AC Not Cooling? 10 Reasons Your Air Conditioner Isn't Cooling Your House

If your air conditioner is running but the house isn't getting cool, the cause is usually one of ten things. Four you can check yourself in ten minutes. The rest need an HVAC technician, and two of them can damage the system if you keep running it.

Check these four yourself first

1. A clogged air filter

This is the most common cause by a wide margin. A dirty filter starves the system of airflow, which drops cooling capacity and can freeze the indoor coil. Pull the filter. If you can't see light through it, replace it and give the system an hour.

2. The thermostat setting or batteries

Confirm it's set to COOL and not FAN or HEAT, that the setpoint is below room temperature, and that the fan is on AUTO. Dead batteries in a battery-powered thermostat cause exactly this symptom. If a smart thermostat recently updated or lost its schedule, reset it.

3. Blocked outdoor unit

The outdoor condenser needs to dump heat. Grass clippings, cottonwood fluff, mulch and overgrown shrubs choke it. Shut power off at the disconnect, clear a two-foot perimeter, and gently rinse the coil fins from the inside out with a garden hose — never a pressure washer.

4. Closed vents and blocked returns

Too many closed supply registers or a return blocked by furniture raises pressure in the ducts and cuts airflow. Open the vents. All of them.

These six need a technician

5. Low refrigerant from a leak

Air conditioners don't consume refrigerant. If it's low, it leaked. Signs are weak cool air, ice on the line set, or a hissing sound. Topping it off without finding the leak means paying for refrigerant twice — insist the leak be located and repaired.

6. A frozen evaporator coil

Ice on the indoor coil or the copper line means airflow or refrigerant is wrong. Turn the system to OFF and the fan to ON to thaw it — several hours — then call. Running a frozen system can destroy the compressor.

7. A failed capacitor or contactor

Classic symptom: the outdoor fan doesn't spin, or you hear a hum or click and nothing starts. These are inexpensive parts and a common summer repair, usually a few hundred dollars.

8. A dirty condenser coil

Years of dust and pollen build an insulating blanket on the outdoor coil, so the system runs constantly and barely cools. A professional coil cleaning restores much of the lost capacity.

9. Leaky or undersized ductwork

If cool air comes out of the vents but rooms never catch up — especially rooms far from the unit, or over a garage — you may be losing a large share of your cooled air into a hot attic or crawlspace. Duct sealing pays for itself here.

10. A failing compressor or an undersized system

A compressor that won't start, trips the breaker, or runs loud is a major repair, and on a system over 12 years old, replacement is usually the better spend. And if the system has never kept up on the hottest days since the day it was installed, it may simply be too small for the house — a load calculation will tell you.

When to shut it off and call immediately

• Ice on the indoor coil or refrigerant lines
• Burning smell, or a breaker that trips repeatedly
• Water pooling around the indoor unit or air handler
• Loud grinding or metallic screeching from either unit

Running the system through any of these turns a repair into a replacement.

What a diagnostic visit costs

Most Middle Tennessee HVAC companies charge $85 to $175 for a diagnostic, often applied toward the repair. Common fixes: capacitor $150 to $400, contactor $150 to $350, condensate pump $200 to $450, coil cleaning $250 to $500, refrigerant leak repair $500 to $1,800. A new system runs $7,500 to $15,000 — see our HVAC cost breakdown.

Prevent most of this

• Change the filter every 1 to 3 months
• Keep two feet clear around the outdoor unit
• Have the system serviced each spring
• Watch the condensate drain line for clogs
• Keep supply vents and returns open and unblocked

Common questions

Why is my AC running constantly but the house is still warm?

Usually restricted airflow, a dirty coil, low refrigerant, or a system too small for the load. On the very hottest afternoons, near-constant running is normal.

Should I keep the AC running if it isn't cooling?

No — especially if you see ice. Shut the cooling off, run the fan, and call a technician.

Why is upstairs hot and downstairs cold?

Heat rises, attic ducts leak, and a single-zone system can't balance floors well. Duct sealing, added return air, or zoning are the usual fixes.

How cool should my house get?

A properly sized system generally maintains about 20 degrees below the outdoor temperature. On a 98-degree day, holding 78 inside is normal performance, not a failure.

Is it worth repairing an older air conditioner?

Under 10 years old, repair. Over 12 to 15 years old with a major failure or R-22 refrigerant, replacement usually costs less over the next few years.

Find an HVAC company near you

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