Home › Troubleshooting › Dirty air filter
Why does a dirty air filter stop your AC from cooling?
A clogged filter chokes the airflow across your evaporator coil. With too little warm air passing over it, the refrigerant can't absorb enough heat to boil off, so the coil keeps getting colder — until the moisture condensing on it freezes solid and blocks the airflow completely. It's the most common AC fault there is, and the cheapest to prevent.
What causes it
The filter clogs with dust and chokes the airflow across the indoor coil.
What it does to the cycle
Too little warm air reaches the cold evaporator, so the refrigerant can't pick up enough heat to fully boil. The coil keeps getting colder until moisture on it freezes into ice — which blocks the airflow even more.
What you'd notice
Weak airflow from the vents, ice on the indoor coil or copper lines, long run times, and creeping energy bills.
What to do about it
- Turn the system off at the thermostat before pulling anything.
- Slide the filter out and hold it up to a light. If you can't see light through it, it's long overdue.
- Note the size printed on the cardboard frame (e.g. 20x25x1) and fit the replacement with the airflow arrow pointing toward the unit.
- If the coil is already iced over, run the fan alone for 2–4 hours to thaw it before you cool again.
- Set a monthly phone reminder to check it. Most filters want replacing every 1–3 months.
The bottom line
Check the filter monthly and replace or wash it every 1–3 months. This is the single most common AC problem and the easiest to prevent.