If you’ve ever needed an AC repair in the middle of a heatwave, you’re not alone—and you’re definitely not the only one making the same AC mistakes over and over again. HVAC professionals see it all the time: small habits or overlooked details that end up causing big problems, higher energy bills, and uncomfortable homes. Whether you’re trying to extend the life of your system or simply stay cool without breaking the bank, it helps to know what the pros wish you weren’t doing.

We rounded up 10 rarely-mentioned air conditioning mistakes homeowners make—and what to do instead. These tips go beyond the usual “change your filter” advice and get into the nitty-gritty things that really make a difference in how your AC performs.

1. Covering Vents with Furniture or Curtains

It’s easy to forget those vents are even there, but blocking them, even partially, makes your system work harder than it needs to. HVAC pros say many cooling issues stem from poor airflow, not broken parts. Move furniture, rugs, or curtains away from vents to let the cold air circulate freely. It’s one of the easiest ways to improve performance without spending a dime.

2. Skipping a Post-AC Installation Checkup

After a new AC installation, many homeowners assume everything’s perfect—but mistakes can happen, especially with duct connections or refrigerant levels. Pros recommend a follow-up inspection within a month to make sure your system is running efficiently. It’s a chance to catch any issues early, which can save you hundreds on energy costs and unnecessary service calls down the line.

3. Not Matching Your Generator to Your AC System

If you rely on a backup generator during outages, you might assume it will automatically power your air conditioner. But many generators—especially portable ones—aren’t sized properly to handle the high startup load of a central AC system. HVAC pros often see systems fail to run during blackouts because homeowners didn’t verify wattage requirements or install a soft start kit. Always make sure your generator is matched to your AC’s demands before an outage hits. Otherwise, you risk overloading the generator or damaging your compressor.

4. Letting Dust Build Up on Indoor Coils

We clean counters and floors all the time, but how often do you look behind the access panel of your indoor AC unit? Dust and pet hair can coat your evaporator coils and dramatically reduce your system’s efficiency. Regular coil cleaning (or at least a visual inspection once a season) is a game-changer for long-term performance.

5. Using the AC with Leaky Windows or Doors

Even the best air conditioner can’t keep up with constant hot air sneaking in through gaps and cracks. Pros often find that sealing leaks with weather stripping or caulk leads to noticeably cooler rooms and lower bills. It’s one of those “hidden efficiency” upgrades that pays off fast.

6. Ignoring Unusual Noises (or Odors)

A clunk here or a musty smell there? Don’t wait. These are often early warnings of more serious problems like a failing motor or mold in your ducts. Addressing them early usually means simpler, cheaper fixes. Pros say many major AC repairs could’ve been avoided if homeowners had acted on those small signs right away.

7. Cranking the Thermostat Instead of Being Strategic

When it’s blazing hot outside, it’s tempting to drop the thermostat to 65°F and hope for fast relief. But that won’t cool your home any faster—it only makes your system run longer and wear out faster. Instead, keep your AC set to a consistent temperature (typically 72–76°F) and use fans to help circulate air more quickly.

8. Skipping Insulation in the Attic or Ductwork

You might not think about insulation as part of your cooling strategy, but poorly insulated attics or ducts let precious cool air escape before it ever reaches your rooms. This often-overlooked mistake costs you big in comfort and energy usage. Adding insulation (or even just sealing gaps in ducts) can improve airflow and reduce strain on your system.

9. Not Cleaning the Outdoor Condenser Unit

Your outdoor unit needs breathing room. Leaves, dirt, and even tall grass can block airflow, forcing your system to overheat or shut down. Cleaning around your condenser—at least once a season—is one of the simplest ways to avoid emergency repairs and keep your AC running smoothly.

10. Relying on One Room Thermostat for the Whole House

If your thermostat is in a naturally cooler area (like a shaded hallway), your system might shut off before the rest of the house gets comfortable. HVAC pros often recommend zoned systems or smart sensors in multiple rooms to balance temperatures better throughout your home. It’s an upgrade worth considering, especially in larger houses.

11. Using the Wrong Thermostat Mode

Most people leave their thermostat in “auto” mode year-round, thinking it’s the most efficient option. But during extremely humid months, switching to “on” for short periods can help with moisture control by keeping air circulating. This little trick can make your home feel cooler and reduce the risk of mold or mildew forming, especially if you live in a humid area.

Final Thoughts

Avoiding these common—but rarely talked about—AC mistakes can help your system last longer, run more efficiently, and keep your home far more comfortable during those scorching summer months. Whether you’re planning a new AC installation, trying to avoid a costly AC repair, or just want to lower your electric bill, these expert tips are easy to follow and super effective. Small changes = big results.