We have a University Science Building that will house research and teaching labs with multiple VAV fume hoods in several spaces. Our design operational procedure for the VAV fume hoods is to maintain 80 feet per minute face velocity across the hood under normal operation and a reduced face velocity of 60 feet per minute when the space is unoccupied or nothing is present near the hood. A zone presence sensor is installed on every fume hood to determine when something or someone is in the vicinity of the hood and increase the airflow to the 80 fpm setpoint. A typical
fume hood would be constant volume or have a constant face velocity whether it be 80 fpm, 100 fpm, or possibly greater. The airflow requirements for the fume hoods are driving the room ventilation requirements. Since ASHRAE 90.1‐2004 Appendix G paragraph G3.1.2.5 requires that minimum outdoor air ventilation rates be the same for the
proposed and baseline buildings, are we able to model the energy savings from reducing the airflow thru the fume hoods during unoccupied periods and if so, what are we to base it against? Can this be modeled thru scheduling or can it be a process load?
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Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5909 thumbs up
December 4, 2012 - 10:59 am
Hi Ralph,
Sounds like it could be modeled thru scheduling and it certainly is a process load. The baseline must be determined by the project team and you will need to submit an exceptional calculation under Section 1.7 of the prerequisite form. The justification of a baseline is based on industry standard practice when building a new facility in your area. So if the sensors are typically not installed then the baseline is the same hoods without them.
Ralph Bicknese
PrincipalHellmuth & Bicknese Architects
21 thumbs up
December 10, 2012 - 6:26 pm
Marcus,
Thank you very much for your response. We have a follow up question. The question now is how one should model the process load for the fume hoods. If we were to say that the baseline fume hoods would operate at 80 fpm all of the time and the proposed fume hoods would operate at 80 fpm (occupied) and 60 fpm (unoccupied), in my opinion the only way to really model that is through scheduling. LEED rejected us last time because we didn’t have the minimum outside air values the same for both the baseline and proposed buildings so I can’t reduce my proposed ventilation or increase the baseline ventilation. If I were to adjust the scheduling of the ventilation for the proposed building does that mean we would need to submit an exceptional calculation? Your help is much appreciated! Thanks.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5909 thumbs up
December 11, 2012 - 9:18 am
Yep one of the reasons for having to do an exceptional calculation is that you need to violate a modeling protocol to show the savings. In this case schedules must be identical so any change in schedules requires an exceptional calculation.