Why Does Your Home Air Conditioner Smell Bad With the First Use in the Summer?

A bad smell coming from your air conditioner is not uncommon the first time it is fired up after the cold season. These odors range from a faint musty smell caused by dampness and mildew to a foul, rotten odor usually caused by mold. Special sprays designed for air conditioners, which kill mold and mildew, can be purchased at appliance stores, but proper maintenance during use and before shutting your air conditioner down for the winter can prevent these start-up odors from occurring.

Odors are more common with portable air conditioners that have been stored away all winter.

Evaporator Coils and Fins

When your air conditioner hasn't been used for a long period, dirt and moisture can collect on the evaporator fins. This eventually leads to mold growth which has a distinct decaying odor. As the air is passed through the unit for the first time each cooling season, it is passed into your home with the cool air. This occurs with both portable window air conditioners and HVAC units, and the odor usually dissipates after the initial startup. You can prevent the odor by cleaning the evaporator fins with a mild detergent and water before winter storage and again before restarting the unit.

Drain Pan

Whether you store your window air conditioner away over the winter or leave it installed in your window, it requires some pre-winter maintenance to prevent odors from developing. The drain pan which collects the condensation when your air conditioner is in operation should be emptied after the last use. If the water is left in the pan it can cause mold to form, and you may not notice it until you turn the unit on again in the spring or summer. Before turning the air conditioner on each season, remove the drain pan and wash it with a solution of bleach and water. Rinse thoroughly before replacing in your air conditioner.


Your air conditioner's filter gathers dust and debris that are in the air around the unit. If this is left in the filter over the months that your air conditioner is not in use, the material can develop an odor. The filter should be cleaned after the final use each cooling season and checked again before startup to prevent odors from forming.

Removing Odors

Remove your air conditioner's filter and clean it with a mild detergent and water. Allow it to dry completely before replacing it in the unit. If the filter cannot be cleaned, replace it. Pour a cup of bleach diluted with water into the drain tube that transfers the condensation to the drip pan and then rinse thoroughly with clean water. This kills the algae and mold that may have built up inside and removes the odor. To clean your coils, make a solution containing a few drops of liquid dish detergent and water and remove the cover and the filter from your air conditioner. Spray the inside to kill any mold hiding in hard-to-reach places and allow it to dry before replacing the cover.