Engineered Systems - March 2008 - (Page 60)

In the conclusion of this pair of articles, the author delivers opinion and documentation with equal force. Making the argument that traditional VAV is too troublesome regarding meeting ASHRAE 62.1 and 90.1, he also explains how those same standards are pushing designers in the right direction. The destination? A new era for VAV and new efficiencies for their applications — but shining a little light down this path reveals just how fraught with peril the designer’s journey can be. BY MARK S. LENTZ, P.E. C lassical VAV systems have a reputation for compromising IAQ. Because they actively reduce ventilation to save energy. The degree to which they compromise ventilation often exceeds that which is permissible under the building codes. However, they are widely perceived to be efficient and cost-effective, to provide superior comfort control, and as a result, remain popular and in wide use. Most VAV installations designed and constructed in jurisdictions subject to the International Mechanical Code (IMC) since 1998 fail to comply with code. This is especially true in ventilation-intensive occupancies like schools, where the required rates of ventilation are high. The mechanism for this problem was explained in Part I (January 2008, Engineered Systems) of this series. These system designs are still accepted by many code officials for two reasons: they seldom have the expertise to recognize and understand the problem, and they seldom know how to address the situation. It is now widely recognized that shutoff VAV systems cannot provide adequate ventilation when used by themselves. The use and modification of this strategy will be discussed in a future article. The purpose of this article is to discuss the challenges of bringing classical VAV reheat systems into compliance with the requirements of ANSI/ASHRAE Standard 62.1-2004 “Ventilation for Acceptable Indoor Air Quality,”1 ANSI/ASHRAE/IESNA Standard 90.1 “Energy Standard for Buildings Except Low-Rise Residential Buildings,”2 and meeting the requirements of Section 4 of the IMC3. All of these requirements must be addressed and met to legitimately qualify for LEED® certification. CHALLENGES TO BE ADDRESSED As the ratio of design space sensible load to occupancy sensible load decreases the need for VAV system sensitivity to the ventilation needs of the occupants increases Occupancies suitable for use with classical VAV reheat are characterized by high sensible heat ratios and often have high internal equipment gains. However, increasing attention to wasteful equipment energy use by manufacturers, reduced lighting system Watt densities, greater building insulation levels, and more attention to glazing systems are effectively reducing sensible heat ratios. This is reducing the number of occupancies where VAV reheat systems are even viable. The actual needs of many spaces, like educational facilities, are now actually dominated by the need for ventilation. These applications play to the weakness of VAV reheat systems — their inability to provide adequate ventilation — making these systems increasingly inappropriate for use in those occupancies. This will be discussed in more detail in future articles. The weakness of the classical VAV reheat system stems from the fact that these systems not only vary the airflow to individual spaces, 60 En gi neer ed S y stem s March 2008

Table of Contents Feed for the Digital Edition of Engineered Systems - March 2008

Engineered Systems - March 2008 Contents Editor’s Note Back2Basics HVAC Challenge Case In Point Commissioning Building Automation Energy Wiz HVACR Designer Tips Application Checklist Exemplary Design = Elementary Success The Modern Unit Ventilator On A Mission VAV Systems And Green Design – Part II Issues & Events Products Glossary Classifieds Advertiser Index Tomorrow’s Engineer

Engineered Systems - March 2008

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