Learn how demand control ventilation can reduce energy use
By Jennie Morton
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Can ventilation requirements and energy conservation go hand in hand? They can if you implement demand control ventilation (DCV).
There’s no reason to waste energy conditioning air for people who aren’t in your building. Instead of supplying air at fixed rates, DCV automatically adjusts ventilation levels based on real-time occupancy measurements. This strategy allows you to meet code and reduce energy use without sacrificing indoor air quality.
EXHAUST YOUR OPTIONS
The problem with traditional ventilation is that it cannot distinguish between actual vs. projected occupancy. As outlined in ASHRAE 62.1-2013, Ventilation for Acceptable Indoor Air Quality, ventilation rates are calculated using two factors: square footage and peak occupancy.
Since square footage is a constant, any fluctuations on the occupancy side of the equation give rise to energy waste. With travel, sick days, vacation, and inclement weather, your building is rarely at capacity. In fact, human resources data shows an average of 75% of workers will be in attendance at any given time.
Without a way to calculate the actual headcount, your HVAC system operates as if maximum occupancy occurs on a continuous basis. If you can eliminate the excess air supply whenever fewer people are present, however, you have an opportunity to capture energy savings.
To have a responsive, intelligent HVAC system, you need to implement demand control ventilation. This strategy recognizes when a space has fewer people than scheduled and drops ventilation levels accordingly, explains Daniel Nall, senior vice president with Thornton Tomasetti, an engineering firm. Air supply is calculated using verified headcounts rather than occupancy projections. DCV is no different than using occupancy sensors to control lights – both ensure energy is conserved when there’s no activity in a space that justifies its use.
For example, offices need to supply 5 cubic feet per minute (cfm) per person in addition to a baseline of 0.06 cfm per square foot, Nall explains. Unoccupied, a 250-square-foot office needs 15 cfm to meet the ASHRAE standard. With one individual present, this increases to 20 cfm. Using DCV to sense when the room is empty, you can scale back the ventilation from 20 to 15 cfm, a 25% decrease in air supply. These savings are then multiplied across any room that has DCV capability.
If your occupancy variations are known in advance, DCV may be as simple as using a basic schedule in a building management system, says Jules C. Nohra, manager for energy efficiency at SourceOne, an energy consulting and management firm. Those with irregular or unforeseen occupancy fluctuations, however, will require sensors that can determine how many people are present. These include education, retail, conference areas, performance venues, lobbies, and offices with a mobile workforce or flex hours.
Carbon dioxide monitoring is by far the most common way to determine occupancy, says Thomas Lawrence, senior public service associate with the College of Engineering at the University of Georgia. The technology is well-established and straightforward to implement. CO2 isn’t treated as a contaminant that needs to have its levels controlled (a common misconception), but as a representation for the number of bodies in a space.
“Carbon dioxide measurements act as a surrogate for occupancy because humans generate an average volume per hour,” explains Nall. “By calculating the concentration differential between internal CO2 volumes and the outside air, you can estimate the number of people in your building. For example, if your CO2 concentration doubles, then occupancy has doubled.”
Occupancy sensors, such as the infrared ones you pair with lighting controls, can also be used. These are the most effective in individual work spaces and private offices, Lawrence observes. For a zone with multiple workers, however, they don’t offer fine enough measurements to calculate total attendance.
For example, think of an open floor plan that houses 30 people. The occupancy sensor will trip when the first person arrives, but it can’t scan the room an hour later to see if all 30 workers showed up that day. It also can’t detect if 15 of those employees move to another part of the building for a two-hour meeting, leaving the space over-ventilated during that period.
Entertainment venues may be able to use ticket sales to confirm a headcount. Other facilities can derive occupancy by counting cell phone signals present in the facility, Lawrence says. It’s also possible to have IT report the number of active computers, assuming that each device fired up represents a person in the space. If you use an access control system and it can interface with your BAS, each card swipe, keypad entry, or turnstile rotation can count toward occupancy.
INSTALL WITH AN AIR OF CONFIDENCE
Integrating demand control ventilation is heavily influenced by your existing HVAC system, such as whether your ventilation is combined with heating and cooling or is a standalone function.
