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Making the Most of Your VFDs

Apr 2

Written by:
4/2/2015 8:57 AM  RssIcon

Variable frequency drives are quickly becoming the energy efficiency standard for ventilation fans and other HVAC components. But without taking steps to maximize their effectiveness, you could still be wasting energy.

“Reducing rotating equipment speed by 20% can reduce input power requirements by approximately 50%.”
(U.S. Dept. of Energy)

First, the good news. A tip sheet about VFDs published by the U.S. Department of Energy explains, “Small decreases in equipment rotating speed or fluid flow yield significant reductions in energy use. For example, reducing rotating equipment speed by 20% can reduce input power requirements by approximately 50%.” The tip sheet provides the following example:

Consider a VFD coupled to a motor that requires 16.4 kilowatts (kW) to deliver 20 shaft hp to an exhaust fan when operated at its full rated speed. At half its rated operating speed, the fan delivers 50% of its rated airflow but requires only 1/8 full-load power.

Even with a reduced motor efficiency of 77.8% and drive efficiency of 86%, with adjustable speed operation the power required by the fan and the VFD is only 2.8 kW. For this example, input power requirements are reduced by 82.9%.

kW 50% = 0.746 kW/hp x (20 hp x (1/2)3 / (0.778 x 0.86) = 2.8 kW

Now the bad news. There are many ways to use a VFD – and how you use it directly affects your energy savings. For example, if your VFDs are operating at occupied settings all day long, they may be making small adjustments, but you are not maximizing your energy savings. 

To fully take advantage of the technology, you need slow them down when rooms are unoccupied during the day. Here are three ways you can slow down your VFDs during the day when rooms are unoccupied. The most effective option will depend on the systems you have in place and the level of automation necessary to achieve your goals.

1. Scheduling Your Building Automation System

You can enter schedules daily in your building automation system or HVAC controls to return equipment to unoccupied settings between events. This may be a good option if you have one building and not too many scheduled events each day. But if you have a large campus and/or many events to schedule with limited manpower, you will need more automation.

A plant manager in Minnesota said that he actually counted how many clicks it took one of his employees to change a schedule in his high-end building automation system. It took 17 clicks to make one change. That knowledge sent him seeking a better solution.

2. Installing and Programming Occupancy Sensors

You can wire your entire building with occupancy sensors, and write some custom code to allow those sensors to trigger commands from your building automation system. This might be a good solution if you already have occupancy sensors for lighting.

However, using this method, rooms will often be uncomfortable for the first 20-50 minutes (depending on room size and equipment) because systems will take time to make rooms comfortable after a presence is sensed. You may also waste energy on drop-in use or triggering systems to occupied settings when a person passes through a room. If you don’t already have occupancy sensors, the cost to purchase, mount, wire, and program them may make this option cost-prohibitive.

3. Using Event Automation Software

You can use software to link your room schedules to your building automation system so that room schedules entered by event planners drive your HVAC equipment. This is a good option if your facility or campus is using room scheduling software. You can use those schedules to return HVAC equipment and VFDs to unoccupied settings between events automatically, and turn them back up for the next event. A pre-start time can be set for each piece of equipment so that rooms are comfortable when users arrive. This option maximizes efficiency – for energy, labor, and project cost.

The graph below summarizes the results of a test performed by an energy analyst at Appalachian State University after implementing event automation software to control an air handler with a VFD in the student union.

App State VFD Results

The graph shows VFD fan speed over time. The blue line indicates fan speed over three weekdays. The red line indicates fan speed over the same three weekdays of the following week, after implementing event automation software.

The data represents a 70% reduction in fan power over the three-day period, resulting in a return on investment of just five months for the entire project. (Read More)

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Events2HVAC is the only event automation software on the market that works with multiple types of room scheduling systems and HVAC controls. To determine if your facility’s systems are compatible, call (888) 320-4277 or email

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