Demand based Ventilation:
What is it? Why do it?
When a building is occupied, there are two primary sources of air contaminants. The building materials and furnishings emit water vapor and other gases, and the people inside do too. Of these, the “people component” is by far the most significant in a typical office building, school, theater, or other similar commercial establishment. The “non-people component” may be heavier in industrial occupancies or places where volatile materials are stored. In either case, the air contaminant level must be controlled by introducing fresh air at an appropriate rate.
When an engineer selects the amount of outdoor air to be provided to a space, he must comply with the governing mechanical codes. Nearly all of these refer to ASHRAE Standard 62-1989, which specifies minimum ventilation rates for the most common types of occupancy on a CFM per person basis. These are the occupancy types mentioned above, in which the “people component” of contamination far outweighs the other sources. Consider a school classroom with 30 desks plus a teacher. ASHRAE 62 calls for 15 CFM of outdoor air per person, so the air handling system must be capable of supplying a minimum of 465 CFM of outdoor air. Depending on the type of construction and exposure, this outdoor air can amount to as much as half of the heating and cooling load!
Now consider that the classroom in question only has 15 students enrolled for a school year. Through a full heating season and a partial cooling season, an unnecessary 225 CFM of outdoor air will be introduced if the damper controls are only designed for economizer with a minimum position. With typical Memphis weather and utility rates*, that is almost $300 lost for just one classroom in one year. In most other areas the loss will be even higher. Demand based ventilation is the answer to this problem. When people exhale, they tend to raise the level of carbon dioxide (CO2) in the space. When the code-required amount of outdoor air is being supplied, the indoor level of CO2 tends to stabilize at about 700 ppm above the outdoor air CO2 level. Thus, if the outdoor air supply is modulated to maintain a consistent CO2 level of 700 ppm above ambient, the code-required level of ventilation per person is being supplied based on the number of people actually present. ASHRAE has confirmed that this technique is valid with a formal interpretation to Standard 62-1989. Note that a true CO2 transmitter is needed, as this is the only valid indicator of occupancy permitted by the interpretation. VOC** sensors (sometimes referred to as “air quality” sensors) do not give a specific enough response to be used in this application. In most locations, the outdoor air CO2 level is very constant, at 350 to 450 ppm. In this case, control can be achieved with return air or wall mounted sensors. A control point of about 1000 ppm indoors will give the required ventilation rate with an adequate safety margin. In large cities with heavy traffic, or next to large parking structures, outdoor CO2 levels can vary. In this case, both indoor and outdoor levels should be sensed, and the control point should be around 700 ppm differential between the two. In the school example above, this type of control would automatically reduce the outdoor airflow from 465 CFM to 240 CFM since the 15 missing people will not be contributing to the CO2 level in the room.
Kele has a broad range of CO2 level transmitters from Engelhard Sensor, Texas Instruments, and R. E. Technologies. Call your Kele Sales Associate for the products you need to minimize ventilation expenses without compromising indoor air quality or code compliance.
* Average rates $0.065/KWH, $0.50/CCF ** Volatile Organic Compound. See the article “What’s Controlling Your Ventilation?”