An evaporator coil packed with dust in Century City: the filter hadn't been changed
A maintenance visit in Century City, May 2025. The brush in the photo shows what came off the coil.
In this guide
What we found on a maintenance visit
In May 2025 we were in Century City for a maintenance visit, and the evaporator coil was completely clogged with dirt. The cause was plain: the customers hadn’t been changing the air filter.
The evaporator coil is the indoor coil, the one that gets cold. House air moves across it, the coil pulls heat out of that air, and the cooled air goes back to the rooms through the ducts. All of that air has to squeeze through the narrow gaps between the coil’s fins. It doesn’t take much dust to get in its way.
What the photos show
In the photo at the top of this page, the brush in the gloved hand has just come off the coil. Its bristles are buried under a thick black mat of dirt, closer to felt than to powder. Behind it you can see the coil’s tube bends, and the walls of the cabinet around them are stained with dirt as well.
Another photo from this Century City maintenance job shows a coil face partway through cleaning. A strip of bare fins shows through in the middle. Around it, dust still covers the coil like a dark blanket, and a dead insect is stuck in it at the upper left.
How does a dirty filter clog an evaporator coil?
The filter’s job is to catch the dust in the air coming back from the rooms before it reaches the blower and the coil. Left in too long, it fills up. Air takes the easiest path, so once a filter is loaded, more of it slips around the edges and carries dust along. A missing filter, or one of the wrong size, lets dust through even faster.
Next in line is the coil, and while the AC is cooling, it’s cold and wet. Moisture from the air condenses on the fins and drips into the drain pan. Dust that lands on a wet coil sticks. Layer by layer, it builds into the mat you see in these photos, right on the face where the air comes in.
Dust that gets past the filter can pack the blower motor too. On two jobs in Northridge and Bel Air, dusty blower motors overheated: one kept shutting off on long runs, and the other burned out. When a blower looks like that, the coil needs checking as well.
What a clogged coil does to an air conditioner
A coil packed like this lets less air through. Less air across the coil means less heat taken out of the house, so cooling falls off and the system runs longer trying to keep up. With too little air, a coil can also get cold enough to freeze.
We’ve seen how far that can go. On a rooftop unit at a clinic that had no filter at all, the dirt-packed coil froze solid, and the water ended up coming through the ceiling.
This Century City visit was maintenance. The work was getting the dust out.
How we cleaned the coil
We started mechanically, brushing all that dust off the coil with the help of several cleaners. You brush a coil along its fins, not across them. The fins are thin metal that folds over at a touch, and a folded fin blocks air just as well as dust does.
It’s slow, hard work, and that’s what makes it expensive. You’re paying for the time it takes to get dust out of a coil that a filter changed on time would have kept much cleaner.
Change the filter on time
That’s the lesson from this job. A filter is inexpensive, and swapping one is a quick job. Cleaning a packed coil is neither.
What you can check yourself
Start by finding your filter. It sits where the return air enters the system, either at a grille in the wall or ceiling or at the air handler itself. Look at it often while the AC is working hard, and if it’s gray and clogged, replace it. For the regular schedule, follow what’s printed on the filter or in your system’s manual.
When you buy a new one, match the size printed on the old frame, and put it in with the airflow arrow pointing toward the unit. Don’t run the system without a filter, even for a few days. If yours is a washable filter, wash it and let it dry completely before it goes back in.
Leave the coil itself to a technician. It sits inside the cabinet behind panels with wiring close by, and its fins are easy to crush.
When to call
Call if the airflow at your vents is weak even with a fresh filter, if you see ice on the copper lines or the indoor unit, if water shows up around the indoor unit, or if the system runs and runs without bringing the temperature down. If there’s ice, switch the cooling off and let it thaw first. Don’t chip at it.
AC maintenance in Century City
We do AC maintenance and repair in Century City and across the Westside. Coil cleaning is part of our AC tune-up, and we price a coil like this one after we’ve seen it on site. The $85 service call is credited toward the repair or the cleaning if you go ahead, and repairs carry a 30-day labor and 90-day parts warranty. CA BEAR registered (#A 50636), EPA Section 608 Universal certified, $1M insured. See our HVAC services or call (213) 205-2055.
FAQ
Can a dirty air filter clog an evaporator coil?
Yes. A filter left in too long loads up, and more of the air slips around it, carrying dust to the coil. While the AC is cooling, the coil is wet, so the dust sticks and builds into a mat. On this Century City job, the filter hadn't been changed and the coil was completely clogged.
How do you clean an evaporator coil this dirty?
On this job we started mechanically, brushing the dust off the coil with the help of several cleaners. The brush goes along the fins, not across them, so the fins don't fold over. It's slow, hard work.
Why does cleaning a clogged evaporator coil cost more than a filter?
Because of the time it takes. Getting packed dust out of a coil by hand is slow work, and a filter changed on time would have kept most of that dust out. We price the cleaning after we've seen the coil on site.
How often should I change my AC filter?
Follow the schedule printed on the filter or in your system's manual, and look at it more often while the AC is running hard. If it's gray and clogged, change it. A washable filter gets washed and dried completely before it goes back in.
Do you service air conditioners in Century City?
Yes. We do AC maintenance and repair in Century City and across the Westside. The $85 service call is credited toward the repair or the cleaning if you go ahead. Call (213) 205-2055.
AC, heater or mini-split acting up? We cover LA and the Valley.
Call (213) 205-2055 or text a photo of the unit and its model sticker.