What Causes a Mildew Smell From Central Air Conditioning Systems?

Written by kay wagers
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If your air conditioning system is blowing the musty, unpleasant odour of mildew into your home along with cold air, it is a warning that you have a mildew problem within the system. Ignoring the mildew odour or hoping it will go away could lead to an extensive mildew problem that will require professional help to clean.

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The mildew smell from a central air conditioning system is caused by mildew growing in the system. Mildew can grow on the condenser coil inside the main air conditioning unit or inside the ducts that carry cool air into your home. Mildew growth is triggered by too moist conditions inside the air conditioning unit. This can happen when the condenser pan does not drain properly, leading to water building in the bottom of the unit and promoting mould and mildew growth. Mildew spores then become airborne. Some spores land in the air ducts, where they can take root and grow.


Getting rid of the mildew will get rid of the mildew smell, too. If the cause is mildew growing on the condenser coil inside your air conditioning unit, you can remove it. Create a solution of 1/2 cup bleach and 1 gallon water. Wearing gloves to protect your skin, use a sponge and the cleaning solution to wash away the mildew. Dump out any pooling water in the condenser pan. If the mildew smell is coming from inside your air ducts, contact professional duct cleaners and mould remediators to have them cleaned.


You can prevent mildew growth inside your air conditioning system through proper maintenance of the system. Ensuring that the condenser pan drains properly will reduce moisture build-up inside the air conditioning unit. If you have had trouble with mildew growth in the past, cleaning the condenser coil every month or so with the bleach solution will kill any mildew spores before they have a chance to take root and grow. Preventing mildew growth inside the main unit will prevent mildew growth inside your home's air ducts.


If you have had mildew odour problems with your air conditioning system for an extended period, the mildew smell can be trapped in the filter for your air conditioner. If you clean up the mildew but do not change the filter, the odour of mildew can linger inside your home. Replacing the filter will get rid of the final source for the smell.

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