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HVAC - Guide to Demand Control Ventilation

A. Bhatia, B.E.


Course Outline

The ventilation rate inside a building is directly dependent on the number of occupants in the space. If we had the ability to economically count each person as they entered and exited from a space, then we could provide exactly enough ventilation air to meet the needs of the quantity of people in the space. But in most buildings occupancy is not tracked in real-time, and therefore we have sought out another indicator- carbon dioxide (CO2).

Since people produce CO2 as a direct result of respiration, the use of CO2 has been a "logical" choice to control the ventilation rate. Unfortunately, many have not fully understood the relationship between CO2 levels and ventilation. This 4-hr course provides the framework necessary for integrating DCV into a HVAC system and includes background on ASHRAE ventilation standards, as well as California's Title 24 energy code. The course will take reader through the design process and calculations needed to understand, how to properly implement DCV strategy.

The course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

At the conclusion of this course, the reader will understand:

Intended Audience

This course is aimed at Mechanical and HVAC engineers, Architects, Building designers, Energy Auditors, Facility managers, Property & Estate managers, Operational & Maintenance Personnel, Sales & Marketing personnel, and General Audience.v


Course Introduction

Measurement of CO2 concentrations is an accepted scientific methodology to control the actual ventilation rate in a building. If a space is unoccupied, the CO2 controller will set the air intake volume at a minimum setting that allows established ventilation rates to be maintained, while reducing over-ventilation and saving energy. The ASHRAE 62-2004 standard, which is adopted by many local codes, explains how to calculate minimum outside air quantities using an equation that incorporates both occupancy and an area based component. Demand-controlled ventilation (DCV) is essentially applied - especially in applications where contaminant levels result primarily from people (or their activities) and where population varies significantly.

DCV offers great potential for new and retrofit projects to reduce outside air ventilation and therefore reduce heating, cooling and dehumidification energy consumption, but if improperly applied, it can create a negative building pressure environment and lead to undesirable infiltration, building envelope degradation, and indoor air quality problems.


Course Content

The course content is in a PDF file HVAC - Guide to Demand Control Ventilation. You need to open or download this document to study this course.

Course Summary

The concept of demand control ventilation (DCV) makes complete sense in today's energy conservation scenario. Demand controlled ventilation is a control strategy that varies the ventilation rate based on occupancy and currently, the most effective indicator of high or low occupancy is through the use of carbon dioxide (CO2) sensors.

The ASHRAE 62-2004 standard, which is adopted by many local codes, explains how to calculate minimum outside air quantities using an equation that incorporates both occupancy and an area based component. The area based component of the ventilation air requirement shall be used as a lower minimum fresh air quantity when applying DCV, subject to the minimum outside air needed to makeup for any exhaust, to ensure a positively pressurized building. The area based component plus full occupancy component should be used as the upper limit of the minimum fresh air quantity required. The DCV strategy then modulates the minimum fresh air amount between these two values, when not in economizing mode, based on suggested CO2 levels of 100 ppm to 700 ppm above ambient respectively. The lower minimum fresh air quantity based on area is important, because it ensures a positive pressure in the building to prevent uncontrolled infiltration.

Demand-controlled ventilation can reduce the cost of operating the HVAC system - especially in applications where contaminant levels result primarily from people and where population varies significantly (in gymnasiums, large meeting rooms, and auditoriums, for example).

The real energy savings with DCV will depend on the climate being "severe" enough, and the required ventilation rate being large enough so that the cooling load reduction saves enough energy costs to offset the first cost of the CO2-sensing equipment. It is important to look into each area individually before incorporating the DCV strategy. Carefully study all of the requirements in ASHRAE -IC 62-2001-34 before you adopt a CO2-based DCV control strategy.

Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

Take a Quiz

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.