[Bldg-sim] ASHRAE question

James V Dirkes II, PE jvd2pe at tds.net
Tue Jun 22 10:23:17 PDT 2010

Dear Bldg-Sim folk,


I think that overdesigning / oversizing is rampant in the engineering
community.  Largely, this is a response to concerns about litigation or
worries about non-technical clients who don't know how they'll use a
facility.  VSD- equipped fans and pumps go a long way toward reducing the
energy penalty associated with oversized systems, but it can still be


I think I've read recently that many systems are 50% larger than actually
required.  This, of course, means that duct, pipe, and water, gas and
electrical service are also oversized and more costly than needed (not to
mention the equipment itself).  I recently reviewed some load calculations
where the designer used ~5W / sq.ft. for internal loads in an office space
that was pursuing LEED certification.  The same space required ~1W / sq.f.t
for lighting in order to meet the ASHRAE 90.1 requirements and had normal
plug loads, all of which probably didn't add up to anything near 5W/sq.ft.
On top of that, the zone HVAC equipment was heat pumps, so I have serious
doubts about the quality of humidity control for oversized DX equipment.


Doing the math shows that 1% of the summer hours is , well, 1%!  99% of the
time, loads will be met; that's a lot! In addition, the 1% hours do not
occur all at once; the HVAC has a chance to catch up at night.  They don't
occur at the beginning of the day, either. Comfort is maintained for the
bulk of the day in most cases and the "unmet loads" may even occur after
most occupants have gone home due to the normal thermal lag.


Early in my career, I UNDER-sized a heating system in the Midwest and
learned that I was TOO close to the capacity line,  Since then, my
philosophy has been somewhere between "Make sure capacity is plenty, with
good part load performance" and "Act like an engineer and believe your own
calculations (without padding them)!"  Lately, I'm leaning more toward the
second one.


The Building Performance Team
James V. Dirkes II, P.E., LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653


From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Nick Caton
Sent: Tuesday, June 22, 2010 12:16 PM
To: Rob Hudson; Seth P. Spangler
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] ASHRAE question


I can confirm: I learned during and after schooling that the choice to use
any set of columns is the designers' preference.  Some designers will flip
between based on elements of the project at hand, and others will always
stick to one set of columns based on their personal tolerance/perception of
the stringency of each set of conditions.


I don't know if ASHRAE intended this, but personally I always use the most
stringent figures as a rule when sizing up equipment.  My reasoning is I'm a
young designer who hasn't fully grasped where less conservative criteria are
acceptable, and I choose to control the potential for oversizing via final
"rule-of-thumb" / comparative design checks and the occasional investigative
energy model.




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From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Rob Hudson
Sent: Tuesday, June 22, 2010 10:29 AM
To: Seth P. Spangler
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] ASHRAE question


My original response to the question was it was designers preference.  This
seems to be not entirely wrong, as it could be clients preference or space
use that has some influence on the designers choice of design conditions.
Thanks for you input.

On Tue, Jun 22, 2010 at 11:29 AM, Seth P. Spangler <sspangle at rmf.com> wrote:



I have never read specific conditions of when to use one over the other
however typically the conditions used are dictated by the type of building.
In my experience buildings with low tolerances for temperature change use
the 99.6%/0.4%. These buildings would include hospitals, laboratories, R&D
etc. Buildings with a high level of tolerance will use 99%/1%. These
buildings are typically office buildings, higher education, retail etc.


In the past six months I have seen a huge change in design conditions. The
latest owner has specified design conditions of ASHRAE 99%/1% with Summer
indoor of 78 deg F/50%RH and Winter indoor of 68 deg F. However, they want
the thermostat set point to be 73 deg F year round.


Seth Spangler, LEEDR AP 


RMF Engineering, Inc 

Ph: (843) 971-9639 ext:1497

Fax: (843) 971-9641 

sspangle at rmf.com


From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Rob Hudson
Sent: Tuesday, June 22, 2010 11:15 AM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] ASHRAE question


I was asked what influences an engineers decision over design conditions,
and after looking for some textural support in the books, i could not find
anything to give guidance over when to use 0.4/1/2 or 99/99.6 % design
conditions.  Does anyone know if this text exists in the ASHRAE books and if
so which chapter?  I also looked into ASHRAE 90.1, but that only addresses
efficiencies of equipment and not the design conditions/capacity

Thanks in advance,

Rob Hudson

Rob Hudson

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