Variable-speed technology has become an essential component in the effort to improve HVAC equipment efficiency. The term “variable-speed” applies to two different components in an air conditioning or heat pump system: the blower motor and the compressor. A variable-speed blower can be matched with a single-speed compressor, but a variable-speed compressor must always be paired with an adjustable-speed blower.

In the early search to improve commercial HVAC equipment efficiency, manufacturers often incorporated two separate compressors in their high efficiency equipment. The smaller compressor was used for lighter loads, but when severe summer temperatures arrived, the larger compressor would take over to satisfy the heavier demand. Unfortunately, the controls for these systems were complicated and difficult to troubleshoot if a problem developed. To satisfy the need for better efficiency, comfort and simplicity, the industry developed two-stage and variable-speed compressors.

A Guide to Two-Stage Compressors

A single-stage compressor in a conventional air conditioning or heat pump system operates at full capacity regardless of changes in the prevailing load. This means the cooling equipment is either running at full speed during a cooling cycle or it is completely shut off. These capacity limitations result in significant wasted energy and lower levels of comfort since the system often “short cycles” by satisfying the thermostat call too quickly.

To address these deficiencies, manufacturers developed two-stage air conditioners and heat pumps that incorporate a single scroll compressor modified with a solenoid control and a plate with two different sized holes drilled into it. The solenoid shifts the position of the plate based on the mode the thermostat calls for. The bigger hole is used for higher demand, and conversely, the lower hole is used for lesser demand. During mild days, the compressor operates at reduced capacity and uses less energy as the building cools more gradually. When the temperature rises, the unit runs at full capacity to meet the demand.

A Guide to Variable-Speed Compressors

Variable-speed compressors are the latest innovation in HVAC energy efficiency. These units regulate the output of the compressor though frequency modulation guided by a Variable Frequency Drive (VFD). [1] The compressor can modulate up or down to match the cooling or heating requirements of the building. Modern variable-speed compressors provide 70-700 different speeds of operation. [2] The compressor has the capacity to change speeds in increments as small as 1/10th of one percent, so the actual indoor temperature varies by less than one-half degree of the thermostat setting.

When used in combination with a multi-speed blower, a variable-speed compressor offers a wide array of benefits:

  • Improved Efficiency: Multiple-speed compressor output options can result in substantial cost savings throughout the year. These savings are maximized during relatively mild weather when the unit runs longer cycles with far less power consumption. In commercial applications, installing a VFD into HVAC systems can result in annual savings up to 30 percent. The addition of electronically commutated motor (ECM) technology can raise unit efficiency by 50 percent or more. [3]
  • Reduced HVAC Equipment Repair Costs: The greatest wear and tear on a compressor happens during the start cycle. Since units equipped with a variable-speed compressor run longer cycles at much lower speeds, the effects from hard starts when the oil isn’t evenly distributed throughout the unit are reduced. Over time, this can substantially decrease wear and tear, which can lower repair costs and downtime.
  • Better Comfort: When matched with an adjustable-speed blower, a variable-speed compressor provides exceptional temperature and humidity control. In mild weather, running longer cycles at lower speeds helps to maintain an even temperature throughout the entire building, eliminating hot spots and stale air pockets. In many instances, occupants experience improved comfort even at a higher thermostat setting.
  • Longer Equipment Life: Conventional air conditioners and heat pumps are subject to extreme wear and tear resulting from repeated hard starts that place a lot of stress on vital internal components. Longer cycles at lower speeds help reduce the number of hard starts, which can extend the life of the equipment over time.

The Importance of a Variable Speed Blower

If you’re considering improving your HVAC equipment with a variable-speed compressor upgrade, it’s important to remember that adding a variable-speed blower is essential. When installed in an air handler or furnace, the speed of the fan is changed in tandem with the compressor to provide the best combination of comfort and efficiency. Variable-speed fans matched with variable-speed compressors provide a variety of compelling benefits:

  • Improved Efficiency: Variable-speed fans improve efficiency, which lowers operating costs. In fact, using a variable-speed fan can raise a unit’s EER by 1.25 points since a reduction of 10 percent in fan speed reduces electrical consumption by 25 percent. [4]
  • Quieter Operation: Running the blower at a lower speed without sacrificing comfort will result in much quieter operation. In addition to reduced motor noise, occupants won’t be annoyed by the “wind noise” effect from air traveling through the air distribution system.
  • Temperature and Humidity Control: When a fan runs at a lower speed, it typically takes longer to cycle. This helps provide better control over temperature and humidity. In many instances, the thermostat can be raised without any noticeable compromise in comfort.

Lower Costs and Better Comfort With Variable Speed Cooling

HVAC systems equipped with variable-speed compressors and adjustable blowers are designed to integrate with other energy efficiency upgrades like zoning systems, smart controls and indoor air quality products. The design experts at Therma can demystify the technical aspects of variable-speed compressors and explain how the technology can help you save money and improve the comfort experience for your employees and customers.

By William Busse

William Busse has more than 30 years experience in the construction industry in sales, contracting, distribution and manufacturing. He has an extensive background in the mechanical contracting industry, largely from the distribution and manufacturing sides of the business. He has had deep involvement with a variety of large-scale commercial/industrial mechanical contractors, particularly related to value engineering. This includes process piping, high-pressure steam, cryogenics, thermal insulation and commercial/industrial HVAC.