“Air coils in any HVAC system are major refrigeration circuit components,” he said. “Considering the financial burden on the customer, it is wise to make a good-faith effort to repair if a coil is repairable and has several years of useful life left.”

The repair discussion begins when a coil’s heat transfer function is diminished or lost, according to Greg Kohler, engineering section manager, Modine.

“This can be a result of a number of reasons, including external fouling, where dirt and debris or even corrosion byproduct degrades air flow and/or airside heat transfer; refrigerant leaks caused by corrosion, pressure, or thermal fatigue; or physical damage,” he said. “The decision to repair or replace depends on many factors. Obviously, it is almost always more economical to repair a coil than it is to replace. However, the likelihood of success in attempting a repair will vary with the coil construction, contractor skill, and failure type.”

According to Kohler, a field braze repair on a copper tube coil can be accomplished fairly simply, as long as access to the damaged area is available. He noted that when it comes to microchannel coils, there are proven repair methods, but technicians must follow manufacturer procedures to mitigate the risk of further damage.

WHY REPLACE THE COIL?

The financial difference between repairing and replacing a coil can make an unsure HVAC technician look like a snake oil salesman. There are specific reasons to replace coils, however, and technicians should be able to explain these reasons in a manner that customers are able to understand.

According to Singh, coil compatibility is a prime reason to replace the coil or perhaps the entire system. He explained that old original coils have a habit of being obsolete or unavailable. To avoid mismatched systems, Singh said that replacing the entire system instead of just the old coil would be the best option.

“As for refrigerants, R-22 coils are not suitable for R-410a refrigerant,” he said. “When retrofitting, system coils must be replaced to make sure they meet pressure and capacity requirements for the new refrigerant.”

Another reason to replace the coil stems from certain types of leaks within the system.

“Leaks internal to the coaxial coils and in the slab or fin pack areas of a fin and tube coil are not considered repairable leaks,” said Singh. “In such conditions, the coil must be replaced. Coil surface area in a fin and tube coil is a key factor to measure performance. Damaged fins will restrict air flow through the coil. If the fin pack area is damaged beyond repair, it is advised to replace the coil. Hail damage on outdoor condensing coils is a prime example as well.”

King Tong, residential product manager, Goodman Manufacturing Co. L.P., Waller, Texas, said that when a leak is present, technicians need to examine the coil to see if the leak is at the fin pack or at the braze joint.

“If it is at the fin pack, the best practice would be to replace the coil,” he explained. “Overall, check to see if the coil is in good operating condition. If there is clear degradation in performance or efficiency, it may be time to replace the coil.”

Singh said that contractors should be able to decide whether to repair or replace based on their situation and mode of failure.

“Simply put, physical damages generally are repairable,” he said. “If repairable, the coils should be repaired. Corrosions, such as formicary or any other caused by environmental conditions (as in a seacoast application, for example), are not repairable. A replacement is the only option in such a situation.”

IMPROVING COIL LIFESPAN

There are steps that contractors can take to help improve the lifespan of a coil. According to the three manufacturers in this article, the lifespan of a coil can be anywhere between 10 and 20 years, depending on the environment of the application.

“The market expectation may be as high as 15+ years; however, this may be lessened in highly corrosive areas such as coastal or industrial areas,” said Kohler. “Manufacturers of coils and HVAC products continually strive to improve product life through advanced materials and coatings development and greater understanding and prevention of design failure modes, all while working to optimize cost and value to the market.”

With this in mind, he suggested that proper application and maintenance of coils are two specific factors that contractors can focus on with their customers.

“Overall system performance, within design and manufacturers’ specifications, is the key to avoid premature component failures, including condensing and evaporator coils,” said Singh. “High operating pressures, for example, will cause fatigue due to high temperature and vibrations, which results in leaks in refrigerant circuits, including coils.”

Contractors can also suggest regular coil cleaning and professional inspection to help prolong the life of a coil.

“This is the most efficient way for problems to be identified, diagnosed, and rectified,” said Tong.

He pointed out that simply replacing air filters can have a positive performance effect on the evaporator coil.

“When debris and particles in the air cannot be captured anymore due to old, clogged air filters, they instead can build up on the coils,” he said.

KEEP AN EYE OUT

If there are ways to lengthen the life of a coil, then there are ways to shorten the lifespan of the coil as well. Two of those factors have to do with exposure and system operation.

According to Singh, exposing coils to freezing conditions may cause collapsing or burst tubes in the coaxial or brazed plate heat exchanger coils.

“Partially deformed or collapsed coils due to freezing will reduce capacity and will be less efficient.”

As for the system operation, if the unit is functioning outside of the operating envelope, it is possible to shorten the life of the unit’s components, like the coil, said Singh.

“High pressure and high temperature will cause excessive expansion and contraction of the coil,” he explained. “This results in poor bonding between fins and tube, resulting in poor heat transfer.”

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