It is 92 degrees in Huntsville, the AC is running, and the air from the vents feels warm. Some causes take five minutes to fix yourself. Others need a technician. Here is how to tell the difference, starting with the simple checks.
A quick test first: on a working system, air coming out of a supply vent is usually about 15 to 20 degrees cooler than the air going into the return grille. If you have a kitchen thermometer, check both. A small difference confirms the system is not cooling, not just struggling with the heat.
1. The thermostat is set wrong
Make sure it is set to Cool and the set temperature is below the room temperature. Check the fan setting too. On On, the fan blows all the time, even between cooling cycles, so you feel room temperature air for minutes at a time. Set it to Auto. If the screen is blank or flickering, replace the batteries.
2. The air filter is clogged
A dirty filter chokes airflow across the indoor coil. Less air means less cooling, longer run times, and in bad cases a frozen coil. Pull the filter and hold it up to a light. If you cannot see through it, replace it. Most homes need a new filter every 1 to 3 months, more often with pets.
3. The outdoor unit is not running
This is the most common pattern behind “running but not cold.” The indoor fan blows, but the outdoor unit is silent, so nothing is removing heat. Check that the outdoor fan is spinning. If it is not, look at the breaker panel. The indoor and outdoor units usually have separate breakers. Reset a tripped breaker once. If it trips again, leave it off and call.
Also check the drain safety switch. Many indoor units have a float switch that shuts the system off when the condensate drain backs up. If you find water in the drain pan, see our guide to an AC leaking water inside.
4. A capacitor or contactor has failed
If the outdoor unit hums but the fan does not spin, or it clicks and does nothing, a failed capacitor or contactor is a likely cause. These parts work hardest on the hottest days, which is why they often fail in July. Please do not touch them. A capacitor can hold a dangerous charge even with the power off.
5. The outdoor coil is dirty or blocked
The outdoor unit gets rid of the heat pulled out of your house. Grass clippings, leaves, pollen and dryer lint can coat the coil fins and trap that heat. Keep about 2 feet clear around the unit. With the power off at the disconnect, you can rinse the coil gently with a garden hose. Do not use a pressure washer. It bends the fins.
6. The indoor coil is frozen
Ice on the copper line at the outdoor unit, or weak airflow at the vents, can mean the indoor coil has frozen. Turn cooling off, set the fan to On so the ice melts, and replace the filter. Our guide on why an AC freezes up covers what to do next.
7. Refrigerant is low because of a leak
Refrigerant does not get used up. If the level is low, there is a leak. Signs include a hissing or bubbling sound, ice on the lines, long run times and lukewarm air. Only a technician with gauges can confirm it. The right fix is to find and repair the leak, not just add more. Older systems that use R-22 cost more to recharge, because R-22 has not been produced or imported in the U.S. since 2020.
8. Ducts are leaking or disconnected
Many North Alabama homes have ductwork in a hot attic or crawlspace. If a duct comes apart, cold air pours into the attic instead of your rooms. If one or two rooms are warm while the rest are fine, a duct problem is likely. See signs your ductwork needs repair.
9. It is very hot, or the system is too small or worn out
On the hottest afternoons, a healthy system can run for long stretches. That alone is normal. It is not normal if the system never shuts off in mild weather, the indoor temperature keeps climbing, or the house is always humid. An undersized or aging system may not keep up. Our guides on AC sizing and how long systems last can help you decide what to do.
How to reset your AC
- Set the thermostat to Off.
- Turn off the breakers for the indoor and outdoor units for about 5 minutes.
- Turn them back on and set the thermostat to Cool, a few degrees below room temperature.
- Wait. Many outdoor units have a 3 to 5 minute delay before they start.
If it trips again or still blows warm air after 15 minutes, stop resetting it and call. Repeated resets can damage the compressor.
When to call North Alabama Heating & A/C
- The outdoor unit hums, clicks or will not start
- A breaker trips again after one reset
- Ice on the lines or coil that comes back after thawing
- Hissing, buzzing or a burning smell
- Warm air after you have checked the thermostat, filter and breakers
Call North Alabama Heating & A/C at (256) 534-2941. We have served Huntsville, Madison and North Alabama since 1977, Monday to Saturday, 7 AM to 7 PM. Learn more about our AC repair service.
Quick answers
Why is my AC running but not blowing cold air?
The most common causes are an outdoor unit that is not running (tripped breaker, failed capacitor or contactor), a clogged filter, a dirty coil, a frozen coil, or low refrigerant from a leak. Check the thermostat, filter and breakers first, then call a technician.
How can I tell if my AC is low on refrigerant?
Common signs are ice on the larger copper line or indoor coil, a hissing or bubbling sound, long run times with lukewarm air, and higher power bills. A technician has to confirm it with gauges. Low refrigerant always means a leak that should be found and fixed.
How do I reset my AC?
Set the thermostat to Off, switch off the indoor and outdoor breakers for about 5 minutes, turn them back on, then set the thermostat to Cool. Allow 3 to 5 minutes for the outdoor unit to start. If it trips again, leave it off and call.
