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

Robert Towell btowell at cxegroup.com
Tue Oct 29 14:04:36 PDT 2013


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<mailto: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<mailto:jim at buildingperformanceteam.com>]
Sent: Tuesday, October 29, 2013 9:48 AM
To: Christian Kaltreider; bldg-sim at lists.onebuilding.org<mailto: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<http://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<mailto:ckaltreider at sudassociates.com>]
Sent: Tuesday, October 29, 2013 9:21 AM
To: Jim Dirkes; bldg-sim at lists.onebuilding.org<mailto: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<tel:828.255.4691> | F 828.255.4949<tel:828.255.4949> | www.sudassociates.com<http://www.sudassociates.com>


-----Original Message-----
From: Jim Dirkes [mailto:jim at buildingperformanceteam.com<mailto:jim at buildingperformanceteam.com>]
Sent: Tuesday, October 29, 2013 9:11 AM
To: Christian Kaltreider; bldg-sim at lists.onebuilding.org<mailto: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<http://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>
[mailto: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<mailto: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<tel:828.255.4691> | F 828.255.4949<tel:828.255.4949> | www.sudassociates.com<http://www.sudassociates.com>







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