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How To Fix Mold In Air Conditioning Ducts

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How To Fix Mold In Air Conditioning Ducts


Indoor air quality (IAQ) has an enormous impact on our health and the comfort of our homes, and one of its biggest corruptors is mould. Apart from being unsightly, it releases spores into the air we breathe, which can aggravate those with allergies, asthma, and weakened immune systems.

Unfortunately, mould is unavoidable in air conditioners, because where there is moisture, there is an opportunity for bacteria and mould to grow. So what causes mold in air conditioning ducts? How do you fix it, and when should you call in the professionals?

What is mould?

Mould is a fungi that lives on animal and plant matter. It thrives in damp and poorly ventilated areas, and reproduces by making spores, which are distributed on air currents. Mould is not easy to recognise, but it often appears as a smudge, stain, discolouration, or “fuzz.” 

The most common moulds are green, black, or white, but they can change their appearance depending on their life-stage or age. Airborne mould spores can be found in both indoor and outdoor environments, and when they land on damp areas, can begin to grow and spread.

How does mould grow?

Mould grows wherever there is sufficient humidity in the air, a water source nearby, or moisture present in an environment with limited sunlight. It can be present in homes where there is water damage, leaks, or water source issues, including:

  • Rising damp
  • Condensation from showering, cooking, and clothes drying
  • Roof, wall, and plumbing leaks
  • Blocked gutters and downpipes
  • Areas with poor circulation
  • A lack of ventilation in wet areas like bathrooms and laundries

If a property has a water leak, you are likely to see mould growth at the source of the leak. If the cause is not due to a water leak, mould may grow as patterns on the ceiling, or along external walls. This is due to temperature differences on surfaces, and high humidity levels that produce condensation.

What causes mould in air conditioning units?

All sites of mould growth share one thing in common—moisture—and there is no better location for airborne mould spores than in air conditioners. Because units operate by circulating air from outside your property, the air isn’t purified and is packed with airborne mould spores and bacteria. When your unit is turned off, the interior warms up, and any residual condensation and moisture within the unit and its ducting creates the ideal conditions for mould to thrive.

What issues arise from mould in ducted air conditioning?

Moulds are a microscopic fungus, but not all of them are the same—there are good and bad moulds. For example, good moulds give cheeses like brie and camembert their distinctive texture and flavour, and the drug penicillin is derived from a type of mould.

But when it comes to air conditioners, the mould that might be present isn’t useful or friendly, and is likely to cause you more harm than good. Mould reproduces through mould spores, which  become airborne and  spread throughout your home—on your clothes, on your food and on you— and will be ingested via eating or breathing. Some of the issues resulting from mold in air conditioning ducts include:

Health problems

If you suffer from asthma, respiratory conditions or have an allergic sensitivity to mould, it can trigger a significant reaction. Mould can also lead to nasal congestion, irritated eyes, wheezing, coughing, sneezing, and a sore throat.

In extreme cases, people who suffer from weakened immune systems, immune deficiencies or chronic or obstructive lung diseases can have increased allergic or asthmatic reactions. Some moulds are also thought to be responsible for the production of mycotoxins. These can contribute to serious and even life-threatening illnesses such as pneumonia, particularly in the elderly, in children, and in the immune-suppressed.

Mould growth

Mould can easily create new colonies in air conditioners, and easily spread through their vents and filters. Once mould finds alternative places to settle and grow, it can also start new colonies—including in carpets, closets, cupboards and clothing.

System performance

Because we often have unpredictable weather in Australia—from cold snaps to heatwaves—it can be highly inconvenient when your air conditioning system malfunctions or stops working altogether. Bacteria and mould can cause significant issues with an air conditioner’s overall performance and affect the filters, insulation, or the ducts. This can decrease its efficiency and lead to higher running costs. Mould in air conditioning vents and ducts can also lead to blockages within the air conditioning drains, resulting in inconvenient and typically expensive breakdowns.

How do I know if I have mould in my air conditioner?

If you have mould in your unit, it may not be immediately apparent. However, the first sign is usually a musty smell when the unit is in operation. Once a mould colony is well established, you might then notice distinct grey or black mould deposits that look like circular marks. 

These usually appear on the interior surfaces of an air conditioning unit, and can sometimes even spread to your ceiling and walls. Your unit may also start to become noisy. Ticking, rattling, buzzing, screeching, and grinding noises can mean an internal mechanism is clogged or the fans have a build-up of dirt, dust, or mould.

