[Bldg-sim] Adjusting Capacity for Unmet Cooling

Jorge Torres-Coto jtorres-coto at mbo1.com
Sun Nov 21 18:40:43 PST 2010


I deal with unmet hours by supplying more air rather than upsizing capacity,
and it has worked.

 

Jorge E. Torres Coto   

4830 Viewridge Ave.

San Diego, CA 92123

p              858.751.0933

f              858.751.0937

c              858.688.6088



  _____  

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Carol Gardner
Sent: Friday, November 19, 2010 4:48 PM
To: Nick Caton
Cc: bldg-sim at lists.onebuilding.org; Michael Collarin
Subject: Re: [Bldg-sim] Adjusting Capacity for Unmet Cooling

 

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

 

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 @  <http://www.smithboucher.com>
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:
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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