Your air conditioner is running — the fan is blowing, the outdoor unit is humming — but the house isn’t getting cool. This is one of the most common HVAC complaints we receive in Atlanta during summer, and the causes range from a simple DIY fix to a repair that needs a certified technician.
Here are the seven most common reasons an AC runs but doesn’t cool, roughly in order from easiest to check yourself to most likely to need professional attention.
Working through this list before calling a technician can save you a service call fee — or at least help you describe the problem accurately when you do call.
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1. Dirty or Clogged Air Filter
A clogged filter restricts airflow over the evaporator coil. With insufficient air movement, the coil can’t transfer heat effectively, and the system blows air that’s barely cooler than room temperature. Check your filter first — if it’s grey and dense, replace it and run the system for an hour to see if cooling improves.
2. Thermostat Set to “Fan” Instead of “Cool”
When the thermostat fan setting is switched to “On” rather than “Auto,” the fan blows continuously — including when the compressor isn’t running. The result: room-temperature air blowing from the vents. Set it to “Cool” mode and “Auto” fan and try again.
3. Frozen Evaporator Coil
Restricted airflow (dirty filter, closed vents, low refrigerant) can cause the evaporator coil to ice up. A frozen coil can’t absorb heat. Signs: visible ice on the copper lines near the air handler, weak or warm airflow from vents. Turn the system off, switch the fan to “On” to thaw the coil, and call us if the problem returns.
4. Low Refrigerant (Refrigerant Leak)
Refrigerant is the working fluid that carries heat out of your home. AC systems are sealed — they don’t “use up” refrigerant. If yours is low, there’s a leak. Low refrigerant causes weak cooling, ice on the copper lines, and eventually compressor damage if ignored. This requires a certified technician — it’s illegal to purchase or handle refrigerant without EPA Section 608 certification.
5. Dirty Condenser Coils
The outdoor condenser unit rejects the heat pulled from your home. If the condenser coils are caked with dirt, grass clippings, or cottonwood seeds (very common in Atlanta), the unit can’t release heat efficiently. You can gently rinse the coils with a garden hose — but avoid pressure washers and don’t bend the fins.
6. Oversized or Undersized Unit
An oversized AC cycles on and off too quickly (short cycling) — it cools the air near the thermostat before conditioning the whole house. An undersized unit runs constantly but can’t keep up with the heat load. Both result in uneven comfort. This is a design issue that requires a Manual J load calculation to properly diagnose and fix.
7. Failing Compressor
The compressor is the heart of the AC system. A failing compressor may still run — the fan keeps blowing — but it can’t compress the refrigerant effectively. Symptoms: system runs constantly, outdoor unit is warm on top but not hot, little temperature difference between supply and return air. Compressor replacement is typically the most expensive AC repair; depending on the age of the system, full replacement may be more cost-effective.
When to Call a Technician
If you’ve checked the filter and thermostat and the system is still not cooling, call HVAC Pro at (404) 555-0192. Our technicians carry refrigerant, coil cleaner, and most common electrical components on every truck — most of these issues are diagnosed and resolved in a single visit.
