Existing building HVAC modeling

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I'm modeling a building for LEED EAc1 compliance, and was wondering how
you are supposed to model existing building HVAC systems if you are
adding square footage to the building.

Let's assume you have a 4-story building with a 2-chiller HVAC plant and
dual temp boxes. If you were to add a 5th floor to the building, what
would your baseline model look like? I know that the 1st four floors
would use the existing HVAC system, but would your new 5th floor be
treated as a new 15,000 square foot area with packaged RTUs, or would
you add a new chiller to the existing HVAC chilled water plant, and have
that chiller meet all the requirements of App G, and have shutoff VAV
boxes for the 5th floor in lieu of dual temp boxes?

For reference, under our proposed design, we are doing just that:
adding a frictionless chiller to the plant to make up the extra load.

James Hansen, PE, LEED AP

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James,

If you are only adding an additional floor - and are not renovating any
portion of the existing 1st-4th floors then I would model the 5th floor
as if it was a new building by itself according to App. G guidelines.
See Table G 3.1 item no. 2 in 90.1-2004

If the entire building is undergoing a renovation but the existing HVAC
systems are existing to remain then the building would be modeled to
reflect the existing system types and capacities. Refer to the LEED
manual for these guidelines.

Hope this helps.

Seth Spangler, LEED(r) AP

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James,

Per part b of Seth's reference, his modeling solution only works if you do not cross the line between new and existing with any HVAC services (air, chilled water, hot water, dhw). Once you do that, things get complicated.

Based on past experience, I highly suggest a through reading of Appendix G, existing CIRs and then writing your own to avoid being forced to remodel by the LEED review.

Paul

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Thanks for the response Seth. The entire building is undergoing a
renovation (new windows, insulation, lighting)...and I'm going to keep
the existing HVAC system in the EXISTING portion of the building for my
baseline model. I just don't know how to treat the NEW portion of my
building in the baseline model. The plant is interconnected, so Item No
2 in Table G3.1 doesn't apply. I want to include the existing building
in the model (and I have to).

Any other advice on how to properly set up the baseline HVAC system for
a new portion of an existing building if the proposed chilled water
plant is being expanded to accommodate new load but the existing airside
systems in the existing building are NOT being touched?

Common sense would say that my baseline building also has a new App G
chiller to accommodate new load and the new airside systems are shutoff
boxes....but if there is a chance that the new portion of the building
is treated as a standalone 15,000 square foot addition, packaged RTUs
may make the proposed system even more efficient.

Paul, thanks for the email...I'm going to look thru CIRs and see what I
can find. Can't wait for LEED v3.0 where this convenient Option goes
away.

Thanks again,

James Hansen, PE, LEED AP

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>From table G3.1:

"The HVAC system(s) in the baseline building design shall be of the type and
description specified in G3.1.1, shall meet the general HVAC system
requirements specified in G3.1.2, and shall meet any system-specific
requirements in G3.1.3 that are applicable to the baseline HVAC system
type(s)."

the only time the existing building properties are used are for the building
envelope.

--
Karen

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Karen, this seems to contradict your earlier statement. Or more likely,
I'm just not comprehending it.

As Cam pointed out, the user's manual says that when an existing system
exists, as in an existing building that is being built out for a tenant,
the HVAC system in the baseline may match the proposed. This makes
sense to me, and it was how I understood the modeling requirements.

However, it doesn't go into more depth than this, and neither do the
CIRs, if there are modifications to the existing plant to increase
efficiency, but that do NOT alter the system type.

As another example:

LEED building, 10-stories, existing water-cooled chilled water plant
serving dual duct terminals.

The manuals and CIRs basically say:

For modified areas/systems: The Proposed Building Performance is
calculated as per design. The Baseline Building Performance is
calculated as per ASHRAE 90.1-2004 Appendix G requirements

For areas/systems not modified: Both the Proposed and the Baseline
Building Performance must be modeled as per the existing conditions.

This is straight out of a CIR ruling dated 2/9/09.

But what if we took our example building, and made a minor change to,
say, the AHUs (as part the LEED project, we installed VFDs to replace a
pressure bypass configuration).

Is this "modifying" the existing systems enough so that we would then
have to model the Baseline HVAC system per the App G requirements
(System 7 or 8) in lieu of modeling the baseline HVAC system as the
existing system (dual duct system)? Or could we just bring the AHUs up
to App G requirements (temperatures, VFD, fan HP sizing, etc), and leave
the chilled water plant and air distribution system as it currently
exists since we aren't modifying those?

Thanks for any clarification.

This is particularly important for this project because the Client does
not have sufficient funds to upgrade the chilled water plant, so all
existing equipment is to remain. If I have to bring the Baseline HVAC
system completely to App G standards, that's a huge penalty since the
proposed building chillers are older.

James Hansen, PE, LEED AP

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