unit out well, also agree w/the other poster that it would not hurt to put some disinfectant in the water.
Check out Corbett Lunsford’s YouTube: 2024 AHR HVAC Show: Testing, Mini-Splits, and Why Manual J is Pissing Me Off at minute 17:02. iFlo condensate line cleaner.
How do you clean the coils in a horizontal ducted unit?
Robert: If you’ve got airtight ducts and good filtration, you shouldn’t have to clean it.
I periodically spray 3M TB Quat Disinfectant on the blower blades which seems to be effective in addressing the mold component after brushing as much as I can without disassembling.
Whatever you do to clean the coil or blower, be sure you rinse it thoroughly with clean water. Some cleaners are corrosive to coils, especially copper ones.
RoyC: Good point. We used just plain water when we cleaned mine.
Allison,
A few more things that we have seen good results with:
1. The penetration behind the unit is often poorly sealed – an infiltration source at the worst possible place behind that cold, wet coil.
2. The fan does not turn off at set point causing re-evaporation off the wet coil. Hack: You can get a conversion modules for conventional thermostats, This moves the indoor temp sensor from the blower return airstream to the thermostat (also helps with comfort), and this configuration shuts the fan off at set point, allowing coil to drain and maintain lower RH.
Side note: I’ve discussed this with Mitsubishi on many occasions, and they know it’s an issue in humid climates. I would love a dip-switch or setting for a blower-off delay to allow drain time or auto blower for all wall-mount controls. However, they don’t seem to have much desire to do anything about it. I think the US market is the only place where people set a temp and leave it on 24/7. Internationally, users tend to turn the AC off when not home or in that room. It’s no wonder the power button is the largest button on the remotes, and our conventional thermostats don’t even have a dedicated “off” button.
I think mini split manufacturers do not have an obvious option for turning the blower off as a way of gaming their efficiency ratings. They do often have a “dry” mode that turns off the blower when the setpoint is reached, but the dry mode usually also reduces the coil temperature and fan speed when the compressor is running which might not be what you want.
There are some downsides to ductless mini-splits compared to ducted systems that are not obvious to many people. By not having to supply external static pressure to force air through ductwork, they have reduced blower power requirements that give them an advantage on overall efficiency. If they made room for better filters, the indoor units would be larger and the blower power would be higher resulting in lower system efficiency. Since the larger indoor units have to distribute the air to the entire space from one supply duct, they have to have higher supply air velocities as compared to ducted systems with multiple supply ducts in a comparable space. Since the space air temperature for a ductless mini-split is controlled by a temperature sensor in the indoor unit rather than at a remote thermostat location, it is difficult to accurately sense the space temperature when the blower is not operating. I have heard of one ductless mini-split manufacturer who had a feature where the system would periodically turn off the indoor blower during cooling and let ice form on the coil. Then it would go into a defrost mode to clear the coil of ice with the intent that this cycle would “clean” the coil. They claimed that the icing of the coil would loosen the dirt so that it would be washed away with the condensate. I don’t know if this product has been successful or not. I doubt that this “feature” would have any impact on dirt and mold accumulation on the other components in the indoor air stream.
Install 5 of these in a home. Save several hundreds in utilities a year and pay twice that in maintenance.
There are uv lights by rgf and perhaps others that supposedly helps mitigate this but I have never used one.
I have never understood why these systems do not have provisions for more effective air filtration. The flimsy screens that they call “filters” are pathetic, compared to the air filters that are used with conventional residential HVAC systems.
The conventional system in my house has a five-inch deep pleated filter inside the air handler housing, and two separate wall-mounted return-air filters. But the mini-split heat pump system for my workshop has only a thin plastic screen for air filtration.
Given this meager level of air filtration, there is no way that the mini-split system would NOT get nasty inside. This appears to me to be an obvious design flaw, but I have yet to see or hear of any concern about this “feature” of mini-split systems.
Lunsford’s Youtube: 2024 AHR Show at minute 10:00 has a Sharp ductless head with MERV 14 filters. The filters are at the top and the head sticks way out. I think he said it isn’t available yet.
With this talk of thin/weak filters on these minis, is it not possible to ADD another layer to whats there?
With low external static pressure blower assembly’s I would doubt descent filters would allow proper airflow. It would take quite a bit of filter area to provide adequate esp.
I have seen photos of some projects that add additional filtration to a mini-split unit. The projects involved building filter housing boxes that are attached to the upper part of the indoor unit, with provisions for installing two or more conventional 1″ air filters to “pre-filter” the air entering the mini-split unit. A search for “mini split filter box” or “mini split dust filter” should return some web pages or videos on this subject.
I have tentative plans to do this with my workshop unit, but I haven’t yet taken the time to figure out the details. It seems obvious that such a pre-filter system would need to provide sufficient filter surface area, so as not to restrict air flow too much. If a single 1″ pleated filter is too restrictive, then the design could provide room for 2, 3 or 4 filters.
The most common pleated filters are 1″ thick, but thicker filters are available. A pleated filter that is 2″ thick will have much more filter surface area (and less airflow resistance) than a 1″ filter of the same width and height, and there also a number of products on the market that are 4″ and 5″ thick .
Filters with lower MERV numbers will pass air more freely, and cheap fiberglass filters are less restrictive than pleated filters, so those are other considerations. The fact is that any type of pre-filter should clean the air much better than the flimsy screens included with mini-split units, but care must be taken to avoid excessive restriction of air flow through the unit.
Personally, I think just adding in some additional free-standing filtration elsewhere should lower the overall dust “burden” that might find its way into the mini-split. Strategically place in areas where particulates might be generated or near the mini-split.
I just finished cleaning out my 3 year old unit and the water in the bucket was pretty dirty!
The shroud/ funnel worked well and really contained the mess. I used a decent pump sprayer with the highest pressure tip and that seemed to do the trick, although I’m sure more pressure would have helped.