Beyond On/Off: How Variable Speed Redefines Efficiency

Variable-speed technology is one of those game-changers that is slashing energy bills, fine-tuning comfort, and reducing wear-and-tear on equipment. 

As the variable-speed compressor continues to assert dominance over its fixed-speed counterpart, the future of HVAC might already be here.  

 

True Energy Efficiency Improvement 

The phrase “energy efficient” is slapped on nearly every piece of new HVAC equipment, but does it always hold up? Are there marked improvements between new and old technology?  

In the variable speed versus fixed-speed dynamic, the answer is a resounding “yes.” 

“The greatest source of energy waste in HVAC systems stems from the inability of conventional equipment to match the real-world load of a structure,” said Reid Goyert, sales engineer at Fujitsu General America. “This not only reduces energy efficiency but also compromises indoor comfort and shortens equipment lifespan.” 

Defined efficiency is achieved when the fans, pumps, and compressors operate with just enough capacity to satisfy the operating conditions, said Tom Gallant, vice president, engineering and technology, commercial HVAC Americas – Trane Technologies.  

“Energy is wasted when equipment is selected to operate at one condition, but most operating hours happen at different conditions,” Gallant said. “In most HVAC systems, the equipment must operate within a range of conditions.” 

With most HVAC systems being sized to handle the hottest and coldest days of the year, it’s all the days in between where that conventional system is essentially oversized because they aren’t reaching those extreme conditions. All of that on/off cycling means more energy usage. However, a properly sized fixed-speed system can meet base-tier efficiency standards. 

“If a compressor is undersized, it will run continuously. If it’s oversized, it will cycle more frequently. Likewise, systems should be maintained on a proper maintenance schedule (i.e., changing filters, cleaning, and fine-tuning) to ensure peak efficiency,” said Brad Gross, product marketing manager, Copeland.  

Beyond proper sizing, some manufacturers offer intermediate solutions to boost efficiency without fully abandoning fixed-speed technology, like the two-stage compressor.  

“Copeland two-stage scroll technology operates similarly to a fixed-speed compressor but mechanically modulates at either 67 or 100% capacities to better match load requirements — without the need for an inverter drive,” Gross said. “The results are much-improved comfort along with higher seasonal efficiency ratings.” 

Like conventional systems, variable speed must be properly designed, but they have much more leeway.  

“Unlike traditional equipment, variable-speed systems use inverter-driven compressors, allowing them to adjust their capacity in real-time to match the building’s actual load,” Goyert said. “While conventional equipment operates in an ‘on’ or ‘off’ state, inverter-driven systems continuously modulate between zero and 100% capacity.” 

Gallant added that while variable speed does have the upper hand in a wider range of operating conditions, that’s not the whole story.  

“While variable-speed equipment may achieve better efficiency at off-design conditions, fixed-speed equipment benefits near design conditions because there’s no power consumption from variable-speed drives,” Gallant added.  

 

Where Variable Speed Dominates 

Frequent short cycling, the on/off common in oversized equipment, is a huge source of wasted energy and unnecessary stress on vital components.  

Goyert said to think of inverter-driven, variable-speed systems like cruise control in a vehicle — adjusting the throttle input to maintain steady speed, whether going up or downhill.  

“In an HVAC system, these ‘hills’ represent changes in load, such as fluctuations in outdoor temperature, solar heat gain, or internal loads,” Goyert said. “Just like the cruise control system receives direct feedback from the engine, transmission, and speedometer, the inverter-driven variable-speed compressors receive direct feedback from thermistors (temperature sensors), pressure sensors, and power supply.” 

By providing more consistent output, they also help to deliver more consistent temperatures and humidity levels, thereby creating a more stable and comfortable environment.  

“For these reasons, retrofitting with an inverter-driven system typically reduces energy consumption by approximately 30%,” Goyert said. “While many factors influence actual savings, this estimate provides a reasonable expectation — without even considering the significant comfort benefits of upgrading to a variable-speed system.” 

Beyond just energy and comfort, Derryl Wright, P.E., manager, platform management for LG Air Conditioning Technologies, said there are other benefits as well.  

“Variable-speed inverter drive technology allows for quieter operation due to inverter compressors, which allow for more precise control over the refrigerant flow, and because the system is able to run at more consistent and lower speeds, rather than turning the system on and off to maintain desired temperatures,” Wright said. 

Josh Souders, product marketing manager, Copeland, said while two-stage systems offer a step toward modulation, variable speed takes it further — for example, with a two-stage scroll compressor running at 100% or 67% capacity, the second stage enables the system to turn down, use less power, and still fulfill the cooling requirement. 

“Variable speed takes that modulation to the next level, enabling compressor speed and capacity to precisely match load demand,” Souders said. “This allows the compressor to run for longer periods at lower energy levels.” 

