In Times Of Low Occupancy, Variable Speed HVAC Aids Efficiency

It’s a bit of an understatement to say that the COVID-19 pandemic has changed the way we work, especially in offices nationwide where HVAC systems are essential. 

Currently, many offices either remain empty or are operating at a fraction of their intended occupancy. Remote work, once thought of as a drag on productivity, has proven so successful that it’s likely here to stay in some capacity irrespective of the pandemic.

And, as people slowly return to the office, those spaces are experiencing major changes in how they operate. Of course, wearing masks and social distancing are givens, but facility managers and building owners are also investing in new hardware to mitigate the spread of the virus. Adding upgraded air filtration systems and ultraviolet lighting are recommended, but the most important mitigation feature is increasing the supply of outdoor air in the building, which helps increase airflow and disperse virus particles in the air.

This combination of fluctuating building occupancy coupled with the need to increase the flow of outdoor air presents facility managers and owners with an unusual challenge: How to maintain a steady flow of air to properly ventilate a space that only a small number of people are using. The vast majority of U.S. offices utilize constant-speed HVAC systems to heat and cool these occupied spaces. Whether we’re talking about large commercial office buildings using chillers and boilers or low-rise office suites with rooftop units, these systems are by and large fixed speed, meaning they operate at either 100% or 0% capacity with no in-between. In this scenario, these systems turn on when they condition the space to the thermostat’s set point and click off when that set point is reached. When the system is off there is no airflow occurring.

This approach worked in the pre-pandemic world because offices were mostly full and ventilating beyond what building codes required was unnecessary. In the middle of a pandemic, however, it presents problems. These constant volume systems are designed to heat and cool during the hottest and coldest days of the year, which occur roughly 2% of the time. The other 98% of the time, the system is oversized. Bringing more outside air into the building, which then needs to be heated or cooled, drives up system usage and utility bills. Not to mention that this oversized system is operating more than usual to condition outside air and ventilate the building for only a handful of people. In this context, it makes sense that even as office occupancies cratered by 70% or more in 2020, energy use in those offices had decreased by just under 25% (according to at least one April 2020 snapshot of energy use in offices across the U.S.).

About HVAC And Variable Speed Technology 

Variable speed technology, which is available in office applications large and small, provides some solutions to this distinct set of challenges. Unlike constant speed systems, variable speed ones can run between 20% and 100% capacity, effectively matching HVAC load to the building’s actual demand. And, as opposed to the on/off operation method of their fixed-speed counterparts, variable speed systems operate continuously, providing a steady stream of conditioned air to the space.

These two features of variable speed systems give facility managers distinct advantages during this pandemic. First, as people slowly return to the workplace and while future lockdowns are possible, the ability to ramp a system up and down depending on demand is vital. Even if an office is totally empty, many leases require tenant spaces to be minimally conditioned at all times. Many HVAC systems, particularly hydronic ones involving water, require regular usage to avoid corrosion. Variable speed systems can be run continuously at lowered speeds to match low demand and maintain system usage. When more people return to the office, the system ramps up to meet their needs. Second, since a variable speed system is continuously operating, even at a lowered rate, it is also continuously ventilating the space. Facility managers no longer need to increase systems operating hours to increase ventilation. They need only increase the flow of outdoor air, which the system conditions and moves around the space or raise the system’s minimum speed to allow for more airflow.

Under normal circumstances, variable speed HVAC systems offer attractive paybacks and a low total cost of ownership. During this pandemic, however, these next-generation systems prove essential. They provide facility managers and owners with the flexibility needed to manage in uncertain times.

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