ASHRAE Journal - August 2008 - (Page 41)

high outdoor humidity, the low sensible load results in unitary air conditioners cycling on-off frequently with short on cycles. During the off cycle, moisture on the coil can reevaporate, causing further deterioration of the net latent capacity2,3 and driving up the indoor RH. Changing Building Ventilation and Efficiency Standards During the past 30 years, two trends have affected commercial building cooling loads and the ratio of latent to sensible load. The first of these is a drive to improve the energy efficiency of the U.S. economy, reflected in tightened efficiency standards for automobiles, appliances and buildings. The second trend has been the recognition of the need for minimum levels of outdoor ventilation air to maintain reasonably healthy conditions inside buildings. ASHRAE Standards 62 and 90 (they became 62.1 and 90.1 years later) were initially introduced in the early 1970s, and created to set minimum standards for building ventilation and for energy-related aspects of building design, respectively. Since then, major revisions have been published at regular intervals. During the past 30 years, the outdoor ventilation airflow rates required by Standard 62 were first reduced then restored to earlier levels. In most commercial and institutional buildings outdoor ventilation air is the main source of humidity load, so the net effect has been to leave latent loads unchanged. Standard 90.1 has mandated reductions in various contributions to the sensible cooling load of a building, but has left latent loads largely unaffected. Building Envelope 90.1 defined climate zones. Instead, the maximum Standard 90.1 U-value can be tracked for particular cities, as the successive versions of Standard 90.1 apply to the specific climate characteristics (e.g., design heating degree days and cooling degree days) of each city. Figure 2 plots Standard 90.1’s maximum wall assembly U-value for seven cities that represent the range of climate conditions in the continental U.S., from cold to moderate to hot-humid to hotdry. The required maximum U-values have decreased by a factor of three to four, which reduces the wall conduction contribution to the sensible load by a like amount. Figure 3 plots the Standard 90.1 maximum roof U-value over the period during which Standard 90 has been in effect. Modest (~30%) reductions in the maximum U-value have occurred during this time period. Figure 4 plots the maximum solar heat gain coefficient for window glazings over the life of Standard 90. It controls the fraction of incident solar radiation that enters the conditioned space. The key point is that each of these measures—wall assembly and roof U-values, solar heat gain coefficients— addresses contributors to the sensible cooling load only. Equipment Loads Standard 90.1 specif ies minimum envelope thermal performance by specifying maximum U-values for wall and roof assemblies. Variations in climate are accounted for, so that U-values are cost-effective relative to the ambient conditions in the cooling and heating seasons of a given locale. Over time, the scheme for defining climate zones has changed, so it is not possible to track the changing requirements in U-value over time through particular Standard August 2008 Internal heat gains from energy dissipated by equipment within the building are a significant fraction of the building cooling loads (and contribute to heating the building during the cold periods of the year). Standard 90.1 specifically controls two of the contributors to internal heat gain: lighting and motors. Minimum motor efficiencies are set at the EPAct minimums for general purpose motors, a modest increase in minimum efficiency compared to earlier revisions of 90.1. Maximum lighting power density is plotted in Figure 5. It has been reduced by 40% since the first version of Standard 90. Standard 90.1 does not specify efficiencies or power levels for office equipment, but power densities have been increasing as the usage of computers, printers, etc. has increased, offsetting www.info.hotims.com/16017-65 ASHRAE Journal 41 http://www.ruskin.com/ERV http://www.ruskin.com http://www.ruskin.com http://www.info.hotims.com/16017-65

Table of Contents Feed for the Digital Edition of ASHRAE Journal - August 2008

ASHRAE Journal - August 2008 Contents Commentary Industry News Letters Meetings and Shows Maintain to Sustain—Delivering ASHRAE’s Sustainability Promise Ultraviolet Germicidal Irradiation: Current Best Practices Improving Humidity Control With Energy Recovery Ventilation Single- or Two-Stage Compression Data Center Cooling: Using Wet-Bulb Economizers Building Sciences InfoCenter Practical Pointers Products Emerging Technologies Washington Report People Special Products Classified Advertising Advertising Index

ASHRAE Journal - August 2008

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