In this article I'm going to clarify the different operating characteristics of an HVAC system as well as explain when aftermarket parts are acceptable. This article should help you understand when and where aftermarket parts are acceptable.
There are essentially two sides of all HVAC systems when it comes to operation and they are the control side and the mechanical operation side. The easiest way to think about this is you have a thermostat (controller) that sends a signal to other controls, that allow voltage to be applied to motors, that actually provide us with heating and cooling. Examples of control side components in typical HVAC (residential) equipment are contactors, thermostats, relays, switches, sensors, and relay boards (printed circuit boards). Examples of the mechanical side are compressors, metering devices, accumulators, refrigerant valves, and coils (sometimes referred to as heat exchangers).
All companies carry aftermarket parts on their vehicles and the most common are capacitors, contactors, relays, and thermostats. Often times these are parts that can be fairly universal on most common HVAC equipment. The exception to that last statement is going to be higher efficiency equipment and inverter driven equipment. There are considerations when deciding to apply these aftermarket parts especially in terms of contactors and relays. Contactors and relays are often used to switch high voltage on and off to a motor and these motors have current ratings based on size and starting loads. If the contacts on the aftermarket part are not properly rated for the load, it can lead to premature failure of the relay and sometimes even damage the motor it's controlling. Even when it comes to thermostats, not all of them fit every application. The most common types of equipment in the industry is fuel fired appliances, heat pumps, electric heat only, air conditioning only, heat pumps, hybrid systems, multi stage systems, boilers... the list goes on and the number of thermostats on the market that can control all of these is very few. Something as simple as a thermostat can be a tricky one to get right if you are not familiar with the equipment operation and type. Though selecting the right aftermarket part for the control side can be difficult, it's possible and appropriate in most situations so long as certain guidelines are followed.
When it comes to the mechanical side of operation, these components are specified and designed by engineers to render a very meticulous and specific operation. These components have a dual job in the system, to deliver the designed capacity as well as protecting the compressor (one of the most expensive parts to repair in a system). Compressors are cooled by the refrigerant in the system, not just by temperature, but also by the volume of refrigerant flowing back to it. If the volume is decreased because of a faulty part then the compressor can overheat even if the refrigerant is cool, leading to compressor failure. I'm going to surprise some of you when I say that a fan motor falls into this category. You might be thinking, "a fan motor has nothing to do with the refrigerant flow". Actually a fan motor has just as much effect on the compressor as the compressor itself. Refrigerant changes temperature and physical state only because of the velocity and volume of air flowing over the coils in the system. If the air is too little or too much, it can heat the refrigerant too much causing the compressor to overheat. If the air flow is not enough, the refrigerant may not change physical state from a liquid to a vapor. Liquid refrigerant will kill a compressor as fast as any other improper operation. The coils and metering devices are equally prudent to the process as they control the amount of heat entering or leaving the refrigerant, as well as they control the flow/volume of refrigerant moving through the system. These components are very specific to equipment operation and protection of vital (expensive) parts. Fan motors can fall into a gray area as there are "close enough" options available for purchase, but I always recommend exercising extreme caution when selecting aftermarket fan motors.
There's so much more than meets the eye when it comes to replacing fan motors. Some very important specs to pay attention to are the full load amps, revolutions per minute, starting torque, bearing type, horsepower, and applied voltage. The average tech/company has some rules of thumb for aftermarket motor selection that has been learned from experience to facilitate selecting a decent motor replacement as opposed to the factory motor. My personal thought on this is to always go with the factory when reasonable. Only the factory motor will be perfect for the application, though perfect isn't always plausible, it should at least be considered and investigated prior to aftermarket fan motors being installed.
My final thoughts and suggestions are this. In the hands of a competent individual, aftermarket controls and fan motors are acceptable and often times cheaper and faster solutions to factory components. Factory is best, but you may have to wait for them and typically will cost a bit more than aftermarket parts. When it comes to refrigerant side components such as compressors, metering devices, coils, accumulators, receivers, I personally feel that they should NEVER be substituted unless the unrealistic scenario of a design engineer approves it. Basically, these components should not be replaced with aftermarket parts and keeping the factory design is imperative to the equipment performance.
Kenneth Casebier
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