the air coming out smells musty and gross, like a damp basement. cleaning the filter didn't help.

Diagnosis

The "damp basement" smell in your Costway EP24618US is caused by biological growth—specifically mold, mildew, and bacteria—colonizing the internal moisture-rich environments of the unit. While cleaning the external mesh filter removes dust, it does nothing for the evaporator coils or the internal condensate collection tray. As the unit cools, moisture condenses on the aluminum fins; if this water sits stagnant or if dust bypasses the filter and sticks to the wet coils, it creates a nutrient-rich sludge that smells putrid when the blower fan pushes air across it. This issue is most common in the EP24618US because the internal drain pan can develop a biofilm layer that traps water even after the unit has been drained. You can confirm this by placing your nose directly against the air intake while the unit is off; if the smell is stronger there than at the vent, the growth is rooted deep in the evaporator coils. If the smell only appears during the first five minutes of operation and then fades, the issue is likely concentrated in the condensate tray and the drain line.

How to Fix It

To eliminate the odor, you must chemically neutralize the organic growth on the internal components. Follow these exact steps:

1. Disconnect the power cord from the wall outlet completely to avoid electrical shock.
2. Remove the rear filter panels and locate the evaporator coils (the silver aluminum fins). Use a vacuum with a brush attachment to remove any loose surface dust first.
3. Prepare a cleaning solution of 50% distilled white vinegar and 50% warm water in a spray bottle. Vinegar is essential here because it penetrates the porous biofilm that soap cannot reach.
4. Spray the solution liberally onto the aluminum fins, working from top to bottom. Ensure you saturate the areas where the coils meet the plastic housing, as these corners trap the most moisture.
5. Let the solution dwell for 20 minutes. Do not rinse with a hose, as you risk shorting the control board; instead, use a soft-bristled nylon brush to gently scrub the fins in a vertical direction (never horizontal, or you will bend the delicate aluminum).
6. Locate the drain plug at the bottom rear of the unit. Remove the plug and place a shallow pan underneath. Pour 2 cups of the vinegar solution directly into the internal condensate tray through the coil gaps to flush out stagnant slime.
7. Reinstall the filters and plug the unit in. Set the mode to "FAN" at the highest speed setting for 2 hours. This forces dry air through the system to evaporate any remaining moisture and clear the scent.

If That Didn't Work

• Use a Commercial Coil Cleaner: If vinegar fails, the buildup is likely a hardened scale. Purchase a "no-rinse" alkaline evaporator coil cleaner foam. Spray it onto the coils; the foam will expand, lift the grime out of the deep fins, and liquefy it, allowing it to drain out through the bottom plug.
• Deep Clean the Blower Wheel: The cylindrical blower fan inside the chassis often collects "fuzzy" mold clumps. Use a flashlight to look through the vents. If you see grey clumps on the blades, use a can of compressed air and a long pipe cleaner to manually remove the debris while the unit is unplugged.
• Check the Exhaust Hose: Sometimes the smell isn't coming from the unit, but from the flexible exhaust hose. If the hose has a leak or is sagging, moist air can settle in the ridges of the plastic, growing mold. Wipe the inside of the hose with a bleach solution or replace the hose entirely if it feels slimy.
• Install a UV-C Light Strip: For permanent prevention, some users install a small, low-voltage UV-C germicidal lamp near the evaporator coils. This kills airborne spores before they can settle and colonize the wet surfaces.

Heads Up

• Avoid Bleach on Coils: Never use concentrated bleach on the aluminum fins; it can cause corrosion and pitting, which permanently reduces the cooling efficiency of the EP24618US.
• The "Dry-Out" Routine: To prevent the smell from returning, always run the unit in "FAN" mode for 30 minutes before turning it off completely. This dries the coils and prevents stagnant water from sitting in the pan.
• Drain Frequency: Even if you are using the auto-evaporation feature, manually drain the unit via the bottom plug once every two weeks during high-humidity months to remove sediment.
• Filter Replacement: Replace or deep-wash your filters every 14 days. Dust is the primary food source for the mold that causes the "basement" smell.