A CO2-based DCV system utilizes an air quality sensor (carbon dioxide) to determine the occupancy level for the building space. If there is one person in the space, the outdoor dampers can cut down on the amount of outside air being brought in. Obviously, if there are more people in a space, the system brings in more outside air. This system can save energy for the building owner as well as improve indoor air quality in the building by bringing in outside air when it is needed by the occupants.

But some remain unconvinced about the benefits of a DCV system. Concerns exist over whether a DCV system can meet ASHRAE ventilation standards, as well as how well the system performs over the long term, especially when periodic monitoring is not performed. In addition, there is no widely accepted, published baseline rate of ventilation, so engineers often have to guess the minimum levels of ventilation required by a building. That leaves some asking whether DCV can be a help or a hindrance.

Figure 1. DCV can provide the right amount of ventilation according to standards without overventilating the space and wasting energy. (Courtesy, Telaire).

All Eyes on Energy Savings

The reason is that buildings often provide two to three times more air than the design ventilation rate of the building. All that additional ventilation means more air that needs to be heated and cooled, resulting in higher energy costs. "The advantage of using DCV here is that you can provide the right amount of ventilation according to standards but you're not overventilating the space and wasting energy. The primary benefit is energy savings," says Mike Schell, director of marketing and business development, Telaire (Goleta, CA).

Mary Milmoe, vice president, marketing, Commercial Systems and Services, Carrier Corp. (Syracuse, NY), adds that many buildings are over- or under-ventilated, wasting significant amounts of energy and creating uncomfortable environments. "One of the main reasons for overventilation is that hvac systems typically operate at a fixed maximum occupancy level, a level of occupancy which is rarely reached in a building. If the system cannot detect changes in occupancy, it will overventilate, bringing in much more outdoor air than is required."

This not only results in unnecessary utility costs, but also can cause coils to freeze in colder climates and result in unacceptably high humidity in warmer, southern regions. On the other hand, notes Milmoe, underventilation can result from overzealous attempts to reduce energy costs, as well as poor maintenance.

Advocates of DCV say that the technology can help engineers design systems that don't waste energy, which is what everyone is looking for these days. Whether or not engineers use DCV, they are obliged to use one of two methods to calculate the amount of outdoor air they have to bring into a building. Most engineers use the prescriptive method, which is outlined in Standard 62.1-1999 and usually calls for 15 to 20 cfm per person.

If the building in question has varying occupancy rates during the day, there is an opportunity to avoid bringing in that outdoor air by using DCV. If you only have a building that is half full, why would you bring in 100% of your design outdoor air, asks Hugh Crowther, director of applications, McQuay International, and a member of the ASHRAE Board of Directors.

In fact, ASHRAE Standard 90.1-1999 requires DCV for all ventilation systems with design outside air capacities greater than 3,000 cfm serving areas having an average design occupancy density exceeding 100 people/1,000 sq ft. "DCV should be used when it makes sense to use it. It doesn't have to be CO2-based DCV, though," adds Crowther.

Figure 2. ASHRAE Standard 62.1-1999 indicates there shall be no greater than 700 ppm differential between indoor and outdoor levels of CO2. (Courtesy, Telaire).

But Does It Meet the Standard?

"That's when it was clarified that CO2 wasn't a pollutant or a contaminant, per se, but was really just an indicator of the ventilation rate and occupancy in the space," says Schell. The interpretation clarifies that CO2 DCV can be used under the ventilation rate procedure in ASHRAE Standard 62.1-1999 as part of the provisions for variable and intermittent occupancy. In this section it is possible to reduce ventilation levels below design, as long as the target cfm per person provided in Table 2 of the standard is maintained (e.g., 15 cfm/person). The interpretation also makes it clear that CO2 control is for control of occupant related ventilation and that a base ventilation rate to control other non occupant related sources is necessary. While ASHRAE has never stipulated what that base should be, our experience shows that a base ventilation rate of 20 to 30% of the design ventilation rate works well.

ASHRAE Standard 62.1-1999 indicates there shall be no greater than 700 ppm differential between indoor and outdoor levels of CO2. So for example, a typical office space or conference room has a ventilation rate per person of 20 cfm. Assuming an outdoor CO2 concentration of 400 ppm, the differential CO2 concentration between indoor and outdoor would be 530 ppm (which is well below the 700 ppm maximum differential). This would provide a maximum 930 ppm setpoint in accordance with 62.1-1999, notes Milmoe.

DCV continually measures outdoor and indoor CO2 concentration levels. By calculating the occupancy level of the space and the differential between outside and inside CO2 levels, the ventilation rate per person can be determined and compared to the standard's requirements. (As an alternative to actual measurement, the standard allows for an established outdoor air CO2 value to be used.)

It is important to note that in using CO2 sensing for DCV, CO2 is an indicator only of occupancy and is not considered as a contaminant. For this reason, indoor concentrations are used more for comparison and calculation purposes than as gauges of indoor air quality.

"To use the same example, if the outdoor CO2 concentration were higher than typical - say 550 ppm due to high vehicle traffic near the building - the target 20 cfm per person could be achieved, yet the indoor CO2 concentration would be higher, about 1,250 ppm. This is an acceptable scenario within the guidelines of the standard," says Milmoe.

But there are some other issues that need to be considered, says Crowther. Yes, it is true that DCV can meet the requirements of 62.1-1999, but there's still a huge question over what the baseline rate of ventilation should be for a building. Unfortunately, that information is not published. ASHRAE tried to include a baseline rate in Standard 62R at one point, however, this standard was put on continuous maintenance and the information is not yet available.

