[Bldg-sim] New Rules of Thumb for Design Loads

Christian Kaltreider ckaltreider at sudassociates.com
Wed Oct 30 06:10:06 PDT 2013


I was afraid the conversation would go in this direction.hence the
disclaimer at the end of my original post.  But I think this is a good
string, I appreciate all of the comments and I agree with most of what is
being said.  On the value/appropriateness of rules of thumb for design:  I
fully agree that rules of thumb are not appropriate as a basis for actual
design.  I run computer simulated loads on a regular basis.  These often
take many hours of effort.  They are tailored specifically to what is
needed, whether it is plant size (chiller/boiler), AHU coil and fan sizes,
or VAV box/duct/diffuser sizes.  Each of these design needs require a
different modeling approach.  I can't imagine suggesting an equipment
purchase based on rules of thumb, and we never have.  It is even going
quickly out of style among HVAC contractors in the residential world. 

 

However, I do think that rules of thumb are very valuable and appropriate in
certain situations, and I would argue that almost every engineer uses them
whether or not they are conscious of it.  For instance, if I told you that I
designed an office building that consumes 2 kBTU/sf/yr and doesn't have any
renewable energy system, probably every person on this list would want a
little explanation.  That's because, based on years of accrued experience,
you know the reasonable range of annual energy consumption that can be
expected from an office building.  Further, your assessment that a 2
kBTU/sf/yr building needs some explaining took you about 1 second of
analysis.  That can be done on the fly, in a phone conversation with a less
technical or less experienced person who may not have any idea that this
value is unusual.  And the more experienced you gain, the more reliable your
mental assessments can be.  We have an engineer here that always seems to
know (within a few percent) what the results of my calculations will be long
before I get a final result.  And he will know immediately if there is
something fishy about my assumptions.  There are many times when it is
useful to know a reasonable range of values for design loads for a
particular building type without needing to do a block load or a full
simulation.

 

I was just hoping there might be a document which had compared design loads
between different versions of 90.1, similar to the energy comparisons that
have been done, so that I could have it in my back pocket.

 

Thanks for the lively responses!

 

Christian 

 

 

 

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Jim Dirkes
Sent: Tuesday, October 29, 2013 8:45 PM
To: Joe Huang; Robert Towell
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] New Rules of Thumb for Design Loads

 

..and now that Joe makes me think a bit more..

A block load calculation can be done VERY quickly.  Why bother with rules of
thumb when a few minutes more gives you a fully customized load?

 

James V Dirkes II, PE, BEMP, LEED AP
 <http://www.buildingperformanceteam.com/> www.buildingperformanceteam.com 
Energy Analysis, Commissioning & Training Services
1631 Acacia Drive, Grand Rapids, MI 49504 USA
616 450 8653

 

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Joe Huang
Sent: Tuesday, October 29, 2013 6:40 PM
To: Robert Towell
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] New Rules of Thumb for Design Loads

 

I think there's been some conflating of annual energy simulations with loads
calculations. To me, the big difference is not in their techniques, but
their objectives, and it bothers me when I read posts suggesting that loads
calculations can be done with rules of thumb, simple manual calculations,
etc., which reminds me of what a DOE officer opined in a meeting one time
that residential equipment were sized with the "door method", i.e.,
"whatever fits through the door".  

In my opinion, both should be (and are being) done with equal rigor.  In
fact, most of the dynamic calculation methods (cooling load temp.
difference, response factors, radiant time series, etc.) were developed for
load calculations, and only later extended to annual simulations.   From
what I know, there are plenty of load calculation tools that rely on dynamic
simulations, the main difference from annual simulations being what they
assume for the building internal conditions and the outdoor conditions.  For
an annual simulation, those are designed to be as representative or typical
as possible of actual conditions.  For load calculations, these are the near
worst case conditions (e.g., for heating - no internal loads, 99.6% climate
conditions, no sun, etc.) to derive the peak loads, to which safety factors
are often added.

