ASHRAE 90.1 Definition of "Conditioned" - The PRM Report for LEED EAc1 and Appendix G Energy Model

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Dear All,

I would like to know if and how you consider ASHRAE's definition of
conditioned space on page 13 of the 2004 version.

Specifically, I am interested to know if you abide by this definition when
listing the conditioned vs unconditioned area? The definition seems really
specific to me, explaining how the system serving a conditioned area must
have a certain capacity per square foot of building area served. It even
goes on to explain how for some spaces one must consider the sum of the
products of various U-factors and wall areas. To me, it does not seem like
one can eyeball all these comparisons and definitions and therefore one has
to go ahead and measure the capacities of systems and compare them with the
area they serve, as well as do computations for rooms that might be
indirectly conditioned.

The PRM report for LEED requires you to list unconditioned and conditioned
areas separately. Do any of you actually go through the calculations to find
out if each single space in your project complies with the definitions put
forward in the 90.1 document? It seems to me like more effort than what it's
worth. I don't recall ever using this classification of unconditioned vs
conditioned anywhere else in the LEED certification process. Especially, I
don't recall ever having to use this definition to do the PRM model itself
under Appendix G. Has anyone come across another place than the PRM report
where the efforts of classifying their spaces accoring to the 90.1
definition above has been rewarded? Please also let me know if any one of
you have ever experienced trouble getting your project LEED certified
because of lack of compliance with this definition.

Lastly, the definition explicitly states that "Crawlspaces, attics, and
parking garages with natural or mechanical ventilation are not considered
enclosed spaces." Do most of you usually leave out car parks from the PRM
area report? I am working on a building where about 60% of the floor area is
parking space, meaning the sum total in my Space Summary will be much less
than stated otherwise in the documents submitted to USGBC. I know that they
are looking for all the total areas to be pretty consistent, and that some
people experience problems when thes total areas don't match up. Has anyone
ever dealt with this issue? If so, how did you deal with it?

I appreciate any response you may have to any or all of my questions.

Sincerely yours,

*Ulrik Welle-Strand Horn *

Ulrik Welle-Strand Horn's picture
Joined: 2011-10-01
Reputation: 0

Dear Ulrik,

Regarding "conditioned space":

"Conditioned space" seems pretty straightforward to me. If, for example,
the cooling system for a 10,000 sq.ft. space has 10 tons cooling capacity,
then the capacity is 12Btu/hr/ft2 (10tons x 12,000 Btu/hr/ton / 10,000 ft2)
This exceeds the ASHRAE threshold of 5 Btu/h/ft2, so it's "conditioned". If
the cooling system was only 4 tons, it would not be "conditioned" (only 4.8
Btu/hr/ft2).

The definition depends only indirectly on "U" values and wall areas. For
any given construction, you choose to provide an air conditioning system or
not. You can also choose to cool a lot or a little. Each choice affects
whether you end up with an air conditioning system greater than 5 Btu/hr/ft2
or not.

Heating works the same way, except the threshold varies by climate zone.

James V. Dirkes II, P.E., LEED AP

James V. Dirkes II  P.E.'s picture
Joined: 2011-10-02
Reputation: 0

Dear Mr. James V. Dirkes II:

Could you please clarify what you mean by "The definition depends only
indirectly on "U" values and wall areas?" I am not designing the mechanical
system, I am only reviewing it. Therefore I can't chose which spaces to
condition. The building has many rooms that don't have any cooling coils,
nor do they have any cold air entering the rooms. These must surely be
indirectly conditioned spaces?

For these indirectly conditioned spaces, I am wondering if I need to perform
the U-factor and surface area calculations and comparisons? The spaces in
question surely are not ventilated with 3 ACH from conditioned spaces. As
there are quite a few of these spaces, I just wanted to know if it is really
common practice to perform these calculations?

Thank you for a quick reply.

Sincerely yours,

*Ulrik Welle-Strand Horn *

Ulrik Welle-Strand Horn's picture
Joined: 2011-10-01
Reputation: 0

Now your question becomes more clear! If your principal concern is
"indirectly conditioned space", I'll say that:

*

I'm making some assumptions without seeing your floor plan, mostly
because I'm not sure why there would be a significant number of spaces which
have no ventilation, heating or cooling.
*

Any "non-conditioned" space within an otherwise conditioned area
will become cool or warm because of heat transfer from the adjoining spaces.
Effectively this means that the main heating / cooling plant serves this
"non-conditioned" area regardless of whether there is ductwork, fan coils,
etc. Therefore, if you want to properly predict the heating / cooling
energy, you must include the heat transfer to and from these
"non-conditioned" areas. I think that's all ASHRAE is trying to account for
in their definition.
*

The energy model, as a result, should include these
"non-conditioned" areas with appropriate definitions for the wall
construction and internal mass. The airflow and heating / cooling capacity
can be zero or perhaps the setpoints can be chosen such that no heating or
cooling will ever be required...

p.s., I don't know about "common practice" since I'm one consultant among
many and have never discussed it with others. If the "non-conditioned"
spaces are a very small portion of the overall area, perhaps they can be
ignored in the energy model. If not, they should be included. Regardless,
the USGBC reviewer will have the final say on the matter. I hope this
helps!

James V. Dirkes II, P.E., LEED AP

James V. Dirkes II  P.E.'s picture
Joined: 2011-10-02
Reputation: 0