ASHRAE question

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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
calculations.

Thanks in advance,

--
Rob Hudson

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Rob,

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, LEED(r) AP

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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.

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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.

NICK CATON, E.I.T.

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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
severe.

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.

James V. Dirkes II, P.E., LEED AP

James V. Dirkes II  P.E.'s picture
Joined: 2011-10-02
Reputation: 0

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
severe.

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.

James V. Dirkes II, P.E., LEED AP

James V. Dirkes II  P.E.'s picture
Joined: 2011-10-02
Reputation: 0