Why is my AC running non-stop but not cooling below 78°F?

The 20-degree rule: a properly-sized Phoenix AC can only pull indoor temperature about 20°F below outdoor ambient. On a 115°F afternoon, holding 78°F is the practical ceiling for most homes. If you set the thermostat to 72°F, the system will run continuously and never reach setpoint — that's expected physics, not a broken unit. Raise the setpoint to 78–80°F during peak heat, add ceiling fans, and close blinds on sun-facing windows.

Beyond the 20-degree rule, what else makes an AC run constantly?

Common Phoenix culprits: (1) clogged filter, (2) dirty outdoor condenser coil, (3) low refrigerant from a slow leak, (4) failing capacitor causing the compressor to run at reduced capacity, (5) undersized system for the home's load, (6) leaky ducts dumping cold air into the attic, (7) poor insulation. Any of these can double your electric bill in July.

Is it bad to let my AC run 24/7 in Phoenix summer?

No — in fact it's normal. Phoenix AC systems are designed to run long cycles during peak heat, and cycling them off/on repeatedly wastes more energy than continuous operation. What's not normal is running non-stop and losing ground on setpoint.

Does high-pressure lockout cause an AC to keep running?

Opposite — a high-pressure safety lockout will trip the outdoor unit off. If your indoor blower is still running but no cold air is coming out and the outdoor fan is silent, the compressor may be locked out. Turn the system off and call for service.

What temperature should I set my thermostat to in Phoenix summer?

78–80°F during peak sun (2–7 PM) is the sweet spot for efficiency and comfort. Set it lower and your bill climbs steeply — every 1°F below 78°F adds roughly 3–5% to cooling cost. Use ceiling fans to feel 3–4°F cooler at the same setpoint.

Why Is My AC Running Constantly (But Not Cooling Below 78°F)?

The one-line answer: the 20-degree rule. Every honest Phoenix HVAC contractor will tell you the same thing. Here's what it means, and the six other issues that mimic it.

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The 20-degree rule

A properly-sized residential AC in the Phoenix metro can pull indoor temperature roughly 20°F below outdoor ambient. When it's 115°F outside, 95°F is the physical limit at the evaporator coil delta-T; realistic indoor hold with a good envelope is 78–82°F. Setting the thermostat to 72°F on a 115° afternoon will just make the system run non-stop and never catch up — that's expected physics, not a broken unit.

If you're above the 20°F ceiling, check these next

Clogged filter. Swap for a fresh pleated 1" or 4" filter.
Dirty outdoor coil. Cottonwood and dust choke heat rejection. Rinse gently with a garden hose (power off).
Low refrigerant. A slow leak drops capacity. EPA-regulated — call a licensed tech.
Weak capacitor. The compressor can start but run at reduced output. $180–$280 to replace.
Undersized system. If your home was built < 2005 and never had a Manual J load calc, you may simply be short 0.5–1 ton.
Leaky ducts. 20–35% of cool air dumped into a 150°F attic. Fixable with Aeroseal or a duct replacement.
Envelope losses. Old single-pane windows, weak attic insulation, gaps at top-plates. A BPI home energy audit finds these.

Why Is My AC Running Constantly — FAQ

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