 

New Construction And Retrofitting 

As for whether variable speed can be retrofitted, it’s a more nuanced answer, as careful consideration must be given to the existing ductwork to ensure proper performance.  

“One of the best options for a commercial variable-speed retrofit is a DX (direct expansion) kit,” Goyert said. “A DX kit includes one or more loose coils and updated controls. In this process, the existing coils within the air handler are removed and replaced with a properly sized new coil, effectively converting a conventional air handler into a variable refrigerant flow (VRF) system.” 

A similar approach can be taken in residential applications using a loose coil, Goyert added.  

“In this setup, a specifically selected refrigerant coil is installed on top of the home’s existing furnace to provide both hybrid heating and cooling,” Goyert said. “The existing furnace blower then circulates air through the new coil and existing ductwork, giving homeowners the flexibility to heat their space using either the existing furnace or the high-efficiency heat pump. In many cases, homeowners can take advantage of rebates for high-efficiency heat pumps while still retaining the furnace as a backup heating source if needed.” 

In ducted systems, the size of the zones plays a crucial role. Larger zones average out the load across the space, while smaller zones allow for more responsive temperature control in specific areas. Whenever possible, large zones should be subdivided into multiple smaller zones during a retrofit to maximize performance benefits. 

“One of the biggest challenges in a retrofit is the design of the existing ductwork. Assuming the duct system was properly designed, sized, and installed for adequate static pressure when originally installed, it can lead to suboptimal performance if paired with improperly selected new air handling units,” Goyert said. “Therefore, a thorough survey of the existing ductwork should always be conducted before beginning the design phase.” 

Another challenge, particularly in commercial applications, Goyert added, is ensuring compliance with fresh air requirements per ASHRAE 62.1.  

“Older ductwork is often oversized due to greater CFM per ton values and larger fresh air requirements. While modern systems typically move less total air, that may correlate to a higher percentage of fresh air,” Goyert said. “Since building codes have evolved in recent years, many existing systems were not designed to accommodate these changes in fresh air loads. For this reason, it is essential to verify that the existing ductwork can support the retrofit without compromising system performance.” 

Chris Rannells, product manager, Copeland, said to get the full benefit of variable speed, the complete system has to be considered.  

“For example, if contractors replace only the outdoor condenser (i.e., chop-and-drop), it won’t be integrated with the rest of the system,” Rannells said. “However, you’ll have much better results if you install a complete system as designed, enabling communication between the outdoor condenser, indoor air handler fans, and thermostat. This is primarily because OEM systems are designed and integrated to communicate holistically and optimize system changes accordingly.  

Upgrading to a condenser with a modulating compressor is also always an option, but contractors must be fully aware of what impact this may have on overall system operation.  

“We recommend installing a fully integrated variable-speed system and running it as designed and intended for optimal performance and reliability,” Rannells said. “In any case, contractors need to be prepared to present building owners with options that meet their cost, comfort, and efficiency preferences — whether that’s a fixed-speed, two-stage, or variable-speed system type.” 

 

The Future Is Now 

With SEER2 standards now in effect, most don’t see how the industry could avoid variable-speed technology becoming the standard. Goyert said the only real question is how quickly it will happen.  

“Regulations may accelerate the shift, but with rising energy costs, widespread adoption of variable-speed systems is inevitable,” Goyert said. “These systems will always outperform traditional on/off operation.” 

While variable speed leads the charge, Vijay Deshmukh, vice president, product management, residential HVAC, Trane Technologies, notes that fixed-speed systems aren’t standing still. 

“SEER2 standards are providing significant energy savings for homeowners,” Deshmukh said. “Variable-speed technology is not the only place in HVAC where we see improvements. Many fixed-speed solutions deliver significantly higher value as compared to an older system that is being replaced in a home, but also qualify for tax credits or incentives.” 

But, as the shifts take hold, some technicians will face a steep learning curve.  

“Mastery of solid-state controls is essential, as variable-speed systems rely on precise feedback between components, necessitating more sophisticated control systems. The next generation of HVAC professionals must become proficient in electronics to meet these demands,” Goyert said. “Technicians will need to understand more than just amperage and voltage — they must also be familiar with Ohms, DC voltage, and variable DC voltage. Computer literacy will be increasingly important, as modern HVAC systems are controlled by complex algorithms embedded in printed circuit boards.” 

“Technicians and contractors should determine what programs and incentives are available for both homeowners and businesses,” Wright said. “Additionally, to help prepare for the upcoming evolutions and future of the industry, they should prepare for the adoption of A2L safety requirements and an increase in systems that support electrification, such as dual fuel furnaces.” 

Souders said contractors need to understand that there is no one-size-fits-all approach or standard per se, as solutions must be tailored to suit a variety of regional climates, energy costs, rebates/incentives, and end-user preferences.