# System Fan Power in Appendix G

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Hi All,

I have a specific question about using Appendix G3.1.2.9 to calculate
baseline system fan power (I have already searched previous posts and
could not find the answer I'm looking for). So, the Appendix G section
reads as follows: System fan electrical power for supply, return,
exhaust, and relief (excluding power to fan powered VAV boxes) shall be
calculated using the following formulas. I understand the formulas,
which essentially use a fan CFM to determine a brake horsepower BHP,
which is then used to calculate the fan power (Watts), but my question
is this: Supposing you have an HVAC system with supply, return, and
small exhaust fans (in restrooms, for example) - how do you calculate
the power to each fan?

Option 1: Use your supply fan cfm to calculate a power, say it comes to
10 kW, and use this as your total "system" fan power. This 10 kW, being
your whole system power, is then distributed among all of your system
fans however you desire (For example, supply fan power = 5 kW, return
fan power = 4 kW, exhaust fan power = 1 kW, for a total of 10 kW for the
fan system)

OR

Option 2: Use each supply, return, and exhaust fan cfm to separately
calculate an individual kW for each fan (For example, supply fan power =
10 kW using the supply cfm, return fan power = 9 kW using the return
cfm, exhaust fan power = 1 kW using the exhaust cfm, for a total of 20
kW for the whole fan system)

Obviously, you will have much more fan energy in a baseline system
calculating fan power with Option 2. Does anyone have a specific
example they can share of a fan system calculation that was accepted by
USGBC for an EAc1 energy model?

Thanks for your help!

Mike Kaufman

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My understanding is option 1. The fan energy in that formula is the sum of all the fan horse power. Search the board, I think there is a previous email from Michael Rosenberg talking about the fan energy. Hope this helps.

Ming Zeng, PE, LEED(r) AP

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Option 1 ? the user?s manuals are a bit more clear, and specify that the its
is the supply cfm that should be used to calculate the fan power. This fan
power includes supply, return and exhaust.

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Make sure you look at Addendum AC if you are using A90.1-2004. The calculation and pressure credits have changed, but it is still consistent with your Option 1 description.

Bill Talbert, PE, LEED? AP

Option 1 ? the user?s manuals are a bit more clear, and specify that the its is the supply cfm that should be used to calculate the fan power. This fan power includes supply, return and exhaust.

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Section G3.1.2.8 says "if return or relief fans are specified in the
proposed design, the baseline design shall also be modeled with fans
serving the same function and sized for the baseline system supply fan
air quantity less then minimum outdoor air, or 90% of the supply fan air
quantity, whichever is larger."

This would lead me to believe that you need to specifically model
return/exhaust/relief fans the same as supply fans and not group
everything together. Section G3.1.2.9 is written poorly because it
calls out "supply CFM," but should be applicable to all types of fans.

Sam Mason

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One quick comment is that eQuest cannot currently model relief air
fans. This would be a nice feature to add, but for now you would have
to approximate separately in a spreadsheet, and then add to building
electric meter as a direct load.

Regards,

James A. Hess, PE, CEM

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Sam,

Thanks for the response. So, you are suggesting Option 2 is
appropriate. Have you had this method accepted by USGBC? I feel that
Option 1 is more consistent with the intentions of Appendix G, because
every time I use Option 2, I get unreasonably large fan energy savings.
I do agree that the section is written poorly, and the wording can be
interpreted either way, but I wouldn't want to overestimate fan savings
and later have this decision questioned/rejected. Also, I interpret
section G3.1.2.8 to specify the required cfm of the return and exhaust
fans, not the power to each fan.

Thoughts?

Mike

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Mike,
Option 1 does seem to be the correct way to model. However, for a few
lab buildings following this method, the fans used almost twice as much
energy in the proposed model as the baseline, so option 2 was used.
Also, if you bunch all of the fan energy into the supply, would it be
possible to model 24/7 constant volume supply or exhaust (fume hoods or
clean rooms) on the same system?

