# Appendix G Fan Power

G3.1.2.9 Gives you a formula for calculating system fan electrical power
for supply, return, exhaust and relief. Lets sat I have a zone with a
PSZ-HP that has a supply volume of 1500 cfm. Using this formula along
with table G3.1.2.9 I get a fan horse power of 1.29 and kW of 1.17. This
equates to 0.00078 kW/cfm - which seems really high.

The second part to my question is assuming I have 500cfm of exhaust in
this zone, do I apply the same kW/cfm to the exhaust fans as well as the
return fans? It seems that the lower the fan volume the more inefficient
the Appendix G values get from these tables.

Any guidance would be much appreciated.

Vikram Sami, LEED AP

You have calculated the kW/cfm correctly according to the 2004 version
of Appendix G. The allowance actually goes up a little in 2007, for your
case. As for the second part of your question, no you do not apply the
same kW/cfm to the exhaust fan. The calculated allowed fan power the
total allowed power for all fans that are part of the system, including
the exhaust fan. The cfm component is based on the supply fan cfm only.
Unfortunately, Appendix G gives no direction on how to divide the
allowed fan power up among the various fans in the system.

Michael Rosenberg

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At my firm, we proportion the baseline total fan power determined as an
aggregated W/CFM based on the hp assigned to the fan motors (supply,
return, exhaust) in the design documents. Of course this assumes that
correspondence.
Ellen

Ellen Franconi, Ph.D., LEED AP
Senior Energy Analyst
Architectural Energy Corporation
2540 Frontier Avenue
Boulder, CO 80301
tel. 303-444-4149
fax 303-444-4303
efranconi at archenergy.com
http://www.archenergy.com/

You have calculated the kW/cfm correctly according to the 2004 version
of Appendix G. The allowance actually goes up a little in 2007, for your
case. As for the second part of your question, no you do not apply the
same kW/cfm to the exhaust fan. The calculated allowed fan power the
total allowed power for all fans that are part of the system, including
the exhaust fan. The cfm component is based on the supply fan cfm only.
Unfortunately, Appendix G gives no direction on how to divide the
allowed fan power up among the various fans in the system.

Michael Rosenberg

From:bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Vikram
Sami
Sent: Wednesday, July 01, 2009 3:03 PM
To: Bldg-Sim
Subject: [Bldg-sim] Appendix G Fan Power

G3.1.2.9 Gives you a formula for calculating system fan electrical
power for supply, return, exhaust and relief. Lets sat I have a zone
with a PSZ-HP that has a supply volume of 1500 cfm. Using this formula
along with table G3.1.2.9 I get a fan horse power of 1.29 and kW of
1.17. This equates to 0.00078 kW/cfm ? which seems really high.

The second part to my question is assuming I have 500cfm of exhaust in
this zone, do I apply the same kW/cfm to the exhaust fans as well as the
return fans? It seems that the lower the fan volume the more inefficient
the Appendix G values get from these tables.

Any guidance would be much appreciated.

Vikram Sami,LEED AP

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Reputation: 0

This issue will start to become a problem for anyone running energy
models for LEED projects - reviewers on the new review teams are asking
to see the total fan power for the baseline broken into individual
components and reported as such in the template for Energy and
Atmosphere Credit 1. The 90.1-2004 User's Manual is also silent on the
issue; however it raises another issue. In the example (Example G-J on
page G-28) the calculations use total building supply CFM for an
80,000SF medical office building. Since the baseline is System 5 in
that example and there would be multiple zones for the baseline, it
appears that the equation should be used for the whole building only and
not individual systems. Is this how you are applying the formulas? The
user's manual notes that fan powered VAV boxes are NOT included in the
total fan power number as calculated from G3.1.2.9.

Jeremy R. Poling, LEED AP

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The equation is meant for each HVAC system. The example in the User's
manual is probably not a very good one. If it was an 80,000 sf single
story building it would only have one baseline Packaged VAV system and
the example would be correct. If it were a multi-story building, there
would be one baseline Packaged VAV system for each floor and the example
would be incorrect.

The User's manual is correct that the power of the parallel fan-powered
terminal units in systems 6 and 8 is not included in the calculation.
The power of those fans is specifically called out in G3.1.3.14 at 0.35
W/cfm.

Michael Rosenberg

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Interesting to hear the request for fan energy delineation in the template.

The old ECB had a largely one to one relationship and Ellen's approach worked.

Appendix G is no longer one to one and that is key to understand.
Section G3.1.1 in its varied & modified versions prescribes single zone or multi-zone system types and provides constraints on system delineation.

