Why Your AC Runs Constantly in July (And What It's Costing You)

Your AC ran from 1 PM to 9 PM without stopping. Is that normal? In the Valley, the answer is "partly." Here's how to tell how much of it is the weather and how much is your system.

What's normal

An air conditioner is sized to hold your set point at a "design temperature" — for Phoenix, roughly 108–110°. On a 115° day, a correctly sized system will run almost continuously in the afternoon and still might lose a degree or two. That's not a failure. That's physics, and it's why oversizing "for the hot days" is a bad idea (see below).

Normal: long run times on 110°+ afternoons, catches up by evening, holds the set point overnight.

Not normal: runs constantly in the morning, can't hold the set point on a 100° day, or the bill jumped 25%+ with no change in habits.

What's not normal, and why

Dirty outdoor coil. After a dust storm, the coil is insulated with grime. The unit can't reject heat, so it runs longer to do the same job. This is the most common cause we find, and a coil cleaning fixes it.

Low refrigerant. A slow leak means less cooling capacity. The system runs longer every week, and you don't notice until the bill lands. See AC blowing warm air.

Clogged filter or restricted return. Airflow drops, the coil can't move heat, run time goes up.

Duct leaks in the attic. A Valley attic is 140° in July. If 20% of your cooled air is leaking into it, your system is cooling the attic. Common in homes 15+ years old.

Aging compressor. Efficiency drops as the compressor wears. A 12-year-old unit may run 30% longer than it did new for the same cooling.

Thermostat placement. A thermostat on a wall that gets afternoon sun, or near a kitchen, calls for cooling the rest of the house doesn't need.

What it's costing

A 4-ton system pulls roughly 4–5 kW while running. At Valley summer rates, an extra 2 hours of run time a day is roughly $50–$90 a month, depending on your plan. A dirty coil or a slow leak can easily add that. Over a summer, that's a tune-up several times over.

Four fixes, in order of return

1. Change the filter. Rinse the outdoor coil. Free. Do it this weekend.
2. Tune-up with real readings. $59.95. Catches refrigerant, capacitor, and airflow problems before they cost you a July.
3. Duct sealing. If your ducts are in the attic and the house is 15+ years old, ask for a duct leakage check. Often the biggest single bill reduction available.
4. Thermostat strategy. Pre-cool in the morning on off-peak rates if you're on a time-of-use plan. Don't set it to 68 at 4 PM and expect it to get there.

The oversizing trap

Someone will tell you a bigger unit would "keep up." It won't. Oversized units short-cycle — reach the set point fast, shut off, never run long enough to pull humidity, wear out the compressor from constant starts. Right-sized systems run long on the hottest days by design. That's the correct behavior.