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Why are my HVAC coils corroded?

HVAC coil corrosion can be a costly problem for building managers, and it can strike at any time – from a matter of weeks to years following installation. To make matters worse, symptoms aren’t always immediately apparent to the naked eye so it’s a tricky defect to spot. By the time corrosion is apparent, it’s usually too late and replacement is required.


It’s often a matter of location, as coils are more prone to corrosion when installed in buildings near the coast or close to industrial applications such as laundries and water treatment works. However, on the whole corrosion occurs simply because coils operate in an ever-changing environment of air and water.

There are two common types of coil corrosion – pitting and formicary.

Pitting corrosion is caused by the presence of chlorides or fluorides and can normally be seen on the outside of copper tubing. Pitting occurs when condensation forms on coils and coils and negatively-charged chloride or fluoride ions attack the metal surface. Pits, or small pinhole-type undulations are formed which eventually corrode through completely and cause a refrigerant leak.

Formicary corrosion occurs as a result of organic acids including acetic and formic acids. Formicary corrosion is less obvious than pitting as it tends to take the form of a network of tiny tunnels beneath the surface of the copper. Eventually one of these passages breaks through the surface and allows refrigerant to seep out. Look out for blue/grey deposits on coil surfaces as a sign that formicary corrosion is taking place.

Refrigerant leaks and burst coils, like any HVAC system fault, lead to discomfort for building occupants and require prompt attention. You can rely on Gibbons to attend to burst or leaking coils urgently and have your system functioning properly in the shortest possible time. Our flexible working systems mean our engineers will operate around your schedule to minimise disruption to your business.

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

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