We Didn't Think We Needed to Say This, But ... Please Stop Running Your AC With the Windows Open

By Missy Nolan

June 11, 2025

Do you enjoy hearing birds chirp while cold air flows from your home's vents? Running your AC with the windows open may seem like the best of both worlds, but don't unlock your windows just yet. We're here to explain why you shouldn't run your AC with windows open, especially if you're wondering how to save money on your AC bill.

This May Also Interest You: 10 Ways to Make Your Central Air Conditioner More Efficient

Why Is It Bad to Run the AC With Windows Open?

Your air conditioner is designed to cool closed spaces, so you shouldn't run the AC with windows open. You're wasting AC energy, and your HVAC system must work harder. This can wear down your air conditioner quickly, resulting in costly repairs. You may even have to replace your HVAC equipment if you consistently use the AC with windows open.

Inefficient AC use can also drive up your utility bills. Energy bills typically skyrocket when your air conditioner works unusually hard to cool your home.

Leaving the windows open makes your AC work harder for several reasons, including:

  • Increased indoor humidity levels. Leaving the windows open can increase humidity in your home, especially on hot or rainy days.

  • Loss of cold air. Cold air can escape through the open windows, taxing your HVAC system and wasting AC energy.

  • Decreased indoor air quality. Pollutants like pollen and dust can enter your home, clogging your AC's filter.

Running the AC with windows open is hard on your HVAC equipment. It's like filling ice cube trays and setting them in the freezer with the door open. Your water may eventually turn into ice, but closing the freezer door would speed up the process.

How Much Energy Do You Waste Running AC With Windows Open?

Air leaks from unsealed cracks or poorly installed insulation can waste more than 20% of the energy that cools your home. This number may jump even higher while running the AC with windows open. Stop wasting AC energy by shutting your windows before running the air.

Opening the windows at bedtime may also be wasting AC energy, but it depends on how hot and humid it is outside. If you're trying to figure out how to save money on AC bills, consider increasing your thermostat's temperature rather than opening the windows. Try decreasing the temperature by 7 to 10 degrees when you go to bed. A skilled HVAC professional can offer personalized AC efficiency tips for your budget and climate.

More Related Articles:

How Can You Run an Air Conditioner and Still Get Some Fresh Air?

Does closing your windows make your home feel stuffy or stale? Using ceiling or box fans can help circulate air throughout your home. The gentle breeze of a fan can help the air feel fresher in your home.

If your home smells bad, try running air purifiers in each room. Air purifiers remove tiny particles that can cause odors or trigger allergies. Depending on your climate, a dehumidifier or humidifier may also mimic the benefits of outdoor air while your windows are closed.

Sometimes, poor indoor air quality comes from HVAC problems. Make sure to schedule HVAC maintenance that includes filter changes, duct cleaning or vent cleaning when needed. An HVAC tech can help you determine the best maintenance schedule for your equipment.

Air conditioners generally work by cooling the air that's already in your home, not pulling air from outside. However, some units have features that bring fresh air into your home. Consider having a professional add a fresh air intake system to your central air. You can also ask your HVAC provider about PTAC units, which are pass-through air conditioners. You can often find these self-contained air conditioners in hotel rooms, but they can also be used in small homes.

View Plans in Your Area
Search Icon

Sign up for HomeServe emails!

Get the latest news, tips and promotional messages, including special offers.