January 2015

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Demand Controlled Ventilation

Proper management of ventilation is an essential task for any BAS, and when properly applied it can result in a building that is both healthy and efficient.

Paul Ehrlich, Ira Goldschmidt & Angela Lewis
Building Intelligence Group

As published
Engineered Systems 
January Issue - BAS Column

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One of the challenges we have with commercial buildings is the fine line between providing a safe and healthy indoor environment and maximizing energy efficiency. This challenge is exasperated when it comes to the topic of the “V” in HVAC or ventilation.  Ventilation air (or minimum outside air) is essential for balancing building pressurization and for removing, or diluting, the impact of indoor pollutants including various chemicals such as VOC’s as well as CO2.  Of course during part of the year, we can also use ventilation air for free cooling or economizer.  Bringing in too little outdoor air can result in building pressure issues and can jeopardize the health of the building occupants.  On the other hand bringing in too much ventilation air can have a significant impact on the amount of energy required to heat, or cool and dry the makeup air. 

Ideally we want to bring in just enough outdoor air to keep the building at a net positive pressure and to maintain healthy indoor environmental quality.   Of course this is not as easy as it sounds since the distribution of contaminants is not uniform.  Fortunately there has been extensive research and discussion conducted by industry over the last several decades, much of which is reflected in ASHRAE Standard 62 and eventually adopted into state and local mechanical codes.  While you should consult your local code for more details, the general trend for ventilation management is as follows:

Ventilation Design:

The codes and standards for ventilation design have changed significantly over the last 20 years.  We often find existing buildings that may still be operating as originally designed (back when indoor smoking was common) or those that over the years have gone too far in reducing ventilation levels.  For any new or existing building, the first step in evaluating ventilation is to look closely at the building pressurization, occupancy, code when constructed, and the current code requirements.  Often we find that just re-balancing to the current code can have a significant impact on energy efficiency.

Demand Controlled Ventilation:

Many state codes now allow for the use of demand controlled ventilation or DCV.  The concept of DCV is to measure and control the amount of ventilation air based on indicators of occupancy.  ASHRAE Standard 62 has a series of tables that outline the required levels of building area and occupant ventilation for various building types.  The area rates are intended to deal with the impact of chemicals and odors and must be provided at all times when the building is occupied.  The people portion of ventilation can be varied based on occupancy using counts or an indicator such as CO2.  In order to properly apply DCV you need a method of measuring and controlling ventilation air as well as a way of measuring occupancy.  The use of DCV is a natural for buildings, or areas of buildings, that have highly variable occupancy.  For example a church or an auditorium is a perfect application.  Properly applying DCV in facilities with VAV systems is more complicated to do properly.

Proper management of ventilation is an essential task for any BAS, and when properly applied it can result in a building that is both healthy and efficient.

About the Authors
Paul and IraPaul and Ira first worked together on a series of ASHRAE projects including the BACnet committee and Guideline 13 – Specifying DDC Controls. The formation of Building Intelligence Group provided them the ability to work together professionally providing assistance to owners with the planning, design and development of Intelligent Building Systems. Building Intelligence Group provides services for clients worldwide including leading Universities, Corporations, and Developers. More information can be found at www.buildingintelligencegroup.com  We also invite you to contact us directly at Paul@buildingintelligencegroup.com or ira@buildingintelligencegroup.com


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