Even the simplest HVAC devices have gained sophistication

From VAV boxes to DCV Systems

I was eager to use my newfound - but still limited - knowledge about building energy consumption in the consulting engineering field. In 1986, however, hiring a recent engineering graduate who was dedicated to saving clients energy wasn't a priority for many consulting engineering firms. Building energy use was important in 1986, but 13 years had passed since the 1973 energy crisis, world energy supply had stabilized, a prolonged housing market crash had just bottomed out in 1982, commercial construction was on the rebound, and the industry was happily building cheap and fast to keep up with resurging demand. Low energy use buildings were not a priority. Although I was more interested in whole building energy, I accepted a job as an application engineer for a manufacturer specializing in variable air volume (VAV). My responsibility was not the entire VAV systems as we know them today, but rather, I was responsible for VAV “boxes”, a simple but essential component of a VAV system.

The key feature of a VAV box is a simple internal damper which opens and closes to allow more or less air into a room. The term “box” refers to the rectangular shape of the discharge, which connects to like-sized rectangular distribution ductwork (the photo at the top of this article shows a typical VAV box installation). Other than installation convenience, the box part of a VAV box adds little value – a simple damper in a sleeve provides the essential air control functionality. Sound attenuation provided by the box is similar to the attenuation of standard ductwork. The work of controlling the airflow to a single occupied zone is accomplished by the damper, whether the damper is in a rectangular box or in a round duct sleeve.

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Upstream, the VAV system includes a pressure-controlled duct system, enabled by air handlers set up to control duct pressure, and pressure control dampers.

Ventilation Controls Strategy Transformation

Back to that first job. At the time, it seemed I was working in a remote, commodity-driven end of the market, far from the excitement of doing big things to save lots of energy. Little did I know that I was just beginning my participation in at least four ventilation transformations related to building energy use: from Constant Air Volume (CAV) to Variable Air Volume (VAV), from VAV to Demand Control Ventilation (DCV), from CO2-based DCV to multi-parameter DCV, from pneumatic controls to analog and then digital electronic controls. None of the newer control strategies have any value if the VAV damper doesn't respond properly. And even the VAV damper has broken into new territory, finding another home in traditionally constant volume chilled beams.

Most of my early work was application engineering – explaining to customers how VAV boxes worked, and how to select the box with the right controls to meet project requirements. It was a new era – applying a simple sheet metal box with a damper to control air volume now required knowledge of controls that measured and responded to the zone temperature changes and to ductwork pressure fluctuations, to achieve pressure independence. The need for controls knowledge has only grown since then. VAV dampers do the important work of the larger VAV system that saves energy by preventing space over-ventilation and the resulting wasted fan energy. These dampers also enhance comfort by opening when necessary to deliver more warm or cool air.

In the mid-1980’s, most VAV boxes were shipped with pneumatic controls. Compressed air in a network of tubes powered thermostats, VAV box controllers and their damper actuators. Even the air handling unit was controlled by compressed air.

By the late 1980’s, electronic VAV box controls emerged, along with the concept of controls-by-others. Electronic controls enabled a leap in the way VAV boxes could be used, responding not only to room temperature, but also to carbon dioxide (CO2) in the air as an indicator of the need for fresh air in the space. These earliest DCV systems responded to CO2 levels, by delivering more fresh air when CO2 levels were too high, by delivering less fresh air when CO2 levels were below the target level (setpoint), in turn allowing the supply air fan to consume less energy. VAV boxes made it possible to adjust the volume of air flowing to a zone, based on the needs (“demand”) of that zone. DCV systems thus save more energy than a VAV system that always provides air on some calculated minimum airflow. DCV is now well-established, thoroughly studied and well-understood. CO2-based DCV emerged in part from the sophistication of electronic VAV box controls, but made possible because a simple damper controlled the volume of air traveling down a duct. Today, ASHRAE Standards 62.1, 90.1, 189.1 and others provide guidance for applying DCV in buildings to provide safe amounts of air for breathing while saving energy on the basis of occupancy.

The same dampers that reduce airflow to save fan energy when conditions are right can also beneficially increase airflow under other conditions. For instance, if a building is set up for a daily pre-occupancy air flush, opening a damper is an efficient way to quickly deliver the prescribed volume of fresh air with the lowest use of energy.

Today, DCV is sometimes still based on controlling CO2 levels, but may also be set up to control volatile organic compounds, airborne moisture, sensed occupancy or other demand inputs to deliver just the right amount of airflow without energy-wasting over-ventilation. No matter what the input, however, the desired action resulting from even the most sophisticated DCV control system is the adjustment of a simple VAV damper to control the flow of air.

The next transformation

Building science isn’t stagnant. The occasional reflection on my own career leads me to pay more attention to building science transformations happening now. From low energy use to low carbon emissions. From unknown product ingredients to red list free certification. From throwaway components to design choices informed by a circular economy. From buildings designed to meet current minimum energy use requirements to Passive Buildings designed to maximize energy savings and comfort while minimizing carbon emissions and payback periods.

Which emerging building science trends are you observing?

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