“For example, adding DCV to a packaged rooftop unit may be as simple as including the CO2 sensor with a controller that has the DCV control logic built into it. Such a system likely serves only one or a few occupied zones, making it simpler to control CO2 levels,” explains Lawrence. “A larger building with central air handling, however, may serve many occupied zones. Determining the proper amount of outdoor air to bring in at the central air handling unit is also complicated by the variable occupancy patterns within the multiple zones.”
Say your VAV system supplies air to a large conference area and a group of private offices. To scale back the ventilation when the conference room is empty means that you risk the possibility of underventilating the offices at the same time. To avoid this scenario, you will need air flow sensors that measure the amount of air going to each space as well as the outside air that’s being drawn through the air handling unit, says Nall.
CO2 sensors are typically installed in the occupied space instead of ductwork because return air is an average of all conditioned spaces rather than an individual area, state ASHRAE members Mike Schell and Dan Inthout in their article Demand Control Ventilation Using CO2. Duct sensors can be used if all ventilated spaces share common occupancy patterns; otherwise, sensors should be wall-mounted.
“Avoid installing in areas near doors, air intakes or exhausts, or open windows,” advise Schell and Inthout. “Because people breathing on the sensor can affect the reading, find a location where it is unlikely that people will be standing in close proximity (2 feet) to the sensor. One sensor should be placed in each zone where occupancy is expected to vary. Sensors can be designed to operate with VAV-based zones or to control larger areas up to 5,000 square feet.”
Switching to DCV will typically require additional building management system points, new setpoints, and new control codes for dampers, Nohra notes. This may include a controller or DDC programming to communicate either directly with the economizer controller or a central control system, specifies the DOE in its 2012 report on demand control ventilation.
You should also make sure outdoor dampers are operable and not stuck in fixed positions, stresses Lawrence. It’s not unusual to find air intakes blocked in a misguided attempt to save energy. There may also be missing equipment, such as economizer controls with modulating air dampers that were specified but never installed.
Once the DCV sequencing has been established, the system requires minimal maintenance. CO2 sensors should be recalibrated periodically as their accuracy will drift over time. Consult your manufacturer guidelines, which may recommend recalibration every five years, annually, or every six months. Lawrence also recommends sensor testing prior to launch in case the product is deficient or was damaged during installation.
A BREEZY SOLUTION
Demand control ventilation isn’t a flashy energy efficiency project, but it consistently generates payback under five years or less. Paybacks can also be achieved more quickly if the system incorporates lighting and electrical outlets (vampire energy) controls. For upfront investments, owners can expect to pay less than $100 for occupancy sensors, Nall estimates. CO2 sensors can cost several hundred dollars per unit, adds Lawrence.
“The installation costs for a DCV project vary significantly depending on building size, existing infrastructure, and control requirements. An owner can expect to pay approximately $1,000 to $2,000 per point on average,” Nohra adds.
Nall was recently involved with a renovation project that incorporated DCV by using occupancy sensors. A series of perimeter offices and those adjacent to an atrium were paired with a dedicated outside air system and variable speed fan coils.
Each 160-square-foot office has a two-position damper. The default setting for an unoccupied office delivers 10 cfm of outside air. Anticipating occupant diversity when the office is in use, the secondary position is configured for three guests at 25 cfm.
“This ensures that we’re providing the minimum ventilation for the maximum expected occupancy,” Nall stresses.
Whenever the system senses the room is unoccupied, it can scale back ventilation to 40% of peak flow. The project cost less than $1,000 per office and since the occupancy sensor controls ambient lighting and power receptacles, the payback is under five years. The DCV capability also meets the LEED credit for increasing ventilation by 30%.
Lawrence also oversaw a DCV project at the University of Georgia. The retrofit converted a single classroom, but has seen great success since its installation. Payback was achieved in less than two years and there are plans to adapt more areas in the future.
“Regardless of the actual design standard, energy savings with a DCV retrofit should focus on a comparison to the existing ventilation patterns, even if they do not match current codes or standards,” emphasizes Lawrence. “If a building is not providing ventilation that meets existing standards, then the primary benefits of DCV are indoor air quality.”
Jennie Morton firstname.lastname@example.org is senior editor of BUILDINGS.