Hello,
Our project scope consists of designing a ~60,000 ft², 7 floor addition to an existing 75,000 ft² building that is also 7 floors. Our LEED boundary includes the existing building because we are modifying parts of the existing building as part of the scope of work. The existing mechanical system that serves the existing building is a 100% outside air system that is sized for 6 air changes per hour in all of the spaces (constant volume, variable temperature). On 3 out of 7 of the existing floors, we are rebalancing the existing ductwork to only provide the code required ventilation and will be adding fan coil units for heating and cooling decoupling ventilation and space loads and saving significant energy. The other 4 floors will remain as is and only be balanced to ensure the same airflow is being provided. We are not doing any work to the existing air handling units aside from installing new control valves and rebalancing.
We are currently modeling the baseline system serving the addition as System 7 and the baseline system for all of the existing areas as the existing system (100% OA, 6 ach to all existing spaces). Is this the correct way to model the baseline, or should the baseline system for the modified existing spaces be one of the Appendix G systems, and if so which one?
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5909 thumbs up
October 23, 2017 - 5:42 pm
The baseline should be a system 7 for the entire building. The proposed is as designed and as exists.
Eric Krieg
SmithGroupJJROctober 23, 2017 - 6:07 pm
The proposed (final state) includes portions of the building that are not being renovated at all. If LEED requires the existing system that isn't being modified to be compared to a new VAV with reheat system for those same spaces not being renovated, the proposed system will consume more energy than a baseline system. This would be because the baseline system would include scope and construction cost that doesn't exist in the proposed system.
I could see LEED making the case that areas being renovated should be compared to a system 7, but allowing the existing spaces not being renovated to be modeled the same in both baseline and proposed. Is that how LEED has viewed this in the past, or has it required comparing existing to remain to a baseline system?
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5909 thumbs up
October 24, 2017 - 10:49 am
LEED has flipped/flopped back and forth on this issue.
What I shared with you is the official USGBC/GBCI position on this subject. It is not necessarily a position I agree with but here are some of the details. I think 90.1-2007 is very clear that existing systems should be modeled identically. The 2007 standard is silent on the issue but the User's Manual provides clear guidance. USGBC/GBCI has allowed the existing systems to be modeled identically for projects under 90.1-2007 and allowed a full Appendix G baseline. 90.1-2010 and 90.1-2013 do not provide much clarity. However, I think under 90.1-2010 USGBC/GBCI is requiring an Appendix G baseline. You can certainly try to make the case to the reviewer that modeling them identically should be allowed and you would likely be able to do so because it remains unclear. You may run into resistance and have to argue your case. 90.1-2016 does seem to clarify that the baseline is always Appendix G. Since this is where the standard is headed USGBC/GBCI is of the opinion that the baseline should be Appendix G.
In my opinion existing systems should be modeled identically when they have not been modified. This aligns with the principle of the whole code that anything you do not modify you do not need to bring up to code compliance. The 90.1 Appendix G committee apparently does not agree however.