Variable speed compressor technology brings to the HVAC industry whole new levels of efficiency, comfort, reliability and versatility. The major change happening now is the adoption of variable speed technology to compressors in ducted style systems in the U.S. This is important because according to the U.S. Census, over 75% of the 132 million homes in the U.S. have ducted HVAC systems and approximately five million of them get replaced on an annual basis. These replacement opportunities, combined with approximately one million new homes, which the National Association of Home Builders (NAHB) forecasts to be built in 2014, also with a high percentage of ducted AC systems, makes up the six million units of addressable market for superior efficiency systems such as those featuring variable speed compressors.

Though only a small fraction of those homes will come with a variable speed system next year, as this technology is increasingly adopted into new and replacement installations over time HVAC energy costs will decrease while improving system comfort and reliability for homeowners.  In order to enable the early adoption of variable speed compressor technology into the U.S. HVAC market, U.S. OEMs began introducing new, ducted systems with variable speed compressors as early as 2006 and more have followed since. Within the next few years it is expected that all of the major OEMs will be offering high efficiency ducted variable speed systems in the U.S.

What is Variable Speed Technology and How Does It Work?

Variable speed systems work by connecting a motor control drive to the compressor and then connecting the line input power from the utility to the drive. The drive uses frequency modulation to adjust power output of the compressor motor enabling it to speed up or slow down according to the heating or cooling load in the home. This ability to modulate compressor capacity enables many of the advantages of variable speed technology including improved efficiency and comfort.

Improved Efficiency

The ability to adjust power output to the compressor, rather than merely on-off functionality in traditional systems, allows homeowners to achieve significant cost savings over the course of a year. In some cases these savings amount to 40% of the overall annual energy costs.

Variable speed technology enables this kind of efficiency for three main reasons:

1. Precise load matching
2. Less cycling on and off
3. Low amp gradual compressor motor startup

For example, if the temperature outside is 80 F and the thermostat setpoint is 72 F, the compressor only needs to overcome the 8 F delta to maintain optimal comfort for the homeowner. In these light-load conditions, the compressor may only need to run at 30% or 40% of full speed to maintain 72 F indoors. In traditional fixed-speed systems the compressors are either on 100% or off completely. In those situations, the system will cycle the compressor on and off as needed to match the setpoint as close as possible. This leads to power losses because the energy required to start up a non-variable speed compressor is much more than the energy required to gradually start a variable speed compressor and keep it running at 30% of full speed. This high efficiency operation during light-load conditions is the major efficiency advantage of variable speed technology.

Improved Comfort

The great thing about variable speed technology is that its inherent ability to precisely match compressor output to load in the home also means improved comfort for the homeowner.

This improved comfort is achieved because of three main things:

1. Precise temperature control
2. Precise humidity control
3. Extra capacity during extreme hot or cold weather conditions (in heat pumps)

With less cycling and longer run times comes the added benefit of maintaining precise temperature control (roughly within 1/10th of a degree) whereas in traditional fixed systems temperatures can fluctuate anywhere from 0.5 degrees to 1.5 degrees F. For homeowners who are sensitive to temperature swings, this can be a significant factor in purchasing a variable speed system over a traditional system.

Temperature differences are not the only source of discomfort in a home. Humidity also plays a big role—especially in climates that tend to be both hot and humid. Anytime the compressor is running in air conditioning mode it will also be pulling humidity out of the home. Given that variable speed systems run much longer (though slower) than traditional systems, humidity levels also reduce to more comfortable levels—a difference that homeowners in the southern part of the U.S. can clearly perceive.

Another way variable speed improves comfort is by providing extra capacity during extreme weather days—either hot or cold. On hot days over 110 F, a variable speed air conditioner is able to overspeed the compressor faster than a traditional system would run and provide extra capacity for those few days out of the year. Rather than struggle to keep up, variable speed systems that have been properly designed and sized are able to keep the homeowner comfortable and dry all year long.

These comfort advantages, tighter temperature control, reduced humidity and better air flow (which also improves air quality through better filtration) all contribute to healthier and happier living within the home.

Improved Reliability

With the addition of power electronic controls, variable speed technology is able to protect itself from failure in ways traditional fixed systems are not capable of. Because the motor control drive receives power input from the utility line before sending power to the compressor, the drive acts as a shield to protect the compressor from harmful circumstances. Additionally the drive is constantly monitoring and adjusting the power required from the compressor motor to ensure optimal compressor and system operation.

For example, if the drive senses an unusual torque load on the motor because of adverse outdoor conditions—which if not corrected can lead to failure—it will reduce power to the compressor and proactively prevent it from damage.

Versatile Applications

Because capacity modulation is a universal performance benefit to all HVAC systems, variable speed technology can improve the efficiency and comfort of commercial applications as well.

 As the technology matures and applications proliferate, the benefits of variable speed will expand into other areas including advanced diagnostics, energy use monitoring and demand response programs from utilities.

In the case of demand response, variable speed offers the unique benefit of allowing companies and utilities to increase and decrease electricity demand gradually rather than the blunt on-off method traditional systems allow.

 Even though the fundamentals of variable speed technology have been a part of the HVAC industry for years, the opportunities for improving home efficiency, comfort and reliability in homes have never been better.