Why is my AC blowing warm air?

In a Phoenix summer, an air conditioner can run for most of the day, month after month, and homeowners notice quickly when the air coming out of the vents stops feeling cool. When your AC is blowing warm air, the useful first step is to understand how the system makes cold air, because the cause usually sits in one of a few places along that path.

How your AC is supposed to cool

A central air conditioner has two halves. Inside, a blower pushes your home’s air across the evaporator coil, which Trane describes as the indoor coil paired with your furnace or inside your air handler. Outside, the condenser unit holds the compressor and the condenser coil, which release the heat pulled out of your house.

Refrigerant carries that heat from the indoor coil to the outdoor unit. If any part of the loop stops working, whether that’s power to the outdoor unit, the refrigerant, the compressor, or airflow across the coils, the blower can keep running and moving air that was never cooled. Lennox notes that if the indoor unit is running but the outdoor unit has lost power, the system can’t complete the cooling process.

Common reasons an AC is blowing warm air

Carrier and Lennox point to the same group of causes:

• Thermostat settings. The thermostat may be set to Heat or Fan Only, or its batteries may be low, according to Carrier and Lennox.
• Dirty air filter. Carrier says accumulated dust blocks airflow and forces the system to work harder. The U.S. Department of Energy says clogged filters “significantly reduce a system’s efficiency.”
• No power to the outdoor unit. A tripped breaker or a switched-off disconnect near the outdoor unit leaves the indoor blower running with no cooling.
• Electrical parts. Carrier says faulty contactors or capacitors can keep the compressor from starting.
• Low refrigerant. Lennox says low refrigerant is often due to a leak. The Department of Energy adds that if a system leaks, simply adding refrigerant is not a solution.
• A frozen coil. Trane lists warm air from the vents as one sign of a frozen evaporator coil. See why an AC freezes up.
• Compressor problems. If the compressor fails or overheats, Carrier says the refrigerant cycle stops and the system circulates only warm air. The Department of Energy notes that when a central air conditioner’s compressor stops on a hot day, the high-pressure limit switch may have tripped.

Phoenix conditions add to several of these. Heavy summer use means more wear, and the Department of Energy says compressor and fan controls can wear out, especially when the system turns on and off frequently. Dust is the other factor. The Department of Energy says outdoor condenser coils can become very dirty in a dusty environment, and monsoon season, which runs June 15 to September 30 according to the City of Phoenix, brings dust storms.

What you can safely check yourself

Several causes take a few minutes to rule out:

1. Check the thermostat. Set it to Cool, set the fan to Auto, and lower the temperature by several degrees. Replace the batteries if the display is dim or blank.
2. Check the filter. Turn the system off, pull the filter, and replace it if it’s dirty. Carrier and Lennox recommend changing filters every 1 to 3 months. See how often to change your AC filter in Phoenix.
3. Check the breaker and outdoor switch. Carrier says to confirm the breaker hasn’t tripped and the outdoor unit’s power switch is on. Lennox says to reset an HVAC breaker only once.
4. Look at the outdoor unit. Clear debris and plants away from it; the Department of Energy recommends trimming foliage back at least 2 feet. Lennox suggests checking for ice and listening for unusual sounds.
5. Check the vents. Make sure supply and return vents are open and not blocked by furniture.

When to call a pro

Turn the system off and call for service if:

• The breaker trips a second time. Lennox says to call a technician rather than keep resetting it.
• You see ice on the coil or refrigerant lines
• The outdoor unit is silent or makes unusual noises while the indoor blower runs
• The air stays warm after you’ve fixed the thermostat, filter, and power

Refrigerant, capacitors, contactors, and compressors are work for a licensed technician. Lennox says low refrigerant “is not something you can refill like windshield washer fluid.” Don’t open the electrical panels on the indoor or outdoor units.

In a Phoenix summer, if the house is getting too hot to stay in safely, find a cooler place while you wait for service.