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Forum discussion

NC-2009 EAp2:Minimum Energy Performance

outdoor air unit system type

I have a project that is classified as a high rise residential building. In the proposed design, the living units are conditioned by water source heat pumps and ventilated via natural ventilation (operable windows). Four 100% -outside air units (each about 10,000 cfm) supply outside air directly into the corridors. Since we don’t plan to claim any energy savings from natural ventilation, I intend to model zero ventilation air in the living units both in the baseline and proposed case and the same amount of outside air (a sum of minimum exhaust rates for residential kitchens, bathrooms and dryers) introduced into corridors in both the baseline and proposed case. Question 1: What would be the appropriate baseline system for dedicated outdoor air units? Since our main baseline system is PTAC, I would use a constant volume system with DX cooling/air cooled condenser and hot water fossil fuel boiler as a heating source. Our design selection is a 100% outdoor air unit with DX cooling /evaporative-cooled condenser and natural gas heating. Are we allowed to take credit for evaporative-cooled condenser, or we are required to model the same condenser cooling source in the baseline and proposed case? For example, if we switch to 100% outside air water cooled heat pumps, should our baseline dedicated outdoor air system remain DX cooling/ air cooled condenser and hot water coil? Question 2: Can we use min exhaust rate per ASHRAE Table 6-4 and equation per Table G3.1.2.9 to size exhaust fans for unconditioned parking garage and take credit by comparing it against the proposed design BHP. I would greatly appreciate your opinion.

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Marcus Sheffer
LEED Fellow
7group / Energy Opportunities
LEEDuser Expert
4454 thumbs up

Mon, 03/28/2011 - 13:28

For the baseline use system 1 or 2 for the entire building unless the exception in G3.1.1(a) applies. If it does reenter Table G3.1.1A and select the appropriate system. The baseline exhaust fans in the garage should be sized based upon the design rates and design exhaust fans power. Credit can be taken for improved motor efficiency. Table G3.1.2.9 only applies to fan power in conditioned spaces.

Mon, 03/28/2011 - 15:10

Marcus, Thank you for your input. Do you agree with me that the corridors providing access to residential apartments fall under "residential conditions" for the purpose of G3.1.1 (a)? I assume the baseline condenser cooling source should be identical to the one used in the proposed case. As opposed to ASHRAE Ch11, App G is not specific about this. Am I correct? Thank you.
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Marcus Sheffer
LEED Fellow
7group / Energy Opportunities
LEEDuser Expert
4454 thumbs up

Mon, 03/28/2011 - 21:28

Certainly debatable and it could be argued either way. I think the corridors are not residential since they do not fall in a living space and are common space. Table G3.1.1A is entered based on the heating source. So use the system it tells you to in the baseline and use the system you have designed in the proposed. Appendix G is a bit confusing on this subject. I have found that looking at 90.1-2010 can often help clarify the intent of the 90.1 committee related to 90.1-2004 or 2007 since some of these issues have been addressed.

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