Last summer, my window AC started blowing this weird musty smell — like someone left a wet towel in a gym locker for a week. I cleaned the filter (like I always do), turned it back on, and… same smell. I was frustrated. I thought I was doing everything right.
Turns out, I was only cleaning about 20% of what actually needed cleaning.
After some trial and error, a conversation with an AC repair guy who came over for something unrelated, and a lot of YouTube rabbit holes, I discovered there’s so much more to window AC cleaning than just rinsing the filter. And honestly? These steps changed everything — my unit runs quieter, cools faster, and that funky smell? Completely gone.
Here’s what I wish someone had told me years ago.
1. The Filter Is Just the Beginning — Clean the Evaporator Coils Too
Most people stop at the filter. I did for years. Pull it out, rinse it under the tap, let it dry, done. It feels thorough. It’s not.
Behind that filter sit the evaporator coils — a set of thin aluminum fins that actually do the heavy lifting of cooling your air. Over time, dust, pet hair, and moisture create a grimy paste on those coils that no amount of filter-cleaning will fix.
When I finally looked at mine with a flashlight, I was genuinely shocked. It looked like someone had applied a thin layer of grey felt to every surface.
Here’s how to clean them properly:
- Turn off and unplug the unit completely. Non-negotiable.
- Remove the front panel and filter.
- Use a soft bristle brush or an old toothbrush to gently loosen the buildup between the fins. Go with the fins, not across them — they bend incredibly easily.
- Spray with a no-rinse coil cleaner (I use Nu-Calgon Evap Foam, available on Amazon or at most hardware stores). It foams up, breaks down the grime, and drips into the drain pan.
- Let it sit for 10–15 minutes, then wipe away any residue from the accessible areas.
The difference in airflow after this step alone was noticeable within minutes of turning the unit back on. Cooler air, less strain on the motor, and significantly less noise.
One mistake I made: I used a pressure washer attachment the first time thinking “more power = cleaner.” Bad idea. I bent about a dozen fins and had to use a fin comb (a cheap tool that straightens bent coil fins) to fix the damage. Gentle is the word here.
2. The Drain Pan Is Basically a Bacteria Hotel — Flush It Out
This one genuinely grossed me out when I learned it.
Your window AC pulls moisture out of the air and that water collects in a drain pan at the bottom of the unit. In a properly tilted unit, it drains outside. But even then, the pan never fully dries. It stays damp, and damp + dust + warm air = mold and algae growth.
That musty smell I mentioned at the start? Yeah. It was coming from a dark green slime coating the bottom of my drain pan.
To clean it properly:
- Remove the unit from the window (or at least tilt it to access the back/bottom panel — check your model’s manual).
- Use a wet/dry vacuum to suck out any standing water or debris first.
- Mix a solution of 1 part white vinegar to 1 part warm water and pour it directly into the drain pan. Let it sit for 20 minutes.
- Scrub with a stiff brush (an old dish brush works great).
- Rinse thoroughly and make sure the drain hole is completely clear. A clogged drain hole is one of the top causes of water leaking into the room — if you’ve ever had that problem, now you know why.
- Drop in a pan tablet (look for AC drain pan tablets on Amazon — brands like Nu-Calgon make them). These slow algae regrowth for months.
I do this at the start of every cooling season now and once mid-season if I’ve been running the unit heavily. It takes maybe 20 minutes and completely eliminates the mold smell.
If you’re dealing with water dripping inside your home, check out 10 Easy Window AC Cleaning Maintenance Guide Fixes for Water Leakage — it covers that specific issue in much more depth.
3. Clean the Condenser Coils (The Outside Part Nobody Touches)
Okay, be honest — have you ever cleaned the outside coils of your window AC? Not wiped the plastic casing. The actual metal coils on the exterior side that sit facing the outdoors?
I hadn’t. Not once in four years.
The condenser coils are on the back of your unit (the part that sticks outside). They release the heat pulled from your room. When they’re clogged with leaves, cottonwood fluff, insects, and outdoor grime, your AC has to work significantly harder to do its job. Your electricity bill quietly creeps up and you have no idea why.
Here’s the cleaning process:
- With the unit still unplugged, go outside and look at the back of the unit.
- Use a garden hose on a gentle spray setting and rinse from the inside out if possible — meaning from the coil side toward the outside — to push debris out rather than deeper in.
- For stubborn buildup, use a condenser coil cleaner spray (available at HVAC supply stores or online). These are foaming cleaners specifically designed for this purpose.
- Let it foam and rinse clean.
- While you’re there, check if any of the fins are bent. A fin comb can straighten them and noticeably improve airflow.
| Coil Type | Location | Cleaning Frequency | Tool Needed |
|---|---|---|---|
| Evaporator Coils | Inside (front/cold side) | Once per season | No-rinse coil cleaner, soft brush |
| Condenser Coils | Outside (back/hot side) | Once per season | Garden hose, condenser cleaner |
| Drain Pan | Bottom interior | Twice per season | Vinegar solution, wet/dry vac |
I’ve started doing this every spring before I install the unit back in the window. It makes a noticeable difference in how quickly the room cools down during the first hot days of the year.
4. Disinfect the Fan Blades — This One Gets Overlooked Every Time
Here’s something that surprised me: the fan blades inside your AC unit collect a coating of dust and moisture that’s almost invisible until you actually touch them. Run your finger along a blade you haven’t cleaned in a year and it’ll come away with this grey, slightly damp residue.