Sam Mason

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Sam,

Option 1 is the correct way to model. It is painful for a lab building, yes. Designs are often 50-100% higher in horsepower than what is allowable per Appendix G. It's very important to help the design team understand this and emphasize the importance of finding ways to reduce the fan bhp.

Paul Erickson

Mike,
Option 1 does seem to be the correct way to model. However, for a few
lab buildings following this method, the fans used almost twice as much
energy in the proposed model as the baseline, so option 2 was used.
Also, if you bunch all of the fan energy into the supply, would it be
possible to model 24/7 constant volume supply or exhaust (fume hoods or
clean rooms) on the same system?

Sam Mason

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I agree with you, Sam. The supply, return and relief fans need to be modeled
separately, which I believe
fits the intent of comparing the efficiencies of the fans between "Proposed"
and "Baseline" and also
informs you where to improve for your "Proposed" design in terms of fan
power consumption.

George Hu, PE, LEED AP

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George and Sam,
I may just misunderstand what you are saying, but I suspect you may not
be reaching a correct conclusion. Whether the fans are modeled
separately or together, the baseline fan power allowance covers all the
fans in the system (I divide this among the various fans based on the
proposed system fan power ratios of supply to return, etc).

As Bill Talbert mentioned earlier in this thread, look at Addendum ac to
90.1-2004. Specifically read through the definitions of "fan system bhp"
or "fan system motor nameplate hp" therein which should clarify that the
hp allowance values are for "all fans that are required to operate at
design conditions to supply air from the heating or cooling source to
the conditioned spaces and return it to the source or exhaust it to the
outdoors".

I hope this helps to clarify things.

Kind Regards,

Molly Curtz, P.E.

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Molly,
Thanks for your input. I agree with your interpretation of the code.
Supply fan CFM should be used, with applicable pressure credits, to
determine the entire baseline HVAC system fan brake HP, which is then
split amongst the supply/return/exhaust fans. My interpretation was
trying to get at lab building modeling which in some cases have separate
AHUs for return systems.

Another question to everyone then: How do you deal with labs which have
extremely high fan HP to overcome the large static pressure typical of
this building type? This has been a topic of conversation before but
would like to know from experience what the calculations have shown.

My example: a current lab building has several large AHU providing
80,000 CFM each with 200 HP fan motors. The return systems are the same
size, so each AHU has a total of 400 HP of fan power. Following
90.1-2007 and using total static pressure credits of 9.5 in. wc, the
baseline system fan power is only 288 HP. There is no way to meet code
and overall the proposed building is barely saving any energy over the
baseline building.

Sam Mason

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Sam,
Addendum ac includes an exception for fans exhausting air from fume
hoods (6.5.3.1.1 exception c). A LEED CIR submitted on 6/15/2007, ruled
on 8/13/2007 allows use of the Addendum ac fan power calculation
approach, including the fume hood exemption. Does this help for your
application?

Kind Regards,

Molly Curtz, P.E.

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I would submit (without knowing anything about the project, of course)
that `There is no way to meet code' simply isn't true. Perhaps no way to
meet it economically - which rests on the definition of `economical'.

There are many strategies that can be employed to reduce system pressure
drop, even in demanding lab situations. Duct sizing, control losses
(selection of the pressurization control valves in the supply/exhaust),
coils, filter and system effects, fan efficiencies, duct run
(distributed systems), etc. There is no one silver bullet but a lot of
little things add up quickly.

We often see the same results - as-designed fan power in excess of App G
calculations but when you look into it, it's because the designer uses
the same approaches they've been using for the last ten years. If you
want efficient systems, you have to put more effort into the design, and
probably more money into the construction.

I'm sure I'll piss off a few designers saying this, but complaining that
you can't meet code is a cop out.

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I would submit (without knowing anything about the project, of course)
that `There is no way to meet code' simply isn't true. Perhaps no way to
meet it economically - which rests on the definition of `economical'.

