Len Damiano
Contributing Editor

What does the term "demand controlled ventilation" (DCV) mean?

Very simply, it is "any" method used to control ventilation that modifies intake rates based on changing "demand". The intention is to control ventilation rates based on occupancy within a predefined space (assuming it varies over time). Because ventilation rates are normally associated with occupancy levels, we conclude that the "demand" for ventilation is due to a measured change in the occupancy level for the space.

DCV is a method of measurement that approximates the number of people that occupy a space, and thereby allow the intake rates to be reset based on the indicated occupancy. It is only with this data that one can optimize the rate of outside air intake, to something less than maximum capacity. It is this method that most CO₂ sensor makers and ASHRAE 62 refers to.

However, many readers misinterpret technical articles on CO₂ measurement touting the energy benefits of DCV, but the authors rarely provide sufficient information allowing the reader to apply the methodology. Seldom do they provide supporting details on how it allows the user to comply with ventilation requirements. Can we conclude that even the authors are unsure how to justify it?

Problems occur when methods of control and their terminology are used interchangeably. "Demand Controlled Ventilation" is not synonymous with "CO₂ measurement for control".

Let's unravel some of the persistent confusion that surrounds the application of Demand Controlled Ventilation.

CO₂ Measurement and Ventilation Rates

First, let's get one simple truth out of the way. There is no direct relationship between interior CO₂ levels and intake rates. At best, an indirect relationship exists that relies on numerous assumptions, most of which are not valid in most dynamic commercial building environments.

Because ASHRAE initially used odor control and comfort as the minimum criteria for ventilation effectiveness, many have assumed that the use of CO₂ measurement is supported by the Standard. This conclusion is not valid.

Andy Persily best explained the usage and relationships of CO₂, in his paper for the Indoor Air '96 conference. Now the current Chairman of ASHRAE SSPC 62.1, Mr. Persily presented his conclusions for: The Relationship Between Indoor Air Quality and Carbon Dioxide.

The relationship between CO₂ and outdoor air ventilation rates is well understood and is based on the consideration of CO₂ as a tracer gas. ….However, to make quantitative estimates of ventilation parameters based on measured CO₂ concentrations one must employ a specific tracer gas technique [e.g. ASTM] that is appropriate to the conditions that exist in the building.

………The Ventilation Rate Procedure in the standard is based on outdoor air ventilation rates requirements, not on the maintenance of indoor CO₂ levels……...maintaining CO₂ levels below [a specific level] does not mean that a building is in compliance with the standard.

While indoor CO₂ concentrations have been shown to be reliable indicators of the acceptability of a space in terms of human body odor, there is little justification for using CO₂ as a comprehensive indicator of indoor air quality. ….. Also, many contaminant sources are not associated with occupancy levels, and their concentrations will not be associated with CO₂ levels. The analysis of CO₂ concentrations can be used to obtain information on building ventilation performance based on a number of tracer gas techniques, but the assumptions associated with these techniques must be understood by the user.

This citation succinctly summarizes many of the misunderstandings and the correct usage of CO₂. It can be used as a tracer gas to estimate the outside air intake rate, at a single point-in-time, but only within the guidelines and procedures required in the ASTM Standard D6245-98. The Mass Balance equation in the Appendix of ASHRAE Standard 62 is NOT an endorsement of CO₂ measurements as a direct control for ventilation rates. It is shown to allow engineers that under very specific circumstances, it can be used to approximate occupancy and from that data ventilation can be reset to an amount calculated to be required by the indicated occupancy level.

The following assumptions are required by the Mass Balance equation referenced in ASHRAE 62 to make calculated ventilation estimates useful:

1. CO₂ measurements should be taken when the space reaches a "steady-state". Interior CO₂ concentrations should not fluctuate. Outside CO₂ concentrations are assumed to be constant in the calculation.
2. CO₂ measurements are used in calculation without measurement or sampling error. CO₂ sensors are assumed not to drift and do not require maintenance or recalibration over time, or between measurements. 
3. Human respiration is the same for all building occupants, regardless of: age, sex, size, diet, health, etc. Human activity is assumed the same for all building occupants. Human activity is assumed to equal that of a seated person.

