Smart Demand Control Ventilation

Overview
Historically, building owners and operators have viewed building ventilation as a requirement, giving limited attention to its critical role in an overall efficiency strategy. Operators have been managing ventilation in primarily two simple ways—either by providing constant ventilation airflow and then adjusting accordingly for tenant complaints, or by implementing a discrete sensing system in the spaces to increase airflow in response to reaching a pre-set level of CO₂.

The opportunity for energy waste and cost overruns in these two scenarios is obvious and significant. Properly and safely ventilating your building, facility or lab in the most sustainable, most energy efficient and healthiest way requires new thinking and new approaches. Building owners and managers should focus on this often overlooked opportunity to both increase energy efficiency and improve indoor environmental quality (IEQ), given that technology now exists that can do both.

The status quo regarding building ventilation is changing.

Not a One-Size-Fits-All World
Commercial building ventilation varies dramatically depending on use, type and requirements of the facility. Sure, there are regular and repeated patterns, but there are also so many variables that it is impossible to safely and efficiently ventilate a building based on design occupancy or “industry standard” peak usage levels. Those that try and manage ventilation based on these design standards typically end up with an over-ventilated, inefficient and costly building.

Ventilation air is one of the most expensive HVAC costs due to the conditioning requirements. Knowing that buildings consume 40% of all electricity produced in the US¹ and account for the vast majority of greenhouse gas emissions, this is becoming a cornerstone measure to consider in any efficiency program.

For many highly variable occupied buildings, such as libraries, recreation centers, arenas, airports and teaching buildings, the “right” amount of air will depend not only on occupancy, but on other factors such as time of year, activity, or even construction-related events. Logically, some of these situations will demand greater amounts of “fresh air” than others. In the end, ventilation is and should be a variable demand requirement and not a fixed one, as truly no one pre-determined ventilation rate can satisfy all these conditions.

Traditional DCV: Long Promised, Seldom Delivered
The most common approach to solving this issue is to employ demand control ventilation, or DCV. With DCV, sensors are distributed throughout a building to continuously measure and analyze levels of CO2 in the room and adjust ventilation as needed.

However, this approach has many shortfalls, including sensor accuracy, stacking errors, and the cost of maintaining and replacing sensors as they degrade and fail outright.

In a recent study performed by the National Building Controls Information Program at the Iowa Energy Center², several manufacturers had their building information sensors tested for accuracy. Eight of the fifteen manufacturers had none of their three tested sensors meet specified transmitter accuracies—and furthermore, none of the fifteen had all three within specification.

Also, noticeably flawed is that CO2 measurement in general is (and should be) considered a differential measurement application. It’s critical to consider what is being supplied to the space vs. what is happening in the space. Utilizing discrete sensors with individual errors will result in further stacking errors that can greatly and significantly create energy penalties. Even if one were to ignore the stacking errors and out-of-box variations of these sensors, the cost of regular calibration and maintenance is prohibitive to most buildings owners.

In order for DCV solutions to be accepted and implemented in any meaningful way, the issue of calibration and its associated life cycle costs needs to be addressed. Certainly, measurement of CO2, when done properly and analyzed in a life cycle cost manner, is a great start for more energy efficient and healthy buildings. But this approach should only be considered a “better” way of doing things—not the “best.”

Going Beyond CO2
Adjusting ventilation levels to properly reflect actual occupancy rates is a great energy efficiency measure, but ventilation should not be reduced if it leads to a less productive or unsafe workplace. The presence of airborne contaminants, non-human pollutants (such as cleaning products or construction dust) can adversely affect the indoor environmental quality and therefore should be treated as a measured application similar to that of CO2. Proper ventilation should be decreased for savings, but also increased as needed for maximum occupant productivity and health. The balance of providing best in class IEQ as efficiently and cost-effectively as possible is the true goal of sustainable “best in class” indoor working environments.

What is often overlooked in DCV systems is the need to accurately sense and analyze outdoor levels of pollutants as well. CO2 levels, particulate levels and such things as CO and volatile compounds can and will be brought into buildings as a part of what is considered to be clean air. The levels of CO2 and other containments vary during the course of the year, month and day—so it is important to consider this when working to provide proper IEQ. Understanding the outdoor environment can also provide meaningful insight into filtration effectiveness and proper mechanical system operation. The bottom line is that good indoor environments start with a sound understanding and analysis of outdoor environments.

In all cases, a multi-parameter approach is ideal and necessary, so that smart DCV implementations can still cut ventilation rates and save energy when occupancy levels are low and the air is “clean”—but can also know exactly when to increase the fresh air intake and dilute all types of contaminants.

[an error occurred while processing this directive] Next-Generation DCV: Economically Solving the Calibration Problem
If traditional DCV systems do not perform accurately, it stands to reason that increasing the number of sensors measuring CO2 and multiple other parameters would only exacerbate the issue. The key to solving this problem is to manage the number of sensors needed and allow for regular and cost-effective calibration.

Multiplexed sensing is a practical approach to solving this problem. Using this approach, packets of air samples are routed from up to 30 different locations sequentially to a shared set of higher grade and easily accessible sensors. Using one set of sensors to make multiple indoor and outdoor measurements makes true differential measurements possible and eliminates accuracy concerns, while proving economic multi-parameter measurement viable. Stacking errors for all parameter measurements are eliminated by using the same sensor to measure both indoor and outdoor conditions.

Multiplex sensing can be slightly more expensive to install initially, but over time will save the building owners and operating significant amounts over other approaches. By sharing sensors, less are needed within a building, which reduces the calibration and replacement costs over the life of the building. Facility owners can now save energy, ensure high quality indoor environments, and maintain these conditions consistently and cost effectively year after year.

Summary
Commercial buildings are the largest consumers of electricity produced in the U.S. by a long shot. Building owners and managers are continually challenged to find an economically-friendly way to become “greener” and more sustainable, attract and retain tenants and improve productivity for workers. Optimizing building ventilation is a cost-effective, high-reward way to accomplish all of these objectives. Green buildings are easier to sustain, are in demand by potential tenants and have been proven to increase property values over time.

Instituting smart multi-parameter demand control ventilation and designing systems based on demand and the analysis of what actually makes up the indoor environment is a better way of control and can lead to the creation of truly sustainable buildings. Moving away from prescribed assumptions based on the failings of previous technologies can no longer be accepted as the magnitude of the efficiency improvement opportunity is simply too great!

About the Author
Daniel Diehl is the vice president of sales and marketing for Aircuity. He brings to the company close to twenty years of industry expertise across a wide variety of vertical markets and disciplines in commercial and light industrial building markets. Prior to Aircuity, Dan led the new business development and energy services teams at Lutron Electronics, an industry leader in lighting controls. More recently, Dan was a partner for six years with Synergy, an energy services company that delivers energy efficiency programs for national clients in the light industrial sector. Dan began his career with Johnson Controls, Inc. working in sales, branch and regional management positions of increasing responsibility over an eleven year career. He earned his degree in mechanical engineering from the University of Maryland and an MBA from Villanova University.

¹ CleanTech Group, “As Energy Efficiency Booms, Buildings Get a Brain,” March 2010.
² Product Testing Report, Wall Mounted CO2 Transmitters, National Building Controls Information Center, June 2009

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