Modeling Same Airflow on Proposed and Baseline Models

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Have any of you run into a review team telling you that you must model the baseline fan supply volume and baseline fan power the same in the proposed design? I have a project under review that is a distribution center primarily served by heating only make up air units. I have modeled cooling in both models and modeled the setpoint high enough that it would never turn on.

My interpretation of the requirement to model cooling the same in both models if cooling does not exist on the proposed design is just that, you only model the cooling the same. However, this is the response we received after our initial design comments review:

2. The response narrative to preliminary comment 5 states that the baseline case was modeled with a fan supply volume determined using Section G3.1.2.8 and a fan power determined using Section G3.1.2.9; however the proposed case must also be determined using these parameters. Per Table G3.1.10(d) in the proposed building column, where no cooling system exists, it must be modeled identically to the baseline case. Because the fan supply volume and fan power are cooling driven, it is unacceptable to auto-size the baseline case fan supply volume and fan power and not auto-size the proposed case fan supply volume and fan power. Currently, the baseline fan supply volume is sized to meet cooling needs and the proposed fan supply volume is sized to meet heating and ventilation needs. If appealing this credit, revise the Proposed case fan supply volume and fan power so that they are determined using Section G3.1.2.8 and Section G3.1.2.9. Note that it is appropriate for the proposed fans to be modeled as VAV since HVAC System 7 applies. Additionally, provide SV-A reports confirming the changes.

Another comment we received claimed that we must model the 0.4 CFM/SF requirement on the proposed model because the airflows are sized based on cooling and therefore, the airflow rules from System 7 apply (i.e. 20 delta T, min flow requirements, etc.)

3. The response narrative to preliminary comment 12 states that the full load equivalent hours are so high due to the 0.4 cfm/sq.ft flow rate; however the 0.4 cfm/sq.ft rate applies only when the building is occupied or when the interior fans are cycled on during unoccupied hours to meet heating or cooling loads. Based on the information provided in Section 1.2 of the Prerequisite form the occupied hours are only 40 per week. If appealing this credit, revise the schedules in both cases to reflect the actual anticipated schedules of the interior fans installed in the building. Additionally, revise the baseline case (and proposed case as necessary for equipment auto-sized per Table G3.1.10(d)) so that the 0.4 cfm/sq.ft minimum flow rate applies only when the building is occupied or when they are cycled on.

I already took care of the scheduling of the when the fans turned on. However, I honestly feel like the rest of both comments step over the line in terms of what we are suppose to model on the proposed design. The changes we would need to make (primarily forcing the airflows to be sized the same as the baseline and 0.4 cfm/sq ft) do not represent the design intent of the system shown on the drawings.

What do you guys think?

William Mak, LEED AP BD+C

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Will,

This reviewer appears to be seriously confused and has applied baseline rules to your proposed design model.

You can do whatever you want with the fan sizing in the proposed model, so long as your have use identical space conditioning set points and schedules and results are within stated limits for maximum unmet load hours and you do not exceed the maximum difference in unmet hours between the baseline and proposed. This is to prevent you from having a proposed design that is under-cooled or under heated relative to the baseline.

Similarly, the 0.4 CFM/SF requirement is for the baseline model, not the proposed.

Regards,

Timothy Moore

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Will,

I actually think that the reviewer has a point here. Your proposed design
has no cooling, therefore, the proposed design needs to have a cooling
system that is the same as the baseline system. The airflows don't have to
be equal, but they have to be determined in the same fashion (auto-sizing
with a minimum flow of 0.4 cfm/sf). HOWEVER, these rules should only apply
to the system in cooling mode. When in heating mode, the system can reflect
the designed values.

--
Karen

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Joined: 2011-09-30
Reputation: 200

Will,

First, note that the 2010 version of ASHRAE 90.1 PRM includes two new systems for heat & vent only, and these apply to certain warehouse/storage space types. This is intended to address projects like yours and may apply to certain areas of your distribution center. However, as you're already committed to an earlier version, you're stuck with a challenging situation.

Karen does have a point, however... While I was reading fast and missed the bit about there being no cooling system in the proposed design or certain portions thereof, the reviewer is still not correct in asking you to model your proposed design with cooling supply airflow and minimum flows the same as the baseline model. Requiring the presence of a cooling system with suitably sized equipment is different than requiring you to run a fan with a minimum airflow that does not exist.

Indeed, Table G3.1 section 10d does state that "Where no cooling system exists or no cooling system has been specified, the cooling system shall be identical to the system modeled in the baseline building design." Autosizing is an appropriate way to set this up such that equipment is adequately sized to meet all cooling loads present under design conditions and that the cooling airflow can vary between the design flow rate and 0.4 cfm/sf. However, when it comes to modeling the actual proposed fan power and supply airflow in the simulation, if the building has only heating and ventilation, and no cooling system, it should most definitely not be simulated with 0.4 cfm/sf minimum supply airflow, or you will clearly not account for the benefit of having a heat & vent only system in the proposed design. Your ventilation system provides the continuous operation during occupied hours that is required by section G3.1.2.4 Fan System Operation. The 0.4 cfm/sf flow rate should be the minimum during all occupied hours for a VAV system where required in the baseline model, and it should be the minimum in the proposed model when cooling is required to avoid unmet load hours.

