How to Diagnose a Frosty Air Conditioner

Diagnosing frosty air conditioner


Diagnosing frosty air conditioner
This Old House Production

Ask This Old House plumbing and heating expert Richard Trethewey discusses the possible causes and solutions for a frozen air conditioner.

Steps for Diagnosing a Frozen Air Conditioner

Usually, ice on an air conditioner indicates that the airflow between the coil and the condenser unit is out of balance. Follow these steps to properly diagnose this problem.

  1. To start, replace a disposable air filter or clean a reusable one to maximize airflow on the indoor unit.
  2. Check for obstructions on the condenser unit outside. Clean off any leaves or droppings that may be on or near the condenser, but work very carefully to avoid bending or damaging the fins. A vacuum with the brush attachment can usually do the trick.
  3. Be sure to install a unit that is properly sized. If you add insulation to the house, your air conditioning unit may need to be changed for a smaller one.
  4. If none of these solutions solves the problem, there could be a refrigerant leak. Hire a professional to investigate further.


Air filters on HVAC equipment should be changed or cleaned periodically. Replacement air filters come in many sizes and can be purchased at home centers. Reusable filters should be cleaned at least once per season, more regularly in homes with pets.

Outdoor units should be cleaned with a vaccum brush attachment and kept clear of bushes and other obstructions.

A professional HVAC contractor can help assess if a system may be oversized and need replacement or if the refrigerant is recharged incorrectly.


  • HEPA vacuum