According to the 20/20 rule, you shouldn’t have your air conditioning set more than 20 degrees higher or lower than the outdoor temperature. In other words, if it’s a blistering 100 degrees outside, the lowest you should set your AC to is 80 degrees. But is that realistic during a heat wave? And, will it hurt your air conditioning unit (or cost you more money in energy bills) if you surpass the 20-degree temperature difference?

To find out whether the is rule is legitimate or just a myth, we turned to Andy Fox, a filtration and indoor air quality specialist. Ahead, Fox weighs in on the rule and shares tips for how to maximize the unit’s efficiency to keep summer utility bills down.

Andy Fox is a filtration and indoor air quality specialist at Filtrete.

Legit Rule or Just a Myth?

The 20/20 rule states that you shouldn’t have your air conditioning set outside a 20 degree window, but is that a good rule of thumb or just a myth? “There’s no universal rule that says your thermostat should never be set more than 20 degrees below the outdoor temperature, but it can be a useful rule of thumb,” Fox says. The expert explains the general idea: air conditioners may be “sized” to your home to provide a maximum 20 degree cooling compared to the outside air temperature, and if the temperature is set lower than that, the AC compressor may run continuously for hours (or days) on end. Fox warns that this can cause the air conditioning unit to fail prematurely.

“Instead of focusing on the temperature difference between indoors and outdoors, homeowners should focus on setting the thermostat to the highest temperature that still feels comfortable,” Fox advises. Taking that approach helps reduce energy use, while allowing you to maintain a comfortable indoor environment during a heat wave. If you’re not going to be at home during the day, he recommends raising the thermostat by a few degrees and cooling the home again when you return. He notes this is particularly smart to do if your utility company charges higher electricity rates during peak demand periods.

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What Happens If You Set Your AC Outside the 20 Degrees?

If you’re wondering whether setting the AC to more than 20 degrees below the outside temperature will hurt the unit, Fox says not necessarily. “The suitability for setting your AC more than 20 degrees below the outside temperature is influenced by many factors, including the cooling capacity of your home’s air conditioner, the humidity, heating from the sun, shading, indoor heat-generating activities, and more,” he says.

Anytime you find that the AC compressor, which is the outside unit, is running non-stop for extended periods of time—even if the temperature difference is less than 20 degrees—Fox advises that you consult your HVAC professional.

Does the 20/20 Rule Save Money?

Energy bills can skyrocket in the summer, especially if your AC is running around the clock. “Every degree you set the AC lower will cost you more money, since one of the biggest factors that determines the cooling energy required is the difference in temperature between inside and outside,” Fox explains. He notes that roughly speaking, if the indoor-outdoor temperature is twice as big, it will take twice the energy to run the AC.

“A common misconception is that turning the thermostat all the way down will cool your home faster; air conditioners cool at a consistent rate, regardless of how low the thermostat is set,” Fox shares. For example, if you set the AC to the coldest possible temperature and forget to adjust it later, the system may continue running longer than necessary, using more energy and increasing your utility bill as a result.

“To maximize efficiency, choose the highest temperature that still keeps you comfortable, and make sure you have an effective HVAC system and high-performance air filter,” Fox says. Additionally, he stresses the importance of regularly replacing your air filter, keeping the outdoor condenser unit free of dirt and debris, and scheduling routine HVAC maintenance in order to improve the unit’s cooling performance and help reduce energy consumption.

Expert Tips to Maximize Your AC’s Efficiency

To keep your home cool and comfortable during a heat wave, Fox shares these tips to maximize your AC’s efficiency:

• Reduce the amount of heat entering your home by closing blinds and other window treatments during the day.
• Take advantage of cooler nighttime temperatures by opening windows when appropriate.
• Limit activities that generate excess heat—such as baking or running heat-producing appliances during the hottest hours.
• Use fans to help make rooms feel cooler and allow you to keep the thermostat a few degrees higher without sacrificing comfort.
• If you only need to cool one area of the home, use a window or portable air conditioner, as it may be a more energy-efficient option than cooling the entire house.
• For homes with central air, don’t skip regular HVAC maintenance. “A well-maintained system not only cools more effectively during a heat wave but can also help keep energy costs under control,” Fox concludes.