VAV SYSTEM INDOOR AIR QUALITY CONSIDERATIONS FOR OFFICE SPACES

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Marjorie Parker
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The indoor air quality is a significant concern for A system, that provides various supply air rate according to the space, especially in multiple spaces, served by a common air handling unit.

1 Proceedings: Indoor Air 005 A SYSTEM INDOOR AIR QUALITY CONSIDERATIONS FOR OFFICE SPACES PH Zhang * Jacobs Civil, Inc. 60 Madison Avenue, 1 th Floor, NYC, NY10016, USA ABSTRACT This paper discusses indoor air quality considerations for office spaces when using A system. The indoor air quality is a significant concern for A system, that provides various supply air rate according to the space, especially in multiple spaces, served by a common air handling unit. For office space A system design, two procedures are used to ensure the indoor air quality: ventilation rate procedure and indoor air quality procedure. The ventilation rate procedure is a diluting method, widely used in HAC designs because of its practicability. The indoor air quality procedure is a monitoring contaminant concentration method. CO concentration in the office space is an indicator of occupied space contaminant level. This paper simulates and analyzes the ventilation for variable multi-space when the multi-space equation (6-1), ASHRAE 001, would be ineffective or not functioning in the A system at a fixed acceptable outdoor air rate. Also, it provides a design solution and ventilation control strategy at the fixed outdoor air rate for indoor air quality. INDEX TERMS A system, Building ventilation. INTRODUCTION A typical office floor plan, shown in Figure 1, includes enclosed offices around the perimeter, open interior office spaces, and conference rooms in the corner. For HAC system calculations and design, the floor is divided into ten zones from the northeast corner to the central areas. There is a common air-handling unit in the west end of the floor plan to serve the multiple spaces. Figure 1. Typical Floor Plan ENTILATION FOR INDOOR AIR QUALITY An HAC system delivers conditioned air, including the re-circulated air and outdoor air, to the occupied space to ensure human thermal comfort and acceptable indoor air quality. For multiple space ventilation with a common HAC system, the system ventilation should comply with ASHRAE 6, 001 and is adjusted according to the equation (6-1): Y = X /( 1+ X Z ) * Corresponding author pz5356@hotmail.com 1365

2 Proceedings: Indoor Air 005 where: Y = ot / st = corrected fraction of outdoor air in system supply X = on / st = uncorrected fraction of outdoor air in system supply Z = oc / sc = fraction of outdoor air in critical space. The critical space is that space with the greatest required fraction of outdoor air in supply to this space = corrected total outdoor air flow rate ot st on = total supply flow rate, i.e. the sum of all supply branches of the system = sum of outdoor air flow rates branches on the system = outdoor air flow rate required in critical spaces oc sc = supply flow rate in critical space Table 1 presents the results and analysis for outdoor air rates in the system using four methods: entilation Rates by Equation. 6-1, ASHRAE 6, 001, entilation Analysis with Adjusted Critical Spaces, System entilation Rates by CO Concentration and entilation Analysis with Adjusted Critical Spaces by CO Concentration. The first columns in each zone indicate the calculated supply air rates by space for the selected month and times during the design. The maximum air supply rates appear at 3:00 PM, when the ambient temperature appears to be the highest temperature during the day. The second columns in each zone illustrate the ventilation outdoor air rates. Acceptable outdoor air rate, which comply with ASHRAE Standard 6, 001, is brought to office spaces to dilute the indoor contaminants due to human activity and other contaminants. Typically, for office spaces, the required outdoor air rate is 9 L/s (0 CFM) per person, the maximum occupancy density is 7 people per 100 m, and the maximum occupancy density for a conference room is 50 people per 100 m. If there is a person in an office room along the perimeter area and 9 occupants in the conference room, when the conference rooms are considered to be at intermittent occupancy, then 75% of the maximum occupancy is used. Thus the ventilation rate of outdoor air is 07 L/s (440 CFM) for occupants. Zone 1 and Zone for the conference rooms are the critical spaces; the supply air rates are less than the required ventilations by ASHRAE 6, 001. The supply airflow should be 100% acceptable outdoor air; the fraction of outdoor air Z should be 1.0. By equation (6-1), Y should be 1.0. This presents 100% outdoor air would be used in the system for the common air-handling unit because of the high occupancy and low load conditions in the critical area. Based on these results, the great energy cost associated with a 100% outdoor air rate finds the HAC system to be impractical. The system would fail in the small area (fraction of supply airflow in the critical area to total supply airflow seems approximately.4%). However, if the fraction of outdoor air in the critical area is decreased by increasing the space when the required ventilation is constant, the system outdoor air would be greatly decreased. The increase of space is easily completed by the addition of heat from a heating coil or other heating equipment. With the exception of the conference areas, the central area contains the highest fraction of outdoor air. When the central area is chosen as the critical area, and the same fraction of outdoor air is taken in the conference area, the total system outdoor air would be greatly decreased. The results, shown in ventilation analysis with adjusted critical space (Table 1), confirm the approximately 0.30 outdoor air fraction and 1700 L/s (3400 CFM) outdoor air rate make sense with the practice of HAC system. 1366

