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NC-2009 IEQp1:Minimum IAQ Performance

100% OA multiple-zone system

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Michael Wironen
Ecology and Environment, Inc.
LEEDuser Basic Member
16 thumbs up

Hi, I'm a little confused about a review comment we received. We have rooftop AHUs supplying 100% Outdoor Air to multiple zones equipped with terminal fan coil units. The terminal fan coils mix the OA with locally recirculated air to meet heating and cooling requirements. We used the 62MZCalc form to document compliance with ASHRAE 62.1 since it is a multiple zone system. Per the reviewer's comment, we cannot do this as the system is 100% OA. However, the LEED form itself does not allow you to factor in diversity at the level of the AHU. This is a university building with a mix of uses and occupancy schedules and so it will never be 100% occupied at max occupancy. Will LEED reviewers allow you to use a population diversity factor when working with 100% OA systems?

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Julia Weatherby
President
Weatherby Design & Co. Engineers
LEEDuser Basic Member
67 thumbs up

Wed, 09/03/2014 - 18:47

Hello, Michael- The ventilation requirement has to be satisfied for each individual space. Therefore, if you are not recirculating "unused" outdoor air from some spaces, it doesn't seem that population diversity would be relevant for the individual room calculations. You would need to provide enough outdoor air through the 100% outdoor air system to satisfy the ventilation requirements for the design occupancy in each space. For airflow at the AHU level, if you have a way to reduce or shut off outdoor air to individual spaces when the room is at reduced or no occupancy (such as shut off or modulating dampers on the outdoor air duct to the room controlled by CO2 sensors and/or occupancy sensors), then you could explain that your design cfm for the 100% OA AHU does not need to add up to the full OA design cfm sum of all the spaces due to occupant and scheduling diversity. If the LEED form does not provide for this, you can provide your own calculations and explanation under the Special Circumstances section.
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Michael Wironen
Ecology and Environment, Inc.
LEEDuser Basic Member
16 thumbs up

Thu, 09/04/2014 - 14:19

Thanks! That is exactly our scenario - we have demand controlled ventilation and so airflow is adjusted to meet occupancy levels.

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