Troubleshooting an Air Conditioning System with a Frozen Evaporator

When arriving on-site to a frozen evaporator coil and suction line on an air conditioner, you should be aware that at least one of the three following problems exist: Low Refrigerant Charge, Low Airflow, or Liquid Line Restriction.

Even while the evap is frozen and the system is still running, you can identify one of the three as the source of the problem. The low pressure side of the system is the part that is covered with ice or frost, but the high pressure side of the system can still be checked by measuring the subcooling.

If the subcooling is low while the evap coil is frozen and the system is still running, the system is low on refrigerant. An indication of this would be if the subcooling is 3° F or lower. If the system has normal to high subcooling such as 6° F of subcooling or higher, the system is not severely low on refrigerant.

If the system is not low on refrigerant, the ice on the evaporator must be melted to move on to the next step in order to determine the problem. Be aware that water will likely drip outside of the primary condensate pan while melting the ice from the evaporator with a heat gun, blower motor, or while the system is off.

Check the indoor air filter to make sure that it is clean while the indoor blower is off. Once the evap coil is completely defrosted, airflow can be measured at the registers with a rotating vane anemometer or flow capture hood while the outdoor disconnect for the condenser is in the off position and the indoor thermostat is calling for cooling. There are multiple other methods for measuring airflow such as traversing the duct with a hot wire anemometer or a manometer with a long Pitot tube. Another method is the temp rise formula used with a source of heat (These are all explained in greater detail in our book as well as determining the exact source of the airflow problem. https://www.acservicetech.com/the-book). If airflow is the problem, this needs to be corrected before turning on the outdoor unit and trying to measure the saturated temps, total superheat, and subcooling.

If there is proper indoor airflow matched to the capacity of the system (approx. 350cfm-425cfm per 12,000 BTU/HR and 12,000 BTU/HR = 1 ton), the outdoor unit can be turned on and the system’s refrigerant charge can be checked to determine the problem. Make sure that the refrigerant manifold and temp sensors are attached to the system prior to starting the outdoor unit. The charge will need to be checked quickly before the indoor coil freezes to read accurate sat temps, total superheat, subcooling, and Delta T.

When checking the charge, refrigerant does not need to be added in order to determine the problem. Measurements must be taken before the evaporator coil freezes. The following readings can be used to identify the problem:

Low Refrigerant Charge, TXV or Fixed Orifice: Low Vapor Sat Temp, High Total Superheat, Low Subcooling, Low Delta T

Low Airflow, Fixed Orifice: Low Vapor Sat Temp, Low Total Superheat, Normal to Low Subcooling, High Delta T

Low Airflow, TXV: Low Vapor Sat Temp, Normal Total Superheat, Normal to High Subcooling, Normal Delta T

Liquid Line Restriction TXV or Fixed Orifice: Low Vapor Sat Temp, High Total Superheat, Normal to High Subcooling, Low Delta T

Check out our book the “Refrigerant Charging and Service Procedures for Air Conditioning” for a full Troubleshooting Guide for each problem scenario that you can run into when checking a refrigerant charge. We also have a full chapter on Low Airflow Troubleshooting.

Check out the full outline at https://www.acservicetech.com/the-book

If you have already purchased our book, be sure to tell local HVACR Instructors about our book and what you think of it. We would love to get this book into the hands of the next generation of HVACR Technicians!

Published: 8/20/2019

Author: Craig Migliaccio

About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 15 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel