The short answer
Start with the air filter, the thermostat settings, and the outdoor unit. A clogged filter, a thermostat on 'fan on,' or an outdoor unit that isn't running explain a lot of these calls. If the filter is clean and the outdoor fan and compressor are running but the air at the vents isn't about 15 to 20 degrees cooler than the room, it's usually low refrigerant, a dirty coil, or a failing part, and it needs a technician.
What you can check yourself
- The thermostat. Set to "cool," fan on "auto," and at least three degrees below room temperature. Replace the batteries if the display is dim.
- The air filter. If you can't see light through it, change it. This alone fixes a surprising number of "not cooling" calls in a Carolina summer.
- The outdoor unit. Is the big fan on top spinning, and can you hear the compressor? Clear leaves, grass clippings and shrubs back at least two feet.
- The breakers. Heat pumps and air conditioners usually have two: one for the outdoor unit and one for the indoor air handler or furnace. Reset a tripped breaker once. If it trips again, leave it off.
- The vents and returns. Open supply registers and make sure furniture or rugs aren't blocking the return grilles.
- Ice. Look at the copper lines at the outdoor unit and, if you can, the indoor coil. Ice changes the plan: see frozen AC.
What usually causes it
Airflow problems
A dirty filter, dirty indoor coil, a weak blower, or ductwork that's crushed or disconnected in the attic. Leaky attic ducts are a big one in our area: a return leak in a 130-degree attic pulls superheated air straight into the system.
Low refrigerant
A slow leak lowers capacity a little at a time, so it often shows up on the first really hot week of the year. The fix is finding and repairing the leak, then charging the system correctly.
A failed electrical part
Capacitors and contactors in the outdoor unit are the most common failures we see in a hot summer. When the outdoor fan or compressor won't start, the indoor blower still runs, so it feels like the AC is "on" but only moving room air.
A system that's too small, too old, or both
Sometimes the equipment is working as designed and simply can't keep up: an undersized system, a house that's been added onto, or a 15-year-old unit that's lost capacity. That's a conversation about options, not a repair.
When to stop and call
- The outdoor unit hums but its fan doesn't spin, or it clicks on and off.
- A breaker trips again after one reset.
- You see ice on the lines or the coil after changing the filter.
- You smell something burning. Turn the system off at the thermostat and the breaker.
Call Thermal Services at (704) 701-6478 and tell us what you found. It helps us bring the right parts.
Common questions
Why does my AC run all day in July and never shut off?
On a 95-degree afternoon, a correctly sized system may run for long stretches, and that's normal. It should still hold the house within a couple of degrees of the setting and catch up after sunset. If it never shuts off even at night, or the house keeps getting warmer, something is wrong: airflow, refrigerant charge, or a failing part.
Can low refrigerant make my AC run but not cool?
Yes. Low refrigerant lowers the system's capacity, so it runs longer and cools less, and the indoor coil may freeze. Refrigerant doesn't get used up, so a low charge means a leak. Adding refrigerant without finding the leak is a short-term fix.
The outdoor unit is humming but the fan isn't spinning. What is that?
That's often a failed run capacitor or a seized fan motor. Turn the system off at the thermostat so the compressor doesn't overheat, and call for service. Capacitors store a charge even with the power off, so this isn't a DIY part.
Is it bad to keep running an AC that isn't cooling?
It can be. If the coil is freezing or the outdoor unit is humming without the fan turning, running it can overheat and damage the compressor, the most expensive part of the system. Turn cooling off and run the fan until it's checked.