Why Does My Air Conditioner Smell Bad When Turned On?

Why Does My Air Conditioner Smell Bad

Many people have experienced the unpleasant smell that comes from their air conditioner when they turn it on for the first time each year. While the cause of this smell is often harmless, it can indicate a more serious problem and shouldn’t be ignored. 

If your air conditioner is suddenly producing an unfamiliar odour and making you feel uncomfortable, it may be time to check it out. The most important thing you must determine: why does my air conditioner smell bad in the first place?

The most common cause of a bad-smelling air conditioner is dirt and dust build-up on the coils. Other potential issues include mould and mildew, dirty filters, animal droppings or carcasses inside the AC unit, or a refrigerant gas leak. 

In some scenarios, this bad smell could also present your household with unexpected dangers. Therefore, immediate attention is required. By troubleshooting the cause of the smell, you can ensure that your air conditioner is safe and efficient all season long.

Read on to learn about the source of unfamiliar aircon odours, how to remove them, and how to prevent them! 

9 Causes of Bad Air Conditioner Smells & How to Fix Them 

When you turn on your aircon and begin to smell decay, must, burning, chemical fumes or a sewerage-like smell, it is a clear indicator that something is not right. So, what are some causes of these odours, and how can we address them?

1. Animal Droppings in Ductwork

A common cause of unusual odours from your aircon unit is the small animal droppings and urine left by rats, mice, and insects. If your air conditioner smells like pee or something even more disturbing, pests may be the cause. 

Animal waste, which can leave a strong aftertaste in your mouth, can also be a significant health hazard. These odours could contribute to asthma and allergy attacks and damage the ductwork inside the air conditioner’s ducts. If seals and connections are torn apart by animals, their waste can cause mould and harmful bacteria to form. 

To avoid this type of damage, get on to your air conditioner repair team immediately before the damage is permanent. You might also want to investigate pest control options if there is a rodent issue around your home.

2. Mould and Mildew Growth

Another potential cause of a bad-smelling air conditioner is mould growth. If your AC smells musty when you turn it on, a simple cause could be the build-up of air conditioner mould and mildew.

Over time, dust, hair, and moisture can build up in the home, and your AC catches these particles on the interior filter, which fuel mould growth. When your air conditioner is on, and air passes over the evaporator coils, a musty or decaying odour may drift throughout your home.

If you notice musty odours coming from your unit, it is crucial to have it professionally inspected for mould. Left unchecked, mould can cause severe respiratory problems. To prevent mould and mildew from forming and avoid musty smells, ensure you clean or replace the interior filter at the start of each season before turning the aircon on for the first time.

3. Refrigerant Gas Leaks

A foul smell could be caused by a gas leak. If you notice a sulfuric or rotten egg smell coming from your air conditioner, turn it off immediately and call a qualified technician. These types of leaks can be classified as toxic and need fixing urgently by an air conditioning technician.  

The refrigerant gas is what is filtered throughout the aircon system when cooling. If this toxin is released directly through the aircon without filtration, it becomes known as refrigerant poisoning. 

Gas leaks can be hazardous and should always be addressed by a professional. If your AC unit begins to emit this chemical or exhaust-like fumes, act immediately and contact an aircon professional. If you or any family members start to feel faint or have difficulty breathing, seek emergency medical attention immediately.

4. Dirty Air Filters

When your aircon unit runs frequently, it is natural for the air filters to build up over time and collect dust and various airborne particles. If the air conditioner is not used as often as it should, dirt can add to the pile-up.

The last thing needed is for your newly installed AC to suffer a breakdown. So how do you get rid of the dusty smell from the air conditioner? 

To avoid any hindrance to your AC’s performance, replacing your air filters or cleaning them every new season is important. This way, you will manage the care of your AC and guarantee ongoing excellent performance and longer-lasting clean air.

5. Stagnant Water in the Drain Pan

Another possible source of bad AC smells is a build-up of water or other clogged debris in the drain lines. What does this mean? Water can build up in the pan of the AC, and over time this can be a breeding ground for fungal growth, mildew and other unpleasant things. 

When you perform regular maintenance checks and notice a bad smell, get a spare towel, remove the water in the pan, and then clean the space with vinegar or similar cleaning solutions. It would then be a good idea to find the cause of the water build-up, which can be achieved with the help of a professional if needed.

6. Clogged Drainage Pipes

Clogged drains in the condensate line can cause the AC to emit bad odours. This is because it leads to the build-up of various microorganisms like algae, mould, mildew and nasty bacteria.

If contamination in the condensate drain goes unaddressed, this can ultimately lead to water leaks. So, what is the best solution? Clogs can potentially be flushed out of the drainage line, but if the problem continues and the odour remains, seek a professional’s help.

7. Animal Carcasses in the Unit

What happens when your AC smells awful, unlike the mould smells you might be familiar with? If you have a ducted system, it could be that a rodent or lizard has snuck into your ductwork and died there. The odour will most probably come from your vents, and the first step in the removal process is to track it down. 

