Supply Fan Power

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Good Morning All,

I recently received comments back from LEED and one of them was about my fan
power. In the submission I calculated all of my fan power using G3.1.2.9
for the baseline building, and I used the actual fan power in the proposed
building. The reviewer has commented that all independent fan systems have
to be modeled identically in both the baseline and the proposed building.
The supply fans use very high efficiency motors, and as such are
substantially more efficient than the calculation that G3.1.2.9 provides.
Is there no way to take credit for using high efficiency supply fans?

Thanks,

____________________________
Michael Shields

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from your wording of "independent" it sounds like you may have an
application where you have a unit in the proposed building that is
required but not provided in the design? or something similar. is this
the case? if a unit is required in the proposed per the requirement all
spaces be heated/cooled (per the definitions) they are required to be
the same as the units in the baseline building. if this is not the case
if you post the exact comment it may shed some light on the subject.
otherwise i would argue the comment is in error - if all of your zones
are served by units in the proposed and you have matching units in the
baseline (regardless of system types) then your approach would be correct.

Patrick J. O'Leary, Jr.'s picture
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The exact comment is this:

"The fan power spreadsheet for the Baseline and Proposed models indicate
that the fan power of the exhaust fans have not been modeled identically in
each model. All independent fan systems of the HVAC systems in the actual
design must be modeled identically between the Proposed and Baseline models
at actual equipment capacities (fan volume and fan power) as required by
Table G3.1.10 in the Proposed building column, since the fan design air flow
rates and fan power per Sections G3.1.2.8 and G3.1.2.9, respectively, only
applies to system types 1 through 8 in Table G3.1.1A. If appealing this
credit, revise the Proposed and Baseline models so all independent fan
systems of the HVAC systems are modeled identically between the Proposed and
Baseline models. In addition, separate the energy consumption and peak
demand energy for independent fans in Table 1.8.1 and Table 1.8.2 of the
template and provide revised SV-A reports for each model reflecting the
changes."

I initially interpreted this to be that for exhaust systems the bhp/kw needs
to be identical in both buildings and based on the specified equipment from
the mechanical designer, however, my supply fan energy was Ok. After
reading the comment and Appendix G I am not unsure if I also need to specify
the fan power to be the same for both models, ie use table G3.1.2.9 to
calculate fan power for both instead of just the baseline.

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it reads to me that your initial interpretation is what the reviewer is
asking for. on the other hand the question is what is constituting an
"independent" exhaust fan here. if the exhaust fan is part of the
balance of the supply/return/exhaust system it probably should be
modeled as such as it is not really different than having a return fan
installed in the ductwork somewhere and not in a unit itself. per hte
90.1 user's manual (page g-28 in 2004), "System fan electrical power for
supply, return, exhaust and relief fans is calculated based on the
following formula. The power from this formula includes supply, return,
relief, and exhaust fans ..." if the exhaust fan is not part of the
designed system supply/return/exhaust then i would think your initial
interpretation of what the reviewer is asking is correct. whichever
situation applies make sure the response documents the purpose of the
exhaust fans and how 90.1 applies (including section/user manual
references).

Patrick J. O'Leary, Jr.'s picture
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Color me concerned... This reviewer may have an argument but I can't
reason it. I think he/she may be leading us off the path regarding what
the baseline fan power calculations apply to...

You've got exhaust fans in a proposed model matching the construction
documents. Sounds okay so far... Going through this line by line:

* I would start with directing the reviewer to the glossary.
The entry for "HVAC system" reads: the equipment, distribution systems,
and terminals that provide, either collectively or individually, the
processes of heating, ventilation, or air conditioning to a building or
portion of a building."

o Clearly, an exhaust fan providing ventilation to a building
independent of other systems is an HVAC system. The reviewer's phrasing
of "..independent fan systems of the HVAC systems" is a
misinterpretation.

o Building on the understanding that an independent exhaust system is
a complete HVAC system, Table G3.1.10.a is as far as you need to go for
support of following the construction documents.

* I'd again reject the reviewers' choice of words: Exhaust fans
are not "independent fans of the HVAC systems."

* Sections G3.1.2.8 and G3.1.2.9 agreeably apply only to
baseline systems. They have no bearing on the topic at hand (what goes
in the proposed model), unless your baseline fan capacities/efficiency
calcs didn't follow the instructions.

* The request to document the independent fan systems' energies
separately is not new. May as well do that after all's said and done.

That's my initial take. Thoughts, anyone?

~Nick

NICK CATON, E.I.T.