How do I deter the growth of mold in air conditioning ducts?

Mould can be challenging to remove once it starts forming in your air conditioning unit, so you should do everything possible to prevent it from spreading. The most important steps you can take are those that minimise moisture. These include:

Assessing ventilation

Mould loves damp environments, and it tends to grow in places where moisture can’t easily escape—like “wet” areas such as your kitchen, laundry and bathroom. Resolving the issue can be as simple as improving natural ventilation with vents, doors, or windows. Air circulation can also be increased by moving furniture away from the walls, and using the reverse cycle and/or dehumidification settings on your unit. If ventilation is an issue in areas without air conditioning, like living or sleeping areas, consider purchasing a dehumidifier.

Minimising moisture

Wet areas should be dried thoroughly by wiping bench tops, sinks, tiles, floors, windows, shower glazing, and anywhere where moisture is present. This is important because mould can grow in as little as 24 hours! In the bathroom, make sure your exhaust fan is turned on when you shower and keep it running for at least 30 minutes afterwards. In the kitchen, open windows and use the exhaust fan when cooking or using your dishwasher.

Checking indoor plants

Indoor plants can help improve indoor air quality because they filter pollutants, but they can also provide the perfect breeding ground for mould. To alleviate the issue, when you are watering plants don’t let excess water stagnate in the drip trays. You could also consider adding a dash of a natural antifungal solution to your plants, which can hinder mould growth in the soil.

How do I fix mold in air conditioning ducts?

When performing maintenance yourself, it is essential to protect yourself from mould spores by maximising ventilation, and wearing both gloves and a filter mask. You should also turn off the power supply—this includes from the isolation switch near the outdoor unit. Allow the charge to dissipate for half an hour before beginning maintenance. You can then get started! 

DIY maintenance you can safely undertake includes:

Cleaning or replacing the filters

This should be your top priority in terms of removing mould in air conditioning units, as clean filters are the first line of defence against mould spores. Start by determining how many filters your unit has, where they’re located and what type they are. Some units have disposable filters, while others have filters that can be cleaned and reused.

Cleaning the condenser

The condenser is a coil located in the outdoor unit, and it receives hot, compressed refrigerant gas. With the assistance of a second fan, it expels the heat into the atmosphere while condensing the refrigerant gas into a liquid. The condenser should be free from anything that could affect its functioning. This includes plants, dirt, yard debris and leaves. A build-up of these can significantly reduce its efficiency and even cause damage.

Checking and cleaning the ducting

You can clean some parts of your system’s ductwork yourself by wiping down the visible parts of the ducts. While you are there, you can also inspect these parts for water damage, wear and tear, and the presence of mould. However, because most of the ducts in a ducted system are hidden within walls and ceilings, a thorough cleaning and maintenance process should only be undertaken by a professional.

What should I use to clean my air conditioner?

Scented candles, air fresheners, and diffusers will only mask the problems associated with mould in air conditioning units. The mould itself and the smells that go with it need to be eliminated. 

Warm, soapy water should be used to clean components and then a clean cloth used to wipe down all surfaces before they are allowed to thoroughly dry. You should then wipe them again with a purpose-designed air conditioning cleaner, which often takes the form of a foam cleaner or aerosol spray. Finally, apply a mould inhibitor with a clean, damp cloth. Naturally-derived products that are ideal for inhibiting mould include:

  • Tea tree oil
  • Clove oil
  • Vinegar and water
  • Methylated spirits and water

What shouldn’t I use to clean my air conditioner?

Bleach and anti-mould sprays contain a toxic cocktail of chemicals, which will eventually find their way into your unit, become airborne and then be inhaled, so these should be avoided. Other chemical cleaners can be corrosive or caustic and can cause long-term damage to the internal components of your air conditioner. This will leave your unit vulnerable to breakdowns and may mean your warranty is voided.

When should I call in the professionals?

All air conditioning units should be maintained regularly, and although you can do some maintenance yourself, it’s vital you have your unit professionally serviced at least every 12 months. The best time to schedule this is after winter when humidity and temperatures start to rise again.

A professional technician has the equipment and experience to repair or upgrade your system to keep it running efficiently, and address any issues before they become larger problems. When it comes to detecting and eliminating mould in air conditioning units, they will also have the working knowledge of the anatomy of your particular unit.

Many of the components of your air conditioner where mould may form are also in areas that should never be accessed by anyone other than a trained and licensed specialist because of the risk of injury or electrocution.

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