It’s one of the more confusing AC problems a homeowner can face: your outdoor unit is clearly running, but inside, it’s warm—and getting warmer. “Your thermostat says it’s cooling, and you can hear the outdoor condenser humming—but your home isn’t getting cooler. So what gives?” If you’re asking yourself, “Why is my AC running outside but not inside?” you’re not alone. This issue can point to a few different problems—some minor, some more serious. In many cases, it means it’s time for professional AC repairs. But before you call, let’s break down the most common causes and what you can do next.
Confirming What’s Running—And What’s Not
Is the AC unit running but not cooling? Your central air conditioning system has two main parts:
- The outdoor condenser unit, which releases heat from your home
- The indoor air handler, which circulates air through your vents
For this post, we’re going to consider a situation where there’s no airflow coming from the vents. That points to a problem with the indoor air handler, and it is the scenario most people mean when they ask, “Why is my AC running outside but not inside?”
Note: If air is blowing but it’s warm, your system likely has a cooling issue—such as low refrigerant, a frozen evaporator coil, or a problem with the condenser. But if no air is blowing out of your vents, it’s a different category of problem.
Common Reasons for AC Running Outside But Not Inside
1. Thermostat Issues or Communication Failures
What’s Happening? The thermostat may be calling for cooling, but your air handler isn’t getting the message, which could be caused by:
- A malfunctioning thermostat
- A tripped safety switch
- Faulty or loose control wiring
What to Do: Try changing the batteries, resetting the thermostat, and checking its settings. If the fan still doesn’t kick on, it’s time to call a technician.
2. Dirty Filters or Blocked Airflow
What’s Happening? A clogged air filter can reduce airflow so severely that the system shuts down or the coil ices over. When this happens, even if the outdoor condenser continues running, your indoor unit may remain off or ineffective.
What to Do: Replace dirty filters immediately, but if you see ice, you should immediately turn off your system to prevent further damage and call for AC repairs.
3. Condensate Backup
What’s Happening? When your AC cools your home, it also removes humidity from the air. That moisture collects in a drain pan and exits through a condensate line. But if the line becomes clogged, the drain pan can fill up. Many modern systems include a safety switch that shuts down the indoor blower to prevent water damage. In this case, your outdoor condenser keeps running, but the indoor unit quietly turns off—resulting in no airflow.
What to Do: In most systems, the drain pan is tucked inside the air handler or near the coil, making it hard to access without removing panels. You will likely need an HVAC technician to flush the line and inspect the system for long-term drainage issues.
4. Indoor Blower Motor or Capacitor Failure
What’s Happening? The indoor blower motor pushes cooled air through your ducts and into your home. If it fails, or if the capacitor that helps it start and run goes bad, your system can’t circulate air. Meanwhile, the outdoor unit may continue running normally, which is why it may seem like your system is working when no air is actually coming out of the vents. Blower motor and capacitor failure is one of the more common causes of the AC running outside but not inside during high-demand summer days.
What to Do: Stand near your indoor unit and listen for unusual sounds. A humming noise without airflow, clicking, or a faint burning smell can indicate a blower issue. If air from the vents feels weak or nonexistent, the motor may be struggling to start or completely inoperable. You’ll need a licensed HVAC technician to diagnose the problem and replace the motor or capacitor.
5. Electrical or Control Board Issues
What’s Happening? Your AC system relies on a control board to coordinate communication between the indoor and outdoor units. If the board malfunctions or if there’s a failure in an electrical relay, the outdoor unit can keep running while the indoor blower remains off.
What to Do: If your thermostat appears to be working, your filter is clean, and there’s no sign of a drain or airflow issue—but your vents still aren’t blowing air—an HVAC technician should inspect the system’s electrical controls and components.
FAQ
Why is my outside AC unit running but not blowing air inside?
When the AC unit is running outside but not inside, it usually points to an indoor air handler issue—like a failed blower motor, tripped safety switch, or frozen evaporator coil.
What if the outside AC unit fan is not running, but the inside is?
That’s the opposite problem—and it usually means the condenser isn’t working. You might have a failed fan motor or capacitor on the outdoor unit.
Can a bad filter cause my indoor unit to stop working?
Yes. A clogged filter can restrict airflow, which leads to coil freezing or blower motor overheating. Always check and replace filters first.
How can I tell if my AC condenser is not running?
Step outside and listen. If you don’t hear the fan spinning or a consistent hum, your condenser may not be working. If it’s hot outside and your system is “on” but silent outside, call for service.
Your Local AC Repair Experts
Still wondering, “Why is my AC running outside but not inside?” If you’ve ruled out filters and thermostat settings and your vents are still quiet, it’s time for a professional. Precision Comfort Systems’ experienced technicians are ready to diagnose and repair your system—fast. Whether it’s a failing blower motor, electrical issue, or something more complex, we’ll get your comfort back on track. Call us at (317) 867-2665 or schedule service online today.