Is it normal for my AC to run all day in a Phoenix summer?

Yes. In a Phoenix summer, an AC running constantly through the afternoon is usually doing exactly what it was built to do. Home systems here are sized to run almost nonstop on the hottest days, so hours of runtime on its own is not a sign of trouble. It becomes a problem when the air from the vents isn’t cool, the house keeps getting warmer anyway, or the system runs nonstop in mild weather too.

Why AC running constantly is normal in a Phoenix summer

Contractors size home air conditioners with a load calculation called Manual J, from the Air Conditioning Contractors of America (ACCA). It plans around a local “design temperature”: the outdoor temperature a place is expected to exceed only 1% of the hours in a year. ACCA lists 108°F as the cooling design temperature for Phoenix Sky Harbor, with an indoor target of 75°F.

A system sized for those conditions is supposed to run for long stretches when it’s near 108°F outside. Building scientist Allison Bailes of Energy Vanguard writes that at design conditions “your air conditioner should run continuously,” and that long runtimes are the best way for a properly sized system to pull moisture out of the air. That matters during monsoon humidity.

Recent summers have gone past that line far more often than the long-term average it’s based on. The National Weather Service counted 70 days of 110°F or hotter in Phoenix in 2024, and 143 days of 100°F or hotter. On days like those, a working system may not shut off all afternoon.

In milder weather it should cycle on and off. Trane describes a normal cooling cycle as about 20 to 30 minutes and says it’s normal for a unit to run longer than usual in the hottest months. If the house can’t hold its setting at all on the worst afternoons, see why an AC can’t keep up when it’s 115°F outside.

What makes it run longer than it needs to

Some of that runtime comes from the house and the equipment, not the weather:

• A dirty filter. The U.S. Department of Energy (DOE) says clogged filters reduce airflow and efficiency, and that replacing a dirty filter can lower an air conditioner’s energy use by 5% to 15%.
• Dirty outdoor coils. DOE notes that condenser coils get very dirty in dusty surroundings, which cuts performance. Look at the outdoor unit after a dust storm.
• Leaky ducts. ENERGY STAR says about 20% to 30% of the air moving through a typical duct system is lost to leaks, holes, and poor connections. It flags ducts in an attic or garage, rooms that are hard to cool, and high summer bills as reasons to have ducts checked.
• Sun on the windows. Afternoon sun on glass adds heat the system has to remove. DOE recommends awnings on south- and west-facing windows and white shades or blinds to reflect heat away.
• Low refrigerant. DOE says a system low on refrigerant was either undercharged when installed or has a leak, and that a trained technician should fix the leak and recharge it.

What you can do to shorten the runtime

• Raise the thermostat a little. SRP suggests 78°F to 80°F when you’re home and up to 85°F when you’re away. It estimates about 2% to 3% savings on cooling costs for each degree you go above 80°F.
• Run ceiling fans. SRP recommends setting fans to blow down in summer and turning them off when you leave a room, since fans cool people, not rooms.
• Shade the glass. SRP says shade screens can cut cooling costs by up to 25%.
• Keep the outdoor unit clear. DOE recommends trimming plants back at least 2 feet around the condenser.
• Pre-cool on a time-of-use plan. APS suggests setting the thermostat a few degrees lower before its 4 p.m. to 7 p.m. weekday on-peak window, then raising it during those hours. If you’re on an SRP time-of-use plan, use the on-peak hours on your SRP bill instead.

When constant running means something is wrong

Long runtime alone is not the problem. These signs are:

• Air from the vents feels warm or only slightly cool.
• Ice forms on the refrigerant lines or the indoor unit. See what to do when your AC freezes up.
• It runs nonstop in mild weather, such as a spring or fall day in the 80s, when it should be cycling on and off.
• It won’t shut off even when you set the thermostat well above room temperature. Trane notes that a contactor stuck closed can keep the unit running.

If you see ice or warm air, turn the system off at the thermostat and call a licensed HVAC technician. If it won’t shut off from the thermostat, switch off the AC breaker in your electrical panel or the disconnect next to the outdoor unit, then call. Don’t add refrigerant yourself or open electrical panels.

Extreme heat is dangerous. If the house is getting hot and a repair is hours away, go somewhere cool. Maricopa County opens heat relief sites, including cooling centers, from May 1 through September 30, and 2-1-1 can point you to help. Watch for dizziness, nausea, headache, heavy sweating, or weakness; if they appear, move somewhere cooler, drink fluids, and seek medical help. Symptoms can escalate quickly.