Indoor Cooling Coil Freeze Up

COOLING COIL FREEZE-UP: The indoor coil develops either a coating of thick, white frost or thick ice.

• May only freeze a portion of the indoor coil or the entire coil.
• Ice may extend from the indoor coil all the way to the compressor in the outdoor unit.
• Generally causes a loss of airflow from the registers.

CHECKOUT:

1. Check Contactor

• Turn the thermostat system switch to the "Off" position.
• Check the outdoor unit to make sure that it quits running.
  • Units Still Running --> Check for 24V across the contactor coil
    • Contactor Pulled-In + No Coil Voltage = Stuck Contactor
    • Coil Voltage = Shorted Low Voltage Wire

2. Check Indoor Coil

• DO NOT attempt to check the refrigerant charge with ice on the coil.
• If possible, remove the coil access panel to check for ice.
• If ice is still present, set the thermostat fan switch "On" and wait for the coil to thaw.

3. Check the Blower

• Make sure that the fan motor is operating.
• Check the blower wheel for correct rotation.
  • A blower wheel can be turning backwards and still blow air from the registers.

4. Check the Air Filter

• Make sure the air filter is clean.
• In some cases, it may not be possible to remove the filter until the coil thaws.
• Be cautious of "Ultra Allergen" air filters - highly restrictive.

5. Check Refrigerant Charge

Expansion Valve

Check Subcooling & Superheat

• If a Liquid-Line Sight Glass is installed, observe for bubbles in the sight glass (after the system has been operating for at least 5 min.)
  • Low Subcooling = Bubbles in Sight Glass
  • High Subcooling = Clear Sight Glass

Fixed Orifice

Check Superheat

6. Check Supply/Return Air Temperature

• Check the Return Air Temperature at the furnace or air handler.
  • Minimum acceptable return air temperature = 64 deg.
• Check the supply air temperature and calculate the coil temperature drop.
  • Normal Temperature Drop = 15 - 18 deg.
    • Low Temperature Drop = Low Refrigerant Charge or Bad Metering Device (Expansion Valve)
    • High Temperature Drop = Low Airflow
      • Dirty Air Filter
      • Dirty Coil
      • Dirty Blower Wheel
      • Low Blower Speed
      • Dirty Re-Coup Coil (90% furnaces)
      • Mis-Matched Equipment