Are Cottonwood Trees Clogging Your AC in Houston?

By Chris Crawford, Owner of Spring Branch AC

Cottonwood fluff can hurt your AC once it mats over the outdoor coil, the thin metal fins wrapped around the unit outside your house. When the cotton gets thick enough that you can’t see the metal, less air gets through the coil. The unit can’t get rid of heat as well, so the system works harder, cools less and wears out sooner. While the cotton is falling, you can keep the fins clear yourself: turn the power off, then rinse them gently with a garden hose. Spring Branch AC has been servicing air conditioners around Houston since 1956, and a gentle hose rinse is what I tell homeowners to do for their outdoor coil.

Does Cottonwood Fluff Hurt Your AC?

Cottonwood fluff hurts your AC by blocking the air your outdoor unit needs. That unit’s job is getting rid of the heat your AC pulls out of the house. The refrigerant picks up that heat indoors and carries it out to the coil (U.S. Department of Energy). The fan on top pulls outside air in through the fins on the sides and blows the heat out the top. Down inside the unit is the compressor, the pump that moves refrigerant through the whole system. It’s really the heart of your AC.

Every time that fan runs, it also pulls in whatever is floating around the yard, and in cottonwood season, that’s cotton. Once enough of it mats on the fins, less air gets through, and the unit can’t get rid of heat as well. The refrigerant’s temperature and pressure climb, so the compressor has to work harder to pump it. It draws more power and runs hotter (ACHR News). You get less cooling, and the AC runs longer to keep the house comfortable (ENERGY STAR).

I filmed a coil like that for our YouTube channel (Maintenance is Cheaper than Replacement). We’d just pulled the outdoor unit off a home with a cottonwood tree in the yard, and the coil was completely covered. As I said in that video, “If I can’t get airflow through this coil, this thing really has to work.” All that extra work made the system fail early. My read is the homeowner probably would have gotten another three or four years out of it with yearly maintenance.

When Do Cottonwood Trees Shed in Texas?

In Texas, cottonwood trees release their cotton, the fluff that carries their seeds, from May through mid-July (Native Plant Society of Texas). By then, your AC is already running most of the day, so the fan pulls air through the coil while the cotton is floating around.

How Do I Know If My AC Coil Is Clogged?

A coil is clogged when cotton covers the fins so thickly that you can’t see the metal underneath. Two other signs show up inside the house:

• The AC runs longer than usual to cool the house.
• The house doesn’t get as cool as it used to.

Those two can have other causes, so start with the coil. It’s a quick check you can do just by looking.

How to Clean Cottonwood Out of Your AC Unit

You can rinse cottonwood off the outside of the coil yourself with a garden hose, as long as the power is off first and the spray stays gentle.

1. Turn the power off at the disconnect, the small box near your outdoor unit. Inside, you’ll find either a breaker to flip off or a pull-out block to remove. If you’re not sure how yours works, switch off the AC breaker in your home’s main breaker panel instead (Family Handyman). The outdoor unit runs on 240 volts, so you don’t want the power on while you’re spraying water around it. Turning it off also keeps the fan from starting while you work.
2. Pull the loose cotton off the fins by hand, or use a soft brush or the brush attachment on a shop vacuum. Brush up and down, not side to side, and go easy. The fins are thin aluminum and bend easily.
3. Set your hose nozzle to the fan setting, not the stream setting, and never use a pressure washer. If your nozzle doesn’t have a fan setting, use the mist or shower setting. Too much pressure can bend those little aluminum fins and really damage the coil, and bent fins block airflow (University of Illinois). Plain water is all you need. When University of Nebraska-Lincoln researchers washed dirty coils with detergent, it did no better than plain water on most of them, and some performed worse afterward.
4. Spray the water up into the fins and move the nozzle up and down, nice and easy. Don’t go across the fins. Work your way around the whole unit. That washes out the cotton, dirt, dust and pet hair stuck in the coil and lets a lot more air through.
5. When you’re done, turn the power back on wherever you switched it off, at the box or at the main breaker panel.

How Often Should I Clean Cottonwood Off My AC?

During shedding season, look at your outdoor coil often, and rinse it whenever you see cotton building up on the fins. A tree right next to the unit or a heavy shedding year means more rinsing.

When Should a Technician Clean the AC Coil?

A technician should clean the coil when cotton is still stuck in it after you’ve rinsed it. Getting it out means taking the top off the unit and washing the coil from the inside out. Call a technician in two other cases, too:

• The fins are bent. A technician can straighten them.
• The coil looks clean, but the house still isn’t cooling. Something else is going on, and that’s an AC repair call.

Your rinse takes care of cotton while it’s falling. A spring AC maintenance checkup cleans out the dirt the coil picked up running all last summer, and our technician checks the rest of the system while they’re there. Book it for March, April or May, before the real summer heat. That unit sits outside in the elements all year, and I can promise you that coil is dirty. I know everybody says, “Really, Chris? I got it checked last year.” I get it, but getting it checked every single year makes a huge difference in how long the system lasts. If you’re wondering what that visit costs, I broke down AC maintenance cost in Houston in another post.

Can Grass Clippings Clog Your AC Too?

Your outdoor unit pulls in grass clippings the same way it pulls in cotton, especially when someone mows right next to it while it’s running. On one tune-up, the unit was only about two years old, and there was grass packed all over the outside of it (Landscapers vs A/C System). The homeowner had a landscaper cutting the grass every week.

The fix is turning the unit off at the same disconnect box you use before rinsing. With the unit off, it’s no longer a vacuum cleaner sucking grass up into the coil. Ask your landscaper to:

• Turn the unit off at the box before they mow or trim around it.
• Mow so the clippings blow away from the unit, not toward it (Mississippi State University Extension gives the same advice for HVAC units).
• Turn the unit back on when they’re done.

How Do I Keep Cottonwood Fluff Off My AC?

You can’t keep every bit of floating cotton off your AC, but a clear space around the outdoor unit keeps cotton and debris from piling up where the fan pulls them in.

• Trim shrubs and plants back so there’s about 2 feet of clear space around the unit, which is what Amana recommends.
• Clear away cotton, leaves and grass that collect around the base so the fan doesn’t pull them into the coil.
• Never cover or wrap the unit while it’s running. A cover blocks the air the same way a mat of cotton does.

If you’d rather leave the coil to us during shedding season, let us know and we’ll come out and clean it. For the yearly check, our maintenance agreement is $169 a year and covers two checkups, one in the spring for the AC and one in the fall for the heating. Our technicians aren’t on commission, so nobody’s going to push you to buy anything. If you ever need us, give us a call at (713) 937-6739.