[Trace-users] Load Calcs vs Energy Model for LEED

Tue Oct 7 10:44:03 PDT 2014

Scott/Craig –

I have two ideas that might help get a workaround – it may not work on every system type, but it did on a FPVAV system I just experimented with:

1.      To address Scott’s issue re: a the difference between block load and sum of peak zone CFMs, Trace offers an option to size the fan (and/or the main cooling coil) based on the sum of the peak airflows, rather than the block airflow. On the “Create Systems” dialog, “Advanced…” button there are fields that allow you to modify the parameter used to determine the fan size and the coil size (as below). Changing these from “block” to “peak” changed the flow of the “Tutorialtrc.trc” file from 16160 cfm sum of peaks / 13778 block cfm to 16160 sum of peaks / 16160 block CFM.

[cid:image009.jpg at 01CFE234.C0F03C20]

2.      On the Create Systems – Heating and Cooling Over-rides tab, the main cooling coil can be oversized by a specific percentage “by adjusting airflow.” I believe this would allow you to maintain the same discharge air temp and modify the system airflow as desired. It may not offer unconstrained combinations of leaving air temp, coil capacity, and system airflow, since Trace is auto calculating entering air conditions, but it may offer you an additional degree of freedom to get close at the system level:

[cid:image010.jpg at 01CFE234.C0F03C20]

There may be unexpected consequences that make these ideas less than the best for your specific situations – but it seems like I’m able (at least in this case) to get the fan CFM to match the scheduled CFM, even if the load calc block load would show a different total CFM.

Hope this helps,

Aaron Dahlstrom, PE, LEED® AP
In Posse – A subsidiary of AKF| 1500 Walnut Street, Suite 1414, Philadelphia, PA 19102
d: 215-282-6753| m: 267-507-5470| In Posse: 215-282-6800| AKF: 212-354-5656
e: ADahlstrom at in-posse.com | in posse web: www.in-posse.com<http://www.in-posse.com/> | akf web: www.AKFGroup.com

From: Trace-users [mailto:trace-users-bounces at lists.onebuilding.org] On Behalf Of Craig Gann
Sent: Tuesday, October 07, 2014 12:55 PM
To: Scott Parker; Steve Jacobs; 'Caballero, Catalina'; trace-users at lists.onebuilding.org
Subject: Re: [Trace-users] Load Calcs vs Energy Model for LEED

I communicated with CDS about this a couple of months ago. It's a royal pain in the @#$ to do this! What you have to do is go to the rooms and manually enter the CFM for each room such that it totals up to the desired CFM at the system level. This means you have to first figure out which rooms are assigned to each system then figure out how much to change each room CFM such that it totals to what you want then print out the calculated room CFM values. Not easy if you have hundreds of rooms. I wound up putting it all in Excel so I could figure out a room CFM multiplier that changed each room CFM by the percentage needed. It took me a half day to mess with this and is a major deficiency in the Trace software IMHO.

Regards,

Craig J. Gann, P.E.; LEED AP
On 10/7/2014 11:18 AM, Scott Parker wrote:
I agree that the proposed system model should have the same capacity as the actual equipment.  However, the challenge is that when you fix the leaving air temperature of the air handler (which matches typical design) then Trace does not allow any method for fixing the air handler CFM capacity.

For example, below I have just listed some numbers for reference to illustrate the point:

Sum of Peaks = 19,458 CFM
Block load = 18,902 CFM (actual maximum hourly airflow in Trace will typically be even less than this value)
AHU size = 20,000 CFM (scheduled on plans)

AHU size in trace = block load (18,902 CFM) – no way to force it to be 20,000 CFM.

Therefore, when the air handler (in Trace) is at 18,902 CFM it is at peak fan power (i.e. top of fan curve).  However, it actually is already part way down the true fan curve since the unit is scheduled for 20,000 CFM.  If you use the theoretical fan laws, then the fan power at 18,902 CFM is 84% of the scheduled break horsepower - (18,902/20,000)^3 = 0.84  -- even though the fan is operating at 94.5% of peak fan power.

If anyone has a work around for this I would like to hear it.  For me I have had to manually change the kW/CFM value to match the actual scheduled fan at the block airflow calculated in Trace.

Scott Parker  PE
LEED AP BD+C
Mechanical

AEI | AFFILIATED ENGINEERS, INC.
1414 Raleigh Road, Suite 305 | Chapel Hill, NC  27517

P: 919.419.9802 | F: 919.419.9803
sparker at aeieng.com<mailto:sparker at aeieng.com>  |  www.aeieng.com<http://www.aeieng.com/>

From: Trace-users [mailto:trace-users-bounces at lists.onebuilding.org] On Behalf Of Steve Jacobs
Sent: Tuesday, October 07, 2014 11:13 AM
To: 'Caballero, Catalina'; trace-users at lists.onebuilding.org<mailto:trace-users at lists.onebuilding.org>
Subject: Re: [Trace-users] Load Calcs vs Energy Model for LEED

Your proposed inputs need to match what is actually being installed in the building. If they are installing oversized equipment, you need to model oversized equipment. The model will account for part load efficiencies.

-          Steve

From: Trace-users [mailto:trace-users-bounces at lists.onebuilding.org] On Behalf Of Caballero, Catalina
Sent: Tuesday, October 7, 2014 9:04 AM
To: trace-users at lists.onebuilding.org<mailto:trace-users at lists.onebuilding.org>
Subject: [Trace-users] Load Calcs vs Energy Model for LEED

Hello experts,

We recently received comments back from LEED  and one of the comments says the following:

It appears the equipment capacities (fan volume, fan power, cooling capacity, etc.) for the HVAC systems in the Proposed model
are inconsistent with the equipment capacities in the actual design when comparing the LEED Energy Performance Summary Report
to the mechanical schedules provided for PIf4: Schedule and Overview Documents
G3.1.10 in the Proposed building column requires that the Proposed model reflect all HVAC systems at actual equipment capacities
and efficiencies. The HVAC equipment capacities cannot be autosized in the Proposed model. Revise the Proposed model to reflect
all HVAC systems at actual equipment capacities. In addition, update Table 1.4.7B, and provide a revised LEED Energy Performance
Summary Report and the System Enterd Values reports for the Proposed model reflecting the changes. Further, if the equipment
capacities and efficiencies are based on updated mechanical schedules and/or HVAC submittal sheets, provide the updated
mechanical schedules and/or HVAC submittal sheets.

The only question that I have is why would they require for the airflows, to be exactly the same, to the original load calculations when the load calculations looks for the worst case scenario (using ashrae basic envelope, lighting, values), while the energy model looks for the most energy efficient model (actual installed envelope, occupancy, etc). It make sense that they are looking for something similar but it’s definitely not going to match the capacities and or airflows, (load calcs tend to be oversized).

I would greatly appreciate your opinion.

Thanks.

Catalina Caballero.  AIA. Assoc., LEED GA.
Sustainability Coordinator

Johnson, Avedano, Lopez, Rodriguez & Walewski Engineering Group, Inc.
Engineering for High Performance Buildings.
MEPF - BIM - LEED - Cx

2510 NW 97 Ave, Ste 220, Miami, FL 33172.
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