Vent, Valve & Volt
AC running but blowing warm air

You can fix this

Likely cause: a clogged air filter

A clogged filter starves the system of air, so it can't cool properly. A new filter is the cheapest fix in HVAC.

Why this happens

Carrier and Trane both put a dirty filter at the top of their lists for an AC blowing warm air. When the filter clogs, less air crosses the indoor coil, cooling drops off, and the coil can eventually freeze. Trane recommends replacing the filter every 1 to 3 months.

How to fix it

  1. Set the thermostat to Off before you pull the filter.
  2. Note the size printed on the edge of the old filter and the arrow showing airflow direction.
  3. Slide in a new filter of the same size with the arrow pointing toward the furnace or air handler, away from the return grille.
  4. Set the thermostat back to Cool, fan on Auto, and check the air at a vent after several minutes.
  5. Write the date on the new filter's edge so you know when it's due again.

When to stop. If the air is still warm a day after a fresh filter, the cause is elsewhere. Work back through the questions or call an HVAC technician.

If this does not fix it, tell the HVAC technician

  • When the filter was last changed and how dirty it was.
  • Whether the air got cooler after a new filter.
  • Whether you saw any ice on the lines or coil.

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

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