Why does my air conditioner smell mouldy?

Why does my air conditioner smell mouldy?

Why does my air conditioner smell mouldy?

With summer coming soon and another round of la Nina forecast this summer, conditions are ripe, once again for an explosion of mould growth in homes and businesses across south east Queensland.

In short, a mouldy/musty smell can indicate bacterial growth build-up, which can form in the drain, drain pan or evaporator coil (indoor coil). Your evaporator is a dark and humid location, so all sorts of mould and mildew can grow, which causes that unpleasant, musty smell. 

Living in Queensland means we deal with our fair share of humidity, which can directly cause excess moisture (and as a result, mould) to build up within your air conditioning unit or home. There are a number of factors that may cause your air conditioner to emit a mouldy or musty smell. The most likely culprits (for air conditioners) include the following.

Blocked Filters

All air conditioners have filters. These filters trap dust and small particles from the air and allows "cleaner air" to be recirculated into your home or business. Neglect to maintain filter cleanliness can lead to a reduction in air flow, air volume, energy efficiency and is also the leading cause of mould growth in Brisbane.

Solution: To prevent this from happening, you should clean your filters at least twice per year. We recommend in Spring (September - November) right before the "heavy usage period" in summer and in Autumn (March - May) after the heavy usage period.

Blocked Drain(s)

Not only do air conditioners heat and cool, they are also very efficient dehumidifiers. Air conditioners remove humidity from rooms as well as heating and cooling them. Your system has a drain installed that removes the excess humidity from the room and drains it outside. Sometimes a blockage can occur in the drain line so this excess water has no way of escaping. Overtime the blockage causes water to build up in your air conditioner, which is an ideal condition for mould to grow.

AC System Is Too Big

Not a week goes by that we find improperly sized air conditioners in peoples homes or businesses. That's how common the issue is! We have been conditioned to make smarter choices in regards to choosing more energy efficient home appliances, and most air conditioning technicians know this. "What's the star rating of the system"?, "how much will this cost to operate over a year"?  are common question asked by home or business owners.

The response is generally, "let's go the next size up" meaning the contractor is offering the client a larger system because

  • A larger system will heat and cool the space faster than a smaller system, reducing the run time of the major power consuming motors and costing less overtime to warm or cool your home or business.

Going the next size up and oversizing the air conditioner elates the client. The client assumes the larger air conditioner will do the job better, quicker and be more energy efficient. The contractor is jubilant because he has ticked two boxes by offering the larger air conditioner. Cooling the space quickly and alleviating the clients energy efficiency concerns.

If you're curious about the size of your air conditioner, check here

One Solution Creates another Problem

If your air conditioner is too big for your home or business, it can cause the air to be cycled throughout your home too quickly. The result is improper dehumidified air going through your air conditioning system , which can cause excess moisture throughout your home and create ideal conditions for mould to grow. So the larger system has solved one problem, but creates another in the form of potential moisture control and mould issues.

As an experiment, you can use our "dew point calculator" below to see the effects an oversized system has in your home. Dew point is the temperature at which air at a constant pressure has to be in order to change from a gas to a liquid (condensation) anything at or below the dew point temperature will begin to condensate, creating damp conditions, ideal for mould growth. Set the "relative humidity" gauge to 70% (general humidity levels in Brisbane) and play with the "room temperature" settings (this is the temperature you want the room to be) for example. 22 degrees Celsius and 70% relative humidity gives a "dew point" calculation of 16 degrees Celsius. Changing the temperature from 22 to 25 degrees gives a dew point calculation of 19 degrees Celsius, meaning anything in the room at 19 degrees or lower will start to form water droplets in your home and increase the potential for mould growth. For energy efficiency reasons, most air conditioners will supply the room with temperatures at or below 12 degrees Celsius in cooling, so anything directly in the air path will be cooled to this temperature.


If you're experiencing constant mould issues and need help, contact us today for a solution to a healthier home and work place. 


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