The conditions described by these assumptions can only occur at a very specific single point-in-time. The corollary is also true - that the assumptions cannot occur in a dynamic, fluid and changing environment.

It is assumed that CO₂ is measured with the use of a single, highly accurate instrument and that the calculations needed to usually assume no measurement error. Therefore, any of the possible CO₂ methods for the evaluation of ventilation effectiveness cannot be valid, when applied to ventilation control in a dynamic building system, without making numerous and questionable assumptions.

ASHRAE Standard 62-1999

What does the ASHRAE Standard 62-1999 really say, with regard to CO₂ measurement, ventilation rate control and IAQ?

Here are some direct quotations with explanations.

FOREWARD (not part of standard)
… Addendum 62f addresses a lack of clarity in ANSI/ASHRAE Standard 62-1989 that has contributed to several misunderstandings regarding the significance of indoor carbon dioxide (CO₂) levels. The Standard led many users to conclude that CO₂ was itself a comprehensive indicator of indoor air quality and a contaminant with its own health impacts, rather than simply a useful indicator of the concentration of human bioeffluents.

More directly, the Standard states explicitly what is preferred under Section 5: Systems and Equipment.

5.1 … When mechanical ventilation is used, provision for airflow measurement should be included ...

Although a recommendation and not a requirement, the committee indicated their intention and concern with interpretations that incorporated indirect "rate" measurement and control.

6.1.3 Ventilation Requirements: Indoor air quality shall be considered acceptable if the required rates of acceptable outdoor air in Table 2 are provided for the occupied space. … Comfort criteria with respect to human bioeffluents are likely to be satisfied if the ventilation rate results in indoor CO₂ concentrations less than 700 ppm above the outdoor air concentration

Could this be interpreted to mean, "CO₂ measurement can be used to calculate ventilation rates"? I don't think so.

Many researchers and engineers have demonstrated that the use of CO₂ sensors is only an indirect indicator of occupancy, and that a base ventilation rate must be continuously maintained to dilute contaminants which can not be effectively measured or removed from within the building. This conclusion is also reflected in the interpretations published by ASHRAE on the standard.

If a design can continuously maintain the ventilation rates as required by Table 2 of the ASHRAE Standard, indoor air quality would be "considered acceptable". CO₂ control will satisfy the ventilation requirement for human bioeffluents only, not other contaminants. Therefore, a base ventilation rate should be set to handle "non-occupant sources" of contamination. The differences in the requirement between "base" and "variable" rates is reflected in the pie charts below, which depict the diversity in pollution sources generated by people and the building.

Figure #1, Typical Office Building                        Figure #2, Low Emission Office Building

Significant and specific limitations are specified by ASHRAE before a user is allowed to use CO₂ input for control. [See "Engelhard Interpretations" in 62-89 and INTERPRETATION IC 62-1999-03, -04, et. al.]

6.1.3.4 Intermittent or variable occupancy: … When contaminants are generated in the space or conditioning system independent of the occupants or their activities, supply of outdoor air should lead occupancy…

CO₂ measurement to detect occupancy in transient spaces may not provide adequate lead-time to assure acceptable air quality. The lag in timing between detection and effecting a change in CO₂ concentrations is well known and is reflected in ASHRAE 62 paragraph 6.1.3.4 above.

All major ventilation codes and our national standard (ASHRAE 62-1999) rely on specific intake rate tables to establish mechanical ventilation compliance. None of them allow the substitution of controlling CO₂ concentrations. Regulation of CO₂ concentrations does not insure compliance with ASHRAE Standard 62 on Ventilation for Acceptable indoor Air Quality. Neither can it be used as a surrogate for other indoor contaminants.

The popularity of "DCV" and all of its misinterpretations have led to further confusion on the proper use of CO₂ measurement and of the interpretations published on its application regarding ASHRAE 62.

"Less" CO₂ is not necessarily "Better" Ventilation, nor Energy Efficient

The basis of authority that most use to justify CO₂ measurement as a direct control for ventilation rates, originates with the CO₂ Mass Balance Equation, referenced in the Appendix of ASHRAE Standard 62 - 1999. This is NOT the method used to estimate ventilation rates referred to in ASTM Standard D6245-98, Guide for Using Indoor Carbon Dioxide Concentrations to Evaluate Indoor Air Quality and Ventilation.

