Fan power when changing baseline system per LEED District Energy rules

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I'm modeling a multifamily building that uses a campus plant for cooling.
I'm familiar with the USGBC document for modeling DES plants, and am
currently looking at Option 1, which is an energy neutral purchased chilled
water plant. Because the building is residential, the normal baseline
system is System 1-PTAC. The USGBC document says to change this to a 4-pipe
fan coil in order to make use of the purchased chilled water. Fair enough.

My problem is how to calculate fan power for this new baseline system. FCUs
are not a normal baseline system, so there's no direction in Appendix G.
Has anyone come across this before? If so, what did you do to calculate the
fan power?

For what it's worth, I see three options:
1) Since I'm replacing System 1, use the formula for System 1,
Pfan=CFMs*0.3. The resulting fan power is much lower than my proposed,
which is a bummer, though it's primarily due to 5 DOAS fans that of course
aren't in the baseline.

2) Allow TRACE to calculate the fan power using its built-in 90.1 features.
This number comes out much higher than the proposed fan power, which is
what tipped me off to this issue. I'm currently trying to back out how
Trace arrived at the fan power it did.

3) The proposed building uses fan coils. Since the new baseline also does,
set the fan powers equal and only receive credit from reduced run hours due
to improved envelope and such. This sounds like a nice way to guarantee
reviewer comments.

Any thoughts?


*Jason Kirchhoff*'s picture
Joined: 2013-04-19
Reputation: 0

I am not familiar with the Campus Option 1. I have only modeled 16 LEED Silver/Gold buildings and one Net-Zero indoor firing range. So I don't have a direct answer, but I will say what is in your head already...If it doesn't feel right, then at least minimise the worry of the USGBC reviewer. What is the most conservative approach that can be explainable and supportable? If you proposed design and baseline building has the same system Fan Coils, then it would only make sense to model them similarly, that's not to say equal, but will probably turn out to be so. Remember that your baseline building may shift by a few hours on your design day which may increase/decrease fan kWh.

So your reasoning in 1) seems the most inline with 90.1 App-G for that system. This is essentially saying that you will make up in efficiency elsewhere in your system, energy recovery on your DOAS? I'm assuming you're using a more realistic horsepower for your 5 DOAS units that the nameplate (bhp is closer to the actual max kW the fan motor is using).

Good luck

Be Sustainable -- Never let today use up tomorrow!.

Bobba_Fett's picture
Joined: 2010-10-01
Reputation: 229

Per the "Treatment of District or Campus Thermal Energy..." document, p. 10, "Any system parameters not specifically referenced in Table 3 are modeled as specified in Appendix G." I would say that applies to fan power, so your Baseline FCUs should use the System 1 fan power Pfan(w)=CFMS*0.3.

Bill Bishop's picture
Joined: 2012-02-25
Reputation: 7

I agree with Bill.

James V Dirkes II, PE, BEMP, LEED AP

James V Dirkes II, PE's picture
Joined: 2011-10-02
Reputation: 203

If the DOAS fan power is what is driving you over the top on your proposed
model, how are you dealing with outside air in the baseline? If the added
energy consumption for the FCU's handling all of the OA without heat
recovery in the baseline is not offsetting the DOAS fan power then look at
how the FCU fans in the baseline are bing controlled. I would assume since
it is a residence or dorm the FCU fans have to run 24/7 at constant volume
to handle the required ventilation and the district chilled water and
district hot water (if that is your heat source) have to expend a lot more
energy to treat the ventilation than the DOAS with heat recovery. Just a

Dennis Knight's picture
Joined: 2012-11-12
Reputation: 0

Hi, Jason.

With all due respect to Trane TRACE, I have not found their built-in PRM Fan Calculator to accurately reflect the equations and methods in ASHRAE 90.1, Appendix G, particularly related to the way they handle the 'A' fan power credit. Verify it for yourself, but I would calculate baseline fan power outside the software.


Clark Denson, PE, CEM, BEMP, LEED AP BD+C

cdenson's picture
Joined: 2011-05-17
Reputation: 1