I'm working on a LEED project with the following conditions:
1) The proposed system is a 4 pipe Fan Coil system. Outside air is
brought into the space through an central ERV and independently ducted
to each Zone.
2) Appendix G requires the Baseline case to be Packaged VAV for
each floor, with terminal re-heat for each zone
In my simulation, I have the outside air being calculated based on the
"critical" zone for the baseline case. In order to meet the needs of the
critical zone, outside air for each floor is increased to meet the
critical zone needs. Outside air for the proposed system is simply the
sum of all the zones, because each zone is supplied individually.
The net result is that the OA in the baseline and proposed simulation
are different, because the PVAV system has to adjust the OA upward for
the critical zone.
The reviewer has commented that the outside air must be the same for
both simulations.
This seems like a paradox to me. It is physically impossible to make
15% OA go to one zone and 20% OA go to another zone in a PVAV system,
where the OA is mixed in the AHU prior to distribution through a common
duct. Forcing my simulation to do this in order to meet the reviewers
comments seems to violate the spirit of LEED. Why would we purposefully
compare a proposed system to baseline which cannot be built?
Likewise, artificially raising the OA used in the proposed system
simulation, to match that of the baseline system makes no since.
Has anyone else run into this before? Do you have suggestions?
All help greatly appreciated!
Regards,
Chris D. Mullinax, P.E., LEED AP, CEM
I see your point, although I usually just toe the line and force the OA
percent to be different in each zone. If you want to pursue your approach,
calculate the outdoor airflow for your baseline system using the ASHRAE
62.1 spreadsheet provided for the IAQ credits. Use the minimum amount of
system ventilation air required. You'll find that for a multi-zone system,
the outdoor air is shared between zones, with some penalty, but it's not as
bad as forcing the whole system to be 50% outdoor air simply because one
zone requires it. I won't gaurantee that the reviewer accepts this, but you
need to calculate ventilation this way in order for your argument to be
consistent.
Karen
On Saturday, August 18, 2012, Chris Mullinax
wrote:
brought into the space through an central ERV and independently ducted to
each Zone.
floor, with terminal re-heat for each zone
?critical? zone for the baseline case. In order to meet the needs of the
critical zone, outside air for each floor is increased to meet the critical
zone needs. Outside air for the proposed system is simply the sum of all
the zones, because each zone is supplied individually.
different, because the PVAV system has to adjust the OA upward for the
critical zone.
simulations.
15% OA go to one zone and 20% OA go to another zone in a PVAV system, where
the OA is mixed in the AHU prior to distribution through a common duct.
Forcing my simulation to do this in order to meet the reviewers comments
seems to violate the spirit of LEED. Why would we purposefully compare a
proposed system to baseline which cannot be built?
simulation, to match that of the baseline system makes no since.
Just change the system OA-SIZING-METHOD in the baseline to SUM-OF-ZONE-OA, like it is in the proposed system. I would argue that the G3.1.2.5 requirement that ventilation rates be the same applies first and foremost to the building as a whole, then to the systems, and then to the zones if possible. The intent is to make the ventilation load neutral between the baseline and proposed designs, not to simulate effective ventilation distribution in the baseline.
Regards,
Bill
[Senior Energy Engineer 28Jun2012]