[Equest-users] Unwanted Reheat hours in Baseline Model

[Jeremy Poling]

One of the old CIRs (not as relevant anymore, but still a good strategy reference) allowed modelers to set the thermostats at an artificially high cooling setpoint or artificially low heating setpoint.  This way, the system is included in the model but it never actually uses any energy when modeled, balancing the Appendix G requirements with the need/desire to use the same building model for calibrated verification of the building after completion.  It seems like most reviewers are still accepting this strategy - it's a lot less stress than trying to figure out how to get the two systems to use the same amount of energy in two different models or worrying about the unreal savings generated by a non-existent system.
 
It would be great if anyone from the the ASHRAE 90.1 committee (who is on this list) could enlighten this group on why this particular provision has persisted?  The User's Manual makes it clear that the provision is intended to reflect the fact that buildings without a heating system or without a cooling system are often renovated to include the missing system after construction; however, Appendix G and the LEED rating systems that use it are strictly looking at projected performance based on the current design.  Modeling a system that may or may not be installed in the future without a specific design document describing it seems to detract from the broader goal of modeling: arriving at a reasonably accurate projection of how a given building should perform against a benchmark.
 
Jeremy Poling

________________________________

From: equest-users-bounces at lists.onebuilding.org on behalf of Fred Betz
Sent: Tue 12/14/2010 5:57 PM
To: Walson, Kristy; equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Unwanted Reheat hours in Baseline Model



My only recommendation is to leave it as is if you are comfortable with the inputs reflecting reality to the best of your ability. Remember to make your temperature set points, schedules, and dead bands realistic for the client. If the client is ok with a few 65F hours at night, have the inputs reflect that. It just has to be the same in the Baseline and Proposed. 

 

As long as the systems are the same, electric reheat, then you should be ok. I have had a few projects that utilize a strong passive design (shading, natural ventilation, etc.), which are meant to eliminate the entire heating or cooling system. I could easily see eliminating reheat in the Caribbean quite easily through a smart passive design and expanded thermal comfort range. If your passive design eliminates reheat in the real world, then the simulated design should mimic that for the most part in operation if not in inputs. 

 

These passive features are missing in the Baseline, which is why it needs the reheat. In my mind the model is doing exactly what the design is supposed to be doing. If you have a poor Baseline design, you'll need reheat. 

 

You'll have to decide if the Baseline model is realistic in its use of energy, and you can only really get that out of analyzing the hourly reports and referring to ASHRAE and your engineering experience. 

 

Hope that helps, 

 

Fred

 

 

Fred Betz  PhD., LEED AP 
Sustainable Systems Analyst

 

AEI | AFFILIATED ENGINEERS, INC.  
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From: Walson, Kristy [mailto:kristy.walson at tlc-eng.com] 
Sent: Monday, December 13, 2010 3:15 PM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] Unwanted Reheat hours in Baseline Model

 

Hi all,

 

I am modeling a building located in the Caribbean that does not have any heat in the design.  Per ASHRAE 90.1-2004, even though the building has no heat, I am supposed to model both the Baseline and the Proposed model with electric heat (with all of the same properties between the two models).  My Baseline system is a Packaged, Constant Volume, DX system.  I find that the Baseline building is using a ton of electric heat for reheat because the system is constant volume and it seems to be overcooling without the reheat.  I was able to correct this in my Proposed model by reducing the Minimum Flow Ratio, but since my Baseline building is constant volume I can't reduce the minimum flow ratio to fix the reheat problem.

 

Does anyone have any suggestions on reducing this reheating?  Because the building doesn't even have any heat, I artificially inflated the heat in the Proposed to make the same as the Baseline because I was getting an imaginary benefit otherwise.  Thanks for the help.

 

Kristy M. Walson, PE, LEED AP

Mechanical Engineer / Sustainable Design

 

TLC Engineering for Architecture
Your 2030 Challenge Partner

 

255 South Orange Avenue, Suite 1600

Orlando, FL 32801-3463

 

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