That residue gets blown directly into your room every time the unit runs.
The fan is usually a squirrel cage blower — a cylindrical wheel with lots of small fins. It’s tucked behind the evaporator coils and most people never see it, let alone clean it.
To clean the fan blades:
- After removing the front panel and filter, look for the fan assembly behind the coils (you may need to remove additional screws depending on your model — refer to your manual or a quick YouTube search for your specific unit).
- Use a long, flexible brush — the kind sold for cleaning dryer vents works perfectly here — to reach between the fan blades and dislodge the buildup.
- Wipe each blade with a microfiber cloth dampened with a 50/50 water and white vinegar solution.
- For heavier buildup, a spray bottle of diluted all-purpose cleaner (like Simple Green) works well. Spray, let it sit two minutes, wipe clean.
- Make sure everything is completely dry before reassembly and powering on.
This step alone reduced the noise level of my unit by what felt like 30%. A dirty, unbalanced fan has to work harder and vibrates more. Clean blades spin smoothly.
For more detailed troubleshooting on airflow problems that cleaning can fix, 8 Proven Window AC Cleaning Maintenance Guide Tricks to Fix Airflow Problems has some really practical follow-up steps.
5. Seal the Gaps Around the Unit — It’s Part of Cleaning Up Your Cooling System
This last one isn’t about scrubbing anything, but it made such a big difference that I’d feel wrong leaving it out.
When I finally deep-cleaned my unit and put it back in the window, I noticed warm outside air was seeping in around the accordion side panels and along the top and bottom edges. On a hot day, that means your freshly cooled air is constantly battling hot air sneaking in — your AC runs longer, uses more electricity, and the room never quite gets as cold as it should.
Dirty or missing insulation foam around the unit is basically undoing all the work you just did cleaning it.
Here’s how to address it:
- Pull the unit slightly forward and inspect the side accordion panels. Are they cracked, warped, or full of gaps?
- Use foam weatherstripping tape (available at any hardware store for a few dollars) to seal the top and bottom edges where the unit meets the window frame.
- For the side panels, foam insulation cut to shape or purpose-made window AC insulation kits work brilliantly.
- If the accordion panels themselves are warped beyond fixing, replacements are available for most major brands (LG, Friedrich, Frigidaire, GE) either from the manufacturer or on Amazon.
Here’s a quick checklist I now go through every time I install my unit for the season:
Pre-Season Window AC Checklist
- [ ] Evaporator coils cleaned
- [ ] Drain pan flushed and tablet added
- [ ] Condenser coils rinsed
- [ ] Fan blades wiped down
- [ ] Air filter washed and dried completely
- [ ] Window seals and foam weatherstripping checked/replaced
- [ ] Unit tilted slightly backward for proper drainage
The Mistakes I Made So You Don’t Have To
A few things I got wrong before I figured all this out:
Cleaning while the unit was still plugged in. Even just to “quickly check something.” Don’t do this. It’s genuinely dangerous.
Using too much water near electrical components. Some areas of the unit should only be wiped with a damp cloth, not sprayed directly. The control board and any wiring should stay dry.
Reassembling before everything was fully dry. I once turned the unit on with a slightly damp filter because I was impatient. It started smelling burnt for about 20 minutes. Scary enough to never rush again.
Skipping the fin comb. Bent fins on the coils look minor but they genuinely restrict airflow. A fin comb costs about $8 and takes five minutes to use. It’s worth it.
Only cleaning once every two years. I used to think “it still works, why bother.” Then my electricity bill one August was $40 higher than the previous year for no obvious reason. A dirty AC runs longer to achieve the same temperature. Clean it every season — beginning and mid-season if you run it heavily.
How Often Should You Be Doing All This?
Here’s a simple schedule that’s worked well for me:
| Task | Frequency |
|---|---|
| Air filter rinse | Every 2–4 weeks during active use |
| Evaporator coil cleaning | Once per season (start) |
| Drain pan cleaning + tablet | Twice per season |
| Condenser coil rinse | Once per season (before installation) |
| Fan blade wipe-down | Once per season |
| Window seal inspection | Once per season |
| Full deep clean | Annually (end or start of season) |
Final Thoughts
Honestly, I spent years doing the bare minimum with my window AC and wondering why it smelled weird, ran loud, and never cooled as well as it seemed like it should. Turns out it just needed a proper clean — not a quick filter rinse, but an actual thorough cleaning of every part that accumulates grime.
None of these steps require special skills. You need a screwdriver, some basic cleaning supplies, about 90 minutes, and the willingness to actually look inside the machine.
The reward is real: better cooling, lower electricity bills, cleaner air, and a unit that’ll last years longer.
Start with the drain pan and evaporator coils first — they’ll give you the biggest immediate improvement. Then work through the rest before next summer hits.
For more on building good habits around your AC to prevent future issues, this guide on 7 Smart Window AC Cleaning Maintenance Guide Habits That Prevent Breakdowns is a great read that pairs well with everything covered here.
Want to go even deeper on keeping your unit running at its best all season? Check out 9 Essential Window AC Cleaning Maintenance Guide Tips I Learned After Repairs — it covers what happens when you skip maintenance too long and how to bring an older unit back to life.
