Quick Answer: Clean a window air conditioner’s filter every two weeks during cooling season — a dirty filter can raise energy use by 5-15%, per the US Department of Energy. Wipe the front-facing evaporator coil and drain pan with a 5% white vinegar solution once or twice a season to stop mold before it starts (never bleach — it corrodes the aluminum fins). The part most owners skip is the condenser coil on the back half of the unit, the side that faces outside; it collects the most grime of any component and usually requires removing the unit from the window to reach properly.
A window AC that’s cooling weaker or smells musty after a season in place usually isn’t failing — it’s dirty in a spot you can’t see from inside the room. Unlike a portable unit, where every serviceable part sits behind one front grille, a window unit has two separate coils split across the cabinet: one facing into the room, one facing outside through the wall or sash. That back half is where most of the buildup happens, and it’s also the part owners most often skip. Here’s the real cleaning schedule.
Last updated 2026-08-14: reconfirmed the DOE 5-15% dirty-filter energy stat and the vinegar-over-bleach coil/drain-pan guidance — both unchanged.
Cleaning schedule at a glance
| Component | How often | What happens if you skip it |
|---|---|---|
| Air filter | Every 2 weeks (heavy use) | 5-15% higher energy use, weaker airflow |
| Front evaporator coil | 1-2x per cooling season | Musty smell, reduced cooling capacity |
| Rear condenser coil | 1x per season (often needs removal) | Restricted heat rejection, higher energy use, premature compressor wear |
| Drain pan | Monthly in humid climates | Mold growth, standing-water odor, indoor leaking |
| Cabinet exterior & louvers | Monthly | Dust recirculated into the room |
| Full interior dry-out | Before removal or off-season storage | Mold sets in over winter storage |
The filter math is identical to a portable unit’s: the Department of Energy says a dirty air filter can increase a cooling system’s energy consumption by 5-15%, since a clogged screen forces the compressor and fan to work harder for the same cooling output. On a window unit, the filter usually slides out from behind the front grille or lifts out from the top — check your manual for the exact latch, since it varies more by brand than a portable unit’s does.
1. The filter — every 2 weeks
Slide or lift the filter out and hold it up to a light. If you can’t see light through it clearly, it’s overdue for a rinse. Wash it under lukewarm running water, working from the back side so debris flushes out the way it came in, then let it air-dry completely before reinstalling — a damp filter sealed back into a warm, humid cabinet is exactly the environment mold needs to establish.
Replacement window AC filter
- Washable filters handle dozens of cleaning cycles, but replace one that's torn, warped, or won't come clean after rinsing.
- A spare filter means you're never tempted to skip a cleaning cycle because the unit's mid-use.
2. The front evaporator coil — 1-2x per season
The coil right behind the filter, facing into the room, is reachable without pulling the unit. Use a soft brush or a fin comb to loosen dust and debris first — fins bend easily, so work gently and straighten any that get flattened. Once loosened, spray on a no-rinse foaming coil cleaner, let it sit per the label (typically 10-15 minutes), and it drains away on its own without a water hookup. A coil coated in dust and grime is the most common source of a musty smell, since it stays damp between cooling cycles in a dark cabinet — the exact combination the EPA identifies as conducive to mold growth in air conditioning equipment.
No-rinse AC coil cleaner
- Restricted coil airflow is the root cause behind both weak cooling and the classic musty-AC smell.
- No-rinse formulas foam on and drain away without a garden hose.
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3. The rear condenser coil — the part most people skip
This is the real difference from cleaning a portable unit. A window AC’s condenser coil faces outside through the wall or window opening, and it takes the brunt of outdoor dust, pollen, and — on a street-facing or ground-floor install — cottonwood fluff and lawn-mower debris. It’s also the coil doing most of the actual heat-rejection work, so a caked condenser coil is a bigger efficiency hit than a dirty front coil.
For units mounted where the rear is exposed and reachable, the same coil-cleaner-and-brush process works: brush off loose debris, apply a foaming cleaner, let it sit, then rinse with a garden hose from outside. For most installs, though, the rear coil is wedged against the window frame or wall sleeve and genuinely isn’t reachable without removing the unit. If it’s been a full season or more since the last deep clean, or you can see visible buildup on the exterior fins from outside the house, plan on pulling the unit out of the window for this step rather than trying to reach around it in place — a partial clean on the accessible half doesn’t fix an efficiency problem rooted in the half you can’t see.
4. The drain pan — monthly in humid climates
Standing condensate in the drain pan is where most window-unit mold actually grows, not on the coils themselves. Pour a solution of equal parts water and white vinegar into the pan to kill mold at the root and clear minor debris clogs — the same 5% acidity that EPA-aligned home-care guidance recommends over bleach, since vinegar doesn’t corrode the pan or nearby aluminum fins the way bleach can. If the unit has been leaking or smelling musty, this is very likely why; see our window AC leaking water guide if the pan itself needs attention beyond a routine cleaning.
5. Exterior housing and louvers — monthly
Wipe down the front grille, louvers, and visible cabinet exterior with a damp cloth monthly. Dust that collects here gets pulled back into the airflow and recirculated into the room every time the fan runs, undermining the point of cleaning the coils in the first place.
6. Preparing for removal or off-season storage
Run the unit in fan-only mode for a few hours first to dry out the interior completely, then unplug it, pull and dry the filter, and wipe down the drain pan and cabinet. If you’re removing the unit from the window for winter, pick a dry day — trapped rainwater inside the housing during transport or storage is a common, avoidable mold cause. Store the unit upright in a cool, dry place, never on its side, and a breathable cover keeps dust out without trapping residual moisture the way a sealed plastic bag can.
The bottom line
Clean the filter every two weeks, wipe the front coil and drain pan once or twice a season with vinegar (never bleach), and don’t skip the rear condenser coil — it’s the one component a window unit has that a portable AC doesn’t, and it’s usually the dirtiest one because it faces outside. If it’s been more than a season since a full clean, plan on pulling the unit rather than trying to reach around it in the window.
Cleaning won’t fix a unit that’s genuinely worn out — start with our best window air conditioner guide if it’s time to replace instead of repair.