[Bldg-sim] Adjusting Capacity for Unmet Cooling

[Michael Collarin]

Nick,

 

Thank you for the response. My first energy modeling experience was with eQuest, but have been using Trane’s TRACE 700 more extensively. In addition to the bldg-sim lists, the TRACE support is an excellent resource for those using the software, especially when it comes to unmet hours.

 

As you mention, and as I was alluding to in my third paragraph (starting with ‘Before taking this step,’), there are certainly other areas that should be addressed prior to adjusting the cooling and heating capacities. I was not trying to cover all of them and did not want to get too in depth, as there are numerous areas that could be contributing to the unmet load hours and a review of the model would be necessary to determine what changes must be made to remedy the model. I’d like to note that I said the model and not the unmet hours. As you and Carol have illustrated, I fully agree that we aren’t supposed to be trying to manipulate the model to give us our desired outcome, but rather provide an accurate model. I do not think anyone should be taking liberty with the capacities to such extremes just to meet the requirements for unmet load hours. I even scratched my head the first time I read through that section and thought, “well what’s the point?”

 

My response wasn’t meant to suggest that modelers wildly vary cooling and heating capacities to obtain their desired results, but to clarify that Neeraj’s comment (copied below) was in fact contrary to what ASHRAE requires in Appendix G. Perhaps a few disclaimers were in order J

Correct me if I am wrong but on the contrary I think that ASHARE baseline PRM fixes the cooling and heating capacities to 1.15 and 1.25 respectively and the user is allowed to bump up the supply CFM in increments if necessary.

 

In conclusion, I offer these bullet points:

·         Design airflow rates are set using a 20-degree delta T between supply air and room space

o   These shall not be adjusted to “fix” unmet hours

·         Baseline cooling and heating capacities are initially set to 115% and 125% respectively

o   These may be adjusted incrementally, but should not be relied upon to “fix” a poorly produced model. In fact, their adjustment should be considered last, after all other approaches have failed.

·         It is important to understand what an unmet hour is and why they occur

o   It is of even greater importance to remedy the cause of the unmet hour rather than “fudging” the numbers

o   Refer to your software’s Building Temperature Profile report, if such a report exists, to help find the source (space, system, etc) of the unmet hours

o   Thermostat Drift Points and Utilization Schedules have been primary culprits

§  Other areas affecting unmet hours include but are not limited to the following: Thermostat placement, fan cycle operation, optimum starts, building envelope entries, Ventilation (outside air) schedules

 

Happy modeling!

 

Michael M. Collarin, EIT, LEED AP BD+C | Elm Engineering, Inc. | 212 S Tryon St | Suite 1375 | Charlotte, NC 28281

PHONE 704-335-0396 Ext. 108| FAX 704-335-0399 | www.elmengr.com <http://www.elmengr.com/> 

 

From: Nick Caton [mailto:ncaton at smithboucher.com] 
Sent: Friday, November 19, 2010 5:57 PM
To: Michael Collarin; Nearedge; cjaigath at yahoo.com
Cc: bldg-sim at lists.onebuilding.org
Subject: RE: [Bldg-sim] Adjusting Capacity for Unmet Cooling

 

Hi Michael,

 

I’d like to offer a quick heads up – your interpretation of what 90.1 has to say regarding addressing unmet hours is spot on and well-stated, no disagreements here.  Neeraj however is indirectly making a pretty valid point that I’d hate for anyone to miss…

 

If you only ever approach addressing unmet hours by bumping the oversizing factors, you may well be missing something critical.  I for one was in this train of thought when I started out.  To the letter of the standard, indeed this is all we’re *prescribed* to do to remedy the situation, but this approach will not always work, and may result in baseline models with a very skewed performance – suffice it to say skewed is not always a *good* thing, even if it does make your performance numbers look good.

 

Whenever I have unmet hours, I’ve learned through others on these lists to investigate the “why” and “when” of those unmet hours as a first step.  More often than not, unmet hours in my models, when they do occur, have nothing to do with lacking heating or cooling capacity – fan (airflow) scheduling conflicts, thermostat setpoints and unrealistic deadbands are the primary culprits.  The systems effectively aren’t running as frequently or for a duration as they should be.

 

When this is the case, and I can assure Neeraj is not alone in this experience, fluffing the oversizing factor may appear to “remedy” the issue, but often as not may *not*  be enough to pull your baseline/proposed models into the prescribed requirements for unmet hours.  In some cases, you might even make the situation worse.  I’m pretty sure the spirit of the standard, by specifying a maximum number of unmet hours, is to allow both the proposed and baseline systems to run when they need to satisfy the loads

 

For whatever combination of reasons, and luck may be a part of it, it has been a long while since I’ve created an autosizing 90.1 baseline model whose unmet hours did not ring in at either zero or the single digits, and as such did not require any adjustments.  I personally feel this has had a lot to do with understanding the mechanics behind what defines and can cause an unmet hour – as you’re getting at with consideration to drift points.  

