Frozen evaporator coil: causes, measurements and HVAC troubleshooting guide
Why does an evaporator coil freeze? A field guide to airflow, filters, blower operation, charge, restrictions, saturation temperature and superheat.
A frozen evaporator coil is a symptom, not a diagnosis. Frost means the coil surface remained below freezing long enough for ice to build. Restricted airflow is common, but a starved evaporator, incorrect charge or control problem can create a similar result.
Trane's official frozen coil guide lists dirty filters, dirty coils, blocked returns and thermostat problems among possible causes. Trane advises against continuing to operate a frozen system.
1. Defrost before drawing conclusions
Ice changes airflow and system pressures. Stop cooling, protect the area from water and allow the coil to defrost completely before final measurements. Do not scrape the ice or apply aggressive heat to the coil.
2. Prove the air side
- Filter, returns and registers are open.
- Evaporator and blower wheel are clean.
- Blower operation, speed and rotation are correct.
- Airflow matches the equipment, duct system and load.
Measure return and supply temperatures after stabilization. Temperature split alone is not enough; combine it with pressures, saturation temperature and airflow evidence.
3. Read the refrigerant circuit
Select the correct refrigerant and convert suction pressure to the appropriate saturation temperature. Measure actual suction-line temperature at the proper location to calculate superheat. Low suction pressure alone cannot distinguish low charge, a restriction and low airflow.
4. Investigate a starved evaporator
After airflow is proven, evaluate superheat, subcooling, temperature drops across possible restrictions, metering-device operation, bulb mounting, distributor condition and charge using the manufacturer's procedure. A leak must be repaired before refrigerant is added.
5. Prove normal operation
After correction, stabilize the system and record suction/discharge pressure, saturation temperatures, superheat, subcooling, air temperatures, current and coil condition. Confirm that meltwater and normal condensate drain correctly.
Use FrigoTechPro's free P/T chart and superheat calculator. The full app connects these measurements to nameplate scanning, the HVAC-R AI assistant and service reporting.