pro-rating fan power to supply/return fans per 90.1-2007?

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App G says that if the proposed building has return fans, the baseline
building is to be modeled with them as well, sized either at the minimum
required to ensure ventilation, or 90% of the supply volume, whichever
is greater. And the fan power calculations show that the KW allowance
is for all supply, return, exhaust and relief fans combined.

How do you generally pro-rate the fan power between supply and return
fans in the baseline model? Ultimately it probably doesn't make too
much difference, but is there an actual ruling that tells you how to do


James Hansen, PE, LEED AP

James Hansen's picture
Joined: 2011-09-30
Reputation: 200

Last time I did this, I entered 0.00 kW/CFM for the return fans, and all
of the calculated kW/CFM for the supply fans - better to keep it in one
place imho to avoid the headache of explaining it. Just got news that
project wrapped up review with a silver rating this week =).

If you wanted to distribute the power regardless, I'd do it by weighting
with the airflow amounts. Generally, I don't think it matters, so long
as you can backtrack your steps if asked to explain yourself ;).


Nick-Caton's picture
Joined: 2011-09-30
Reputation: 805


This was discussed recently in the posts. See the attached response
from last Tuesday.

Mike Rosenberg commented that this is being addressed in the upcoming
90.1-2010 as such:

G3. The calculated system fan power shall be

distributed to supply, return, exhaust, and relief fans in the

same proportion as the proposed design.


Andrew Craig PE, LEED AP

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Joined: 2011-10-02
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Mike Rosenberg's section is a welcome improvement but it is still a challenging aspect.
You could be proportioning power (kW) across fans with different operation. For example:

- CAV exhaust fan vs. VFD driven supply & return fans

- "Powered exhaust" fans that only operate to provide relief during economizer mode vs. normal supply fans

In some instances, applying a bit of professional judgment will get better results in terms of fan energy (kWh) and fan heat (DT and resultant heating and cooling loads).
Paul Riemer, PE, LEED AP

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Joined: 2011-09-30
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I think your point about "proportioning power (kW) across fans with different operation" is exactly the intent of this addition to 2010 (though perhaps I'm twisting your angle on it). The intent of it is to provide consistency in the review process and undue penalty with regards to fan power. Using your example, if the Proposed design uses CAV exhaust fans and VFD driven supply & return fans, then the Baseline should as well. Some modelers (because the requirement has been unclear to date) put all of the fan kW on the Supply fan for the Baseline, which would skew the results so as to under-account for the Baseline energy/cost. Others may actually game the system and place the bulk of the allowable kW on the CAV fan and not the Baseline's SA and RA fans. It can go either way so I think this new clarity is helpful.

I wasn't sure about the last part of your comment but assumed you were referring to "professional judgment" when selecting fans in the design process. Did I miss an important modeling insight?

Paul Erickson LEED(r) AP BD+C

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Joined: 2011-09-30
Reputation: 400

Kudos to those who are improving the fan, system, and lab aspects in 90.1. And the intent of the specific section below is good, logical, and apparently needed if some modelers are putting all of the baseline power on the supply fan.

However, given all of the all of the possible model complexities, it is still not does not preclude the need for judgment. As one example, several clauses in G3.1.1 can cause a deviation from a one-to-one relationship between baseline and proposed systems. So the section cannot state " same proportion as the proposed system design." And I highly doubt the intent is "same proportion as the aggregate of all the systems in the proposed design". So it is somewhere in between and hence the modeler needs to apply some judgment.

If reviewers really care about this, then I suggest requiring separate documentation of the power and energy of the supply, return, relief, and exhaust for the building or even for each proposed and baseline system.

Paul Riemer, PE, LEED AP

Paul Riemer's picture
Joined: 2011-09-30
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I do appreciate the heads up - I've been reading the ongoing discussion
with interest!

In the particular case I referenced, it didn't seem a big deal to me at
the time as to my understanding my baseline supply and return fans were
operating in tandem, but as I think it through now, I see this might
skew the results somewhat with regard to fan energies in the airstreams.

This has got me wondering: What, if anything, does 90.1 or similar have
to say regarding fan heat in the baseline airstreams?

I've developed a practice in eQuest of intentionally specifying baseline
fan motors as outside the airstreams. I've done this specifically to
not have their heat incident on the airstreams, because the procedure I
follow for calculating baseline efficiencies (backing out the calculated
fan energy, Pfan, from the prescribed efficiencies), can become a
potentially cumbersome/iterative process otherwise. For example, a
simulated baseline sizing run gives an airflow/capacity, from which I
calculate Pfan, which once entered and simulated affects the calculated
capacity, which affects the airflow, so I have to recalculate Pfan...
and so forth until the reported figures match my calculations. For one
system this is cake, but when I've got a bunch of baseline systems it's
quite a pain.

I recognize some others follow a baseline efficiency procedure that
don't involve the Pfan for baseline heating/cooling efficiency
calculations, for whom baseline fan heat energy is a moot concern, but
for the moment that's really another can of worms I'd rather not open...
I'd specifically like to hear if anyone knows of what guidance is or
isn't out there for whether and to what extent baseline fan heat should
be incident on baseline airstreams.

Thanks again to everyone for providing such continuously supportive and
productive discussions - it makes me proud to be a part of this


Nick-Caton's picture
Joined: 2011-09-30
Reputation: 805

Dear Nick et al,

90.1 does not specify how to split up the fan heat when there is more than
one fan (although I haven't read the 2010 version yet). My goal is to model
something close to reality, so I split supply and return fan power in rough
proportion to their total static pressure in the Proposed system.

If the (proposed) supply fan moves air against 4" TSP and the return fan
against 1" TSP, I allot 80% of the baseline power to the supply fan, 20% to
the return fan. I don't try to be super precise, just reasonably close.
This requires a little more art than science if you have a VAV supply fan
and, say, a constant volume relief fan.

On a related note, I always model Baseline systems with draw-through fans

. ASHRAE is silent on the matter and

. The extra fan heat in the airstream results in a little more
airflow and makes it more favorable for the Proposed system J

. Let me know if anyone thinks that's cheating!

I use EnergyPlus exclusively, but I suspect that it should work for any
software to do the following as a one-shot
get-the-baseline-fan-power-correct approach:

. Knowing that fan power is always related to CFM

. Also knowing (or assuming) fan and motor efficiency

. And knowing any pressure correction

. I calculate the system delta P that will result in the desired fan

. Then I use that delta P, and those efficiencies as my baseline
input values

. Fan power comes out right on the money every time (with

For example, using a System 3 or 4:

CFM * .00094 = allowable BHP (assuming no pressure correction)

Delta P = BHP * (fan eff'y * motor eff'y) * 6356 / CFM

= (CFM * .00094) * (fan eff'y * motor eff'y) * 6356 / CFM

= .00094 * (fan eff'y * motor eff'y) * 6356

= 5.9746 * (fan eff'y * motor eff'y)

Note that CFM is not relevant to this calculation, making it very handy for
autosizing! Also note that I combine the fan and motor efficiencies because
E+ uses their product as the single "fan efficiency" input, rather than
keeping fan mechanical and motor efficiencies separate. Your software may
not do that.

James V. Dirkes II, P.E., LEED AP

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