Owner wanted all fume hoods operating with 100cfm 24:7. After preliminary energy modelling with 60 cfm during working hours and 30 cfm after hours, we reduce air changes, fan energy and with VAV, more energy during unoccupied hours. This upgrade to energy performance is proving convincing - especially in high desert macroclimate. Question: Is it legitimate to use 100cfm in our base model ?
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Joyce Kelly
Architect - Cx Provider - Green Building SpecialistGLHN Architects & Engineers
27 thumbs up
September 29, 2017 - 3:29 pm
Correction: 100 fpm (1180 cfm) was original requirement which we are reducing to 60 fpm. This also results in smaller fan hp required.
Joyce Kelly
Architect - Cx Provider - Green Building SpecialistGLHN Architects & Engineers
27 thumbs up
October 2, 2017 - 1:32 pm
A professional energy modeler just advised that fume hoods are process loads because they are withdrawing chemicals from labs, not conditioning space for people working in the labs. So... is it still possible to explore the energy savings between constant flow in the baseline and VAV in the proposed? Perhaps through the Exceptional Calculation Method?
Tyler Thumma
7GroupLEEDuser Expert
67 thumbs up
October 18, 2017 - 4:12 pm
Laboratory systems are covered by Appendix G and therefore are not process loads, so energy savings are possible as long as you follow the modeling protocol for the Baseline systems. Here are some notes specific to 90.1-2007:
-Exception (d) to Section G3.1.1 requires modeling of system #5 or #7 in Baseline with 50% reduction in exhaust and makeup air volumes during unoccupied periods (for laboratory systems with minimum of 5,000 cfm of exhaust). The User's Manual Appendix G laboratory schedules appear to indicate that this refers to the fume hood being unused (i.e. sash closed), and not to the whole building occupancy conditions.
-G3.1.2.10 Exhaust Air Energy Recovery - exception (h) exempts labs with 5,000
cfm of exhaust (i.e. the systems modeled as Baseline per Exception (d) G3.1.1 from
requirement for energy recovery in the Baseline).
-Table 6.5.3.1.1B Fan Power Limitation Pressure Drop Adjustments include a fume hood
exhaust exception (-1.0 in. w.c. adjustment). This means when applied to the laboratory
system's supply fan power limitation, the Baseline calculation must apply the -1.0 in. w.c.
adjustment, and the fume hood exhaust fan power is separated from the system and
modeled as unregulated process energy, with the same W/cfm as that of the Proposed Case.
Credit may be taken from improved motor efficiency.
Addendum p to 90.1-2007:
-Removes the fume hood exhaust exception within Table 6.5.3.1.1B as noted above, and
adds two new device credits, one for exhaust system serving fume hoods (0.35 in. w.c.)
and laboratory and vivarium exhaust systems in high-rise building (0.25 in. w.c./100 ft of
vertical duct exceeding 75 ft)
-Modifies pressure credit for "fully ducted return and/or exhaust air systems" from 0.5 to
2.15 in. w.c. for lab and vivarium systems