Water freezes at 32°F or 0°F. When a surface, such as an evaporator coil, is below water’s freezing point and contains moisture, frost or ice may form on that surface. In many cases, freezing in HVAC systems starts at the evaporator coil and works its way out; freezing can happen when the suction saturation temperature stays below 32°F for an extended period of time.

Some HVAC/R applications, such as commercial freezers and heat pumps in heating mode, need to operate under below-freezing conditions; the formation of frost or ice will hinder heat transfer over time, and periodic defrosting will be required. Evaporator coils in cooling mode should NOT freeze; when freezing happens, it will block system airflow. In general, evaporator coils will be about 35°F below the return air temperature.

The rate at which freezing occurs will depend on the amount of moisture in the air, the length of time at which the evaporator coil temperature is below freezing, the temperature of the return air, the air velocity, and the overall coil design.

When you encounter a frozen system, allow it to defrost completely before troubleshooting. You may allow the system to defrost by leaving it off with (or sometimes without) the blower running. In some cases, you may be able to use a heat gun or run the heat pump in heating mode for a short period. Be mindful of the water as the ice melts and take steps to prevent water damage.

A low evaporator coil temperature may be caused by low load (low airflow and/or low indoor temperature), refrigerant undercharge, low outdoor ambient temperatures, blower issues, and refrigerant side restrictions. We can prevent low load by advising customers not to set their thermostats below 72°F in cooling mode on standard equipment. We can also prevent low airflow from being a factor by making sure we replace dirty filters and clean the evaporator coil and blower wheels as necessary. When freezing happens due to a low refrigerant charge or a restriction, we may also notice high superheat; in humid climates, freezing of this nature can cause the entire system to freeze up.