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Troubleshooting a FROZEN Evaporator Coil
Troubleshooting Guides

We have partnered with HVAC Help Desk to offer you 100's of "how to" videos for you to learn, troubleshoot, and repair your heating and air conditioner.

HVAC Help Desk

 

A frozen evaporator coil is one of the most common issues for air conditioners and heat pumps during the cooling season.  Use the following check list prior to contacting a licensed and qualifed HVAC Service Technician.

There a many reasons an evaporator or AC coil will freeze; ( If you have a frozen evaporator coil, shut off your system immediately as prolonged operation can and will damage the compressor.)

  • Restricted air flow across the coil will allow the temperature of the coil to drop below 32 degrees F and cause the humidity/condensate to freeze.
    • Check the air filter to ensure it is clean.  If not, change it.   If this is all that is wrong, shut the air conditioner off at the thermostat and turn the fan to the "on" position.  This will keep the blower fan on and will assist in thawing the evaporator coil. 
    • Check the evaporator coil on the intake side and ensure it is clean.  If it is dirty and or clogged, then you need to clean it.  Be sure to use the correct cleaners and brushes to ensure a proper cleaning.  DO NOT USE A WIRE BRUSH.  When using a plastic brush be sure to brush only parallel to the fins.  You can bend the fins very easily and this will resrict air flow.
    • Check your home for any blocked cold air return grilles.  There are instances where a low placed cold air return grille is blocked by a piece of furniture and this restricts the air flow.  Remove any restrictions and allow the evaporator coil to thaw prior to starting operation again.
    • Check your home for too many supply registers that are blocked or shut off.   There are instances where a home owner has shut off too many in order to force air to one part of a home.  The rule of thumb for most homes is each register provides around 80 CFM to 100 CFM of air flow.  Each tonnage of air conditioning requires 350 CFM to 400 CFM.  If you have a 2 Ton air conditioner, then you require 800 CFM of air flow.  If you have 12 vents in the home, you need at least 8 to 10 of them unrestricted.
    • Check the blower motor for proper operation.  If the blower motor is not running the coil will freeze.
    • Check to be sure the outdoor unit shuts off when the thermostat is satisfied and is no longer "calling" for cooling.  A stuck or welded shut contactor will keep the outdoor unit running all the time.  Without the indoor unit blower motor in operation there is no air flow and the evaporator coil can freeze.
    • Lack of refrigerant or a low refrigerant charge is also a symptom of a frozen evaporator coil.  Cotact your local HVAC contractor to charge and conduct a refrigerant leak test.  The easiest and most accurate is by using an electronic leak tester.  Most older air conditioning systems will lose their refrigerant charge.  Sometimes by adding will result in proper operation for the season.  Sometimes it is time to replace either the evaporator coils or the outdoor unit.

*This guide's intention is to provide an understanding of the operation of a furnace.  It is not intended as a hands-on teaching tool, or aide.  All HVAC equipment should be serviced by a licensed HVAC technician. Consult your local, town, city, and state laws, oridinances, and regulations prior to accessing, engaging, troubleshooting, repairing, or servicing any HVAC equipment. 
SAFETY NOTE - ALWAYS TURN THE MAIN POWER AND GAS VALVE OFF BEFORE SERVICING ANY HVAC EQUIPMENT. SERIOUS INJURY CAN OCCUR.

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