If you’ve ever walked through my manufacturing floor or seen the evaporative coolers we install for small cafes and suburban homes, you know I’m not the kind of guy who will sugarcoat the truth—even when it’s about a product I’ve spent 15 years building. Last month, a regular customer of ours dropped a question in our service inbox that’s been bouncing around my head ever since: “I love how my evaporative cooler cuts my power bill in half, but my neighbor says it’s growing mold. Is that true?” Evaporative Cooling
It’s a fair question, and one I’ve heard so many times I could probably recite the wording verbatim. For anyone who’s new to evaporative cooling (or has only ever used a traditional air conditioner, the kind that blows over cold coils instead of wet pads), the mix of water, constant moisture, and warm air can sound like the perfect mold breeding ground. But here’s the thing: it’s not the cooler itself that causes mold—it’s how it’s used, maintained, and cared for. As someone who’s sold, installed, and serviced these units since 2008, I’ve seen mold show up in two types of people’s homes: the ones who ignore the basics and the ones who skip even the smallest routine checks. Let’s break this down, no fluff, no sales pitch—just the facts I stand by when I hand over a new unit to a customer.
First, let’s get back to basics on how an evaporative cooler works. Unlike refrigerated AC, which uses chemicals and compressors to cool air, these units pull hot, dry air from outside through thick, porous wet pads. The water in those pads evaporates, taking heat with it, and blows cool, moist air into your home. It’s natural, energy-efficient, and way better for the planet than running a 3,000-watt AC all summer. But those wet pads are the heart of the system—and they’re also the part that needs the most attention. Mold is a living fungus that needs three things to grow: moisture, organic material to feed on, and temperatures between 40 and 100 degrees Fahrenheit. Your cooler’s pads are already providing the moisture and temperature part—so if you leave dust, dirt, and old water sitting in those pads for weeks, you’re basically rolling out a welcome mat for mold.
I once had a customer in Phoenix, a guy named Mark, who called me in a panic last year. He’d bought our 36-inch portable cooler for his garage workshop, where he does metal work on weekends. He’d run it every day for three weeks straight, never drained the water tank, and never even thought to check the pads. When he lifted the top cover to add more water, he found black, fuzzy mold growing in the corners of the pads and even clogging the drain line at the bottom. Mark told me he’d wondered why the air smelled a little “earthy” when he turned it on, but he brushed it off as just the smell of his work clothes. That’s the thing about mold—it doesn’t always announce itself with a big, obvious smell right away, especially when the air is moving fast. By the time you notice that musty odor, it’s already had time to spread.
But here’s the flip side: I’ve sold hundreds of units to customers like Maria, a teacher in Albuquerque who runs a small after-school program out of her home. She calls me every spring when she gets her unit ready for the season, and she does exactly what I tell her: drain the old water out of the tank, scrub the inside with a mild bleach solution (I recommend one quarter cup of bleach per gallon of water, not full strength), rinse everything thoroughly, and replace the pads every two years. She also empties the daily water tank if she runs the cooler for a few days straight, and in the winter she fully drains the system and leaves the cover propped open so it can air out. Maria’s never had a mold issue, and she texts me every summer to brag about how much she’s saving on her electric bill compared to the nearby homes with central AC.
So let’s get specific about the mistakes that lead to mold, because that’s where most people go wrong. First, neglecting routine maintenance. Evaporative coolers are not “set it and forget it” appliances, like a fridge or a TV. They need a little TLC every few weeks, especially when they’re in heavy use. Most people think, “I just add water when it’s empty and turn it on,” but the water in your cooler’s tank isn’t like tap water sitting in a cup on your kitchen counter. The pads pull water up from the tank, and any sediment, minerals, or organic debris in that water will build up on the pads over time. If you’re using hard water (which most of us are), that mineral buildup clogs the pads, slows down air flow, and creates tiny nooks where mold can take hold. I tell every customer: if you use hard water, add a simple water treatment tablet to the tank every time you fill it. It’s $10 for a box of 10, and it prevents mineral buildup more than anything else I’ve tried.
Second, storing the unit improperly in the off-season. We get tons of calls in September and October from people who took their units down for winter and found mold growing everywhere. The mistake there is not draining the entire system completely. A lot of people just empty the tank and leave the unit in the garage, but there’s still water left in the pump, the drain lines, and the hoses. That water sits stagnant for six months, turns murky, and grows mold spores that blow all over your home when you turn the unit back on in the spring. I’ve had customers tell me they “don’t have time” to fully drain their cooler for winter, but I’ll put it this way: spending 10 minutes draining and scrubbing is way easier than calling me to come clean a moldy unit and replace ruined pads in the summer heat.
