Why Is My House Still Hot With the AC On?
You hear the AC running. You feel air coming from the vents. But your house is still hot, and the thermostat reads 80°F at 3 PM on a July afternoon in Indianapolis. What's going on?
This is one of the most common summer HVAC calls we get at CSO Mechanical. The good news: the fix is often simpler than you think.
7 Reasons Your AC Is Running But Not Cooling
1. Dirty or Clogged Air Filter
This is the #1 cause and the easiest fix. A clogged filter restricts airflow through the system, making it impossible for your AC to cool effectively. The air simply can't move fast enough.
Fix it: Check your filter right now. If it's gray, packed with dust, or you can't see light through it, replace it. Filters should be changed every 1–3 months during heavy use season.
2. Low Refrigerant (Refrigerant Leak)
Refrigerant is what your AC uses to absorb heat from your home's air. If there's a leak and refrigerant levels drop, the system can't cool properly, no matter how long it runs.
Signs of low refrigerant: warm air from vents, ice forming on the refrigerant lines or outdoor unit, a hissing or bubbling sound near the unit, and noticeably higher energy bills.
Fix it: This is a job for a licensed HVAC technician. Refrigerant handling requires EPA certification, it's not a DIY repair.
3. Frozen Evaporator Coil
The evaporator coil inside your air handler can freeze if airflow is restricted (usually from a dirty filter) or refrigerant is low. A frozen coil can't absorb heat, so you get little to no cooling.
Signs: ice visible on the indoor unit or refrigerant lines, reduced airflow, water dripping near the air handler.
Fix it: Turn the AC off and let the coil thaw (this can take 24 hours). Then replace the filter before turning it back on. If it freezes again, call a technician, you likely have a refrigerant issue.
4. The System Is Undersized for Your Home
If your AC was sized incorrectly when installed, or you've added square footage to the house, it may simply not have the capacity to cool your home on the hottest days.
A properly sized AC should reach your set temperature within a reasonable time, even on a 95°F Indianapolis afternoon. If it runs all day and never gets there, sizing may be the issue.
Fix it: A load calculation by a licensed HVAC contractor will tell you if your system is right for your home.
5. Ductwork Problems
Leaky, blocked, or poorly designed ductwork can lose 20–30% of conditioned air before it ever reaches your rooms. If your ducts run through an unconditioned attic or crawlspace, that problem gets worse in the summer heat.
Signs: some rooms consistently warmer than others, weak airflow from certain vents, visible disconnected or crushed duct sections.
Fix it: A technician can test duct pressure and identify leaks. Sealing and insulating ducts is one of the highest-ROI HVAC improvements a homeowner can make.
6. Thermostat Issues
Sometimes the problem isn't mechanical at all — it's the thermostat. A miscalibrated thermostat may read the wrong temperature and cycle the system off too early. A failing thermostat may not communicate properly with the AC at all.
Fix it: Check that the thermostat is set to "cool" (not "heat" or "fan only"), is set below the current room temperature, and isn't in direct sunlight or near a heat source. If those are fine, a thermostat replacement is inexpensive and quick.
7. An Aging, Worn-Out System
An AC unit over 12–15 years old simply loses efficiency. The compressor wears, the coils lose effectiveness, and the system struggles to keep up with Indiana's peak summer heat, even when nothing is technically "broken."
Fix it: At this age, an honest evaluation of repair-vs-replace makes sense. See our guide: How Long Does an AC unit Last?
What to Check Right Now (Before You Call)
✅ Check and replace the air filter if it's dirty
✅ Make sure all supply and return vents are open and unobstructed
✅ Check the thermostat settings and batteries
✅ Look at the outdoor unit — is it running? Is it covered in ice or debris?
✅ Check your circuit breaker, a tripped breaker can cut power to part of the system.
If none of those reveal the problem, it's time to call a technician.
CSO Mechanical serves Indianapolis, Carmel, Mooresville, Noblesville, Westfield, Avon, Greenwood, and surrounding communities.
When your AC is running but your home isn't cooling, we'll find the problem fast and give you a straight answer on what it takes to fix it.
📞 Call (317) 372-1608 we pick up the phone.