Short answer
An AC that runs nonstop and can't keep up is usually short on airflow or short on capacity: a clogged filter, a dirty outdoor coil, a weak part, or low refrigerant from a leak. Replace the filter, rinse debris off the outdoor unit, and make sure vents and returns are open. If it still can't hold temperature in the evening, call before it freezes up or wears out the compressor.
What's probably happening
Upstate summers are hot and humid. July highs at GSP average 90.3°F (NWS 1991–2020 normals), and the system is also pulling moisture out of the air the whole time. A healthy system can run for long stretches on those days. The problem is when it runs and loses ground.
Not enough air across the coil
A packed filter, closed vents, blocked returns, or a dirty indoor coil all choke airflow. Less air means less cooling, and eventually a frozen coil.
A dirty outdoor coil
The outdoor unit has to dump the heat it pulled out of your house. Grass clippings, cottonwood fluff, leaves, and pollen pack into the fins and it can't do that well.
Low refrigerant
Refrigerant isn't used up. If it's low, it leaked. A slow leak shows up as a system that gradually stops keeping up, often with ice on the copper line.
A weak part
A capacitor that's fading, or a fan motor running slow, can leave the system running at part strength without stopping it outright.
Ducts in a hot attic
Many homes here have ductwork in the attic or crawlspace. A disconnected or leaking duct in a hot attic can dump cold air where you'll never feel it, while pulling hot attic air in.
What you can check yourself in five minutes
- Check the thermostat: cool mode, fan on auto, and a setpoint you'd expect it to reach. Replace the batteries if the screen is faint.
- Replace the air filter if it's dirty. This one fixes more calls than anything else.
- Walk the house and open supply vents. Make sure furniture and rugs aren't covering the return grilles.
- Look at the outdoor unit. Clear away weeds, shrubs, and anything stacked against it. With the power off at the thermostat, you can rinse the outside of the coil gently with a garden hose from the outside in. Don't use a pressure washer.
- Look for ice on the copper line at the outdoor unit. If there's ice, turn cooling off and run the fan only until it thaws.
- Check whether the drain pan under the indoor unit is full. A tripped float switch can shut cooling off and on.
- Close blinds on the sunny side in the afternoon. It helps more than people expect.
If the house catches up by evening after those, you may be fine. Watch it for a day or two. It also helps to know that a system can only cool the house so far below the outdoor temperature on the hottest afternoons. Holding steady and catching up after sunset is normal. Losing ground all evening is not.
When to stop and call
- There's ice on the lines or the indoor coil, even after a thaw.
- The house never catches up, even overnight.
- A breaker trips again after one reset.
- You smell something burning or hear loud buzzing from either unit. Turn it off at the thermostat and the breaker.
- Water is collecting near the indoor unit or its wiring. Shut the power off first.
Running a struggling system hard for weeks can turn a small repair into a big one. If you're in Spartanburg or Greenville County, call Alpine Cool at (864) 702-0777.
Common questions
Is it normal for my AC to run all day in July?
On the hottest afternoons, a system sized for the house can run for long stretches and still hold the temperature. What isn't normal is running nonstop while the house keeps getting warmer, or never catching up once the sun goes down.
Why does my upstairs stay hot when downstairs is fine?
Heat rises, and upstairs rooms often sit under a hot attic with ducts running through it. Leaky or poorly insulated attic ducts, closed or blocked returns, and a system that was never balanced are common reasons. It is worth having the ductwork looked at.
Should I turn the thermostat way down to catch up faster?
No. The system cools at the same speed whether you set it a little lower or a lot lower. A very low setting just makes it run longer and can freeze the coil.
Could my AC just be too small for my house?
It can happen, especially after an addition or a finished bonus room. But a system that used to keep up and no longer does has usually changed, not the house. Dirty coils, a weak part, or a refrigerant leak are more likely.