Why Is My AC Not Blowing Cold Air?

Step by step

1. Set the thermostat to Cool, below the current room temperature, with the fan on Auto. Replace the batteries if the screen is blank.
2. Check the air filter. If it's gray or clogged, replace it.
3. Make sure supply vents and return grilles are open and not blocked by furniture or rugs.
4. Check the breaker panel for a tripped AC or furnace breaker. Reset it once; if it trips again, leave it off.
5. Look at the outdoor unit. The fan should spin and the unit should hum. Clear debris and keep plants about 2 feet away.
6. Look for ice on the indoor coil or the larger copper refrigerant line. If you see ice, turn the system off and let it thaw; don't chip at it.
7. Check for water around the indoor unit or a full drain pan. A safety switch may have shut the system down.
8. Still no cold air? Call a technician. Refrigerant and electrical repairs need a pro.

Is the thermostat set correctly?

It sounds too simple, but Trane lists thermostat problems among the main reasons an AC blows warm air. Make sure the mode is Cool, the set temperature is below the room temperature, and the fan is on Auto. With the fan on On, the blower keeps running between cooling cycles, which can push room-temperature air through the vents and feel like a failure.

A blank screen usually means dead batteries or a tripped breaker.

Can a dirty filter stop the cold air?

Yes. A clogged filter starves the indoor coil of airflow, which can drop cooling and even freeze the coil, according to Trane. ENERGY STAR says to check filters monthly, and SMUD notes a dirty filter can make the AC use 5 to 10% more energy. During wildfire smoke, filters clog faster, so check them more often.

Why is the outdoor unit running but the air is warm?

If the outdoor fan isn't spinning, or the unit hums without starting, a failed capacitor or fan motor is a common cause. Capacitors store electrical energy, so don't open the panel; this is a job for a technician. A capacitor replacement typically runs $80 to $400 nationally, according to This Old House.

If the fan spins but cooling is weak, the outdoor coil may be caked with dust, leaves, or grass clippings. Trane lists a dirty condenser coil as a cause of warm air. With the power off, you can rinse it gently with a hose from the top down.

Why does my AC freeze up?

Trane says restricted airflow is the biggest reason an evaporator coil freezes: a dirty filter, blocked return vents, or a dirty coil. Low refrigerant and thermostat problems can also cause it. Once ice builds up, air can't pass through the coil, so the vents blow weak, warm air.

Turn the system off at the thermostat. Trane warns that running a frozen system can cause major damage and that chipping the ice can damage the coil. After it thaws, replace the filter and open blocked returns. If it freezes again, call for service.

Is my AC low on refrigerant?

Refrigerant circulates in a closed loop and isn't used up, so if it's low, there's a leak, Fixr explains. Signs include weak cooling, long run times, and ice on the refrigerant lines. Only an EPA Section 608 certified technician can buy and add refrigerant, and venting it on purpose is prohibited.

Ask the technician to find and fix the leak before recharging. Nationally, leak repair plus recharge runs about $200 to $1,600, according to This Old House and Fixr. Older R-22 systems cost more to recharge because R-22 is no longer produced or imported in the U.S.

Could the problem be the ducts?

If the AC runs fine but some rooms stay hot, look at the ducts. ENERGY STAR says a typical house loses 20 to 30 percent of the air moving through its ducts to leaks, holes, and poor connections, and lists ducts in the attic and tangled or kinked flex duct as warning signs. The Department of Energy notes that disconnected or crushed ducts can keep cooled air from ever reaching a room.

Attic ducts sit in very hot air during Sacramento summers. The DOE says duct sealing should be tested by a qualified professional to ensure combustion safety and proper airflow.

Frequently asked questions

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