We are pursuing LEED certification for small project in which some offices are converted to open offices, break room and reception area in an existing building. Project involves modifying existing HVAC duct work, adding two VAV boxes and new lighting fixture. Area is served by existing AHU. Existing AHU also serves other area of the building which are not part of the project. Entire building is LEED certified.

We want to meet the prerequisite requirement using mandatory and performance (Section 11) requirements. I have perform energy modeling using following approach.

1. Model the entire existing AHU including the area which is not part of the project but served by the existing AHU.
2. Area which is not part of the project is model same in the Design Energy Cost and Budget Energy Cost Model.
3. Envelop is modeled same in both models because nothing is being done to envelop.
4. HVAC system is modeled same in both models because nothing is being done to central AHU. I am confuse here, do I select system based on Figure 11.3.2 HVAC systems map or model the existing AHU.

I was wondering if someone can review my approach and let me know if it is incorrect. I will really appreciate any help.