So determine the number, types, sizes of the systems in the baseline model then go back and solve the fan power for each.

Paul Riemer

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We got a review comment on a recent LEED project asking us to break down the baseline HVAC power into components even though there is no direction, or requirement, from 90.1-2004 to do so.

LEED guidelines say we have to model per Appendix G, but Appendix G doesn't say we have to do this.

I do not think it is necessary to do this as the baseline total fan energy consumption numbers would still be the same.

The equation in Appendix G is meant to give a total allowance (for each system) for supply, return, and exhaust fans. It doesn't matter if we break the allowance down into components or not as the total fan energy consumption remains the same. Therefore, there is no value in doing this.

If we did break the fan energy numbers into components, how would we do that given that there is no direction from ASHRAE or LEED on how to do this?

Thoughts?

Regards,

James A. Hess, PE, CEM

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Hi James,

As far as I?m concerned the main point of having project teams break the fan energy down into components is to ensure that the baseline fan power derived from the fan supply volume using Table G3.1.2.9 is not all attributed to supply fans if exhaust fans, return fans, etc. exist.

Regarding the breakdown, as far as I know you?re correct in that there is no guidance on that and what I do is take the percentages based on total fan power from the proposed case and apply it to the baseline case (e.g. 70% supply, 20% return, 10% exhaust).

Best Regards,

Gregg Liddick, EIT, LEED? AP

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I think a lot of us were making the mistake of applying the total calculated
fan power/cfm to both the supply and return/exhaust fans (mostly due to
ambiguous wording in 90.1). Forcing the modeler to explicitly divide up the
calculated fan power allows the reviewer to more easily check that the
energy was properly accounted for.

I played around on one project and found that unsurprisingly the fan energy
didn?t change whether I put it all in supply air and none in exhaust, or
divided it up based on the relative CFMs.

Nathan Miller

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Gregg,

I appreciate the response.

I see what they are trying to do; I just don?t think it needs to be done. The total fan energy consumption (supply, return, exhaust) is the same for comparison purposes to the proposed design model.

On many projects, the baseline system is different than the proposed --> packaged single zone systems versus packaged VAV RTU, for example. Therefore, there would not be any return fans on the packaged single zone systems to allocate fan energy to. I don?t know if anyone is looking at this to that level of detail. Another separate issue is that the Appendix G fan energy equation way over estimates the fan energy associated with real packaged single zone systems, but I?ve posted previously on that issue and won?t bore anyone with those details.

I guess the bottom line is that if GBCI absolutely insists that the baseline fan energy be broken into components, we?ll come up with some kind of percentage allocation, similar to what you have done, as applied to the total baseline fan energy consumption for the sole purpose of filling out the template.

Thanks! ?

Regards,

James A. Hess, PE, CEM

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Hi James,
I actually feel the baseline model fan energy is too small. Baseline model fan usually gives 0.00078KW/cfm. For proposed fans, typically it is 0.00016-0.00018 KW/cfm/in static. With cooling coil at 1 inch, mid life filter at 1.5 (30% and 85%), energy wheel at 0.8, external static of 1.8 inch, you will get .00081 KW/CFM. And the return fan and general exhaust fans haven?t been included yet.

Do you typically have less pressure drop than listed for the system? Shall we use the clean filter or midlife filter for pressure calc?

Ming Zeng, PE, LEED AP

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Only you could say whether these factors were significant in your own project.

It could be possible that if you allocated all of the power to the supply fan, which might be variable flow with a VFD for example, and operate at part-load conditions much of the time, the energy consumption might be different than if an exhaust fan was included that runs continuously at fixed load with some allocation of the power usage.

Also the program may have some variances in heat gain to the airflows from the motor heat.

I?ll second Ellen Franconi?s suggestion of apportioning by design document sizing when there are equivalent pieces of equipment in both models.

David

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I may be a bit late in responding and haven't gone through all the e-mails about it. That being said,
I think it makes a difference as to how it's divided up. The difference isn't in fan consumption, but where the fan heat is dumped. That will effect the cooling and heating loads. For example, if all the energy were in the supply fan, that would increase supply temperature of require the temperature off the coil to be lowered. If it was all in the return fan, that fan energy would just end up increasing the return temperature. I'm thinking more how that difference affects the economizer cycle and ventilation air. I'd proportion it out based on the relative HP's of your supply, return and exhaust fans. Someone had a 70-20-10 split, which seems to be a good stab at it..

