[Bldg-sim] New Rules of Thumb for Design Loads
Joe Huang
yjhuang at whiteboxtechnologies.com
Tue Oct 29 15:40:27 PDT 2013
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outdated "rules of thumb" derived when buildings were leakier (but cooling intensities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
> <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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