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Michael,

I'm confused by your LEED reviewer's comments. As Patrick mentioned, it
would help to know what the reviewer means by "independent fan systems".
I can think of only two situations where you would need to model
identical fan power in the baseline and proposed models. One is if you
have no heating and/or cooling system in the proposed design, in which
case the proposed system "shall be identical to the system modeled in
the baseline building design" per Table G3.1(10.). Since you mention
that you used the actual fan power in the proposed building, I'm
assuming that heating and cooling systems have been specified for your
proposed design.

The other situation is for process loads. If you have fan power
associated with non-HVAC systems, it should be treated as a process load
and modeled identically between the baseline and proposed designs. An
example of this that has been discussed on this forum previously is
parking garage exhaust.

I rarely model anything but supply fans in the baseline model unless
there are process loads. I see no point in modeling return, exhaust or
relief fans. The fan power calculated in G3.1.2.9 is for the sum of
supply, return, exhaust and relief fans. The baseline systems have to
move the same amount of air regardless of the combination of fan types,
and the fan power is based on, and entered as, kW/cfm, so you will end
up with the same fan energy in the baseline model regardless of how many
fans you model for each baseline system.

I don't know what the reviewer is trying to say with their comment about
system types 1 through 8 and Table G3.1.1A.

Regards,

Bill

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Michael,
I see two paths:

1) Your exhaust fans are separate from your HVAC and they become process equipment. Thus they should be modeled identically between the models with possibly an exceptional calculation method for your fan motor efficiency improvement.

2) Your exhaust fans are part of your HVAC system. They now get a prorated portion of the baseline fan power. Here I think the reviewer expects your total system fan power was different between your models so your exhaust fan power should be different too.
I generally chose 1 for my toilet exhaust since its sizing is independent of any load calculation and can be operated independently of other system. More complicated systems have ended up as 2. I have not bought my 90.1-2007 User's Manual yet. Are there any examples in there? If not, we should craft some. Then what is the best way to get it endorsed or approved by ASHRAE?

Paul Riemer, PE, LEED AP

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Thanks for all of the comments. I think I may have figured out how the
reviewer got confused.

As part of my documentation I provided a table which showed the supply and
exhaust cfm, bhp, and kw as calculated from G3.2.1.9 for the baseline
building. I then showed the same for the proposed building using the actual
equipment specified. The building has stacks of apartment units, each stack
has an exhaust riser which has a continuously running fan on the roof. I am
interpreting everything correctly now, the baseline building should only
list the supply cfm, bhp, and kw as calculated from G3.2.1.9 which includes
the fan power from this exhaust riser. The proposed building would then use
the actual supply fan and exhaust fans specified.

Does this seem like the correct path? Alternatively the exhaust riser could
be modeled separately in both buildings using the proposed equipment as a
process load.

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Michael:

Would not the toilet exhaust be an independent system? It does not return air
to the HVAC system and can operate with the HVAC system off.

I am modeling a residential high rise and am modeling the kitchen exhaust hoods
as an independent system. In my case the toilet exhaust fans have been
eliminated and the toilet/apartment exhaust air is returned to an ERV. Each
apartment is being supplied with ventilation air from a DOAS. In my case, I
will model the toilet exhaust as part of the HVAC system.

Paul Diglio

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I agree the toilet exhaust system would be independent, but I guess I am having trouble coming up with an example of an exhaust system which is not independent other than an ERV which seems to be covered under G3.1.2.10. Thus if everything other than ERVs are independent, why does G3.1.2.9 specify exhaust as one of the systems to be included in that calculation?

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Maybe this is the source of the confusion. I disagree that toilet exhaust fans constitute independent HVAC systems that have to modeled separately in the baseline. To me, toilet exhaust, kitchen exhaust and clothes dryer exhaust are all components of the system that heats, cools and ventilates the zone(s). The air handlers, heating and cooling coils that serve these zones are designed to supply and condition the ventilation air that is exhausted, regardless of whether or not the exhaust is at the air handler or through a zonal exhaust fan. Yes, a toilet exhaust fan can operate with the HVAC system off, but the makeup air has to come from somewhere, and it needs to be heated/cooled (even if indirectly by mixing with the room air). I mentioned previously that I usually omit exhaust fans in baseline systems. Another reason I do this is because eQUEST zonal exhaust fans cannot operate when the system fan is off.

My personal interpretation of G3.2.1.9 is that if you choose to model separate supply and exhaust fans in the baseline system, the calculated baseline fan power is split up into supply and exhaust components so that their total power equals the calculated value. I do not think G3.2.1.9 requires you to model separate fans for any reason. It only adds complexity and modeling effort, not baseline fan energy.

