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residential AC/frozen evaporator

I just purchased a new home and last night the existing AC unit (gas furnace with Dx cooling) had about a 1/4" of frost on the entire length of the suction line, the evaporator coil appears blocked with ice, and the compressor is covered in ice/frost. The furnace fan was running load (fan spinning but no air movement) and the discharge grilles had no air flow. The previous home owners had a service contract from the gas company but acknowledged that the AC was not annually serviced.

Is this an easy fix?

Any ideas of the cause?


  • Paul Fredricks_3
    Paul Fredricks_3 Member Posts: 1,557

    Usually frozen coils are caused by a lack of air flow. Shut the unit off and let it defrost. Check your air filters and make sure they are clean. If the blower is driven by a belt make sure it is in good shape and is adjusted properly. Also check all the registers to make sure they are open and make sure the return grills aren't blocked.
  • JStar
    JStar Member Posts: 2,752

    ...could be undercharged. Maybe a partially restricted metering device. Airflow is the easiest to check first. 
  • Techman
    Techman Member Posts: 2,144
    Along the

    low air flow lines, it could be a dirty evap coil and/or a dirty blower wheel.
  • Eugene Silberstein_2
    Eugene Silberstein_2 Member Posts: 349
    Airflow vs. Udercharged

    Here's a quick way to tell the difference.

    If there is an airflow problem, the entire evaporator coil will be frozen, along with the suction line back to the compressor. The reason for this is that the air is the source of heat that allows the refrigerant to boil in the evaporator. If the refrigerant cannot absorb heat, the saturation temperature and pressure in the coil drop. This reults in a completely frozen coil.

    If the system is undercharged, there will an underfed evaporator condition, resulting in high evaporator superheat. If such is the case, you will notice that the beginning of the coil is frozen, but the majority of the coil is warm.
    CNOTE Member Posts: 1
    Check Returns

    Along the lines of checking airflow, make sure no returns have become blocked.  I ran into two situations this summer that a customer had done some remodeling and drywallers simply eliminated returns, thus straving the system for air.  I have also seen people moving couches over return grills straving the system....Also check supply side dampers make sure they are all opened.  Many units have a variable speed motor which you can rewire to increase speed/airflow.....this can fix a symptom not the problem...if airflow seems sufficient after the system defrosts ur probably looking at an under charged system.......Do you have gauges?  
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