There are many strategies that can be employed to reduce system pressure
drop, even in demanding lab situations. Duct sizing, control losses
(selection of the pressurization control valves in the supply/exhaust),
coils, filter and system effects, fan efficiencies, duct run
(distributed systems), etc. There is no one silver bullet but a lot of
little things add up quickly.

We often see the same results - as-designed fan power in excess of App G
calculations but when you look into it, it's because the designer uses
the same approaches they've been using for the last ten years. If you
want efficient systems, you have to put more effort into the design, and
probably more money into the construction.

I'm sure I'll upset a few designers saying this, but complaining that
you can't meet code is a cop out.

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Joined: 2011-09-30
Reputation: 0

This is a very interesting discussion, and I believe I'll agree with Mark.
An overall summary of my position is that I find it interesting that folks
are saying the code is still difficult to meet, because the code (90.1) was
recently significantly overhauled to provide more allowance for lab systems.

I would say that 400 HP for 80KCFM, or 5 BHP/KCFM is quite a bit of fan
power, either under the older 2004 standard or the newer 2007 standard.

For comparison purposes, in the original ASHRAE 90.1-2004 standard, the base
allowance was 1.2 BHP/KCFM & 1.1 BHP/KCFM for constant volume systems (< 20 KCFM & >= 20 KCFM), and allowances of 1.7 BHP/KCFM and 1.5 BHP/KCFM for
variable volume systems (< 20 KCFM & >= 20 KCFM ).

Then, you could add to the base BHP/KCFM number with some credits. However,
some folks noted that the credits were not sufficient to account for real lab
HVAC systems. Therefore, the folks with the Labs21 program
(www.labs21century.gov) figured out what could be appropriate and published a
draft modeling guideline to modify Appendix G of ASHRAE 90.1-2004
specifically for labs. This document (ashrae_appg_draft_508.pdf) is
available at www.labs21century.gov/pdf . This is a great document for energy
modelers to better understand the demanding requirements of laboratory
systems from a fan energy standpoint. The document basically said, among
other things, that conventional commercial AHU's are designed for ~ 5" of
total static (4" supply, 1" exhaust) while a more realistic value for lab
AHU's is 9.15 inches (5.79" supply, 3.36" exhaust). Appendix A of the
document covers very well the assumptions going into those numbers. I would
say that based on lab systems we have designed, the 9.15 inches is a
realistic number for establishing a baseline fan power value.

The document from Labs21 recommends an allowance of 2.2 BHP/KCFM & 2.0
BHP/KCFM for constant volume systems ( < 20 KCFM & >= 20 KCFM ), and an
allowance of 3.1 BHP/KCFM and 2.8 BHP/KCFM for variable volume systems ( < 20 KCFM & >= 20 KCFM).

I don't know for sure (since I have no affiliation with the Labs21 program)
but suspect that the Labs21 guideline helped to form the basis for a bunch of
changes that eventually made their way into the 2007 version of the ASHRAE
90.1 standard, regarding the fan power limitation and fan power limitation
pressure drop adjustment. The fan power limitation calculations in the 2007
version have pretty much been completely overhauled as compared to the 2004
version. That's a good thing (though I would argue more work needs to be
done for the smaller PSZ systems, more to follow below).

That being said, if you use the 80 KCFM and assume the 9.5 inch total
pressure credit, you get 288 BHP (as Sam mentions) using the new formulas in
table 6.5.3.1.1B of 90.1-2007. This amounts to 3.6 BHP/KCFM. Therefore,
what made it into the final 2007 standard seems to be even higher than what
the Labs21 folks recommended in their draft paper.

I guess my overall question, to anybody who read down this far :) , is
whether people think the new 90.1-2007 fan power limitations are sufficient
to cover labs. It seems like they should be to me, but would love to get
feedback from any designers out there.

