New Rules of Thumb for Design Loads

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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 Kaltreider, LEED AP

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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

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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 Kaltreider, LEED AP

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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

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Don't the DOE reference buildings sort of fit that bill? 16 building types/16 climate zone?

I looked at the results for small office for Baltimore and was able to calculate 650 ft2/ton for cooling.... and I see enough data to do similar for heating, peak demand, etc.

http://www1.eere.energy.gov/buildings/commercial/ref_buildings.html

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You could use the DOE reference buildings, although these are not "designed" buildings, they only exist in model space...so as a designer I wouldn't use the peak loads necessarily, but maybe the ratio of peak load would be interesting across the code versions.

DSE Mobile

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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

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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.

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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

Bob Towell at CxE Group

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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 Huang

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In the case of large commercial buildings the cooling energy depends on
occupancy types & internal loads and winter energy depends on ventilation
rates (hospitals). Weather has little effect because of the envelope
efficiency standards.
It is possible to have cooling energy rules of (many) thumbs for different
types of commercial buildings.
See http://bepan.info/design and open Excel file 1 - eQ-DOE22-AES-Structure
for building types
Varkie

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Christian -

If the key issue is the vintage of the ASHRAE information used to estimate the loads per square foot - I am curious if ASHRAE has updated the loads per square foot check figures published in their ASHRAE Pocket Guide. I have a 2001 and 2009 vintage: the 2009 version updates the Expected Lighting & Equipment Heat Gain (Office goes from 4 / 6/ 9 W/sf to 2 / 2.5 / 4 W/sf, low/med/hi) but keeps the expected Refrigeration density the same (360 / 280 / 190 sq ft / ton).

I wonder if the Refrigeration values in the current edition match the refrigeration values in the reference your client's in-house engineer is using.

Aaron Dahlstrom, PE, LEED(r) AP

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On Tue, Oct 29, 2013 at 7:06 AM, Christian Kaltreider> Hello Group,

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..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

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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

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Joined: 2011-09-30
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This has been a fascinating question and thread, a few thoughts from my
limited experience as a mechanical engineer/energy modeler:

- First off, I am a huge fan of rules of thumbs for initial design
and estimation, I would never sign off on a design that is built around
them. I will use rules of thumbs for early on, then fully anticipate
running a full load analysis/energy model later.

- Yes the rules of thumb are based on building codes and
regulations from 30+ years ago, but at least a vast majority of our work is
renovation work, so in some cases those rules of thumb are not that
outdated. Granted we are typically updating the envelope, HVAC
efficiencies, internal loads such as lighting, but in those initial design
phases and meetings where everyone is looking at you for an answer, a rule
of thumb is often all you have, and experience and knowledge of the project
can hopefully help you hone those numbers in. But initially they are very
helpful.

- I have also learned the owner loves when the price comes down,
not up. So if I overestimate the sizes of equipment with my rules of thumb
and later tell everyone it?s actually lower once I started my detailed
calculations, everyone understands and everyone is happy. Undersizing my
equipment and adding cost to the budget later on in the design process is
awful.

- Finally, with all the modulating technologies we have today
oversizing equipment isn?t as terrible, at least with hydronic systems, DX
is a little tougher. Modulating chillers, boilers, pumps, fans and outside
air intake can all match the load surprisingly well, allowing for slight
oversizing, especially in the case below of the massive conference on the
hottest day of the year. Granted if my fan static pressure is 3? I?m going
to slap 6? on it, but we do have some saving grace from modulation.

[image: cid:image003.png at 01CDAB78.6524ACE0]

*Ben Hollon, EIT, LEED*?* AP BD+C, MBA*

*From:* Joe Huang [mailto:yjhuang at whiteboxtechnologies.com]
*Sent:* Tuesday, October 29, 2013 5:40 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 Huang

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