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LEED - Outdoor air flow cycled

asked 2022-11-14 15:59:40 -0500

lrebello's avatar

updated 2022-11-14 16:26:20 -0500

Hello all,

I have a proposed model with a VRF HVAC system and I have received this question/comment from LEED:

"In addition, verify that all HVAC systems in both the Proposed and Baseline cases are modeled with zero outdoor air flow when the HVAC fans are cycled on to meet unoccupied setback temperatures, unless health or safety regulations mandate in alternate minimum flow during unoccupied periods (in which case, the unoccupied minimum outdoor air ventilation rates must be modeled identically in each model)."

Does anyone know what should I do to reflect this in my proposed model?

Also, what does it mean to "HVAC fans are cycled"?

Thanks a lot

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Hi Irebello

I haven´t done energy simulation for LEED for a while so some things maybe different now.

as far as I remember you must have the same airflow in the baseline and proposed case. If you proposed case have more OA air flow than ASHRAE 62.1 your baseline must have the same flows unless you have DCV.

If you put different OA flows, you are not doing a fair comparison since more OA can have an energy penalty. Honestly, I don’t know how to set fans to cycled to meet setback temp. but you can check this post.

[link text](

obuchely's avatar obuchely  ( 2022-11-30 13:26:48 -0500 )edit

Thank you so much for your answer. I havent fixed everything yet, but working on it.

lrebello's avatar lrebello  ( 2023-02-16 12:35:44 -0500 )edit

I could not open the question link you sent above!

lrebello's avatar lrebello  ( 2023-02-16 12:36:18 -0500 )edit

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answered 2022-11-28 10:57:31 -0500

obuchely's avatar

Basically the reviewer is checking that the proposed model has the same outdoor air flow. The process to check it depends of the software that you are using.

Proposed and Baseline cases are modeled with zero outdoor air flow when the HVAC fans are cycled on to meet unoccupied setback temperatures=> If there is no occupancy in the space the outdoor air flow must be zero and the fans should be set to cycled to meet setback temperatures. for example in Trace700 there is a option in the system section to set it like that. If you have a space that required outdoor air 24/7 make sure that you have the same in baseline and proposed case.

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answered 2022-11-29 14:30:28 -0500

lrebello's avatar

Hello @obuchely ,

I am using Open Studio + Energy +.

My baseline model has the same outdoor air flow that local code requires and the proposed model has a bit more than that (and represents the HVAC project).

The proposed model has a Dedicated VRF system to outdoor air so it gets outdoor air, conditions it and supply to some rooms (thermal zones). The outdoor air system just operates from 8am to 6pm, that is when there will be people inside the building.

The baseline model has Package Rooftop VAV with PFP Boxes and Reheat system and has an Air Loop for Outdooar Air set in each HVAC system to represent that the local code requires.

Does that sounds right to you?

I am not sure if my model represent this "fans should be set to cycled to meed setback temperatures" as I did not understand what it means.

Let me know if I described it clearly.

Thanks for your help.

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Asked: 2022-11-14 15:59:40 -0500

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Last updated: Nov 29 '22