Mold and black spots in a mini-split

What the spots are

Mold starts when its spores land on a wet surface, and EPA says no mold grows without moisture (EPA). Inside a mini-split, the coil gets cold and wet every time it cools, and the blower wheel spins in that wet air right behind it. Fine dust that gets past the coarse filters can stick to the wet blades. A federal building-science site calls dust and mold on the blower wheel a common problem for wall units (PNNL). EPA says condensation on cooling coils can be a major source of moisture in heating and cooling equipment (EPA).

You’ll see it as black or gray specks on the louvers, a dark coat on the blower wheel blades, or black flakes that blow out with the air. Flakes usually mean the buildup on the wheel is heavy enough to break loose.

Black doesn’t mean worse. CDC says mold comes in many colors and you don’t need to know the type to deal with it (CDC). EPA says that when you can see mold growing, testing is usually unnecessary (EPA).

Is it safe to keep running?

EPA’s advice is direct: don’t run a heating or cooling system you know or suspect has mold, because it can spread spores through the house (EPA). In a hot New Orleans summer that can be hard. If you have another unit or a fan, use it for that room until the moldy unit is cleaned. If it’s the only cooling you have, book the cleaning as soon as you can.

Some people react more than others. CDC says people with asthma or mold allergies can have severe reactions, and people with weak immune systems or chronic lung disease can get infections (CDC). If someone in the house fits that, move faster. CDC also says those people shouldn’t do mold cleanup themselves (CDC).

Bleach: what EPA and CDC say

The two agencies don’t say quite the same thing.

EPA: biocides like chlorine bleach are not recommended as a routine part of mold cleanup, though a professional may use one in some cases, such as when someone in the house has a weak immune system. Dead mold can still cause allergic reactions in some people, so killing it isn’t enough; it has to be removed (EPA).
CDC: household products, soap and water, or a bleach mix of no more than 1 cup of bleach per gallon of water can remove mold from hard surfaces. Open windows while you work, and never mix bleach with ammonia or other cleaners (CDC).

Neither one is about pouring bleach into an air conditioner. Inside a mini-split, skip it. LG’s manual says not to use bleach on the unit (LG), and Daikin says store-bought cleaning agents can damage the coil’s resin parts and cause leaks (Daikin). Getting growth off the wheel and coil is a job for a professional deep cleaning. See the cleaning guide. If you’re thinking about a bag kit, read cleaning bags and kits first.

Mold on the wall is a different job

A cleaning only covers the inside of the unit. If you see mold on the wall or ceiling around it, the moisture problem is in the house too, and the water source needs fixing either way. EPA says a homeowner can usually handle a patch smaller than about 10 square feet, roughly 3 feet by 3 feet. For anything bigger, EPA points to its guide for professional cleanup (EPA). For a small patch, EPA suggests an N95 respirator, gloves, and goggles (EPA).

After a flood

If floodwater reached the unit, don’t just turn it back on. CDC says to have a flooded heating and cooling system checked and cleaned by a professional who has mold experience before you run it, because running it can spread mold through the house (CDC). For the rest of the restart, see restarting AC after an outage.

Keeping it from coming back

Keep indoor humidity below 60%. EPA says 30 to 50% is ideal (EPA); CDC aims for no higher than 50% (CDC). Dry mode helps on muggy days.
Wash the filters on your brand’s schedule. See the filters guide.
Keep the drain pan and line flowing. EPA lists clean drip pans and open drain lines as part of mold prevention (EPA).
Use self clean or mold proof if your unit has it. Daikin says its cycle dries the inside to reduce mold, though it doesn’t remove mold that’s already there (Daikin).
Check the blower wheel with a flashlight each time you wash the filters, with the power off. The cleaning guide shows how.