Cooling Coil Freezing: Causes & Prevention

Why Coils Freeze

A cooling coil freezes when the surface temperature drops below 0°C. This happens when the refrigerant in the coil is too cold (low suction pressure) or the airflow across the coil is too low (not enough heat being transferred to the refrigerant). The condensation on the coil freezes, building up as ice that blocks the airflow passages.

Causes

• Low airflow (most common): If the airflow across the coil is too low, the refrigerant doesn't absorb enough heat and the coil temperature drops below freezing. Causes: dirty air filter, blocked duct, failed fan, or closed dampers. Fix: check and replace air filters, verify fan operation, check dampers.
• Low refrigerant charge: Low charge causes low suction pressure, which means the evaporator temperature is too low. The coil surface drops below freezing. Fix: find and repair the leak, then recharge.
• Faulty expansion valve: If the TXV is stuck closed or underfeeding, the refrigerant pressure in the coil drops too low, causing the temperature to drop below freezing. Fix: inspect and replace the expansion valve.
• Low chilled water temperature: On chilled water systems, if the chilled water temperature is set too low (below 5°C), the coil surface can drop below freezing. Fix: verify the chilled water setpoint (typically 6–7°C supply).
• Low load: If the cooling load is very low (cold day, unoccupied space), the coil doesn't have enough heat to absorb and the temperature drops. Fix: install a low-load bypass or a hot gas bypass to maintain minimum suction pressure.
• Dirty coil: A dirty coil (on the air side) reduces heat transfer. The refrigerant doesn't absorb enough heat and the coil temperature drops. Fix: clean the coil.
• Defrost cycle failure: On heat pumps, the defrost cycle reverses the refrigerant flow periodically to melt ice. If the defrost control fails, ice builds up. Fix: check the defrost timer and sensor.

Symptoms

• Reduced airflow from supply grilles — ice blocks the coil passages
• Temperature rise in the conditioned space — the system can't cool
• Ice visible on the refrigerant lines or coil surface
• Frost on the suction line (outside the coil)
• Low suction pressure on the chiller/refrigerant system
• Water leakage when the ice melts (if the drain pan overflows)
• Compressor short-cycling (low pressure switch trips and resets)

Prevention

• Replace air filters regularly — a dirty filter is the #1 cause of low airflow
• Install a freeze stat (temperature sensor on the coil) that shuts down the compressor if the coil temperature drops below 3°C
• Maintain correct refrigerant charge — leak test annually
• Install a low-pressure switch on the suction line to shut down the compressor at low suction pressure
• Verify the fan is operating correctly — check belts, motor, and controls
• Clean coils regularly — both air side and water side
• Set the chilled water temperature correctly — never below 5°C
• Install a hot gas bypass for systems that operate at very low loads