The Benefits of Variable Speed Drives in HVAC Systems

August 30, 2023

No matter what the salesperson or spec sheet says, an HVAC system is only as good as the people who install and service them. Though it might sound hard to believe, the number of support calls logged to service HVAC equipment is on the rise and, in many cases, a common culprit can be identified: the variable speed drive.

If installed or configured incorrectly, a variable speed drive can not only reduce the overall performance of the system as a whole, but it may also end up costing the building owner a fortune in unnecessary energy costs. The purpose of this article is to not simply define what a variable speed drive is and how it is used to benefit an HVAC system, but to answer some of the crucial questions you may have should you pursue a career in HVAC maintenance and repair.

Technician inspects and controls the cooling system of a large factory air conditioner

What is a Variable Speed/Frequency Drive?

A variable frequency drive is a type of motor controller that can manipulate the output of an electric motor by varying the frequency and voltage of its respective power supply. VFDs are used in all manner of electrical devices (pumps, compressors, conveyors etc.), but when it comes to HVAC systems, VFDs are most commonly installed as a more efficient and cost-effective means to control air flow. When installed, the VFDs are configured to turn on/off at predetermined thresholds; this ensures the system operates as intended, and that optimal ventilation is delivered throughout the home or building. Variable frequency drives can also respond to manually executed “zone controls” to ensure system airflow is adjusted to accommodate any heating and cooling settings throughout the space. Where possible – either during new builds or during upgrade projects – constant speed blower motors are being replaced by versatile VFDs. In addition to simply being a more efficient method of controlling airflow, VFD technologies have advanced to the point where manufacturers can take advantage of the economies of scale, which in turn has driven down the cost of production, which of course further makes installing a VFD a financially sound decision. 

What are the Benefits of a Variable Frequency Drive?

Two benefits of VFD ownership, namely cost-effectiveness and efficiency, were briefly touched upon in the previous section. Let’s take a closer look at these benefits and others that have been made possible by VFD technology.

  • Significantly better ventilation – Traditional blowers must operate constantly and at full speed to provide adequate air flow. VFDs allow the fan speed to ramp up/down gradually which reduces strain and pressure on the system while ensuring temperatures remain consistent across all zones. 
  • Better air quality – Since VFDs can be configured to operate differently in response to varying factors such as humidity, CO2 levels, temperature, and pressure, the quality of air within the space is significantly better than that of space relying on a more antiquated system.
  • Energy savings – Since VFDs control the frequency and voltage used by an electric motor, the power consumption will decrease any time the motor is not running at maximum capacity. Depending on the number of VFDs in use and the size of the building, any cost savings over the course of a year can be significant. 
  • Less maintenance – The gradual ramp up and slow down of the fan motor made possible by the associated VFD reduces wear and tear and will prolong the life of the motor. This means less maintenance will be required and fewer impediments to system performance. 

How Can Airflow be Adjusted Using a VFD?

As mentioned previously, VFDs help facilitate the delivery of the required airflow by operating according to the configured thresholds set during the installation process. There are six key steps that need to be followed when adjusting airflow using a motor controlled using a VFD. 

  1. Use an anemometer (a device used to measure air direction and speed) to measure the airflow currently produced by the fan.
  2. Record the current VFD operating hertz. This value typically displays on the VFD and will be used to calculate the new VFD setting. 
  3. Determine the airflow requirement of the associated fan. 
  4. Use the inverse of the Fan Law 1 to calculate the new hertz setting that the VFD will need to be set to in order to deliver the required airflow (in simple terms, the Fan Law 1 dictates that an increase or decrease in volume is equal to the speed change ratio).
  5. Manipulate the VFD setting to match the calculation completed in the previous step. This will increase or decrease the flow as required. 
  6. Verify airflow, fan amperage, and static pressure is within system thresholds.

How to Calculate VFD Hertz Settings? 

This section will help you to calculate the new VFD hertz setting that will be required when adjusting airflow. This process will require the data compiled during steps 1-3 of the previous section to populate the formula of Fan Law 1: 


Hertz2 = Hertz1 x CFM2



Hertz2 = represents the new hertz setting

Hertz1 = represents the current hertz setting (e.g. 50 Hz)

CFM2 = represents the CFM of airflow required (e.g. 2100 CFM)

CFM1 = represents the current CFM of airflow (e.g. 3000 CFM)

Let’s say the current CFM of airflow produced by the fan is 3000 CFM with the initial hertz setting of 50 Hz. Due to some renovations, the adjusted airflow needed for the space is 2100 CFM. The new hertz setting can be calculated by inserting these values into the formula above.

Hertz2 = 50 x 2100


Completing this equation we can see that the VFD needs to be adjusted to 35Hz in order to get the desired airflow of 2100 CFM.

Variable Speed/Frequency Drives Integral to Efficient HVAC Systems

As sophisticated as they’ve become, modern HVAC systems are still prone to performance issues if they are not installed correctly for the space. Variable frequency drives in particular, can significantly alter the efficiency, performance, and operating costs of the system. Knowing how to resolve one of the more prolific issues associated with HVAC service calls can help you to quickly solidify your career as an HVAC technician. If you’d like to learn more about how components like VFDs work, speak to a Program Coordinator at George Brown College today!

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