Whether you own a large commercial building or are homeowner, you rely on heating, ventilation and air conditioning (HVAC) equipment to maintain the comfort level of your building. While this equipment is reliable, it inevitably fails. Then, if you find the culprit is a heating or cooling coil, you may ask yourself, “how did this happen?”
HVAC coils, like other components of the HVAC system, are prone to wear as the system ages. These coils typically last a long time but will eventually wear out. How? The following are common reasons these coils fail; and what you can do to prevent premature failure.
Air is dirty. Particles of dander, pollen, industrial pollution, and many other objects float around in the air we breathe. It doesn’t matter whether you are in an enclosed building or sitting in a park, the air we breathe is contaminated. Our bodies do a great job of protecting us from most of these contaminants. But what protects the coils in your HVAC equipment? Most often, the answer is air filters. Filtration plays a critical role in protecting and extending the life of your costly HVAC equipment, including its coils. However, many do not change filters regularly. Preventative maintenance is a cost-effective way to extend the life of your coils.
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| Figure 1 – Condensate and particulates attach to coils causing corrosion. |
Particulates in the air pass around the filter can stick to the fins of the coil. Coils without proper filtration often become a pretty good filter, too, after the fins clog with the particulates in the air. Once a coil is clogged, the fins and copper that make up a coil can corrode from a combination of condensation and the foreign materials stuck between the fins (Figure 1). Corrosion is a major cause of coil failures.
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| Figure 2 – Freezing temperatures will burst coils filled with water. |
Sometimes air handling units take in large quantities of outside air. If the coil is using 100% water as the heating / cooling medium in the coils, and the outside temperature drops below freezing, you could experience a frozen coil failure. This happens when the water freezes and expands in the coils. This expansion is enough to burst the coil’s copper tubing. (Figure 2) This is very preventable. A developed maintenance schedule and an installed freeze stat sensor that closes the outside damper of an economizer when temperatures drop below freezing can prevent coil freeze-up. Still, the freeze stat sensor or damper operator can fail. The best prevention against a frozen coil is using a glycol water solution or by draining the coil during the winter months if the coil is not in use.
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| Figure 3 – Extruded holes prevent failure caused by vibration. |
Excessive vibration is another common cause of coil failure. The coil housing holding the coil tubes can experience vibration from fans and compressors. This unintended vibration can ultimately result in coil failure. The failure point is located where the coil’s copper tubing passes through its sheet metal housing. The constant vibration from fans, compressors or both creates wear on the copper tube wall and eventually causes it to fail. Therefore, make sure the replacement coil or coils you specify should have extruded holes on the housing, so no sharp edges come in contact with the tubing. (Figure 3)
Another source of coil failures is caused by pressure cycling or spikes in the waterside of the coil. Coils are hydrostatically tested according to the factory specifications, but can still fail when the water pressure exceed specifications. Over pressure or spikes can be the result of a faulty high water pressure limit switch. Always be sure to check this switch when replacing coils to ensure it is in proper working condition.
Coils do fail, but remember, you can prevent many of the common causes of coil failure with proper maintenance. However, if a coil failure occurs, most outcomes only require replacement of the coil and not the entire unit. When you are looking for a replacement coil, always be sure to check the measurements of the old coil versus the new replacement coil drawings and performance to ensure a successful replacement.
Learn more about the many types of coils used in HVAC equipment.
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