If you look at the thermostat settings for your AC, you’ll find the lowest you can set it is 60°F. That’s more cooling than most people are comfortable with. But looking at that setting brings up a question: what is the limit on how much cooling an air conditioner can provide for a home? Is the 60°F limit on the thermostat arbitrary, or is it possible for an AC to make a home even cooler?
We’ll answer these questions, which will help explain how best to set the thermostat to avoid heavily taxing your air conditioning.
The AC Limit: The Temperature Differential
Yes, a residential air conditioning system has a limit on how cool it can make a house. An AC works by moving heat from the interior of the home using evaporation of refrigerant and releasing it outside the house by condensing the refrigerant. Using this method, the AC can only lower the indoor temperature so much and cannot pump out any more heat. This is called the air conditioner’s temperature differential, i.e. how much the AC can lower the indoor temperature compared to the outdoor temperature.
For standard residential central air conditioning systems, the temperature differential is 20°F. For most days, this is more than sufficient for comfort. When it’s 88°F outside, you can set the thermostat to 68°F and expect your AC to reach it. And 68°F is still considered too low a setting: the US Department of Energy recommends 78°F during the day when people are home as the best energy-saving setting. This means it’s easy to keep a house cool even when the outdoor temperature is 98°F.
If temperatures start to rise to 100°F and above—not common here, but it does happen—we recommend you adjust the thermostat to accommodate it. When it’s 100°F outside, don’t lower the thermostat below 80°F. If you do, the AC will continue to run and run as it attempts to cool the house to a temperature it actually cannot reach. This not only creates energy waste as the AC won’t cycle down, but it will place too much strain on the components and is more likely to lead to the system breaking down. This is one reason the air conditioners tend to break on the hottest days of the year: it’s not a mystical law of bad luck, it’s what happens when the AC is put up against the limits of its temperature differential.
Use Your Thermostat Wisely
The short version of all this is to see that you don’t lower the thermostat setting too low during the summer, no matter how hot it is outside. The 78°F setting for the daytime when people are in the house is a good standard that will manage most summer days without trouble. When people aren’t home or during the night, raise the temperature by 8° to 10° to help conserve energy while still keeping the house cool enough that the AC won’t have to struggle when it needs to go back to the heavier lifting.