I got comments back on the energy model for a building that has no HVAC systems designed at the time of completion of the C&S. One comment was that the cooling energy savings seemed too high. The other was that the system airflow and fan power in the baseline seemed too high compared to the proposed.
For both cases I let the equipment capacities, airflow and fan power calculate out instead of specifying identical values. Since the baseline has a worse envelope, the higher sensible load led to much higher capacity, airflow and fan power. This made sense to me because the worse envelope would demand the higher capacity system. However, the comments from LEED appear to be rejecting this approach and suggesting that I need to use identical values, but it’s not 100% clear to me that this is the case.
The comments do allow for a narrative to justify the method/inputs. I am wondering if my approach is justifiable from LEED's perspective; or if it would be considered absolutely incorrect, in which case I would have to make the capacities, airflow and fan power identical for both cases.
I would greatly appreciate any input you may have. Thanks,
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5907 thumbs up
October 7, 2013 - 12:10 pm
In general you should be able to get credit for a better envelope.
Sounds like the reviewer is perhaps questioning the amount of savings relative to the envelope improvements. While the general approach sounds right perhaps there is something else going on. What percent savings are you showing for cooling and fans?
If the envelope is the only savings perhaps you could make the two models the same and see what your savings look like then. Do they still make sense?
Tony Schafer
24 thumbs up
October 7, 2013 - 12:56 pm
The cooling savings was 30%. Fans were 27%. The only other thing the reviewer caught is that I was using the wrong system type. I was using system #4 PSZ-HP, when it should have been #3 PSZ-AC. But regardless of the impact that might have, my question is really about what LEED views as the correct approach for this. Allowing the coils and fans to be auto-sized and therefore different for each case, or specify them and making them identical for each case?
I know that table G3.1 in 90.1 states that when no system has been specified, the baseline and proposed systems shall be identical which obviously applies to the system type and efficiency, but it’s unclear if that applies to elements that are auto-sized in the calculation such as the coils and fans. Do you think my argument that the worse envelope demands the higher capacity equipment should at least justify this approach? Thanks,
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5907 thumbs up
October 7, 2013 - 1:07 pm
They would be auto-sized in both cases. Identical systems do not imply identical capacities.
Tony Schafer
24 thumbs up
October 7, 2013 - 2:37 pm
Thanks - very helpful!