FTE vs ASHRAE 62.1 occupancy

Edwin Wealend

Director & Principal Sustainable Design Consultant

Cundall HK Ltd.

LEEDuser Basic Member

59 thumbs up

December 12, 2011

Hi All For a C&S project there are default FTE occupancy figures that shoudl be used for water calcs, bicycle racks etc where the final occupancy is not known. These are I believe average occupancy figures that represent an average number of occupants on a typical day and are less than what we would expet to be the peak occupancy. While FTE are ok for the average usage and water/bicycle number types of calcs I don't belive they should be used for the ASHRAE 62.1 fresh air calculations. ASHRAE 62.1 has another set of default occupancy which are what I would descibe as the peak occupancy and the MEP designer will use these numbers in the MEP calcualtions NOT the FTE numbers. Has anyone encountered this issue and dealt with the fact all the online forms are trying to use the same occupancy for Fresh air calcs as the FTE? I think we should be able to use different occupancy in this case (as would the MEP designer) and that FTE should not be used fo this credit. Regards Julian

Post a reply

Dylan Connelly

Mechanical Engineer

Integral Group

LEEDuser Expert

342 thumbs up

Mon, 02/27/2012 - 22:08

The ASHRAE occupancy numbers are numbers you can use for your 62.1 calculations if you don't have exact occupancy numbers (e.g. seats on a floor plan). FTE is strictly a LEED number and doesn't have to be used for the 62.1 calculations. I've never had my occupancy numbers questioned before, but if your quantities are way off of the FTE numbers then you may be asked to explain yourself in a narrative.

J.W. Shin

LEEDuser Premium Member

9 thumbs up

Thu, 06/28/2012 - 09:49

Hi, You have a really important question to calculate required OA of VAV systems appropriatly. Refer to equation in the definitions on the page 22 of ASHRAE Standard 62.1-2007; Occupant Diversity, D = Ps/ΣPz where, Pz: Zone Population, the largest number of people expected to occupy the zone during typical usage, the ASHRAE occupancy numbers you mentioned. Ps: System Population, it should vary by less than 10% from the peak population density reported(FTE+transient) for LEED submittal. Less 'Ps', less 'Vot'. (but Ps is limited not to be over sum of Pz) Regards