Right-Size Your Ventilation Needs

12/30/2013 | By Jennie Morton

Learn how demand control ventilation can reduce energy use

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.

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