As Karen has suggested, you can set up the model so that the "identical system" applies only in cooling mode, including autosizing, fan efficiency, etc. And, if cooling mode is never actually triggered, then this will never run. This requires separate fans, and so forth, and could be a bit messy when it comes to properly modeling ventilation and minimum airflows.

I would suggest, and this depends upon the software you are using, that in your proposed design you model the complete proposed system for heating and ventilation, as designed, PLUS the same system as in the baseline model (autosized and all) serving ALL of the same zones, but with no OA ventilation, no heating, and no minimum airflow. In effect, every zone in the proposed model that does not have a cooling system in the real building would be modeled with two systems serving the same zone: One for heat & vent, the other for cooling. Then, as you have said, if the setpoints are equally high in both proposed and baseline models, the cooling will not need to run anyway and unmet cooling load hours will not be an issue.

Really, this is just a lot of extra work to prove that you do not need a cooling system in the proposed design to provide the indoor environmental conditions set forth in the design documents, or to suitably penalize your proposed design model if you actually do need cooling to maintain the owner's required setpoints. On the other hand, if you are raising your cooling setpoints and the simulated results include indoor conditions or duration thereof that would not be acceptable to the building owner, then you would not be simulating the true as-built design (assuming that the owner would make you change the design if it appears that the building is going to get too hot too much of the time).

Regards,

Timothy Moore

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Thanks for all the feedback! Timothy, you gotta stop working so darn late!!

I was thinking that I could respond to the comments stating that ASHRAE 90.1-2007 states that cooling capacities are suppose to be the same, airflows do not necessary have to be equal. I left out the part of the review that frustrated me the most:

Due to these issues, the fan power consumption energy value in the Proposed model will be set equal to the fan power consumption energy value in the Baseline model; therefore the revised total electric consumption for the Proposed model will be increased by 2,451,577.5 kWh (3,565,295.5 kWh - 1,113,718 kWh), and the total energy cost for the Proposed model must be increased by $171,610.4 (2,451,577.5 kWh * $0.07/kWh)

The revised Proposed total energy cost is $ 367,627.8 ($196,017.42 + $171,610.4). This leads to a total percentage cost savings improvement of 19.8%.

The total predicted annual energy consumption for the project is 4,411,526.5 kWh (electricity) and 66,091 therms (natural gas).

Basically, they went with the assumption that fan energy consumption must be equal on both and forced us to accept results they feel are correct.

However, I'll be a little more constructive and will have to figure out how to resolve this issue on eQuest...

William Mak, LEED AP BD+C

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Will,

At the risk of confusing things even further, I recently received the following LEED review comment for a LEED 2.2 project:

"Any heated-only spaces in the Proposed building must be modeled as heating-only in both buildings or as heated and cooled in both buildings. Please revise the Proposed and Baseline buildings as necessary to model the heated-only spaces identically in both buildings, and update the template accordingly. Although not required, it is encouraged that the ASHRAE 90.1-2010 Appendix G modeling protocols for heated-only spaces be followed using System Type 9."

As Timothy mentioned, System Type 9 would appear to apply to your case, and my LEED Reviewer's comment gives you a basis for modeling both baseline and proposed as heating only if you choose to, even though that option was not available until 90.1-2010.

Regards,

Bill

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Bill,

Seems like that will be the new default advice on how to deal with a lot of these warehouse/storage type facilities that have heating only. It makes a lot more sense than manipulating with the airflows on the proposed design.

Since the project has already been submitted and we are already at the Final Design Review stage, I probably won't change the system types in order to prevent even more confusion for the review team.

William Mak, LEED AP BD+C

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I don't believe that the airflows between the proposed and baseline
designs need to be equal. System sizing and fan sizing needs to be
determined in the same way, but there are many things that affect
airflow and peak demand that may very well be different in the
proposed and baseline models, LPD, glass types, etc, although sizing
in the end may be determined by the 0.4 cfm/sf number. Also, when the
system is in heating mode, the as-designed fan power and flow should
be used. The last comment you just shared with us is definitely
incorrect.

--
Karen

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Reputation: 200

FYI.

The DOE2.1e manual had extensive discussions on how to create curve fits and it would even run the regressions for you. In this section in the manual there is a table that explains the basic curve types.

You can also roll your own curve-fits using a spreadsheet by performing a multiple, non-linear regression and plug the coefficients back into DOE-2. Care should be taken to properly hand cross-variables, etc.

The final report fro ASHRAE RP1007 also discussed curve fits.

Jeff S. Haberl, Ph.D., P.E., FASHRAE

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