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Why are my HVAC system evaporator coils frozen?

An effective HVAC system relies on electrical, mechanical and plumbing elements all operating properly and efficiently. There are many areas where problems can develop and one of the most common reasons for a HVAC fault is when an evaporator coil freezes up.

The initial signs of a freeze-up include a sudden drop in performance, high indoor temperature and reduced airflow. But why do HVAC coils freeze up? It’s due to restricted air flow meaning the coil drops below 0°C, resulting in moisture on it to freeze. Here are some of the many reasons why this occurs:

Defective blower motor

The fan within an air handling unit (AHU) is commonly known as a blower and is essential for providing airflow across the evaporator coil. Should the blower motor not be operating correctly or is broken down completely, the coil will freeze up. Regular motor checks and maintenance should be carried out and motors rewound or replaced when needed.

Blocked cold-air return grills

Obstructions to low cold-air return grills such as furniture or equipment will significantly impede air flow. These should be moved and the coil allowed to thaw before powering up again.

Insufficient refrigerant

A lack of refrigerant can result in low pressure within the system which will mean the refrigerant becomes colder than the dew point. When the coil operates below the dew point, air moisture freezes to it. Refrigerant levels should be monitored and topped up as necessary and users should be vigilant about leaks.

Blocked air filter

Air filters must be regularly changed or cleaned to allow unrestricted airflow. The frequency of filter changing can vary, so check with your installer.

Coil dirty on intake side

If air filters in your HVAC system are neglected, then the result may be dirty evaporator coils. Dirt builds up on evaporator coils and eventually stops air getting to them altogether and they’ll inevitably freeze up. So when changing filters it’s also worth checking that the coils themselves are clean.

A properly-managed HVAC maintenance routine can be difficult to implement when there are so many other things to worry about. Gibbons can look after this for you with a comprehensive programme of servicing, maintenance and repair for HVAC systems. Our engineers are experienced in a huge range of industries and our flexibility means we’ll fit around your schedule to minimise downtime.

For more information, call Gibbons HVAC Services Manager Andrew Knight on 07850 204915 or email

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