Exhaust, outside air and infiltration for LEED

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The mechanical engineer for a campus laboratory building with 100% OA
wants to claim energy savings for reduced exhaust from more efficient
fume hoods. (Established design practice uses hoods with 100 fpm flow -
they are installing 70 fpm hoods. This reduces peak exhaust from 700 to
490 CFM per hood.) My approach has always been to keep outside air,
exhaust and infiltration flows identical between the proposed and
baseline models (except for DCV). (This was not easy for this model with
proposed VAV and baseline constant volume packaged rooftops.)

Has anybody successfully claimed OA/exhaust/infiltration savings for a
LEED project?


William Bishop, EIT, LEED(r) AP

Bishop, Bill's picture
Joined: 2011-09-30
Reputation: 0

You are correct that Appendix G requires outside air to be the same for both
as-designed and baseline models. You would not be able to claim savings for
reduced ventilation airflow from a more efficient fume hood. Similarly you
cannot claim credit for reduced ventilation airflow on displacement
ventilation and UFAD systems.

However I see no reason why you wouldn't be able to claim the fan energy
savings associated with a more efficient fume hood.

Michael Tillou, PE, LEED

Michael Tillou's picture
Joined: 2011-10-02
Reputation: 0

The hoods are more "effective" really, not more efficient. They have no
fans, but are designed to effectively remove fumes with a lower flow
rate through them. All the hoods are connected to a central exhaust fan

Now that you mention it, I guess I need to go back and add fan energy in
my baseline. The baseline has several systems (one per zone as required
in 90.1) where the proposed only has two air handlers and a common
exhaust fan. I will add a return fan with Pfan power (per G3.1.2.9) for
every system that serves a zone that, in the proposed design, is
connected to the common exhaust.


Bishop, Bill's picture
Joined: 2011-09-30
Reputation: 0

My two cents...

The same OSA for baseline vs as-designed makes sense for occupant-required
OSA, such that there is no reward for compromising occupant health to gain
LEED points.

However, that fraction of the fumehood OSA over and above occupancy-required
is *process* OSA, an opportunity where LEED *should *be encouraging
savings. Now I'm not saying they do, just that they *should...*

And after rereading the requirements it shakes out that LEED really doesn't
give credit for reducing process outside air loads, keep the parametric run
in your model -- the local utility may see things differently, and
'recognize you' with a big fat rebate check.


Brandon Nichols

Brandon Nichols2's picture
Joined: 2011-10-02
Reputation: 0

This is a fascinating convergence of issues.

Appendix G's requirement for the same minimum ventilation rate in the baseline as the proposed is true but also a bit incomplete when considering a design of a 100% OA VAV systems serving fume hoods.

We have a similar scenario of a small building with likely VAV hoods, possibly lower flow, likely served by a 100% outside air system BUT with my favorite added complexity of district heating and cooling.

Alas, while I was scratching my head, the client abandoned their LEED goal.

So I have not fully solved it for myself and cannot solve for it your project but I do suggest the following actions:
1) Read the 90.1-2004 prescriptive requirements for certain systems serving fume hoods to be VAV or have heat recovery.
2) Read the 90.1-2004 Appendix G base system selection section
3) Now read those same sections in 90.1-2007 and ponder which changes represent revisions and which ones represent clarifications of original intent
4) And maybe read the User's Manuals too
5) Consider the exceptional calculation method as the venue to claim energy savings, that you consider real but are not explicitly allowed or defined by the document, for consideration by the LEED reviewer
6) Research the existing CIR's
7) Before spending numerous hours on an approach that may or may not be accepted, buy a CIR and propose your tact to the USGBC itself. If they reject yours they almost certainly will dictate a new tact that their reviewer would be obligated to accept for your project.

And lastly, do not wade too much further through this on a Friday afternoon unless you have to do so. For me, tasks like this should be tackled early in the day and early in the week.

