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

[Volkan Ozdemir]

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.

G3.1.2.9.1 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, G3.1.2.10.1 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 Fans.



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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