Regards,

Bill

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The makeup air can come from operable windows or infiltration and does not
necessarily need to be conditioned.

Paul Diglio

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Paul,

True, makeup air can come from windows or infiltration, but it still constitutes a load that is met by heating and cooling coils if there is a thermostat. Are you going to model the baseline toilet-exhaust-only system and zone with those coils to handle the loads? If yes, use the same G3.2.1.9 system type as the rest of the project and be done with it. If no, you are either going to have a problem with unmet heating/cooling hours, or you?re considering the zone unconditioned and the toilet fan is a process load that should be modeled identically between the baseline and proposed models.

Bill

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Good morning, all,

Try looking at it this way. Exhaust fans (separate fans or components of the proposed HVAC unit) that operate continuously during occupied periods in conjunction with the HVAC systems serving the adjacent areas should be included in the Baseline system fan power calculated using Section G3.1.2.9. Exhaust fans that operate intermittently during occupied periods should be considered process loads and modeled with the same airflow, power, and schedule as the proposed case fan. If the fan motor efficiency is regulated in Section 10 (1 HP and larger) and the proposed design used higher efficiency motors, savings for the improved efficiency may be claimed.

Hope this helps!

Cam S. Fitzgerald, PE, LEED AP

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I may have missed something earlier, but I think apartment buildings
according to ASHRAE 62.1 require a whole building exhaust to ensure
ventilation. This is usually taken care of by intermittent use of the
toilet exhaust fan system. I think 62.1 says something like you can
achieve adequate ventilation by running the exhaust fan intermittently
at something like 70 cfm for a period of time (which I can't remember)
or running it continuously at 30 cfm. So, if the toilet exhaust is being
used for this purpose, it becomes part of the HVAC system and is not
separate. The laundry and kitchen exhaust fan systems are process
oriented and are part of the process loads. The apartment minimum OSA
requirement is usually provided by a bunch of little holes in a panel
connected to some of the windows.

Kathryn Kerns

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>From my understanding of 62.1 you can use natural ventilation if the windows are
within 25' of the areas that require ventilation.

I have seen designs where windows with trickle vents are specified. I don't see
a whole building exhaust requirement in 62.1-2004. I did notice where bathrooms
require exhaust per Table E-2.

What section is a whole building exhaust specified?

Paul Diglio

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Bill:
For an apartment building with PTHP, the system cycles on temperature so there
is not necessarily a load on the system for the infiltration load due to open
windows or trickle vents.

Per the eQuest Help menu, the zone exhaust fans are modeled with the exhaust air
source as infiltration to simulate residential kitchen or bathroom exhaust and
the exhaust fan will operate independently of the space conditioning system.

Without stirring up the controversy of Thermal Zones again, for an apartment, I
have two zones in each zone group. One is the main area including the bathroom
and one is the kitchen. This way I can add another exhaust fan in the kitchen
with a separate fractional schedule.

The BEPS indicates a significant difference in energy use when adding the
exhaust fans to the model.

How do you account for this energy if you do not model the fans?

Paul Diglio

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Paul,

See my response in red italics, below.

Regards,

Bill

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Bill:

Section G3.1.2.4 also states:

"Fan System Operation. Supply and return fans
shall operate continuously whenever spaces are occupied and
shall be cycled to meet heating and cooling loads during unoccupied
hours. If the supply fan is modeled as cycling and fan
energy is included in the energy-efficiency rating of the equipment,
fan energy shall not be modeled explicitly."

What do you think this means? Does ASHRAE mean cycling during unoccupied
hours? If so, we do not need to breakout the fan energy and can use the unit
EER. If ASHRAE is talking about cycling during the unoccupied hours, how can we
model the fan energy as broken out during occupied hours and part of the EER
during unoccupied hours? If the ASHRAE intent is that all fans must run
continuously during the occupied hours then why the underlined sentence?

Of course in a residential application the PTHP fans do not run constantly. I
can understand the fans running constantly in a commercial application.

I would appreciate your interpretation.

Paul Diglio

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Paul,

I don?t know what the intent of the statement that you underlined is. I suppose it could apply to a space that is always unoccupied, but which has a near-constant cooling load. I ignore it for my models and break out the fan energy. In a heating dominated climate, you would greatly underestimate your system energy if you zeroed-out the fan energy and used EER only with no heating COP (hot water coils for heating), since the system would rarely cycle on in cooling mode.

Regards,

Bill

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