Good luck,


Paul Riemer's picture
Joined: 2011-09-30
Reputation: 0

Bill, Paul, Mike, et al -

We also have not mentioned another item relative to Appendix G, which
requires sizing the flow rate based on a 20-degree delta T. If the
system is a 100% OSA unit, and the baseline is sized for a 20-degree
space-to-coil temperature differential, what happens if the makeup air
unit is not designed with a 20-degree delta-T? Then it is possible the
peak CFM rates could potentially vary. So right there, we could
potentially have a contradiction between 2 lines in Appendix G - (1) OSA
ventilation rates are to be identical, and (2) The baseline AHU flow
rate is specified by a 20-degree delta T - now, which statement in
Appendix G "over rules" the other?

This seems like this should fall under the Exceptional Calculation
methodology to explicitly document where the savings come from, and
whether the savings are justified...

Also regarding an earlier item from this thread, that one cannot take
credit for UFAD and displacement ventilation systems with reduced CFM
rates - as long as the OSA CFM rates are identical, then credit can be
taken from the UFAD/DV system design. You'll be providing the same OSA,
but different Total CFM rates could be calculated based on the different
mixed air temperatures and designed coil leaving temperatures. This can
be either a positive or negative, which truly depends on the climate and
the ability to maximize the economizer hours...

Mitch Dec

Mitchell Dec's picture
Joined: 2011-10-01
Reputation: 0


That issue was addressed in the 2007 version of Appendix G.

G3.1.2.8 Design Airflow Rates. System design supply airflow rates for the baseline building design shall be based on a supply-air-to-room-air temperature difference of 20?F or the required ventilation air or makeup air, whichever is greater

Michael Rosenberg

Rosenberg, Michael I's picture
Joined: 2011-09-30
Reputation: 0

We have successfully taken credit for reduced OSA rates in a number of applications where a "cascading" make-up air system has been designed. In a kitchen hood application for instance, the traditional design might bring in 100% OSA to offset the hood exhaust from a separate intake or unit. In certain projects (schools come to mind), this make-up air can be transferred into the space via adjacent spaces with high OSA requirements (i.e. commons), thus saving all or a portion of the mechanical energy to heat/cool this air. We have always taken the path of the exceptional calculation for LEED purposes because this is definitely a grey area as far as Appendix G is concerned.

Andrew Craig2's picture
Joined: 2011-10-02
Reputation: 0

What would be the most appropriate weather data to use for a building
situated on the waterfront in Halifax, NS? The purpose is to use thermal
simulation to measure code compliance with Ashrae 55 and 129.
Many thanks, Chris

Chris Yates's picture
Joined: 2011-10-02
Reputation: 0



- Denis

Denis Bourgeois's picture
Joined: 2011-10-02
Reputation: 0

Interesting. We recently decided not to "take credit" for substantial cascade/transfer air in a restaurant EAc1 app (i.e. the transfer air from dining to kitchen/hood was included in the baseline model). Our experience is that this mode has been extremely common for many, many years in commercial and school kitchens. (And it's very inexpensive, sometimes decreasing construction costs.) We felt taking credit for it did not meet the spirit of the PRM. Maybe if one needed to build all kinds of transfer ducting in a hotel, then it might seem like an improvement worthy of recognition...
I could go on with a rant about the opacity of the credit review process, loopholes, "consensus" standards, etc. But actual work beckons.


Fred Porter's picture
Joined: 2011-09-30
Reputation: 0

I appreciate everyone's detailed responses to my query. The project I'm
modeling has 100% OA VAV for most of the building, while the App. G
baseline is System 3 - Constant volume packaged rooftops. This is a NY
State project that has to be "designed to meet the criteria for LEED"
but will not likely go through the certification process. One thing I
learned is that, even though OA was identical in my baseline and
proposed models (because "I modeled it that way") - it didn't actually
turn out to be true. Custom hourly eQUEST reports showed nearly twice as
much annual OA CFM in my baseline than in my proposed. Ouch. Glad I



Bishop, Bill's picture
Joined: 2011-09-30
Reputation: 0