A STUDY OF ENERGY USE AND SATISFACTORY ZONE VENTILATION OF DIFFERENT OUTDOOR AIR VENTILATION STRATEGIES FOR TERMINAL REHEAT VARIABLE AIR VOLUME SYSTEMS

Increased building indoor air quality (IAQ) complaints due to reduced outdoor air ventilation rates led to ASHRAE Standard 62-1989. Even tho ugh the stipulated standard total outdoor ventilation flow rate may be drawn into the HVAC system, thermal imbalances in the various zones o f the building can lead to certain zones being starved of the specifie d ventilation flow rate, thereby creating localized IAQ problems. The objective of this paper is to compare the differences in energy use an d ventilation air flow rates supplied to different zones in the buildi ng for three different practical outdoor air ventilation strategies al l of which are identical in performance at design conditions but which differ under part-load operation. A simplified simulation methodology (which past studies have demonstrated to be useful for field evaluati on of actual buildings) has been used to predict the heating and cooli ng energy use of a two-zone terminal reheat variable air volume (TRVAV ) system during part-load operation specified by varying outdoor tempe rature and humidity conditions. The trade-off between outdoor air inta ke and energy use are studied for the following ventilation strategies for a typical 10,000 m(2) commercial building: (i) constant outside a ir intake based on a value 20% higher than the ASHRAE minimum ventilat ion rate, (ii) constant ventilation air intake fraction, and (iii) ven tilation air intake based on the unfavorable zone requirements (even t hough the other zone may be over-ventilated). How this trade-off is im pacted by building size has also been investigated. Finally, we use bi n data for Dallas, TX (a moderately hot and humid location) and Seattl e, WA (a mild location) in order to study the differences in energy us e and zone ventilation flows of different ventilation strategies due t o building location. The effect of economizer cycles and of varying ve ntilation strategies depending upon diurnal building schedules have no t been considered in this study. The results of this study which are b ased on a simplified HVAC simulation approach are consistent with conc lusions reached by other researchers using more detailed simulation mo dels. This suggests that sound and meaningful diagnostic insights of a ctual building performance and operating strategies can be obtained fr om such simplified simulations. (C) 1998 Elsevier Science S.A. All rig hts reserved.