The simple concept of this approach is so seductive, that everyone needs to understand in advance the dangers involved in misusing the methodology or misapplying the principles.

First, examine the relationship between the mathematical components of the Mass Balance equation.

ANSI / ASHRAE Standard 62-1999, provides us with the Mass Balance Equation D -1. (below):

Vo = N / (Cs - Co )

Where,

Vo = Outdoor air flow rate per person
N = CO₂ generation rate per person 
Cs = CO₂ concentration in the space 
Co = CO₂ concentration in outdoor air

The equation [Vo = N / (Cs - Co )] can be converted to volumetric units of CFM and concentrations in ppm. The revised equation becomes:

Vo = 10,600 / (Cs - Co)

When applied to 1,000 ppm set point (700 ppm above a fixed outside base of 300 ppm); Vo = 10,593 / (1,000 - 300) = the familiar 15 CFM/person. If we calculate based on 800 ppm, the result is 21 CFM/person. Carrying this to an extreme, we can calculate for 600 ppm and get a required ventilation rate of 35 CFM/person. Remembering that the outside base was held constant at 300 ppm.

If we look at current CO₂ levels in cities, we find that an average closer to 400 ppm. Readings above 500 are not unusual. Los Angeles and Mexico City have reported readings of 600 ppm or better. This puts a great burden on those who insist on controlling indoor CO₂ to a specific maximum level, especially those that assume "if some is good, more is better". For example, if we had an outdoor CO₂ level of 450 ppm and attempted to control ventilation to an indoor level of 600 ppm, we calculate that 70 CFM/person is required. Not very reasonable, is it?

A minority of authorities feel that increasing the amount of dilution ventilation based on a specific indoor CO₂ concentration may be assumed by some to be "linear and absolute", when in fact it has been shown to be "inverse and relative". ref. Feber, T.R., ASHRAE Journal. Lowering the internal level of CO₂ beyond a certain point does not necessarily provide the positive, energy saving results that are desired.

Effect of CO₂ Measurement Error

Based on the mass balance equation, if we conclude that the ventilation rate required in a space is to be 15 CFM/person, we can calculate the CO₂ differential (inside to outside) to be 689 ppm, when N=0.31 (equal to the respiration of a seated person). The calculation using 20 CFM/person as our objective, provides a CO₂ differential is 517 ppm, using the same parameters.

These calculations assume that measurement accuracy is perfect - no error. What happens when our measurement of CO₂ contains error (% Reading)? What impact does the error in CO₂ measurement have on outside air intake rates?

Holding the outside CO₂ concentration constant, a straight calculation of the CO₂ differential including error rates from +2% to +10% provides us with intake/person errors of -3.1% to -13.6%. From this we conclude that an inverse relationship exists between outside CO₂ error rates and intake rates per person. As the positive error of outside CO₂ measurement increases, the intake rate/person decreases.

Effect of Outside CO₂ Variability

We know that outside CO₂ is not static and that it varies both geographically and over time, throughout a single day, as well as seasonally. Therefore, making the assumption that a single outside concentration level is valid continuously is not a very professional engineering practice.

To determine the potential impact of external variations in concentration on ventilation rate control, we make the same calculations, but this time vary CO₂ from 300 ppm to 500 ppm. This range is also fairly typical. The resulting calculation tells us that the intake error rate per person will range from -12.7% at the lower CO₂ rate to +17% at the high end of this example.

In another example, if we had a local outdoor CO₂ level of 450 ppm and attempted to control ventilation to an indoor level of 600 ppm, we calculate that 70 CFM/person is required to offset this exaggerated and minimal CO₂ differential.

Effect of Applying Multiple CO₂ Sensors

The various applications using CO₂ measurement are not the same (DCV, ASHRAE Mass Balance, ASTM tracer method and "concentration balance" methods). All of the methods compared to DCV are time-specific analytical tools that use the inside-to-outside ratio of CO₂ concentrations, together with the internal generation rate of carbon dioxide per person, to calculate estimates of the amount of ventilation air that is available per person in the space. It is important to also recognize that these methods assume the use of a single measurement instrument to help validate their calculations.

