Proper Charge Is Critical for Microchannel Coils

Microchannel coils were long used in the automotive industry before they crossed over to HVAC equipment in the mid-2000s. Since then, they’ve become increasingly popular, particularly in residential air conditioning equipment, as they are lightweight, offer better heat transfer, and use less refrigerant than traditional fin-and-tube coils.

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• Microchannel Coils

• Coil Cleaning

• Service & Maintenance

However, using less refrigerant also means that more care must be taken when charging a system using a microchannel coil. That's because even a few ounces one way or the other could result in the reduced performance, efficiency, and reliability of a cooling system.

Traditional fin-and-tube coil design has been the standard used in the HVAC industry for many years. Initially, the coils used round copper tubing with aluminum fins, but copper tubing led to electrolytic and formicary corrosion, resulting in increased coil leaks, said Mark Lampe, product manager of furnace coils at Carrier HVAC. To fix this issue, the industry moved to round aluminum tubing combined with aluminum fins to improve system performance and minimize corrosion. And now there is microchannel technology, which can be used for both evaporators and condensers.

"Microchannel technology — called VERTEX technology at Carrier — differs because the round aluminum tubing is replaced with flat, parallel tubes that are brazed to the aluminum fins," said Lampe. "This more evenly distributes refrigerant across a wider area, improving the thermal transfer capabilities, which enables the coil to operate more efficiently. While microchannel technology has been used in residential outdoor condensers, VERTEX technology in currently used only in residential indoor furnace coils."

The microchannel design creates a simplified, single pass "out and back" refrigerant flow, consisting of a superheating tube at the top and a subcooling tube at the bottom, said Jeff Preston, director of technical services at Johnson Controls. In comparison, refrigerant within a traditional fin-and-tube coil flows through multiple passes from top to bottom in serpentine form and requires more surface area.

"The unique design of the microchannel coil creates a superior heat transfer coefficient that increases efficiency and reduces the amount of refrigerant required," said Preston. "As a result, equipment designed with microchannel coils is often significantly smaller than high-efficiency units utilizing a traditional fin-and-tube construction. This is ideal for applications with limited space, such as homes with zero lot lines."

Indeed, with the implementation of microchannel technology, Lampe said that Carrier has been able to keep the sizes of most of its indoor furnace coils and outdoor air conditioning condensers the same, while seeing an approximate 30% weight reduction versus their round fin-and-tube designs.

"If we had not deployed this technology, we would have needed to increase the size of our indoor furnace coils up to 11 inches in height and would have needed to go to larger-sized base pans on the outdoor condensers," he said.

While microchannel coil technology has mainly been used in residential cooling equipment, the concept is starting to catch on in commercial units as well, as the need for lighter and smaller equipment continues to grow, said Preston.

Because microchannel coils hold a relatively small volume of refrigerant, altering the charge by even a few ounces can affect system longevity, performance, and energy efficiency, said Preston. That's why contractors should always verify the charging process with the manufacturer, but that typically involves the following steps:

Carrier's VERTEX technology maintains the same installation, charging, and startup procedures as the round tube technology and requires no added or different steps to the current recommended charging procedures for cooling, said Lampe.

"Approximately 80% to 85% of the charge is in a liquid state, so when in cooling mode, that volume resides in the outdoor condenser coil and line set," said Lampe. "When transitioning to a microchannel coil with a reduced internal volume (when compared to round-tube fin design), the charge difference only affects approximately 15% to 20% of the total charge, which translates to a small, hard-to-measure difference in the field. That is why the recommended way to charge the system is through subcooling, which we detail in our installation instructions."

However, the smaller refrigerant volume in a microchannel coil can be an issue when the outdoor unit of a heat pump switches to heating mode, said Lampe. In this mode, the system coils are switched, and the condenser that stores the majority of liquid charge is now the indoor coil.

"When the indoor coil's internal volume is significantly smaller than the outdoor coil, the system could see charge imbalance," said Lampe. "To combat some of this, Carrier uses the built-in accumulator, located in the outdoor unit, to bleed off and store some of the excess charge during heating mode. This allows the system to maintain proper pressures and prevents flooding the compressor, which can result in decreased performance as oil may build up in the indoor coil."

While charging a system using a microchannel coil may require a little extra attention to detail, charging any HVAC system requires precise use of the correct amount of refrigerant, said Lampe.

"If you overcharge a system, high energy consumption, inefficient cooling, leaks, and early compressor failures can result," he said. "Likewise, if a system is undercharged, coil freeze-up, TXV chattering, compressor startup issues, and false trips may occur. The issues with microchannel coils are no different."

Because of their unique construction, repairing microchannel coils can be a challenge, said Jeff Preston, director of technical services at Johnson Controls.

"Brazing the surface requires an alloy and a MAPP gas torch, which are not commonly used with other equipment types. Because of this, many contractors will opt to replace the coils instead of attempting the repair."

As for cleaning microchannel coils, that is actually easier, said Mark Lampe, product manager of furnace coils at Carrier HVAC, as aluminum fins on fin-and-tube coils are easily bent. Too many bent fins can lower the amount of air flowing across the coil, reducing efficiency.

"Carrier's VERTEX technology is a much more rugged design, as the aluminum fins are positioned slightly lower than the flat aluminum refrigerant tube and are brazed to the tubes, meaning a brush cleaning does not substantially alter the fins," said Lampe.

EASY CLEANUP: When cleaning a microchannel coil, only a mild non-acidic coil cleaner should be used, or in many cases simply water. (Courtesy of Carrier)

When cleaning a microchannel coil, harsh chemicals and pressure washing should be avoided, said Preston, and only a mild non-acidic coil cleaner should be used, or in many cases simply water.

"However, the small refrigerant volume does require some adjustments to maintenance processes," he said. "For example, because of the low volume, the refrigerant cannot be pumped down when other parts within the system need to be serviced. Additionally, gauge sets should only be connected when necessary to minimize disruptions to the refrigerant volume."

Preston added that Johnson Controls has deployed extreme conditions at its testing facility in Florida, which has enabled the evolution of microchannel development.

"These testing outcomes have enabled us to evolve our product development with multiple alloy enhancements, tube thickness and improved chemistry in the atmospherically controlled oven brazing process to limit coil corrosion and ensure optimal performance and reliability levels are achieved," he said. "These steps have been taken to not only support homeowner satisfaction but also help to minimize maintenance requirements."

Joanna Turpin is a Senior Editor. She can be contacted at 248-786-1707 or [email protected]. Joanna has been with BNP Media since 1991, first heading up the company's technical book division. She obtained her bachelor's degree in English from the University of Washington and worked on her master's degree in technical communication at Eastern Michigan University.