You turn on your evaporative cooler expecting a rush of fresh, cool air, but instead, you’re hit with a damp, musty odour. It’s a common complaint among homeowners, especially at the start of the cooling season or after the system has been sitting idle. While the smell can be unpleasant, it’s also a warning sign that something inside the unit needs attention.
A musty smell doesn’t usually mean the cooler is faulty or unsafe, but it does indicate hygiene or maintenance issues that shouldn’t be ignored. Evaporative cooling systems rely on water and airflow, and when either is poorly managed, odours can develop quickly.
What Causes the Odour
The most common cause of a musty smell is stagnant water left inside the system. When an evaporative cooler isn’t used for a while, water can sit in the reservoir or distribution lines. This stagnant water becomes a breeding ground for bacteria and microorganisms that produce unpleasant smells.
Dirty or worn cooling pads are another major contributor. These pads are constantly exposed to moisture, dust, pollen, and airborne debris. Over time, organic material builds up, allowing mould and mildew to grow. When the system starts, air passes through these pads and carries the odour straight into your home.
Poor drainage also plays a role. If the unit doesn’t drain fully after shutdown, leftover water lingers inside the base or sump. Even a small amount of trapped water can produce strong odours once bacterial growth begins.
Why the Smell Is Strongest at Startup
Many homeowners notice that the smell is worse during the first few minutes of operation. This happens because the initial airflow pushes out air that has been sitting inside the unit, ducts, and pads. That trapped air absorbs odours from damp components while the system is off.
As fresh water begins circulating and airflow increases, the smell may fade slightly — but that doesn’t mean the issue is resolved. The underlying cause is still present, and the odour will likely return or worsen over time if nothing is done.
Seasonal startup after winter is when this issue is most noticeable. Long periods of inactivity give bacteria and mould time to establish themselves inside the system.
How Lack of Maintenance Makes It Worse
Evaporative coolers require regular cleaning and servicing to function properly. When maintenance is skipped, dust, minerals, algae, and organic debris accumulate inside the unit. Each cooling cycle adds moisture, creating ideal conditions for microbial growth.
Mineral buildup from hard water can also trap moisture in pads and internal surfaces. This not only reduces cooling efficiency but also makes it harder for the system to dry out completely between uses, prolonging damp conditions.
Over time, these issues compound. What starts as a mild musty smell can turn into a persistent, unpleasant odour that spreads through the home every time the cooler runs.
The Role of Pads, Water Quality, and Airflow
Cooling pads are central to evaporative cooling, but they are also the most vulnerable component. Old or low-quality pads degrade faster and absorb more contaminants. Once mould takes hold, simple rinsing is rarely enough to remove it completely.
Water quality matters as well. High mineral content encourages scale buildup, which holds moisture and promotes bacterial growth. Without proper flushing and cleaning, these deposits remain inside the system season after season.
Restricted airflow can also worsen odours. If vents, filters, or ducts are partially blocked, moisture doesn’t evaporate efficiently, leaving damp areas behind. Poor airflow means the system stays wet longer, increasing the chance of smells developing.
Why DIY Cleaning Often Isn’t Enough
Many homeowners try to fix musty smells by hosing down the unit or adding deodorising tablets. While this may temporarily reduce odours, it usually doesn’t address hidden contamination inside the sump, pump, water lines, or pads.
Surface cleaning can’t always reach biofilm, a slimy layer of bacteria that clings to internal components. Once biofilm forms, it protects microorganisms from simple rinsing and allows odours to return quickly.
Without proper disassembly, sanitising agents, and drainage checks, the smell is likely to come back within weeks.
Role of Professionals
A professional Evaporative air conditioner service goes beyond basic cleaning. Technicians fully drain the system, remove and inspect pads, clean the sump and pump, flush water lines, and sanitise internal components to eliminate bacteria and mould.
During a professional Evaporative air conditioner service, airflow, drainage, and water distribution are also checked. This ensures the system dries out properly between uses, reducing the conditions that allow odours to form in the first place.
Regular Evaporative air conditioner service not only eliminates smells but also improves cooling efficiency, extends the life of the unit, and ensures healthier indoor air quality.
How Often Should Servicing Be Done
Most evaporative coolers benefit from a full service at least once a year, ideally before the cooling season begins. In dusty areas or homes with hard water, more frequent servicing may be necessary.
End-of-season shutdown servicing is equally important. Properly draining and drying the system before long periods of inactivity prevents stagnant water from sitting inside the unit.
Consistent maintenance keeps odours from developing rather than trying to eliminate them after they appear.
Conclusion
A musty smell from your evaporative cooler is almost always a sign of moisture-related buildup inside the system. Stagnant water, dirty pads, poor drainage, and a lack of servicing allow bacteria and mould to thrive, especially during periods of inactivity.
Routine professional care keeps your system clean, fresh-smelling, and efficient. Booking a regular Evaporative air conditioner service is the most effective way to prevent odours, protect your indoor air quality, and ensure your cooler performs as it should throughout the season.
FAQs
1. Why does my evaporative cooler smell musty when it first starts up?
A musty odour typically comes from mould, mildew or bacteria growing in the pads, water reservoir or internal surfaces. When the cooler has been idle, stagnant water and accumulated dust promote microbial growth, and the first airflow pushes that odour into your home.
2. Will the smell go away on its own if I keep running the cooler?
Sometimes the smell fades briefly as fresh water circulates, but without cleaning or drying the pads and water tank, the underlying sources remain. Odours usually return unless the unit is properly serviced or the pads are cleaned/replaced.
3. How often should I have an Evaporative air conditioner service to prevent smells?
Most manufacturers and HVAC professionals recommend an annual service before the cooling season. This includes cleaning the pads, sump and water lines to remove mould and bacteria that cause odours.
4. Are dirty pads the main cause of the musty smell?
Yes. Dirty or old cooling pads trap dust, organic debris, and moisture, creating ideal conditions for odour-producing bacteria. Inspecting and replacing pads regularly is key to keeping smells away.