DCV, on the other hand, is a method attempting to control ventilation intake rates for a building (or space) based on the internal CO₂ concentration, many times by averaging multiple measurement points internally (or the amount in the return duct system) and comparing that to a single outside measurement.

An assessment of demand controlled ventilation was recently expressed in the evaluation of the "concentration balance" technique of calculating outside air intake rates, as described in the contents of ASHRAE Research Project RP-980 (1999). Error Analysis of Measurement and Control Techniques of Outside Air Intake Rates in VAV Systems, conducted at UC-Boulder, Department of Civil, Environmental, and Architectural Engineering. We quote from the report.

The concentration balance airflow measurement technique …. is performed using one sensor to measure all three CO₂ concentration values. Using multiple CO₂ sensors to determine the outside airflow rate is not possible due to the relatively large error associated with the absolute accuracy of commonly available sensors. When only one sensor is used, however, the absolute errors cancel out …... The only source of error associated with the sensor then becomes its repeatability. The use of only one sensor, however, increases the time required to calculate the outside airflow rate. Each airflow must be sampled by the sensor before the outside airflow rate can be calculated, and each airflow typically requires two to three minutes to be measured with reasonable accuracy. However, this requirement for relatively stable CO₂ concentrations limits the applicability of the concentration balance technique. In spaces where large, abrupt changes in occupancy (and, hence, CO₂ levels) can occur, this method may prove unreliable. This fact may rule out the use of this control strategy in spaces such as conference rooms and auditoriums or any building where large transient effects are possible. …

…. The predicted errors indicate that the concentration balance airflow measurement technique may be valid except when occupancy is low or when the difference in the recirculated and outside air CO₂ concentration levels is small. Additionally, when the outside air represents a small fraction of the total supply air provided, errors in the calculated outside airflow may become too large for reliable and accurate use.

Logically, the uncertainty of using multiple instruments must consider the combined error rates of all the instruments used. This would apply to their use in the "concentration balance" method as described above or their use in controlling intake air based on internal CO₂ measurements using the Mass Balance Equation. Neither provides the mathematical results expected.

Going back to the Mass Balance formula again, let's examine the results of instrument error, based only on two measurements. What happens when we calculate for a 0% - 5% opposition in uncertainty between the outside and inside CO₂ measurements? If we hold the error rate equal at both the inside and outside units (±2%, ±3%, etc.), we find that the intake error ranges from -4.1% to -9.8%, as indicated in the table below. As the CO₂ error rate grows, the intake error increases negatively, about twice the single instrument rate.

The data calculated above is the measurement error for a single instrument. The compounding effect of multiple instrument error rates allows much greater uncertainty in the outside air control application.

Effect of Variable Respiration Rates

The final variable that we will examine is "N", the respiration rate, CO₂ generation and their effects on ventilation. Is it realistic to assume that everyone in a building will be seated, is of the same size, sex, health, and consuming the same diet? Can all the other factors that influence respiration rates be held constant?

When we examine the range of activities and their impact on respiration, we find that "N" can easily vary to 0.50 for walking, 0.60 for Light machine work and 0.90 for the upper threshold of "light activity". All of these are cited in the Appendix to ASHRAE Standard 62-1999. These figures assume a target ventilation rate of 15 CFM. When the target is 20 CFM, then the range of "N" varies from 0.40 for office work, to 0.50 for Walking. When these are used to calculate the amount of ventilation per person, we come up with the following range of rates.

Respiration Rate Actual Outside Air Activity N L/sec CFM / person 0.30 7.3 14.5 Seated 0.40 9.7 19.4 Office Work 0.50 12.1 24.2 Walking 0.60 14.5 29.0 Light Machine Work 0.90 21.8 43.5 Upper threshold of "light activity"

The point is that any change from the minimum respiration and intake ventilation target has dramatic impacts on the amount of ventilation required, if 700 ppm above a constant outside CO₂ rate is used.

