Home › Troubleshooting › Dirty evaporator coil
Why does a dirty indoor coil kill your cooling?
Dust that slips past a cheap filter settles on the permanently damp indoor coil and mats into a felt-like blanket. That layer insulates the coil, so the refrigerant can't absorb heat from your air — cooling drops off, the coil may ice, and the damp grime grows the mildew behind that musty 'dirty sock' smell.
What causes it
Dust slips past a cheap or missing filter over months and coats the indoor coil.
What it does to the cycle
The grime acts like a blanket, insulating the coil so it can't absorb heat from your air. Cooling capacity drops and, like a dirty filter, the coil can ice over.
What you'd notice
Weak cooling even though everything is running, a musty smell, and ice on the coil.
What to do about it
- Check the filter first — a dirty coil nearly always means the filter failed or was missing.
- Shine a flashlight at the coil behind the air-handler panel if you can reach it, but don't start scrubbing.
- Have a tech pull and chemically clean it. The fins are razor-thin and crush easily, and a damaged coil leaks refrigerant.
- Upgrade to a filter that actually seals in its track — air bypassing around the edges is what fouls coils.
- Keep the condensate drain clear so the coil isn't sitting in standing water.
The bottom line
Have a technician clean the coil — it's behind the unit's panels and easy to damage. Then use a good filter to keep it clean.