3 Proceedings: Indoor Air 005 entilation Rates by Equation 6-1, ASHRAE 6, 001 Table 1. entilation analysis for A system of office spaces Zone 1 Zone Zone 3 Zone 4 Zone 5 Zone 6 L/s L/s Max. 1.0 L/s L/s Max. 1.0 L/s L/s L/s L/s L/s L/s L/s L/s 8:00 AM :00 AM :00 PM :00 PM entilation Analysis with Adjusted Critical Space Zone 1 Zone Zone 3 Zone 4 Zone 5 Zone 6 Adjusted Adjuste SA d SA L/s L/s Max. 1.0 L/s L/s L/s Max. 1.0 L/s L/s L/s L/s L/s L/s L/s L/s L/s 8:00 AM :00 AM :00 PM :00 PM System entilation Rates by CO Concentration L/s L/s Max. 1.0 L/s L/s Max. 1.0 L/s L/s L/s L/s L/s L/s L/s L/s 8:00 AM :00 AM :00 PM :00 PM entilation Analysis with Adjusted Critical Space by CO Concentration Zone 1 Zone Zone 3 Zone 4 Zone 5 Zone 6 Zone 1 Zone Zone 3 Zone 4 Adjusted Adjuste SA d SA ent. Zone 5 Zone 6 ent. OA/SA OA/SA L/s L/s Max. 1.0 L/s L/s L/s Max. 1.0 L/s L/s L/s L/s L/s L/s L/s L/s L/s 8:00 AM :00 AM :00 PM :00 PM

4 Proceedings: Indoor Air 005 entilation Rates by Equation 6-1, ASHRAE 6, 001 (Continue) Zone 7 X by Eqn.6-1 Critical Z Y st on entilation Analysis with Adjusted Critical Space (Continue) Zone 7 Zone 8 Zone 9 Zone 10 Total OA Total SA, zones, Zone 8 Zone 9 X by Eqn.6-1 Critical Z Y adjusted zones on System entilation Rates by CO Concentration (Continue) Zone 10 Total SA by Loads& Zone 7 Zone 8 Zone 9 Zone 10 Total OA by Eqn.6-1 Total SA, zones, Critical OA/SA st on X Z Y ent entilation Analysis with Adjusted Critical Space by CO Concentration (Continue) Total OA Zone 7 Zone 8 Zone 9 Zone 10 Total SA by Eqn.6-1 by Loads& Total OA Critical OA/SA adjusted zones on X Z Y ent