Building Codes

With regard to the ventilation code issues, specifically the International Building Code, we find no mention of CO₂ in Chapter 4 of the International Mechanical Code - 2000 (sections 401 - 402 - 403) on Ventilation. Nor could we find any mention of CO₂ in Section 1002 of the 2000 Building Performance Code version of the IBC. All references to ventilation are related to dilution "rates" using acceptable outside air.

Our search could find no code authorities that use CO₂ concentration or mass balance control as a substitute for ventilation rate requirements. The most significant misinterpretation is the California Energy Commission Title 24. The California Energy Code allows CO₂ as an input to estimate occupancy level in variable occupancy spaces only and thereby the ability to reset the ventilation rate accordingly. It does not indicate nor endorse CO₂ for use as a direct input for control of intake rates.

Conclusions

The preceding citations, calculations, analysis and comments have attempted to provide authoritative sources and their reasons for dissuading people from using "CO₂ demand-controlled ventilation". The positions represented have been supported by many that recognize the difficulties of providing "rate" control, without a "rate" input.

When stated very simply: ASHRAE 62-1999 is "the" ventilation standard for Indoor Air Quality in the United States. Both the Ventilation Rate Procedure and the Indoor Air Quality Procedure quantify the outside air requirements at each occupied space. This leads us to conclude that the best way to insure compliance is to measure and directly control intake air rates. In fact, it can be argued that the outside air requirement at each occupied space, especially in VAV systems, can only be accomplished reliably by the direct and dynamic control of airflow rates into the building and supply air into each occupied zone.

From the potential errors in measurement to the interrelationship of the variables, simple mathematical analysis has provided us with a clearer understanding of some of the real problems in using CO₂ inputs for direct control of ventilation rates.

From this analysis we can conclude that CO₂ should not be used as the sole and direct determinant for dilution ventilation rates, in most commercial and institutional structures.

Why not? Consider these arguments.

First, because there is no direct relationship between interior CO₂ levels and the rate of air that is delivered to a space. Under limited circumstances CO₂ levels can be used to reset control set points to optimize intake rates, but CO₂ levels cannot quantify nor relate directly to volumetric airflow rates. Inferences that there is a direct relationship must be heavily qualified to hold any truth.

Next, none of the potential impacts examined have considered the effect of sampling errors. Logic alone tells us that it is not possible for a single point to provide an accurate "average" for a space, without the measured element being perfectly distributed throughout the entire space.

Also, remember the strict assumptions required by the ASHRAE Mass Balance Equation, without which any calculated ventilation estimates would be useless.

Lastly, the conditions described by these assumptions can only occur at at a very specific single point-in-time; measured with the use of a single, highly accurate instrument; and with calculations made that usually assume no measurement error. The attempted use of the tracer-gas or mass balance method to evaluate ventilation effectiveness, but used for ventilation control cannot be valid for use in a dynamic building system or operating environment.

Recommendations

Avoid all indirect methods of measurement and control.

The best and most secure method of outside air intake control is direct measurement. With a direct, real-time electronic input any number of HVAC strategies can be realized to optimize energy usage, through dynamic control under changing internal and environmental conditions.

Instrument first-cost is not a valid justification to accept ineffective methodologies.

Many excuses have been levied for the avoidance of direct outside air measurement, the most pervasive being cost and the difficulties found in applying traditional technologies to field conditions. But, these excuses are countered with several products and methods that provide both cost-effective means and reliability in performance, when used for outside air control.

Digital electronic airflow measurement products are available for commercial HVAC applications with acquisition costs comparable to traditional technologies, and several methods of CO₂ control.

Direct electronic airflow measurement possesses a number of other advantages, including:

• sensor accuracy better than ± 2% of Reading 
• permanent factory calibration to NIST-traceable air speed standards, 
• no field calibration requirements, 
• zero maintenance requirements, 
• simple installation for either new or existing HVAC equipment, 
• linear analog or digital electronic outputs, and maybe best of all -- 
• minimal duct placement limitations,

Why risk the energy and liability consequences of indirect measurements, when direct control of outside air intake rates can be accomplished (in most situations): economically, effectively and reliably? 

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