Baseline vs. Proposed Fan Curves

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I have just revised a baseline and proposed model for LEED submission and the fan energy savings has changed dramatically as a result. The first go-round the fan energy savings was 27% between baseline to proposed. The formulas in G3.1.2.8 and G3.1.2.9 were used incorrectly though in the first round, so I had to revise the supply and return fan power calculations. The result is that they both were higher (0.001217556 KW/CFM supply and 0.001049567 KW/CFM return now). The baseline fan curve is the equation from G3.1.3.15 Method 2.

The proposed model fan power matches the actual units purchased, (0.001164 KW/CFM supply and 0.000435 KW/CFM return). This represents a 4.4% reduction in supply fan power and a 58.6% reduction in return fan power. The fan curve used for the proposed model is Variable Speed Drive FPLR.

There is a huge savings on the return side and a slight savings on the supply side, so I can visualize how a 60% overall savings between the two models is feasible, but is the sizing for the return fan under the baseline scenario accurate? I followed the G3.1.2.8 method of sizing them based on the supply flow minus the ventilation (or 90%) but the calculations still come out with a substantially higher return fan power than the proposed model. This may be perfectly acceptable, but USGBC always put the disclaimer on the review comment that "the comments are perceived to reduce projected savings". That would be the case otherwise, but since the fan energy has changed so dramatically after revising the baseline calculations that the savings actually increased from 17.99% to 21.77% solely on the fan energy. Does anyone have experience where, with an explanation of why this occurred, that USGBC will accept the revised results even though the savings is actually higher now?

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Bernie Hont's picture
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Hi Bernie,

until corrected: A single value should be determined for the baseline system
fan power (Pfan). Put another way, Pfan should not be calculated separately
for each fan. The language in Appendix G preceding the Pfan calculation
method specifies this value accounts for supply, return, relief, and exhaust
fan energies for the system (as they may occur). If you wish to explicitly
model both supply and return fan energies for your baseline system, you can
apportion the Pfan quantity/result between those kW/CFM inputs, but the
resulting sum between the two should not exceed the singular Pfan calculated
for the system.

As you touched on, kW/CFM is not the only input affecting fan energy end-use
sums. How those proposed/baseline fans operate relative to each other for
8760 hours can be a much bigger deal. It's almost always a worthwhile QC
check to state how both baseline and proposed system fans SHOULD operate
(VAV/CV? Cycling? On 24/7? Setback behavior?), then run a couple system
and/or zone hourly reports to track the flow rates simulated and confirm
whether they match your expectations. There are enough "gotchas" between
the various system types and enough potential for simple human error between
all the involved inputs to potentially trip up even very experienced
eQuest-ers on this front.

This advice may already be on your radar, but if I've presented a new
perspective it may help you shape/understand where the reviewer is coming
from in anticipating an overall performance reduction.

NICK CATON, P.E.
Senior Engineer

360 Analytics
9750 3rd Ave NE, Suite 405

Seattle, WA 98115
office: 206.557.4732 ext. 205
www.360-Analytics.com

Nick Caton's picture
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Bernie,

Nick is spot on. I would also add that you should have a justification for using a different proposed VAV fan curve than the baseline VAV fan curve. There have been some good discussions on the forums. See these for reference:
http://lists.onebuilding.org/htdig.cgi/equest-users-onebuilding.org/2012-February/034577.html
http://lists.onebuilding.org/htdig.cgi/bldg-sim-onebuilding.org/2014-February/045383.html

Regards,
Bill

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Bill Bishop's picture
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Thanks. G3.1.2.8 indicates that return/relief fans should be included if they are in the baseline design, and sized for either 90% of supply flow or supply less outside air, whichever is greater.

In my case the proposed building has two packaged VAV rooftop units with supply and return fans. The baseline system is System 6, across 5 floors. So there are 5 baseline systems (one per floor). If I understand what you are saying correctly, the sum of all baseline system supply fans should be used to calculate an overall KW/CFM which will apply to all systems. The return fans would also be sized in the same manner, less 10% or minimum outside air.

Would that be correct sizing method for fan power? I ask because that only ultimately has a negligible impact on the total fan power for the baseline system versus sizing each baseline floor individually (less than 2% overall reduction).

Bernie

Bernie Hont's picture
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You're not quite understanding what Nick is saying. The equation applies
to each system individually. A system is made up of supply, relief,
return, and exhaust fans. For each system, you calculate the value of Pfan
based on the supply fan airflow. Then you proportion that Pfan (kW or HP)
to each fan within the system (typically by CFM/total CFM, though not
explicitly stated).

So for instance, if I calculated that a system has 100 HP per the fan power
equations, based only on its Supply Fan CFM value, I might distribute that
horsepower to be 52 HP to the supply fan and 48 HP to the relief fan,
assuming that the relief fan is 90% of the airflow capacity as the supply
and that the system does not have an exhaust or return fan. You would
repeat the same procedure for each baseline system in your model.

Hope that makes sense.

Not sure how much this directly relates to your question, but I found this
ASHRAE interpretation request regarding fan power calculations to be
interesting:
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CCUQFjAB&url=https%3A%2F%2Fwww.ashrae.org%2Ffile%2520library%2Fdoclib%2Fstdsinterpretations%2Fic_90-1-2004-34.pdf&ei=0_pQVJiyOJSfyATq4oDoDQ&usg=AFQjCNFAGI3cy9agug0E7xjSnT-eadlxmw&sig2=NvN0kGk_6IluFRbuc3OOkg&bvm=bv.78597519,d.aWw&cad=rja

-Robby

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Thanks for the clarification, I got it now. The relief/return fan is not sized as a separate entity, the power for it is just portioned from the value calculated for the supply fan. That would definitely account for the vast majority of the overwhelming savings I am seeing between the baseline and proposed models with regard to fan energy.

Bernie Hont's picture
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The baseline fan allowances include all SYSTEM fans ( including supply, return, relief, exhaust). If you have special cases you can use the Fan Power Limitation Pressure Drop Adjustment in the Option 2 equation for allowable SYSTEM bhp.

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Richard-Ellison's picture
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Right!

These kinds of ?perspective shifts? are tricky to address and clearly communicate in email format, sorry for any confusion!

If you have 5 baseline systems, regardless of the # of fans involved, you should calculate 5 separate Pfan values. Those results will each cover ALL fans in the corresponding system.

Regards,

~Nick

NICK CATON, P.E.
Senior Engineer

360 Analytics
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Seattle, WA 98115
office: 206.557.4732 ext. 205
www.360-Analytics.com

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