[Bldg-sim] Adjusting Capacity for Unmet Cooling

[Arpan Bakshi]

Some good discussion about unmet hours, to summarize_

Check_
a. Occupied setpoint, unoccupied drifting point delta
b. Throttling range
c. Fan schedule conflicts with load req.

What about additional considerations like_
- Conflicting mix of zones supplied by AHU
- Extreme zone controlling thermostat
- Perimeter condition not addressed
- Water line sizing inadequate to meet flow
- Missing pumps



----------------------------
Arpan Bakshi
YRG sustainability
Direct 646.704.2880

Sent from my iPhone

On Nov 19, 2010, at 7:48 PM, Carol Gardner <cmg750 at gmail.com> wrote:

> Hi All,
>
> I think Nick is spot on in his discussion re unmet hours. While I'm  
> not saying it's a wrong thing to do, I personally have never used  
> oversizing the system as a way to get rid of unmet hours.
>
> As an example, I have been helping someone off the list with his  
> project, a seemingly very simple building with only 5 thermal zones  
> and one PVAVS air handler with dx cooling and hot water heating -  
> system 5. After running my 4 exposures, and having none of them with  
> any hours of unmet loads, I calculated my kW/cfm, heating load,  
> cooling load, boiler kW and all of that, put the calculated results  
> in the baseline, reran the baseline, and for some darn reason I had  
> over 1000 hours of unmet cooling loads.
>
> I checked all my inputs, made a few changes, reran the 4 exposures,  
> and not much changed. So, I put on my thinking cap and read Nick's  
> email - not necessarily in that order. One of the things that  
> occurred to me is that since the building is turned off all weekend  
> my Monday morning pull down loads during the summer might be the  
> cause of the problem, so I put in an optimum start schedule (this  
> might have been required, I haven't double checked) and my unmet  
> hours were reduced by about 300 hours. I had an adequate cfm/sf,  
> something I'm not shy about raising at all since eQUEST defaults it  
> to 0.5 cfm/sf which is usually too small, so I didn't change it.  
> Then I looked at the throttling-range.
>
> For VAV systems the eQUEST/DOE2 manuals say that the throttling  
> range should be at least 4 degrees to insure stable operation and  
> that is what I had it at originally. After thinking about it and  
> doing some simple math in my head I raised it to 6 degrees. My  
> cooling temperature is set at 75 degrees so this means when my room  
> temperature reads 79, set point temp + 1/2 TR + 1 degree, my cooling  
> coils will be activated. Before I did this I had also looked at my  
> SS-F reports and could see that my room temps weren't that far off  
> from the set point temp when I had under cooled hours reported.  
> Using the new throttling range brought me down to less than 500  
> hours of unmet loads, which may work.
>
> So this is an example of the thinking around what's going on and  
> developing a strategy that makes sense for a specific building. I  
> also looked at my heating/cooling temp schedules, etc., as Nick  
> mentioned. What baffles me is why, after none of my 4 exposures  
> showed any unmet load hours, my baseline did. I, like Nick, have not  
> had that happen before. Perhaps it has something to do with the way  
> the building was zoned, 4 exposures and a core but I don't know for  
> sure.
>
> Any ideas?
>
>
> Carol
>
> On Fri, Nov 19, 2010 at 2:56 PM, Nick Caton  
> <ncaton at smithboucher.com> wrote:
> 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 – suff 
> ice it to say skewed is not always a *good* thing, even if it does m 
> ake your performance numbers look good.
>
>
>
> Whenever I have unmet hours, I’ve learned through others on these li 
> sts to investigate the “why” and “when” of those unmet hours  
> as a first step.  More often than not, unmet hours in my models, whe 
> n 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 eff 
> ectively aren’t running as frequently or for a duration as they shou 
> ld 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 baseli 
> ne/proposed models into the prescribed requirements for unmet hours. 
>   In some cases, you might even make the situation worse.  I’m prett 
> y 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 basel 
> ine model whose unmet hours did not ring in at either zero or the si 
> ngle digits, and as such did not require any adjustments.  I persona 
> lly feel this has had a lot to do with understanding the mechanics b 
> ehind 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
>
>
>
> <image001.jpg>
>
>
>
> 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
>
>
>
> 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:
> 101 Anupam Apartments,
> 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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>
> -- 
> Carol Gardner PE
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