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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