 

As you say, future versions of 90.1 may be more nuanced, but I personally feel the manner in which unmet hours are currently addressed does a disservice to those aspiring to be quality energy modelers – one can easily be misled to believe an unmet load hour is a problem in and of itself with an easy “fix,” without recognizing it’s really a symptom of a problem... bumping capacities without first addressing the cause of unmet hours is something like a doctor only giving you a band-aid for a cut when what you need is a tetanus shot!

 

It may be worth mentioning that my experience is dominantly within the world of  eQuest/DOE2.  I would not be surprised to find other engines/software have varying “typical” causes of unmet hours.

 

Anyway, that’s my two cents, for what it’s worth =) – hope it might help lead others along the path!

 

~Nick

 

 

 

NICK CATON, E.I.T.

PROJECT ENGINEER

25501 west valley parkway

olathe ks 66061

direct 913 344.0036

fax 913 345.0617

Check out our new web-site @ www.smithboucher.com 

 

From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Michael Collarin
Sent: Friday, November 19, 2010 1:46 PM
To: Nearedge; cjaigath at yahoo.com
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Adjusting Capacity for Unmet Cooling

 

Neeraj, it is in fact the contrary. 

 

ASHRAE 90.1-2007 Appendix G requires you to model Baseline Airflow Rate using a 20-degree F temperature difference between the supply air and room air temperatures. Therefore, if your room set point is 75F for cooling, set your cooling leaving air temperature to 55F and vice versa for heating; 72F heating set point requires a 92F leaving air temperature. This establishes your CFM airflow within the model. Do not vary these to adjust unmet hours.

 

If you have unmet heating or cooling hours (over 300 or greater than 50 between baseline and proposed), you may incrementally adjust the baseline cooling and heating capacities (originally oversized 15% and 25% respectively) accordingly to reduce the overall unmet hours (below 300) or the difference between baseline and proposed (within 50).

 

Before taking this step, be sure to look at your thermostat drift points. If the cooling set point is 75, but the cooling drift point is 84, when the space is unoccupied this would allow the space to move toward 84 degrees. When the space becomes occupied and the simulation tries to cool to 75, the system will not be able to achieve this within the allotted time (1 hour) and you will have an unmet cooling hour. 

 

There are some changes to ASHRAE 90.1-2010 that will effect unmet hours, but for the time being, if you are modeling a project using 2007 or 2004, this method should help you eliminate the unmet hours.

 

Regards,

 

Michael M. Collarin, EIT, LEED AP BD+C | Elm Engineering, Inc. | 212 S Tryon St | Suite 1375 | Charlotte, NC 28281

PHONE 704-335-0396 Ext. 108| FAX 704-335-0399 | www.elmengr.com <http://www.elmengr.com/> 

 

From: Nearedge [mailto:near_ej at yahoo.com] 
Sent: Friday, November 12, 2010 12:51 AM
To: cjaigath at yahoo.com
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Adjusting Capacity for Unmet Cooling

 

Hello Jaigath,
Using the performance rating method (PRM), in my experience it is usually an issue of low supply CFM.
Correct me if I am wrong but on the contrary I think that ASHARE baseline PRM fixes the cooling and heating capacities to 1.15 and 1.25 respectively and the user is allowed to bump up the supply CFM in increments if necessary.
I have a feeling that the issue of unmet load hours has been discussed many times before -- so, the archives will be a good resource as well.
Best,
Neeraj

Neeraj Kapoor
t: +91.99581.70018

e: neeraj[at]kalpakrit[dot]com
Kalpakrit Sustainable Environments Pvt. Ltd.
www.kalpakrit.com

Office Address:
610-A Udyog Vihar, Phase-5,
Gurgaon, Haryana - 122016
t: +91.124.430.9490/ 1/ 2
f: +91.124.430.9493

Registered Address:
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Mehrauli-Badarpur Road,
New Delhi - 110068

 

On 11-11-2010 10:30, Jaigath Chandraprakash wrote: 

Hello All,

I would like to know how other increase their baseline capacity when they have unmet load. I have a PSZHP system and in one system, I got more than 400 hrs unmet load. I usually just increase the TR cooling capacity until I get a lower unmet load but in this case, I think the unmet load is due to low supply cfm. Is it ok to adjust also the supply fan cfm? I am thinking that G3.1.2.2 only requires me to adjust only the cooling capacity. Do others adjust both cfm and cooling load?

Thanks,

Jaigath

 

 




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