Switching gears a bit, I think where more work needs to be done to the 90.1
standard regarding fan power is the fan power calculation/limitation for the
smaller PSZ-AC & PSZ-HP systems (Appendix G, table G3.1.1A & 1B). Why is
this important --> because these system types are used for the baseline
buildings in a lot of LEED projects, and the equations used for estimating
the fan power (watts) actually go the other direction and dramatically
overestimate the fan power requirements, which in my opinion which results in
questionable fan energy savings. This happened with the 90.1-2004 standard
and doesn't appear to be corrected in the 2007 version. I'll provide an
example, let's say you have a 3 ton unit @ standard conditions, or 400
CFM/ton (of which I happen to have at my house). The ARI SEER rating
standards would assume 365 watts/1000 CFM, or 438 watts total. Using the
equations in Appendix G of 90.1-2007 ( 1200 X 0.00094*0.746/0.80), assuming
80% motor efficiency (approximate for small single phase motor), you come up
with 1026 watts, which is very interesting. Curiosity got the best of me and
so I put my Fluke 39 power meter on the circuit and actually measured the fan
power --> I get approximately 550 watts. The ARI assumption may be too low,
but the Appendix G assumption seems way too high. If we are using the
Appendix G fan power values for small systems on our projects, this appears
to show that our projects are saving lots and lots of fan power. But, that's
really not true because all we are doing is comparing our proposed design fan
power to a really bad baseline, one that is greatly exaggerated in the first
place. That's crazy, and in my opinion diminishes the validity of the
savings calcs because it shows savings on paper, but not real savings. If
people want to get serious about energy savings, I think we need a baseline
that holds us to a higher standard. Similar to the lab systems, we need
realistic baseline fan power values for small systems, except heading the
other direction (i.e. decreasing). If this can be changed, savings will be
more difficult to obtain, but they will be more real and therefore valid.

I've been wanting to bring this up to the 90.1 committee at ASHRAE. Haven't
found the time yet, but will.

Any thoughts on this topic would be appreciated as well.

Appreciate any additional feedback, and sorry for the really long email.

Regards,

James A.? Hess, PE, CEM

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Reputation: 601

This is a very interesting discussion, and I believe I'll agree with Mark.
An overall summary of my position is that I find it interesting that folks
are saying the code is still difficult to meet, because the code (90.1) was
recently significantly overhauled to provide more allowance for lab systems.

I would say that 400 HP for 80KCFM, or 5 BHP/KCFM is quite a bit of fan
power, either under the older 2004 standard or the newer 2007 standard.

For comparison purposes, in the original ASHRAE 90.1-2004 standard, the base
allowance was 1.2 BHP/KCFM & 1.1 BHP/KCFM for constant volume systems (< 20 KCFM & >= 20 KCFM), and allowances of 1.7 BHP/KCFM and 1.5 BHP/KCFM for
variable volume systems (< 20 KCFM & >= 20 KCFM ).

Then, you could add to the base BHP/KCFM number with some credits. However,
some folks noted that the credits were not sufficient to account for real lab
HVAC systems. Therefore, the folks with the Labs21 program
(www.labs21century.gov) figured out what could be appropriate and published a
draft modeling guideline to modify Appendix G of ASHRAE 90.1-2004
specifically for labs. This document (ashrae_appg_draft_508.pdf) is
available at www.labs21century.gov/pdf . This is a great document for energy
modelers to better understand the demanding requirements of laboratory
systems from a fan energy standpoint. The document basically said, among
other things, that conventional commercial AHU's are designed for ~ 5" of
total static (4" supply, 1" exhaust) while a more realistic value for lab
AHU's is 9.15 inches (5.79" supply, 3.36" exhaust). Appendix A of the
document covers very well the assumptions going into those numbers. I would
say that based on lab systems we have designed, the 9.15 inches is a
realistic number for establishing a baseline fan power value.

The document from Labs21 recommends an allowance of 2.2 BHP/KCFM & 2.0
BHP/KCFM for constant volume systems ( < 20 KCFM & >= 20 KCFM ), and an
allowance of 3.1 BHP/KCFM and 2.8 BHP/KCFM for variable volume systems ( < 20 KCFM & >= 20 KCFM).

