AC Not Cooling: What to Check When It Blows Warm Air

Central air that runs but doesn't cool means, in Carrier's words, that "the system's fan is circulating air but the refrigeration cycle isn't transferring heat out of the home." Carrier, Trane, Lennox, and Bryant all point first to a thermostat setting, a dirty filter, or a blocked outdoor unit, and you can check all three yourself. A refrigerant leak, a failed capacitor, or a bad compressor belongs to an HVAC technician.

Ice on the copper lines or the indoor coil? See AC frozen coil. Water around the indoor unit, or a system that shut itself off, points to a clogged condensate drain, and a blank thermostat to thermostat not working. Carrier notes that a heat pump in cooling mode has the same problems as an air conditioner, so these checks apply to it too; for cold air in winter, see heat pump blowing cold air.

Most Likely Causes

1. A thermostat setting. Trane, American Standard, and Bryant start here: COOL, a setpoint below room temperature, and the fan on AUTO. With the fan on ON, Trane says, the system "will blow warm, unconditioned air even when the AC isn't running." Set it 2 to 4 degrees below room temperature (Trane), or 3 to 5 (Lennox), and replace the batteries if the screen is blank (Trane). Carrier says to wait a few minutes after the system kicks on, then feel the registers.
2. A dirty filter. Carrier calls a dirty filter "a frequent cause" of an AC not cooling: it blocks airflow, reduces cooling, and can shut the system down. Bryant adds that it can freeze the coil. Turn the system off and hold the filter to the light. ENERGY STAR says to check it every month, especially in summer, and change it at least every three months; our filter guide covers the arrow and the size.

3. Closed or blocked registers and returns. Lennox says vents "must remain wide open, or the air conditioner won't work properly," and Trane says to check return grilles for blockages. Move furniture, rugs, and curtains off them.
4. The outdoor unit isn't running. If it stops, Carrier says, "the indoor fan will simply circulate uncooled air." Lennox's check: listen for at least a low hum and look for spinning fan blades. American Standard's: without touching the unit, hold a hand over the top and feel for hot air. If the fan spins but the house air stays warm, Bryant says the compressor may not be running or the refrigerant may be low. If the unit hums but the fan doesn't turn, Bryant points to the capacitor or fan motor, says to turn the system off, and warns against running the AC with the outdoor fan stopped. Check the power next; if the power is on, call a technician.
5. A tripped breaker or the outdoor disconnect. Carrier says to check both the breaker panel and the disconnect box near the outdoor unit, which may have been switched off or pulled out. To reset a breaker, Singing River Electric says never to touch one with wet hands or while standing on a wet surface. Open only the panel's door: removing the inner cover is a licensed electrician's job, and the CPSC notes panels are usually live even with the main breaker off. The ESFi says a tripped breaker can stop close to ON, so push it fully to OFF, then ON; Carrier says to wait about 5 minutes at OFF. Reset it once. If it trips again, Carrier, Lennox, and Bryant say to leave it off and call; our breaker page explains why.
6. A clogged outdoor coil, or too little clearance. Carrier says dirt, grass, and debris can clog the coil's thin fins, causing "reduced efficiency, lack of cool air, or even compressor damage." Trane adds cottonwood fuzz and oak pollen clumps to the list. Trane and Bryant say to keep at least two feet clear around the unit, and Rheem says plants should be no closer than 18 inches. Your model's installation manual sets the actual clearances; Lennox's 14ACX instructions, for example, call for 48 inches above the unit.
7. A frozen indoor coil. Carrier's signs: frost on the copper tubing, poor cooling, and excess water at the indoor unit. Carrier says to shut the system off right away to protect the compressor, and Bryant says to let the ice thaw rather than remove it by hand. Restricted airflow and low refrigerant are the usual causes (Bryant); see AC frozen coil.
8. A clogged condensate drain and a tripped float switch. Trane and American Standard say some indoor units have a safety switch that shuts the indoor unit down when water rises in the drain pan; "this could be why your AC isn't blowing cold air," Trane says. Lennox says a tripped float switch usually means a clogged drain line. Our condensate drain guide has the manufacturers' steps.
9. Refrigerant, electrical parts, or the compressor. If the checks above don't explain it, the likely causes are a refrigerant leak, a failed capacitor or contactor, or a bad compressor or fan motor, all covered in When to Call a Pro.
10. A system too small for the house. Carrier says an undersized unit "will struggle to keep up as outdoor temperatures rise." If it runs constantly while the house stays warm, Carrier and Trane say to have a qualified HVAC professional assess your home's cooling needs.

What's Normal in a Heat Wave

Trane says, "It is normal for your AC unit to run longer than usual in the hottest months," and American Standard notes that "unusually high temperatures for prolonged periods" can keep a system running continuously.

Vent air about 20°F colder than the house. Bryant's "20-degree rule" is that a properly working air conditioner "can typically cool the air about 20°F below the indoor return temperature": in a 78°F house, air near 58°F at the vents. Hold a thermometer at a return grille, then at a supply register once the system has run a few minutes. A much smaller difference points back to the causes above.