Michael Williamson

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The Fan Power Allowance in ASHRAE 90.1 is based on "good" engineering
practice. I have attached a .pdf showing the System Pressure Drops used in
determining the allowances for both the VAV fan system and the CV fan
system. The fan efficiency assumed in both cases is 65% and air velocity is
assumed to be 400 fpm through the coils and filters. Filter pressure drop
assumes dirty filters. The standard allows additional credits for systems
with heat recovery, high performance filtration, lower design supply air
temperatures and the use of relief fans vs return fans.

Also, addendum ac of ASHRAE 90.1-2004 modified the fan power calculation
methodology allowing an expanded list of allowances that better suits a
wider range of building types including hospitals and lab buildings. Those
changes are incorporated into Standard 90.1-2007.

I think that Michael Williamson gave the best reason yet why it is important
to split the baseline fan power among the supply, return and exhaust fan
components. Like many others I distribute baseline fan hp using the same
ratios as the proposed design even though the baseline system configurations
may be slightly different.

Finally, three cheers to GBCI for putting review teams together that will
actually do a decent job of ensuring compliance with Standard 90.1 rather
than just taking the design teams word for it. Maybe folks will begin to
realize that exceeding 90.1-2004 by 20-30% is actually a challenge and
building designs will begin to reflect the changes necessary to meet that
challenge.

Regards,

Michael Tillou, PE

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This is good discussion on this topic. I appreciate everyone's responses, and I'll a bit more to the fire :)

My overall thinking is that I don't think breaking up the fan power adds any "significant" value to the overall process. Yes, it obviously does make a difference but with limited time, resources, and (more importantly) available fees (in my experience most clients want to pay very little for LEED energy modeling) the question we should be asking --> is this of significant importance that justifies us spending time on it? And the part about GBCI putting together review teams that enforce compliance with 90.1, that's great. I am 100% for that. But splitting up the baseline fan power isn't actually a requirement of 90.1 Appendix G so I do not know what they are enforcing here --> their own opinion of how we should be doing the modeling. If they want to see the fan power split up, they need to lobby ASHRAE to get Appendix G changed so that we have more appropriate fan baseline numbers that are broken into components. I think that would be the best approach.

I think we've agreed that splitting up the baseline fan energy into components does not make a difference with regards to overall fan energy consumption. Now whether the fan energy is dumped in the supply or return, the fan energy still has to be removed by the cooling system or outside air via economizers? I don't see how splitting up the fan energy makes a difference with regards to cooling energy consumption. I would say that's true for most applications. For lab applications, I can see how there may be a difference because the lab exhaust fans could be significant. For the vast majority of applications, exhaust fans are fractional horsepower motors (i.e. << 1 HP), therefore very insignificant. Regarding what I was saying earlier about Appendix G and overestimating fan power, this was for the small packaged single zone or split DX systems (i.e. system 3 & 4), which are probably the baseline systems on the majority of LEED projects in the US, hence the significance (may change somewhat now that the requirement for PVAV has shifted from 75000 SF to 25000 SF). The fan power equations in Appendix G used to overshoot power for the smaller systems, but undershoot for the more intense systems like labs. Now, the changes in 2007 version better account for labs, but the problem on the low end has not been fixed (could be in the works). This is because the equations assume 3" to 5" pressure drop while small systems have pressure drops on the order of 1" to 1.25". The example I use is my house --> a 3 ton/1200 CFM unit. Per the equations in Appendix G, my system is estimated to use ~ 946 watts of fan power. When I put my Fluke 39 meter on it and actually measure the power --> 550 watts, or 42% less. I believe that ARI SEER calcs would assume ~ 365 watts/KCFM or 438 watts total (I wonder if they are assuming the power based on the heating airflow because interestingly, my fan does use 440 watts in heating mode). When we do a LEED project and assume PSZ systems for each zone (i.e. proposed design is VAV for example), we show ridiculous amounts of fan energy savings, which are really fake because the fan energy for real PSZ systems would be much less than the Appendix G equations show.

If we want a real challenge in exceeding 90.1 for energy savings --> let's get Appendix G changed to reflect more appropriate fan energy benchmarks for the smaller PSZ systems. This could make a significant difference on LEED projects because meeting LEED requirements will be much tougher to meet. I have personally seen projects that would not meet the LEED minimum 14% savings requirement were it not for the combination of artificially high baseline fan energy values along with a very generous allowance of exterior/site lighting power. These changes could be of significant value to our profession and make our calculated savings numbers more legitimate. That's the kind of stuff I think GBCI and our profession should focus on (versus worrying about breaking baseline fan energy numbers into components).

That's my two cents.

Have a great 4th weekend everyone.

Regards,

James A. Hess, PE, CEM

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