"Your average engineer is going to have a real problem if he wants to use DCV," says Crowther. "Let's say there are 100 people in the building at 15 cfm, so that's 1,500 cfm of ventilation. If they're not in the building, how much ventilation does he need to bring in? There's nothing telling him that. He's got ASHRAE 90.1-1999 saying he's got to do DCV, he's mandated to do that if he wants to be 90.1 compliant, but nobody's telling him how to do it. That's a tough situation that needs to be resolved."

In fact, the user manual for ASHRAE Standard 90.1-1999 states:

"The system must be designed to ensure that a minimum outdoor air intake is maintained regardless of CO2 concentration to account for contaminant sources from building materials, furnishings, etc. Unfortunately, Standard 62.1-1999 and most building codes do not provide any data on what this minimum rate should be."

One of the biggest problems is that the baseline rate of ventilation will vary depending on the building. It's also a litigious issue, and engineers (rightfully so) don't want to be left holding the bag. According to Crowther, what ASHRAE would like to do is switch from tables that provide only the cfm per person to a table that gives both the cfm per person and the cfm per sq ft in general ventilation required.

"The day an engineer can have those kinds of numbers, he'll be able to say, 'Oh, now I calculate my baseline ventilation rate in a second and I'll set that as my minimum, and I can vary the ventilation rate between that number and 100% based on my population. I know what those numbers are and now I can design the system.' Engineers don't have a fear of the technology, it's a fear of being able to design a system that's going to work," says Crowther. "It is important to note that the base ventilation rate is not a fixed parameter of design," said Schell. "It is an operational parameter that can easily be adjusted by the user if there are complaints. I think the liability concerns are overstated."

The ASHRAE interpretation makes it clear that CO2 control is for control of occupant-related ventilation and that a base ventilation rate to control other nonoccupant-related sources is necessary, says Schell. "While ASHRAE has never stipulated what that base should be, our experience shows that a base ventilation rate of 20% to 30% of the design ventilation rate works well. And it is important to note that the base ventilation rate is not a fixed parameter of design, it is an operational parameter that can easily be adjusted by the user if there are complaints."

Figure 3. While ASHRAE has never stipulated what that baseline ventilation rate should be, Telaire notes that a base ventilation rate of 20% to 30% of the design ventilation rate usually works well. (Courtesy, Telaire).

Measuring Another Concern

That's not to say that Bearg doesn't believe in DCV technology. In fact, he specified a system that uses DCV for a space that was occupied by directory assistance operators. In that case, the number of people present varied significantly throughout the day. Bearg performed CO2 monitoring to determine that during peak occupancy the amount of ventilation provided, sometimes as low as 13 cfm of outside air/person, was less than the minimum of 20 cfm desired.

Rather than perform a wholesale increase in the amount of ventilation provided for just those intervals of peak occupancy, Bearg used the DCV approach, which allowed adequate ventilation to be automatically provided when needed. In addition, energy was not wasted during intervals of lower occupancy and the risk of excessive wintertime drying, a concern with a huge investment in computer-based systems, was not increased. "It was an elegant solution," says Bearg.

However, Bearg is a strong believer that some form of independent CO2 diagnostic monitoring should take place to make sure that the DCV system is functioning as intended. He says this diagnostic monitoring can range from a single datalogging instrument that gets moved sequentially to all of the locations controlled by DCV up to the use of a diagnostic monitoring system to assess ventilation performance throughout a building, and not only in those locations where DCV is being used. "This monitoring feedback is important, because how can building operators hope to optimize hvac performance if they don't know how the system is performing with respect to adequate ventilation?" says Bearg. Unfortunately, too many applications - including the one in which Bearg specified DCV - do not perform regular monitoring to ensure proper ventilation is taking place.

It's definitely necessary to measure the ventilation air at some point, says Crowther. "If I said a building needs 1,500 cfm and my minimum airflow rate would be 750 cfm, I have to know that I'm bringing in 750 cfm. Measuring it is what every engineer wants to do, but it's proven to be very difficult to do in the past. Traditional systems have been all over the map, particularly with vav systems. It's really only with modern electronics that we have been able to get reliable numbers," notes Crowther.

CO2 measurement in the space provides us with a unique measure of ventilation never available before said Schell. "Previously we could only tell how much outside air was being introduced at the air intake. With zone level measurements of CO2 we can actually determine how much fresh air is being distributed to each area of the building and ensure that target per person ventilation rates are achieved."

But CO2 measurement in the space provides us with a unique measure of ventilation never available before, adds Schell. "Previously, we could only tell how much outside air was being introduced at the air intake. With zone level measurements of CO2, we can actually determine how much fresh air is being distributed to each area of the building and ensure that target per person ventilation rates are achieved."

Even with the concerns outlined here, it is likely that CO2-based DCV will become more popular. One just needs to look at the energy situation in California to see that there will be more interest in this kind of technology. In addition, the technology has become more mature, and the old concerns of repeatability, accuracy, and drift are all pretty much under control.

"CO2 has two great attributes: Its concentration is pretty much directly proportional to the population, and it's an easy gas to measure," says Crowther. "However, the hesitation is that there is no clear-cut way to do [CO2-based DCV], and that causes grief. Standard 62 and IAQ are very litigious issues." ES