When the AC is running but the house won't cool down, it's usually one of a handful of things: airflow, the outdoor unit not running, a frozen coil, or a drain problem shutting things down. Here's the checklist I'd walk you through on the phone, in the order that finds the problem fastest.
One note for the Triangle: on our hottest, most humid days, even a healthy system can struggle to pull the house down more than about 15 to 20 degrees below the outdoor temperature. If it's 98 outside and 78 inside and holding, that may be normal. If the house keeps getting warmer, start here. For more on causes, see our article on why an AC runs but doesn't cool.
Safety first
- Smell gas (rotten eggs)? Don't touch switches or the thermostat. Get everyone out, then call your gas company or 911 from outside.
- Carbon monoxide alarm going off? Get everyone outside into fresh air and call 911. Don't go back in to check the furnace.
- Turn the power off at the furnace or air-handler switch before you open any panel.
- Never bypass a safety switch (door switch, float switch, pressure switch, limit switch). They shut the system off for a reason.
What to check
Check the thermostat
Make sure it's on Cool with the setpoint below the room temperature, and the fan on Auto. With the fan on "On," the blower runs between cycles and can blow warm, humid air.
Replace the filter
A clogged filter starves the indoor coil of airflow. That reduces cooling and can freeze the coil. If it's gray and you can't see light through it, change it.
Make sure the outdoor unit is running
Go outside while the AC is on. You should hear the compressor humming and see the fan on top spinning, with warm air blowing up out of it. If the indoor blower is running but the outdoor unit is silent, check the breaker and the disconnect box on the wall next to the unit.
If the fan isn't spinning but the unit is humming or buzzing, turn it off at the thermostat and call. A failed capacitor or fan motor is common, and running it that way can damage the compressor.
Check the breakers
The outdoor unit and the indoor unit usually have separate breakers. Reset a tripped one once. If it trips again, leave it off.
Look for ice
Check the copper lines at the outdoor unit and, if you can see it, the indoor coil. Ice or frost on the larger, insulated line or on the indoor unit means the coil is freezing, usually from low airflow or low refrigerant. Turn the system to Off (leave the fan on Auto or On) and let it thaw for a few hours before running it again. If it freezes again, call.
Check the condensate drain and float switch
If the AC shuts off by itself, look at the drain pan under the indoor unit (often in the attic). A full pan or a tripped float switch on the drain line will kill the cooling to protect your ceiling. Clearing the drain from the outside end with a wet/dry vac often fixes it.
Clean around the outdoor unit
Leaves, grass clippings, cottonwood fluff and pine straw packed against the outdoor coil block heat from escaping. Turn the power off and gently rinse the coil from the outside with a garden hose (no pressure washer), and keep plants trimmed back a couple of feet.
Open the vents and returns
Closed supply vents and a blocked return grille reduce airflow across the coil. Open them up, especially the return.
What usually causes it
In my experience, most "running but not cooling" calls come down to a few repeat offenders. The most common is a failed capacitor in the outdoor unit. It's a small part that helps the compressor and fan start, and summer heat wears them out. The indoor blower keeps running, so the system seems to be on, but the outdoor unit isn't doing the work.
Next are airflow problems: dirty filters, a dirty indoor coil, or a blower running slow. Low airflow makes the coil get too cold, it freezes, and once it's a block of ice it can't absorb heat at all.
Then there's refrigerant. A system doesn't use up refrigerant. If it's low, there's a leak somewhere, often at a coil or a fitting. Topping it off without finding the leak just puts off the next breakdown.
Finally, drain problems. Our humidity means AC systems pull a lot of water out of the air. When the drain clogs, the float switch shuts the system down to protect your ceiling, which looks like the AC just stopped cooling.
A spring tune-up checks all of these before the heat arrives: capacitor and contactor, coil cleanliness, refrigerant pressures and the drain. It's a lot cheaper than a 95-degree emergency call, though we don't charge extra on weekends either way.
When to call a pro
- The outdoor unit isn't running, or the fan isn't spinning while it hums.
- The coil or lines freeze again after thawing.
- Breakers keep tripping.
- The house keeps getting warmer even with a clean filter and a running outdoor unit (possible low refrigerant or a failing compressor).
- Water is leaking from the ceiling or the indoor unit.
Still not working?
Frost Fire Heating & Cooling: $59 diagnostic (waived with repair), same-day service, no weekend surcharge, 4.8-star rated. Licensed & Insured. Serving Raleigh, Durham, the Triangle, Clayton and surrounding areas.
Common questions
Why is my AC running but blowing warm air?
The most common causes are a stopped outdoor unit (often a capacitor, contactor or fan motor), a frozen coil, a dirty filter, or low refrigerant. Check the outdoor unit and the filter first.
Should I turn my AC off if it's not cooling?
If you see ice or the outdoor fan isn't spinning, yes. Turn it off to prevent damage, let any ice thaw, and get it checked.
Can I add refrigerant myself?
No. Refrigerant handling requires EPA certification, and if a system is low, it has a leak that needs to be found.
How fast can someone come out?
Frost Fire offers same-day service when we can, with no weekend surcharge. The diagnostic is $59 and it's waived if you go ahead with the repair.
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