What You Need to Know About MicroChannel
Microchannel is a coil type used in many evaporator and condenser coils and can easily be identified by its flat tubes and fins between them that appear as waves between the tubes. The technology was developed for use in the automotive industry and is used for radiators and automotive A/C condenser coils.
These coils are made of all aluminum and are used because of their superior heat transfer due to increased surface contact between the refrigerant and the metal as well as the lighter weight and smaller refrigerant charge.
These coils have come under a lot of criticism by technicians due to an undisputed high failure/leak rate of the condenser coils in some systems. Some have felt these failures occur to inherent issues with the design while others have stated that the leaks were due to specific manufacturing issues on a few coils and that these issues are largely in the past. No matter how you feel it's likely that microchannel coils are here to stay due to the increased heat transfer efficiency and decreased weight. Here are a few things you need to know when installing and servicing microchannel coils.
Don't Pump it Down
MicroChannel condensing units are not sent with the full system charge and must have the charge added to manufacturers specs even with a short (say 15′) line set with more charge carefully added for line length.
You cannot, and must not attempt to pump down a system with a microchannel condenser or you risk causing a catastrophic failure of the coil.
Instead, you must recover the charge when making a refrigerant circuit repair and then carefully weigh the proper charge in after the repair is made.
Use Proper Brazing / Evac / and Refrigerant Practices
It's right in the name “micro” channel. The flat tubes have tiny refrigerant channels in them and they are susceptible to blockage by any solid contaminants in the system. Make sure to flow nitrogen while brazing, install a new liquid line drier after making a refrigerant circuit repair and pull a proper vacuum (as always). You also need to take extra care to keep shavings out of the system when cutting and reaming and keep tubing ends and hoses away from dirt and debris. For example, if you replace a compressor, anything allowed to get in your pipework will hit the condenser coil before it ever reaches the liquid filter/drier and has the opportunity to clog part of the microchannel coil.
Most manufacturers advise against using any cleaner on microchannel coils to avoid damage. Either use a garden hose, low pressure “fan” pattern pressure washer less than 100psi or a cleaner that is approved for use with microchannel and work carefully. The refrigerant channels go all the way to the edge of the coil and can be easily damaged if impacted.
The Charge is CRITICAL
When charging microchannel you will want to follow manufacturers specs and weigh the charge in whenever possible. If you see low suction doesn't just start dumping in charge until you are certain it is a charge issue and not an airflow issue or a restriction. Subcooling on microchannel systems tends to be more erratic due to the lower volume of the condenser coil.
Many manufacturers will swear that microchannel is just as resilient as tube and fin coils, based on my personal experience I would suggest taking greater care to protect microchannel. It may make sense to keep microchannel away from areas of the lawn that will have equipment going near the unit and possibly shooting debris into the surface.
When a microchannel condenser leaks it is often fairly evident by the oil stain that appears on the surface. Because of the channels, these leaks can be quite small, so if you see the telltale oil spot it is best to investigate.
I confess I have never attempted a microchannel repair myself, but there are many who claim to do it regularly. Here is a video showing it being done.
So take extra care when installing and servicing microchannel systems when cleaning, charging and repairing.