As temperatures soar during an intense heatwave, many households instinctively turn their homes into cold havens with the help of air conditioning. However, experts warn that some of our most common habits when using air conditioners during high heat can lead to costly long-term consequences—both for our wallets and for the performance of our cooling systems.

The Risk of “Over-Optimizing”

According to Orlando Penner, a building construction expert and owner of South Country Sheds, many of the worst problems with AC units begin with good intentions. Whether trying to save energy, cool the home faster, or redirect air to specific rooms, people often unknowingly strain their systems. “I’ve seen people burn out their systems trying to optimize them without fully understanding what they’re doing,” says Penner.

With temperatures in some areas expected to hit as high as 45°C, understanding how to properly use your AC could make all the difference in both comfort and cost.

Mistake #1: Setting the Thermostat Too Low

It may seem logical to set your thermostat to the lowest possible setting for faster cooling—but that’s a myth. “The AC cools at a steady rate no matter how low you set it,” explains Brandon Young, CEO of Payless Power. “All you’re doing is making the system work harder, which increases your energy bill without making the space comfortable any faster.”

Young suggests setting the thermostat a few degrees higher than usual and using ceiling fans to circulate the air efficiently. This approach helps cool the space without putting unnecessary strain on the system.

Mistake #2: Neglecting the Air Filters

One of the most overlooked components of AC maintenance is the air filter. “Out of sight, out of mind” often applies here, but ignoring the filter can reduce airflow, force the system to work harder, and even lead to early system failure, says Lisa Purvis, Vice President and CFO of Pro-Tech Heating & Cooling.

Purvis recommends checking filters monthly and replacing them at least every three months—or more often if pets or allergies are involved. During particularly hot periods, such as heatwaves, filters can clog much faster. “If it’s above 35°C outside, inspect the filter every two weeks. It’s the cheapest insurance you can get against heatwave breakdowns,” she adds.

Mistake #3: Closing Interior Doors and Vents

While it might seem efficient to shut off unused rooms or block vents to trap cool air, this can actually backfire. “Closing doors disrupts the balance of the HVAC system, increases pressure in the ductwork, and can lead to leaks or reduced efficiency,” explains Purvis.

Instead, keep interior doors open and avoid blocking vents. Using a programmable thermostat to manage temperature zones is a much more effective strategy.

Mistake #4: Using the Wrong Materials to Seal Ducts

DIY duct sealing can do more harm than good if the wrong materials are used. “People often use tapes or foams not suitable for HVAC systems, which can lead to air leaks or reduced performance,” warns Purvis.

If you suspect duct issues, it’s best to consult a professional to inspect and properly seal your system.

Mistake #5: Ignoring Warning Signs

Unusual noises, inconsistent cooling, or unusually high electricity bills shouldn’t be ignored. “These are often early signs of minor problems that are cheap to fix if caught early,” says Purvis. “Left unchecked, they can turn into expensive repairs.”

Both Purvis and Young recommend scheduling seasonal HVAC checkups—before summer and winter. These checks can identify issues such as low refrigerant, dirty coils, or electrical faults. “Skipping seasonal maintenance before summer is a major mistake,” says Young. “During a heatwave, you want your AC—and your energy bill—to be performing at peak efficiency.”