In CO 2-based DCV, CO 2 is monitored as a byproduct of respiration, rather than as an indoor contaminant. Let's take a closer look at what may be the most common application of dynamic ventilation reset-DCV based on CO 2 readings-to understand how it works and how Standard 62.1 affects its implementation. If a zone requires reheat during economizer operation, this strategy can reduce both fan and reheat energy. For example, when a system is in economizer (free-cooling) mode, the amount of outdoor air in the primary air is greater than is necessary to meet minimum ventilation requirements, so the minimum primary-airflow settings on VAV boxes can be reduced. Lastly, Standard 62.1 allows the resetting of minimum primary airflow at VAV boxes in response to changes in intake airflow. This control strategy is known as ventilation reset. In a multiple-zone variable-air-volume (VAV) system, ventilation efficiency depends on zone- and system-level primary airflows and is higher at part load than it is at design (worst-case) conditions. Standard 62.1 also allows intake airflow to be reset in response to changes in ventilation efficiency. Occupancy sensors, which detect the presence or number of people in a zone.ĬO 2 sensors, which monitor the amount of CO 2 produced by occupants and diluted by outdoor air. Occupancy schedules, by which a building-automation system (BAS) predicts population based on time of day. Ventilation demand can be assessed one of three ways: This control strategy-DCV-responds to the actual need, or "demand," for ventilation by regulating the rate at which an HVAC system brings outdoor air into a building. Standard 62.1 allows the resetting of intake airflow in response to changes in zone population. The standard lists three types of dynamic reset: In Section 6.2.7, "Dynamic Reset," Standard 62.1 permits an HVAC system to "reset the design outdoor-air intake flow (Vot) and/or space or zone airflow as operating conditions change." Although the standard does not provide details for implementation, any system-control approach that responds to varying conditions must be capable of providing at least the required minimum breathing-zone outdoor airflow whenever the zones served by a system are occupied. This article reviews Standard 62.1's requirements for dynamic reset and outlines several methods of implementing DCV using CO 2 sensors. The good news is that DCV remains do-able and practical, especially for spaces such as gymnasiums and meeting rooms, where people and their activities are the main sources of contaminants. But implementing DCV based on indoor levels of carbon dioxide (CO 2) is not as straightforward under the 2007 version of ANSI (American National Standards Institute)/ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) Standard 62.1, Ventilation for Acceptable Indoor Air Quality, as it was under previous versions. 5) edition of Engineers Newsletter, "CO 2-Based Demand-Controlled Ventilation With ASHRAE Standard 62.1-2004." Published periodically by Trane's Applications Engineering group, Engineers Newsletter is intended to aid engineering professionals in the design and application of HVAC systems by providing "reliable, objective, and technologically current information in a non-commercial format." Engineers News-letter is archived at Demand-controlled ventilation (DCV) can reduce the cost of operating an HVAC system. Editor's note: Following is an update of the November 2005 (Volume 34, No.
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