Thanks for your response Richard.
Can anyone confirm whether ASHRAE 90.1-2007 specifically says that your
Baseline model MUST have a system per floor?
I can't seem to find it in 2004, or the user's manual, or in the LEED-NC
reference guide.
James Hansen, PE, LEED AP
James,
90.1-2004 addendum u includes the 'one system per floor' wording for
systems 5 thru 8, but floors can be grouped when the thermal blocks are
similar. For LEED, it is optional to use addenda. 90.1-2007 adopts the
same wording.
Regards,
Bill Talbert, PE, LEED? AP
It seems odd to me that 90.1-2007 requires the baseline building to have one system per floor (system types 5, 6, 7 & 8).
These are all VAV systems, where you have individual VAV boxes responding to different thermal zones. Why would different
number of AHU?s in this case make a difference in heating and cooling energy consumption?
In terms of fan energy, however, different number of AHU?s may make a difference. This is because 90.1 regulates
the fan brake horsepower, which is depending on the size of the fan (Table G3.1.2.9, <20,000 cfm or >20,000 cfm).
For the same sized building, assuming the total supply airflow being roughly the same independent of the number of
AHU?s used, each individual fan?s brake house power would be different depending on its fan size, this may lead to
different total fan energy consumption when you add all individual fan energy up. For this reason, I would argue, it makes
more sense to keep the same number of AHU?s in the baseline case as in the proposed design, for you would then be comparing
?apple to apple?, because the fan sizes in the proposed design would be ?comparable? to those in the baseline case.
George Hu, PE, LEED AP
The difference in energy consumption becomes especially apparent when you have a shutoff VAV system with reheat (like I do with this particular model). If, for example, a ground floor interior zone called for heating after-hours because of heat loss thru the slab, and you only had one system for the whole building that has to operate, the energy consumption is significantly more than if you have a system for each floor.
If LEED gives you the option to use Addenda or not use Addenda, then in my case, I would more than likely NOT choose to use it. As long as this is acceptable, which it appears it is, I can take significant credit for having multiple AHUs in lieu of a single RTU system.
It sort of feels like cheating to do this, but ASHRAE is very specific on how you are to model the Baseline building, and I?m not doing anything ?wrong?, per se, by using a single system for the Baseline building. For what it?s worth, there are a lot of buildings that use package penthouse AHUs?
James Hansen, PE, LEED AP
James,
I agree addendum u seems is the only big direction in Appendix G, and LEED-NC 2.2 applicants can elect to follow it or not. Addendum u changed G3.1.1 but not its exceptions. In 90.1-2007, G31.1 includes addendum u and some changes to the exceptions.
I think the exceptions to G3.1.1 should also be reviewed from which ever version. I have pondered occasionally if they are allowed exceptions or dictated/mandatory exceptions, and I am leaning more toward the latter recently.
(If by some odd chance you were referring to the ECB method, I prescribe a baseline system for each proposed system.)
If you have to or choose to have different system alignments, then be conscious of keeping OA constant and all those parameters that vary by system size like fan power, heat recovery, economizer, package cooling efficiency, etc.
Paul Riemer
This brings up the subject of off-time operation of AHU?s. Having one single zone calling for a large AHU in a (large) building
to run is probably an extreme case. More realistically, probably a small number of zones call for the AHU to run.
With large turn down ratios of the supply fan(s) in the AHU driven by VFD(s), and the ability of DDC control systems to
shut off VAV boxes in no-demand zones, I would tend to believe the number of AHU?s would not make a big difference
in (fan) energy cost. This is, of course, assuming that the simulation software is capable of modeling all these features
accurately.
George Hu, PE, LEED AP
As stated Appendix G, 2007 requires one VAV air handler per floor for
baseline systems 5, 6, 7, and 8. In addition to the differences in
baseline fan power, the number and layout of the air handlers can have a
significant impact on the reset of supply air temperature which is also
a baseline requirement in Appendix G.
Michael Rosenberg