Turn off the AC and check every room to identify the source. If you are lucky, you will find the problem quickly. Otherwise, call an aircon professional with the right tools to assist you.

Split-system air conditioning units aren’t immune to this problem, either. The common house gecko is notorious for getting stuck inside these wall units, so it’s worth checking if there are any intruders in your system causing a bad smell. 

8. Dirty Evaporator Coils

Another example of a bad AC odour is the evaporator coil. The coil is used to turn the liquid refrigerant into gas so it can cool and provide that fresh breeze within the home. As the coils heat up, the dirt and dust begin to break down, releasing an unpleasant smell into the air. 

If excess moisture builds up, it can also cause mould and mildew in the coils. Sometimes these coils can be cleaned but if you are uncertain how to do this yourself, call an expert. As a bonus, a professional air con technician can disinfect and treat your evaporator coil with anti-microbial treatments, hindering future mould growth. 

9. Electrical Wires or Dust Burning

If you smell burning from your air conditioner, you’ll probably need to call an HVAC professional immediately – this can indicate a wide range of potentially dangerous electrical problems.

If your AC unit has not been used for a while, however, the burning smell can occur as the unit is trying to get rid of excess dust that has settled. Giving your air conditioner a thorough clean after it’s been sitting dormant can help prevent this problem. 

Remember, if you’re unsure, calling a professional for safety purposes is always a good idea. 

How to Remove a Bad Smell from Air Conditioner

An unpleasant smell coming from your air conditioner can cause discomfort, which is both unpleasant and difficult to locate. 

To remove a bad or musty smell from your air conditioner, we recommend the following steps:

1. Clear Out the Drain Line 

If you find a clogged drain, the first step is to clean it out before it causes a build-up of bacteria. You need distilled vinegar, a wet or dry vacuum, and a cloth or rag to do this. Start by turning the AC off and heading outside to find the T-shaped cap on the drain line. Removing this cap and locating the blockage, pour distilled vinegar into the pipe, repeating the process for about 30 minutes. Using a wet vacuum, place the hose over the pipe and clear the drain by sucking out the blockage.

2. Clean the Drain Pan 

A drain pan full of water can lead to moisture build-up, which can cause a bad odour that can affect the household’s health due to mould growth. To prevent this build-up, take the drain pan, empty it and wash it with warm soapy water. To kill any possible bacteria present in the pan, wash the pan one more time in vinegar or solution, then leave the pan to dry completely before replacing it. 

3. Replace or Clean Air Filter  

Dust from various microorganisms and hair and skin cells can build up over time, especially in the AC filter. Performing routine maintenance and replacing the air filters at the start of every season will help monitor and prevent any build-up and mould growth. Turning off the air con, gently remove the filter and replace it with a new one or, if it is a reusable one, wash it and air dry it before returning it to the AC.

4. Clean the Evaporator Coils

This problem can be easily remedied by cleaning the coils with a solution of vinegar and water. However, if the coils are excessively dirty, it may be necessary to replace them entirely. To access these coils, open the access panel and find them near the air handler. With a soft brush and a gentle detergent mixed with warm water, you can perform routine AC coil maintenance. 

5. Contact an Air Conditioning Professional 

If you cannot locate the source of the bad odour or are unsure how to proceed, contacting an AC professional is a good step to address the issue sooner rather than later. In addition to troubleshooting the source of the odour, they can thoroughly clean your air conditioner and treat the internal components to prevent future bacterial growth. 

What Should I Do if my Air Conditioner Smells Bad?

It is important to find the source of a bad smelling air conditioner so you can enjoy your fresh air sooner rather than later. Check for the most common causes first-the drain line, the air filter, and ductwork, making sure to turn the aircon off before you begin. 

If you can’t locate the source of the smell or the smell’s cause could be a health hazard, then contacting a professional is highly recommended. Air conditioning technicians can thoroughly clean the air con unit, and treat the coils and components of your air conditioner with anti-microbial compounds. This helps prevent the regrowth of mould and bacteria in the unit for months to come. 

With a solution in mind and a way to fix the odour, your home will be free of undistracted smells creating a safe and fresh home for everyone.

Related Questions

What Does a Refrigerant Gas Leak Smell Like?

If your home smells like rotten eggs or something that has gone off, you may have a refrigerant leak. Refrigerants used in Australian air conditioners have an odorant added to them to make it easier to smell and identify refrigerant gas leaks. 

Why Does my Air Conditioner Smell Like Pee?

Urine smells coming from your air conditioner can be caused by pest waste (such as mice and rats) or may be the result of microbial growth. Bacteria and mould inside your air con unit can give off ammonia, which gives urine its distinctive smell. 


This article is published in good faith and for general informational purposes only. It does not constitute an estimate or quote for any specific service. Platinum Power & Air does not make any warranties about the ongoing completeness, reliability, or accuracy of this information. This article is not intended to replace consultation with a licensed air conditioning technician.

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