5 Proceedings: Indoor Air 005 Alternatively, CO concentration is an indicator of indoor air quality. Human occupants produce carbon dioxide, water vapor, and contaminants including particulate matter, biological aerosols, and volatile organic compounds. Indoor air quality criteria with respect to human bioeffluents are likely to be satisfied if the ventilation results in indoor CO concentrations less than 700 ppm above the outdoor air concentration. The outdoor airflow rate is given (ASHRAE 6, 001): where: = outdoor air flow rate per person O N = CO generation rate per person C = s CO concentration in the space C = o CO concentration in outdoor air o = N / ( C C ) s o For office activity level, the ventilation flow rate is 9 L/s (0 CFM) and the CO concentration in a space maintains about 700 ppm above the outdoor air concentration. The ventilation rate of acceptable outdoor air is determined by CO sensor monitoring space CO concentration produced by human activity. The number of occupants in the spaces and the lag time between when persons enter the space and when CO level reaches the set point are considered. The numbers of occupants is an important variable in the CO concentration, which will conclude the actual ventilation rates. entilation procedure of CO monitoring, including the full occupancy, the part occupancy and the unoccupied, is a dynamic method that responds to how the building is used and occupied. For the multi-space ventilation with the common A system, the CO monitoring in the critical area (conference rooms) is more reasonable. As shown in System entilation Rates by CO Concentration (Table 1), there are four occupied cases in the critical area (conference zone): unoccupied, 50% occupied, 75% occupied and fully occupied. When unoccupied, the ventilation fraction is approximately 30%. When fully occupied, the ventilation fraction is 100%, which requires 100% outdoor air in the system, that is a significant impact on the energy cost. entilation Analysis with Adjusted Critical Space by CO Concentration in Table 1 presents the results for the ventilation analysis when the critical area is chosen and adjusted to the central area (Zone 9). The required ventilation is much less than the system ventilation rates by CO concentration without the critical area adjusted, while the indoor air quality is an acceptable level. It shows the ventilation varies with the number of occupants. When the proper control system is used in the system, great energy savings result. ENTILATION CONTROL In a variable air volume HAC system, the indoor air quality is a significant concern; under-ventilation in a critical area can often occur. As a result, when the critical space is properly ventilated, the other areas are over-ventilated, which can produce inflated energy costs. Proper ventilation control is very important to ensure the indoor air quality and an effective HAC system. DDC (direct digital control) controls system is widely used in variable air volume systems to meet ventilation for indoor air quality and human thermal comfort. The ventilation controls generally have a fixed damper control, a fixed flow rate control, and ventilation reset by ventilation analysis. The simplest fixed damper control method is easy to practice and used in many existing systems. However, the indoor air quality would not be met when the space convert from the full design condition to part- conditions. Except in extreme conditions, the HAC system will work in part-load conditions, there is under-ventilation for occupancy space, especially in critical areas. The second method, fixed outdoor flow rate, is used in the system to produce the acceptable indoor air quality. It is mostly practiced for multiple spaces with the common system, as proven in ventilation analysis (Table 1). The fixed outdoor air rate with ventilation analysis is a solution for the acceptable indoor air quality. The third strategy is the optimal option - ventilation reset. It is a complicated control procedure and requires complex programming, however, it saves a great deal of energy cost while not sacrificing the indoor air quality. entilation rates vary with operating and occupied conditions. The ventilation control system should vary the outdoor air rate. According to the analysis, the outdoor air rate is reset (see results in entilation Analysis with 1369

6 Proceedings: Indoor Air 005 Adjusted Critical Space by CO Concentration, Table 1). The ventilation-reset strategy is performed by a DDC control system. The indoor CO sensors monitor the CO concentration level in zones, then compare and choose the critical areas in order for the control system to respond. The outdoor airflow rate is calculated; the control system resets the outdoor damper, returns damper positions and fan operation to generate the required ventilation. During the operation, the outdoor airflow rate which is a dynamic procedure in the system, is reset by the ventilation requirement in zones. SUMMARY A ariable air volume system is often used in office buildings due to the energy savings. The acceptable indoor air quality through ventilation is achieved by the choice of the critical area. When the highest fraction of outdoor air in the supply is much higher than the other area, the solution is to lower the fraction to the acceptable average level. The strategy would have an advantage when the indoor air quality is at an acceptable level, as significant energy cost would be saved. The ventilation reset optimal control with carbon dioxide ( CO ) monitoring can ensure the superior air quality, substantial energy savings, and enhance occupant comfort. Compared with the fixed ventilation flow rate, excessive over-ventilation is avoided while still maintaining acceptable indoor air quality and providing the required outdoor airflow for individuals. REFERENCE: ASHRAE handbook, Fundamentals, 001. ASHRAE Standard 6-001, entilation for Acceptable Indoor Air Quality,

INTERPRETATION IC OF ANSI/ASHRAE STANDARD VENTILATION FOR ACCEPTABLE INDOOR AIR QUALITY

INTERPRETATION IC OF ANSI/ASHRAE STANDARD VENTILATION FOR ACCEPTABLE INDOOR AIR QUALITY INTERPRETATION IC 62-2001-06 OF ANSI/ASHRAE STANDARD 62-2001 VENTILATION FOR ACCEPTABLE INDOOR AIR QUALITY REVISION APPROVED: 1/12/2002 Originally issued as interpretation of Standard 62-1989 (IC 62-1989-23)