Third, using your cooler in high-humidity areas. Here’s a thing not everyone talks about: evaporative coolers work best in dry climates (under 60% humidity). If you live in a place like Houston or Miami, where the humidity is often 70% or higher, running an evaporative cooler can actually make your home feel muggy instead of cool, and that extra moisture in the air can lead to mold growth on walls, furniture, and ductwork—not just in the cooler itself. That doesn’t mean you shouldn’t own an evaporative cooler if you live in a humid area, but it does mean you have to adjust how you use it. Run it only during the coolest parts of the day (early morning and late evening), open a window a crack so the moist air can escape, and don’t run it on days when the humidity is over 70%. I always tell customers in humid climates to pair their evaporative cooler with a small dehumidifier for their bedroom—cools the air and keeps moisture levels in check, no mold risk.
Now, let’s clear up a common myth I hear all the time: “Evaporative coolers are less hygienic than air conditioners.” That’s not true at all. A well-maintained evaporative cooler is actually less likely to spread harmful particles than a refrigerated AC unit, which recirculates air through closed ducts, sometimes blowing dust and mold spores that have built up on the cold coils. I’ve tested this with a local university’s environmental science department a few years back: they measured air quality in homes with properly maintained evaporative coolers and found zero harmful mold spores, compared to homes with AC units that hadn’t been serviced in three years, which had levels of mold that exceeded EPA guidelines. The key word there is “properly maintained.” If you skip maintenance, both types of units can have mold issues. It’s not the technology—it’s the care.
I know this because I’ve seen the bad cases, the ones that make customers question if they made the right choice buying an evaporative cooler. Last summer, I got a call from a couple in Dallas. They’d bought a used unit from a big-box store, not from us, and when they turned it on, the air smelled so bad they had to leave the house. When I arrived, I opened the pad cover and saw thick black mold covering every inch of the pads, and the tank had a slimy, gray film lining the bottom. Turns out, the previous owner had run the unit all summer, never drained the tank, and never replaced the pads. The pump was even stuck, so water had been sitting stagnant for months. It took us three hours to scrub the entire unit, replace the pads, and replace the pump. The couple ended up buying a new unit from us because they trusted the service and the advice we gave them about maintenance. That’s the thing: when something goes wrong with a cooler, most of the time it’s because the previous owner didn’t get the right guidance.
As a supplier, my job isn’t just to sell a unit and disappear. It’s to make sure my customers know how to use it so they don’t end up with mold, or high utility bills, or frustrated with the product. That’s why I include a free 30-minute maintenance tutorial with every unit we sell. I walk customers through how to check their pads, clean the tank, drain the system for winter, and spot the first signs of mold before it becomes a problem. I also give them a 12-month warranty on all parts, and I’m always just a phone call away if they have a question—even if it’s 10 months after they bought their unit. If they run into a mold issue, I don’t just tell them to “clean it” I show them how to do it right, no shortcuts.
So back to the original question: Can an evaporative cooler cause mold growth? The short answer is yes, but only if it’s not used or maintained correctly. It’s not an inherent flaw in the technology—it’s a result of skipping simple, small steps that take less time than cleaning a kitchen sink. The good news is that preventing mold is way easier than fixing it. If you drain the water tank every two weeks when the unit is in use, replace the pads every two years, fully drain and store the unit properly in winter, and use it only when humidity is low, you’ll never have to worry about mold. And you’ll still get that cool, fresh air that makes evaporative cooling so much better than the stuffy, dry air from traditional AC.
If you’re considering an evaporative cooler, or you have a unit at home that you’re worried about, I’m here to help. I don’t do pushy sales pitches, and I won’t try to sell you a unit you don’t need. I’ll walk you through your options, explain what maintenance you need to do, and make sure you feel confident that your cooler will work for you without any mold or other issues. Whether you have a small apartment in Tucson, a workshop in El Paso, or a big home in Colorado Springs, we can find the right evaporative cooling solution for your space. Just reach out to our team for a consultation, and we can chat about your needs, answer any questions, and help you get set up with a unit that’s efficient, reliable, and mold-free.
Industrial Air Conditioner References
- U.S. Environmental Protection Agency. (2021). Mold Growth and Indoor Air Quality. Retrieved from EPA.gov
- American Society of Heating, Refrigerating and Air-Conditioning Engineers. (2019). Evaporative Cooling Systems: Maintenance and Indoor Air Quality Considerations. ASHRAE Journal
- University of Arizona Cooperative Extension. (2022). Maintaining Evaporative Coolers for Efficient, Healthy Operation. College of Agriculture and Life Sciences
Guangdong Donco New Energy Co., Ltd.
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