[Bldg-sim] ASHRAE 90.1G Exhaust Fan modeling problems

Nicholas Caton ncaton at catonenergy.com
Wed May 20 04:19:54 PDT 2015

A few thoughts:

-          Distributing power between baseline fans in proportion to the
proposed should probably reference proposed fan powers, not flows

-          If you are going to have your exhaust fan operate on VSD and
track the supply, then intuitively the only VSD fan curve provided by 90.1
is as cited under, and I would use the same for each system fan.
I should think you could also logically specify a distinct system exhaust
operation/flow schedule citing & matching the proposed design.

-          A general caution on “gaming” the system:  There are a myriad of
subtle, clever, and diabolical ways one could subvert the intent/purpose of
Appendix G and its ilk… but then what’s the point?   Bruce Lee comes to
mind:  “Obey the principles without being bound by them.”  It’s never a
good feeling to look back on a bunch of hours you’ve invested developing a
simulation that doesn’t really represent anything.

-          #2: I think you are trying to read new meanings into
G3.…?  Generally for LEED projects, I routinely handle garage
ventilation, local/centralized restroom exhaust, and similar standalone
ventilation systems operating independently of space conditioning systems
as “process” ventilation (indicating the associated energy consumptions as
such on the EAp2 template).  Essentially those exhaust/ventilation systems
are defined in the proposed model as actually occurring, then the baseline
model is made to match the proposed case.  I’ll often include additional
sub-metering (eQuest) so that I can further demonstrate/document the claim
they’ve been made to match at submission.

Hope that helps!



*Caton Energy Consulting*
  1150 N. 192nd St., #4-202

  Shoreline, WA 98133
  office:  785.410.3317


*From:* Bldg-sim [mailto:bldg-sim-bounces at lists.onebuilding.org] *On Behalf
Of *Rathna Shree
*Sent:* Wednesday, May 20, 2015 12:12 AM
*To:* Volkan Ozdemir; bldg-sim at lists.onebuilding.org
*Subject:* Re: [Bldg-sim] ASHRAE 90.1G Exhaust Fan modeling problems

Hi Volkan,

I can answer your second question. Exhaust fans in unconditioned zones
should be modeled identical in base and proposed case.



On Monday, 18 May 2015 6:22 PM, Volkan Ozdemir <Volkan.Ozdemir at rencons.com>

Dear All,

I got two questions about exhaust fan modeling according to ASHRAE
90.1G-2010, clarifications from experience people would be great;

1-      Proposed system has an exhaust fan, hence I`ll model exhaust fan
for 7. VAV with Reheat system as well.

G3. says that I should model exhaust fan size as 90% of supply fan.
Fair enough, peak flow rate from exhaust will be 90% of VAV peak supply, so
there is positive pressure in baseline design zones.

Further, G3. says that peak fan power should be split between
supply and exhaust in the same proportion as proposed design. For example,
my proposed design got 1000 CFM for supply and 1000 CFM for exhaust, so
I`ll split “Peak Fan Power Consumption” 50% on baseline design as well.

Finally I`ll model VAV *Supply* Fan curve as mentioned in G3.1.3.15.

However, here is my real question: What about efficiency curve of the
exhaust fan? Appendix G doesn`t say anything about this issue. It only says
that (G3.1.2.1) efficiency of HVAC equipment should be modeled as minimum
as shown on section 6.4. Section 6.4 doesn`t say anything about Exhaust

So that means I can arrange efficiency curve of the exhaust fan in baseline
however I want? This kinda opens up the possibility of gaming the system,
one can see where exhaust fan will usually work and set the efficiency very
low at that point of the curve, hence higher kWh consumption for the
baseline system.

Could someone experience clarify what should be the efficiency curve of
Exhaust Fans according to ASHRAE 90.1G 2010 edition?

2-       As far as I understood, ASHRAE 90.1G says that exhaust fans
shouldn`t be modeled for spaces where no supply fan exists. Or better put,
if HVAC system serving the zone has no supply fan, then you cannot model
exhaust fan.

This seems extremely unfair for large scale projects where garage exhaust
fans, toilet exhaust fans are considerably large.

Has any of you tried to model garage and toilet fans in your baseline model
and pass through LEED review? What is the best way to follow if your
proposed system got garage exhaust and toilet exhaust fans? I really want
to model those in baseline as well because it will affect my energy
efficiency rating.

Kind regards,

*Ismail Volkan Ozdemir*

*Mechanical Engineer, LEED AP BD+C*


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