When people persist in using per floor area loads, is that any different
than estimating the annual consumption using fixed kBTU/ft2-yr numbers?
Does anyone think that either is credible?  As far as oversizing HVAC
equipment, there are several factors at play - use of outdated "rules of
thumb" derived  when buildings were leakier (but cooling intensities might
actually be lower), overly conservative design conditions (as I once said to
the owner of the Agenda 21 Building in Beijing - the peak load derived by
his engineer assumes that you're hosting a conference in your building on
the hottest day of the year), and the proverbial 30% safety factor.  Leaving
aside the first factor, the latter two should explain all the differences
between what a loads calculation program shows and the peak loads from an
annual simulation.

I guess what I'm trying to say is that we should promote better load
calculations rather than simply update "rules of thumb" or develop simple
manual methods for the computer-challenged engineers.  Furthermore, as
buildings improve, such simple methods are apt to be progressively less
reliable. 

Joe

Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 108D
Moraga CA 94556
yjhuang at whiteboxtechnologies.com
http://weather.whiteboxtechnologies.com for simulation-ready weather data
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"


On 10/29/2013 2:04 PM, Robert Towell wrote: 

Sometimes I think we get too caught up in the modeling.  Why not simply
provide a manual block load analysis for the proposed building, with basic
allowances for lighting, equipment, envelope loads, etc.  You could even
provide both old school values like 2 W/SF for lighting vs. 1 W/SF today to
show the building engineer why the values have dropped.  A one page summary
of expected loads would provide all involved a snapshot of where the cooling
loads are coming from, list assumptions, and possibly even identify areas
for additional attention.  The building engineer could help define a "safety
factor" to be included if desired.  Taking another step back in time, I
believe the 1975 (first) ASHRAE energy code only required an average R3
window/wall insulation and R12 roof insulation.  We have come a long way! 

  

Robert L. Towell, P.E. LEED AP, QCxP 

CXE Logo with Text Small

20A Edwardsville Professional Park 

Edwardsville, Illinois 62025-3602 

Phone:  618-307-5882 

Cell:      314-591-6543 

  

  

  

  

From: Dru Crawley [mailto:dbcrawley at gmail.com] 
Sent: Tuesday, October 29, 2013 2:12 PM
To: Christian Kaltreider
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] New Rules of Thumb for Design Loads 

  

Those old rules of thumb can mean significantly higher capital cost. I
remember a project in the DC area about 20 years ago. Their rules of thumb
came up with 10,000 tons (yeah a big building) ... but when a colleague did
a back of the envelope calc (in the days before we even hope for low-energy
buildings), he came up with 3,000 tons... and that was very conservative.
So the owner was about to (and did) pay for 3x the size needed and I'm
willing to bet the building rarely goes over 2,000 tons. 

  

On Tue, Oct 29, 2013 at 2:57 PM, Christian Kaltreider
<ckaltreider at sudassociates.com> wrote: 

David, Dru, Jim,

Thanks.  I will consider the DOE reference buildings as an option, at least
for comparison.

Jim, to answer your questions without getting into project details...We have
been asked to give our opinion on the expected loads for a proposed building
so the owner can have an idea of what impact it will have on their central
plants.  We'll do this based primarily on similar past projects of ours.
However, the owner has an on-site engineer who is using very old ASHRAE
documentation to come up with his own (very high) design load estimates.
Apparently ASHRAE used to publish sf/ton guidance for office buildings?  I
would like to have some more current documentation to show the engineer to
help support my explanation of why our prediction will be lower than his.
Also, I think my wording concerning credibility came out wrong.  I certainly
wasn't implying that my company isn't highly capable/credible.  But it's
always nice to have a DOE report as backup documentation!

