Vent, Valve & Volt
AC running but blowing warm air

You can fix this

Likely cause: a thermostat setting

If the thermostat is on Heat or the fan is set to On, the blower can push air that hasn't been cooled. Fix the settings and give it a few minutes.

Why this happens

Carrier and Trane both list the thermostat as a cause of warm air from the vents, and Carrier calls out a thermostat set to Heat or Fan specifically. With the fan set to On, the blower keeps running even when the outdoor unit isn't cooling, so you feel room-temperature air between cooling cycles. Lennox suggests setting the temperature at least 3 to 5 degrees below the room so the system actually calls for cooling.

How to fix it

  1. Set the mode to Cool and the fan to Auto.
  2. Set the temperature at least 3 to 5 degrees below the current room temperature.
  3. If the thermostat runs on batteries and the screen looks dim, replace them.
  4. Wait several minutes, then feel the air at a supply vent. It should be noticeably cooler than the room.
  5. If it's still warm, go back and work through the rest of the questions.

If this does not fix it, tell the HVAC technician

  • What the thermostat was set to before you changed it.
  • Whether the outdoor unit runs when the thermostat calls for cooling.
  • The thermostat brand and whether it runs on batteries.

Build the full list: what to tell the HVAC technician.

Start this troubleshooter over

References (every fact above comes from one of these pages)
  1. Carrier: Why is my AC blowing hot air?
  2. Trane: AC not blowing cold air
  3. Lennox: Outside AC unit not turning on

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