I don't know for sure (since I have no affiliation with the Labs21 program)
but suspect that the Labs21 guideline helped to form the basis for a bunch of
changes that eventually made their way into the 2007 version of the ASHRAE
90.1 standard, regarding the fan power limitation and fan power limitation
pressure drop adjustment. The fan power limitation calculations in the 2007
version have pretty much been completely overhauled as compared to the 2004
version. That's a good thing (though I would argue more work needs to be
done for the smaller PSZ systems, more to follow below).

That being said, if you use the 80 KCFM and assume the 9.5 inch total
pressure credit, you get 288 BHP (as Sam mentions) using the new formulas in
table 6.5.3.1.1B of 90.1-2007. This amounts to 3.6 BHP/KCFM. Therefore,
what made it into the final 2007 standard seems to be even higher than what
the Labs21 folks recommended in their draft paper.

I guess my overall question, to anybody who read down this far :) , is
whether people think the new 90.1-2007 fan power limitations are sufficient
to cover labs. It seems like they should be to me, but would love to get
feedback from any designers out there.

Switching gears a bit, I think where more work needs to be done to the 90.1
standard regarding fan power is the fan power calculation/limitation for the
smaller PSZ-AC & PSZ-HP systems (Appendix G, table G3.1.1A & 1B). Why is
this important --> because these system types are used for the baseline
buildings in a lot of LEED projects, and the equations used for estimating
the fan power (watts) actually go the other direction and dramatically
overestimate the fan power requirements, which in my opinion which results in
questionable fan energy savings. This happened with the 90.1-2004 standard
and doesn't appear to be corrected in the 2007 version. I'll provide an
example, let's say you have a 3 ton unit @ standard conditions, or 400
CFM/ton (of which I happen to have at my house). The ARI SEER rating
standards would assume 365 watts/1000 CFM, or 438 watts total. Using the
equations in Appendix G of 90.1-2007 ( 1200 X 0.00094*0.746/0.80), assuming
80% motor efficiency (approximate for small single phase motor), you come up
with 1026 watts, which is very interesting. Curiosity got the best of me and
so I put my Fluke 39 power meter on the circuit and actually measured the fan
power --> I get approximately 550 watts. The ARI assumption may be too low,
but the Appendix G assumption seems way too high. If we are using the
Appendix G fan power values for small systems on our projects, this appears
to show that our projects are saving lots and lots of fan power. But, that's
really not true because all we are doing is comparing our proposed design fan
power to a really bad baseline, one that is greatly exaggerated in the first
place. That's crazy, and in my opinion diminishes the validity of the
savings calcs because it shows savings on paper, but not real savings. If
people want to get serious about energy savings, I think we need a baseline
that holds us to a higher standard. Similar to the lab systems, we need
realistic baseline fan power values for small systems, except heading the
other direction (i.e. decreasing). If this can be changed, savings will be
more difficult to obtain, but they will be more real and therefore valid.

I've been wanting to bring this up to the 90.1 committee at ASHRAE. Haven't
found the time yet, but will.

Any thoughts on this topic would be appreciated as well.

Appreciate any additional feedback, and sorry for the really long email.

Regards,

James A.? Hess, PE, CEM

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Joined: 2011-09-30
Reputation: 601

Thank you for everyone's thoughts on the fan power calculations.

I would like to confirm my understanding of the fume hood exemption for
Addendum ac. It appears that the fan power for exhaust fans serving fume
hoods can be calculated separately from the fan system bhp, is this correct?

In doing so, the fan system bhp (supply fan) calculation must take a
deduction of 1 inch for the fume hoods.

If this is the case the actual fan system bhp would be the sum of the two
separate calculations (supply and exhaust).

Please correct me if I am misunderstanding the intent or application of the
fume hood exemption.

Thanks,

Mickey Bush, PE, LEED AP

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