The cooling coil is a critical component of an Air Handling Unit (AHU). It removes heat from the air before it is distributed throughout production areas, offices, and industrial facilities.
However, when an AHU cooling coil starts freezing, its cooling performance can decrease significantly. Ice buildup restricts airflow, affects heat transfer, and may cause unstable HVAC operation.
So, what causes an AHU cooling coil to freeze, and how can the problem be prevented?
Here are five common causes that maintenance and utility teams should understand.
1. Dirty or Clogged Air Filters
Air filters remove dust and contaminants before air reaches the cooling coil. When filters become clogged, airflow decreases.
Insufficient airflow can cause the coil temperature to drop excessively, increasing the risk of ice formation.
Solution:
- Inspect air filters regularly.
- Clean or replace clogged filters.
- Follow the manufacturer’s recommended maintenance schedule.
2. Blower Fan Not Operating Properly
The blower fan ensures sufficient airflow through the AHU cooling coil.
A weak blower motor, loose fan belt, damaged fan blade, or incorrect fan rotation can reduce airflow and contribute to coil freezing.
Solution:
- Inspect the blower motor and electrical connections.
- Check fan belt tension and condition.
- Verify fan rotation and operating speed.
- Measure airflow against the AHU design requirements.
3. Dirty Cooling Coil
Dust and dirt accumulating on the cooling coil can restrict airflow and reduce heat transfer efficiency.
This condition may cause uneven cooling and contribute to ice formation, particularly when combined with insufficient airflow.
Solution: Clean the cooling coil using an appropriate cleaning method and chemical compatible with the coil material.
4. Refrigeration System Problems
For direct expansion (DX) AHUs, refrigerant leakage, an improperly operating expansion valve, or excessively low evaporating pressure may contribute to coil freezing.
For chilled water AHUs, insufficient chilled water flow, incorrect water temperature, or control valve problems can affect cooling performance.
Solution:
- For DX systems, inspect refrigerant pressure, possible leaks, and expansion valve operation.
- For chilled water systems, check water flow, supply temperature, and control valve operation.
- Compare operating parameters with the manufacturer’s specifications.
5. Faulty Temperature Sensors or Controls
Incorrect temperature readings from sensors, faulty thermostats, or malfunctioning control valves can cause the cooling system to operate outside its intended conditions.
As a result, the cooling coil may become excessively cold and develop ice.
Solution:
- Check temperature sensor accuracy.
- Inspect electrical wiring and control signals.
- Verify thermostat and controller settings.
- Check actuator and control valve operation.
AHU Cooling Coil Inspection Checklist
Before replacing any components, perform a systematic inspection to identify the root cause.
| Inspection Point | What to Check |
|---|---|
| Air filter | Dirt buildup and airflow restriction |
| Blower fan | Motor, belt, rotation, and fan speed |
| Cooling coil | Dirt accumulation and ice formation |
| Airflow | Air volume and static pressure |
| DX refrigeration | Refrigerant pressure and expansion valve |
| Chilled water | Water flow and supply temperature |
| Control system | Sensors, thermostat, and control valve |
Important: If the cooling coil is frozen, stop the cooling