Indoors about 20°F below outdoors. Bryant says an AC "can typically cool about 20°F below the outdoor temperature, so if it's 100°F outside, maintaining 78–80°F indoors is reasonable," and American Standard's rule of thumb is a setting no more than 20 degrees below the outdoor temperature. Turning it lower won't help: Bryant says cooling capacity "does not increase simply because you lower the thermostat." Closing blinds and cutting heat sources does, Bryant adds.

What You Can Fix Yourself

• Set COOL, a setpoint below room temperature, and the fan on AUTO; replace the thermostat batteries.
• Replace the filter, open the registers, and clear the returns.
• Switch the disconnect on, and reset a tripped breaker once, with dry hands.
• With the power off at the thermostat and the disconnect (Bryant, Rheem), pull leaves, grass, and cottonwood off the unit without damaging the fins. Trane says you can gently hose the coil from outside, never with a jet setting or power washer; Bryant says to let it dry 30 to 60 minutes before restoring power. Lennox's 14ACX manual leaves coil cleaning to a technician, so check yours first.
• Trim plants back, and mow with the discharge pointed away from the unit (Trane, American Standard).
• Let a frozen coil thaw with the system off, and clear a clogged drain line.

When to Call a Pro

Low refrigerant means a leak, or a charge that was wrong from the start. Refrigerant isn't used up: Trane says, "If your air conditioner is running properly, you shouldn't have to replace refrigerant," and names a leak as the most likely cause of a low charge, with improper charging at installation the other. Lennox says, "Low refrigerant is not something you can refill like windshield washer fluid," and that a leak may hiss. The EPA requires Section 608 certification to recharge a home system, restricts sales of its refrigerant to certified technicians, prohibits venting it, and says to ask the technician to "locate and repair the leak instead of 'topping it off.'" Our DIY vs. hire-a-pro guide draws the same line.

• A failed capacitor or contactor. Carrier says capacitors "store lethal amounts of energy even when the power is off," and Bryant leaves capacitor, contactor, and fan motor diagnosis to a professional. Signs include a hum with no fan (Bryant), or an outdoor unit that won't shut off because its contactor is "welded closed" (Trane).
• A bad compressor or fan motor. Bryant says compressor problems "require professional diagnosis and repair," and lists grinding or screeching from a failing fan motor.
• A breaker that trips again (Carrier, Lennox, Bryant), a burning smell (Trane), or a frozen coil that comes back (Bryant).
• Some rooms cool and others don't, which Lennox links to clogged or damaged ductwork.

Goodman suggests getting the model number from your paperwork rather than opening the equipment, and Carrier's guide suggests telling the dealer what you've checked. Carrier and Bryant say to turn off an AC that isn't cooling, so plan for the heat.

Stay Safe in the Heat While You Wait

• Find air conditioning. The CDC says to use air conditioning or find a place that has it by dialing 2-1-1, checking the National Center for Healthy Housing's Cooling Centers by State page, or contacting your local health department or emergency management agency.
• Use fans only below 90°F indoors. The CDC says to use fans "only if indoor temperatures are less than 90°F. In temperatures above 90°F, a fan can increase body temperature."
• Drink plenty of fluids, and check on family, friends, and neighbors, especially those who live alone or have chronic medical problems (CDC). The CDC counts people 65 and older, infants and young children, pregnant women, and people with heart disease or asthma among those at higher risk.
• Know the warning signs. The CDC lists muscle cramping, unusually heavy sweating, shortness of breath, dizziness, headaches, weakness, and nausea. The National Weather Service lists confusion, slurred speech, a body temperature above 103°F, and fainting among the signs of heat stroke: call 911, move the person somewhere cooler, cool them with wet cloths or a bath, and don't give fluids.

Prevent It

• An annual spring check-up. ENERGY STAR recommends a contractor check the cooling system each spring, including the coils, refrigerant level, and condensate drain.
• Filters, checked monthly and changed at least every three months (ENERGY STAR).
• A clear outdoor unit. The DOE says to clear weeds and debris so air can circulate freely around it, and Lennox's manual says to keep sprinklers and soaker hoses from soaking it, since prolonged exposure to treated water corrodes it.
• Thermostat batteries, yearly if yours has no low-battery alert (Goodman).

Our summer maintenance guide has the rest of the season's list.

Frequently Asked Questions

Why is my AC running but not cooling the house?

Most often a dirty filter, a wrong thermostat setting, or a dirty outdoor coil, Carrier says. Beyond those: a frozen coil, a full condensate pan, a refrigerant leak, or a failed part, which are technician work.

How much colder should the air from my vents be?

About 20°F colder than the air going into the return, Bryant says: near 58°F at the vents in a 78°F house.

Should I turn off my AC if it's blowing warm air?

Yes. Carrier says running a malfunctioning unit strains the compressor, and a frozen coil or refrigerant leak can cause severe damage if it keeps running. Turn it off, call, and follow the CDC's heat advice while you wait.

Can I add refrigerant to my AC myself?

No. The EPA requires Section 608 certification to recharge a home system and restricts sales of that refrigerant to certified technicians. A low charge usually means a leak, which the EPA says should be repaired, not topped off.

The Bottom Line

Thermostat on COOL with the fan on AUTO, a clean filter, open vents, power on, and a clear outdoor unit. Long run times and vent air about 20°F colder than the house are normal in a heat wave. Ice, a hum with no fan, a repeat breaker trip, or a leak is the technician's; meanwhile, follow the CDC's heat advice.

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