Why Is My AC Running Constantly and Never Shutting Off?

If you are asking why is my AC running constantly, the honest answer is that in a Phoenix summer a system that runs almost nonstop through the hot part of the afternoon is often normal and not broken at all. When something really is wrong, the usual causes are a dirty filter, a filthy outdoor coil, low refrigerant from a leak, a weak capacitor or contactor, a thermostat left on FAN ON, leaky attic ducts, or a system that is too small or too old for the load.

Every July my phone rings with folks sure their unit is dying. Plenty of them do not need a repair.

First, It Might Be Normal (And That Is the Truth)

Here is the part most articles skip. Your air conditioner is not sized to hit any number you punch into the thermostat. It is sized to hold your house a set number of degrees below the outdoor temperature, usually 20 to 25 degrees here. On a 115 degree afternoon with the thermostat set at 72, it is not broken. It is losing a race nobody could win.

The test I give homeowners is simple. Hold your hand at a supply vent. If the air feels good and cold, and the house gains ground once the sun goes down, your system is doing its job. Long run times in extreme heat are efficient. Short, choppy cycles are what wear parts out.

The Real Reasons an AC Runs Constantly in Phoenix

Dirty Filter or Blocked Return Air

Airflow is everything. A packed filter starves the blower, so the system cannot move enough air to satisfy the thermostat. In our dust a filter can load up in a month during monsoon season. Check the return grille for blockage too.

A Filthy Outdoor Coil

The outdoor unit dumps your house heat into the air outside. When that coil is packed with dust, cottonwood, or dryer lint, it cannot. Pressure climbs and the compressor runs forever. Easy to prevent with a regular AC tune-up before the heat hits.

Low Refrigerant From a Leak

Refrigerant is not fuel. It does not get used up. If you are low, you have a leak, usually at the evaporator coil or a rubbed line in the attic. A low system still blows air, just not cold enough, so it never shuts off.

A Weak Capacitor or Worn Contactor

Capacitors are the gas pedal for your motors, and our heat is brutal on them. A weak one makes the compressor struggle, so the unit limps along and never catches up. Both parts are inexpensive next to a compressor. My post on 5 signs your AC needs repair in Phoenix covers the rest.

The Thermostat Is Set to FAN ON

This one is free to fix and I see it constantly. If your fan switch is on ON instead of AUTO, the indoor blower never stops, even when the compressor cycles off. Flip it to AUTO.

Leaky Ducts, Sun, and Thin Insulation

Plenty of valley homes have ductwork running through an attic that hits 150 degrees or better. Every leak is cold air you paid for going into the attic. Add west windows with no shade and the system never gets a break.

What You Can Safely Check Before You Call Me

•     Change the filter. If you cannot remember the last time, that is your answer.

•     Set the thermostat fan switch to AUTO, not ON.

•     Open every supply vent. Closing vents just chokes the system.

•     Move furniture and rugs away from return grilles.

•     Clear two feet around the outdoor unit and pull the weeds.

•     With the power off at the disconnect, rinse the outdoor coil gently with a hose.

That is where you stop. Do not open the electrical panel, touch the capacitor, or add refrigerant from a can. Capacitors hold a charge even with the power off.

When to Shut It Off at the Breaker and Call Right Away

If you turn the thermostat completely off and the outdoor unit keeps running, you likely have a contactor welded shut. The compressor has power with no way to stop. That cooks compressors and it is a fire risk. Same goes for a hot breaker, a burning smell, or scorch marks.

Shut it off at the breaker and call. We run 24/7 emergency HVAC service with no after-hours markup, so there is no reason to sit on it.

A Homeowner in Surprise Who Almost Bought a New System

Last August a lady in Surprise called me. Her unit had run from noon until past midnight for two weeks, and another company had already told her she needed a full replacement.

I pulled the filter first. It looked like a dryer lint trap. Then I found the outdoor coil matted solid with cottonwood. I washed the coil, put in a clean filter, and checked her charge, which was fine. Her split went from about 12 degrees to 20 before I left the driveway. Nobody had checked the simple stuff.

Sometimes Nothing Is Wrong, and Sometimes Repairs Are the Wrong Answer

I will tell people to do nothing, and I mean it. If your unit holds a solid 20 degree split, the filter and coil are clean, and the charge is right, you do not have a repair to make. Bumping the thermostat up a couple degrees, closing the west blinds, and running ceiling fans will do more than any part I could sell you.

The flip side is just as honest. If your system is 15 or more years old, undersized, and into its third repair of the season, chasing parts is throwing good money after bad. The smarter move is a properly sized AC installation backed by a real load calculation. Estimates on installs are free.

Either way, do not ignore a system that truly runs nonstop. Even while it is still cooling, constant running wears the compressor down years early and drives your electric bill up all summer.

Still Wondering Why Your AC Is Running Constantly? Call Us

If you have checked the filter, the fan setting, and the outdoor unit and still cannot answer why is my AC running constantly, let us take a look. We are family-owned, licensed under ROC #366148, and 40-plus years into this in the Phoenix metro. You get a flat quote before work starts, a 1-year parts warranty, and a 2-year workmanship guarantee.

Call 602-767-5341 any time, day or night. We answer 24/7 with no after-hours markup, because your AC does not care what time it is. See our HVAC repair services or look around the rest of Western Sky Mechanical.

Frequently Asked Questions

Is it bad for my AC to run all day in the summer?

Long run times on the hottest afternoons are normal here and easier on the equipment than short, choppy cycles. What is not normal is running around the clock while the house keeps getting warmer.

The line I draw is nighttime. If the system cannot catch up and shut off for a stretch after the sun goes down, something needs attention.

Will turning the thermostat down colder make it cool faster?

No. Your AC only has one speed for cooling. Setting it to 65 does not make it work harder, it just guarantees the system runs nonstop chasing a number it cannot hit on a 115 degree day.

Pick a realistic setting and leave it there. Constant thermostat fiddling makes things worse, not better.

My outdoor unit keeps running even with the thermostat off. What now?

Shut it off at the breaker. That is almost always a contactor stuck closed, and the compressor has no way to stop as long as it has power.

Then call us at 602-767-5341. The contactor is an inexpensive part compared to a compressor, but only if we get to it before the compressor overheats.