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HVAC School - For Techs, By Techs

Nov 26, 2020

In this episode, Bryan and Sam discuss freezing evaporator coils. They explain why frozen coils happen and how to address them.

When the coil's surface temperature drops below freezing (32 degrees), the moisture in the air that condenses on the coil can freeze to the coil. In those situations, your suction saturation will probably be in the mid to high twenties.

Generally, freezing evaporators will occur when you have less load on the evaporator. When there is less heat, the evaporator temperature will drop accordingly. The return air temperature is usually around 35 degrees, though that number can fluctuate on older equipment or on systems with dirty coils. Freeze-ups usually happen due to poor airflow or low refrigerant charge, though low refrigerant is usually less severe than airflow or compound airflow-charge problems. Conditions that cause low mass flow can lead to freeze-ups.

When you approach a frozen coil, the first thing you want to do is defrost the coil completely. Then, you will want to check airflow (filter, blower wheel, and coil cleanliness) and then refrigerant restrictions and charge. You'll especially want to make sure you check the liquid line for temperature drops and ensure its temperature is warmer than the outdoor ambient temperature. In addition, static pressure is a valuable reading for determining airflow.

Drain lines can also freeze, though it's a rare occurrence. When that happens, you do NOT want to blow out the blockage with nitrogen! You will break the drain line before any ice comes out.

Sam and Bryan also discuss:

  • Driving the temperature down
  • Low charge as a cause of freezing
  • Considerations for various system types
  • Using a scale for charging
  • Heat pumps in heat mode
  • ECM motor failures

Learn more about Refrigeration Technologies HERE.

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