Why can't my AC keep up when it's 115°F outside?

Phoenix afternoons regularly push past the heat that home cooling systems are planned around. In 2024 the National Weather Service counted 70 days of 110°F or hotter in Phoenix, and the high reached 118°F on both July 5 and July 8. On afternoons like those, many homeowners watch the thermostat creep up for hours while the system runs, and start to wonder whether an AC not keeping up with heat like that means something is broken.

How Phoenix air conditioners are sized

Contractors size home systems with a load calculation called Manual J, from the Air Conditioning Contractors of America (ACCA). It plans around a local “design temperature,” not the hottest day on record. ACCA lists a cooling design temperature of 108°F for Phoenix Sky Harbor and 109°F for Luke Air Force Base, with an indoor target of 75°F.

That design temperature is a line the weather crosses only a small share of the time. Building scientist Allison Bailes explains in GreenBuildingAdvisor that “out of the 8,760 hours in a year, the summer design temperature is the temperature exceeded only 87.6 hours, on average.” He adds that a system should be sized to run almost continuously at the design temperature, with some slack for days a few degrees over it.

So at 115°F, you are well past what the system was planned for. A working AC that runs nonstop and lets the house drift a few degrees above the setting in the late afternoon is often reaching its limit, not failing. For more on how capacity is chosen, see what size AC a Phoenix home needs.

Why an AC is not keeping up with heat in some homes more than others

Two houses on the same street can do very differently on a 115°F day. Common reasons:

• 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. Check 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 and rooms that are hard to cool as reasons to have ducts checked.
• Sun on the windows. DOE recommends awnings on south- and west-facing windows and white shades or blinds to reflect heat away from the house.
• An undersized unit. Trane says an undersized system works overtime to reach the set temperature and wears out faster.
• 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 to do on a 115°F afternoon

• Set a realistic target. SRP suggests 78°F to 80°F when you’re home and up to 85°F when you’re away, and estimates about 2% to 3% savings on cooling costs for each degree above 80°F.
• Pre-cool. APS suggests setting the thermostat a few degrees lower earlier in the day, before its 4 p.m. to 7 p.m. weekday on-peak window, so the house has stored some cooling before the hottest hours. If you’re on an SRP time-of-use plan, use the on-peak hours on your SRP bill instead.
• Shade the glass. SRP says shade screens can cut cooling costs by up to 25%.
• Use ceiling fans. SRP recommends setting fans to blow down in summer; fans cool people, not rooms.
• Keep the outdoor unit clear. DOE recommends trimming plants back at least 2 feet around the condenser.

When it’s a repair, not the weather

Heat alone doesn’t explain these signs:

• Air from the vents feels warm or only slightly cool. See why your AC is blowing warm air.
• Ice forms on the refrigerant lines or the indoor unit.
• The house doesn’t recover overnight, even after temperatures drop.
• The outdoor unit is silent or making new noises while the indoor fan runs.

If you see any of these, turn the system off at the thermostat and call a licensed HVAC technician. 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.