[Bldg-sim] Adjusting Capacity for Unmet Cooling

Carol Gardner cmg750 at gmail.com
Fri Nov 19 16:48:05 PST 2010


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 – 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
>
>
>
> [image: cid:489575314 at 22072009-0ABB]**
>
> * *
>
> *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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