Thanks for your help,
Christian

-----Original Message-----
From: Jim Dirkes [mailto:jim at buildingperformanceteam.com]
Sent: Tuesday, October 29, 2013 9:48 AM
To: Christian Kaltreider; bldg-sim at lists.onebuilding.org
Subject: RE: [Bldg-sim] New Rules of Thumb for Design Loads

Christian,
I suspect that you and your company are very credible and shouldn't hesitate
to claim the truth of that.  Your client probably would not have retained
you if that were not true!  It would be nice to have a 1000 building study
from DOE, though :) I wonder why your client is asking?  There are so many
design possibilities and at the end of the day, design load affects only
equipment size (including the electricity and fuel services).  These, in
turn, can be "managed" by choices for insulation, lighting, etc.
Another thing to consider (after assessing your firm's liability from
lawsuits) is that design loads occur infrequently in most climates, so much
so that a full energy model analysis can sometimes show that the impact on
comfort is minimal for a nominally UNDERsized HVAC system.
It's a tangled web!  I think I'd ask the basis of their concern about peak
load.

James V Dirkes II, PE, BEMP, LEED AP
www.buildingperformanceteam.com
Energy Analysis, Commissioning & Training Services
1631 Acacia Drive, Grand Rapids, MI 49504 USA
616 450 8653 <tel:616%20450%208653> 


-----Original Message-----
From: Christian Kaltreider [mailto:ckaltreider at sudassociates.com]
Sent: Tuesday, October 29, 2013 9:21 AM
To: Jim Dirkes; bldg-sim at lists.onebuilding.org
Subject: RE: [Bldg-sim] New Rules of Thumb for Design Loads

Jim,

Thanks.  Yes, I can do my own little study, and I can also just look back at
results from past projects.  That's a good suggestion.  But I was hoping to
have something credible to reference (outside my own firm) for the owner.  I
have seen DOE/National Lab comparisons of annual energy usage between the
different versions of 90.1...I was hoping there might be something similar
for design loads.

Thanks,
Christian

Christian Kaltreider, LEED AP | Energy Analyst Sud Associates, P.A. | T
828.255.4691 | F 828.255.4949 | www.sudassociates.com


-----Original Message-----
From: Jim Dirkes [mailto:jim at buildingperformanceteam.com]
Sent: Tuesday, October 29, 2013 9:11 AM
To: Christian Kaltreider; bldg-sim at lists.onebuilding.org
Subject: RE: [Bldg-sim] New Rules of Thumb for Design Loads

Dear Christian,
I do not know of any such resources!
What do you think about creating a "typical" building and trying out various
envelope, lighting, plug load and HVAC options in your climate to see the
impact of each? That should not be too daunting a task.

James V Dirkes II, PE, BEMP, LEED AP
www.buildingperformanceteam.com
Energy Analysis, Commissioning & Training Services
1631 Acacia Drive, Grand Rapids, MI 49504 USA
616 450 8653 <tel:616%20450%208653> 


-----Original Message-----
From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Christian
Kaltreider
Sent: Tuesday, October 29, 2013 9:06 AM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] New Rules of Thumb for Design Loads

Hello Group,

Does anyone know of any articles/papers/publications which address peak
design loads for buildings as energy codes evolve?  I am looking for basic
rules of thumb for sf/ton (cooling) and btuh/sf (heating).  People use these
rules of thumb all the time for initial assessments, sanity checks on
simulation results, etc, but I'm not sure that the rules of thumb are being
updated as energy codes become more aggressive.  For example, the engineers
I have been around since I entered the engineering world (granted, that was
only a few years ago) seem to always go back to 400 sf/ton as a standard
value, then adjust up or down depending on building characteristics.  I
haven't seen this  number change as buildings improve, or as I move to
different climate zones.  To be clear, I am interested in design loads, not
annual energy usage.

(Note:  Before anyone lambasts me for even bringing up rules of thumb for
design, I'll go ahead and say that I know they are gross estimations at
best, and should be used with extreme caution and judgment.  Nonetheless, I
see them used all the time, so I might as well get as good numbers as I can.
And a project I am on right now actually requires it (by owner) in
preliminary design.)

Thanks for your help,
Christian


Christian Kaltreider, LEED AP | Energy Analyst Sud Associates, P.A. | T
828.255.4691 | F 828.255.4949 | www.sudassociates.com







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