All,
Should always be the goal but it is soooo hard to define. I have worked on
tiny commercial buildings as well as huge commercial buildings. All types.
Right sizing is always my goal and as far as I know is always the goal of
the ME's. It was mine when I did HVAC design work. Building occupants are
intolerant of too hot and too cold. And, in a building of any size with more
than one person in it you will find disagreement about whether it is too hot
or too cold at least some of the time. So what do you do? Especially if you
are just the hapless, overworked, under paid energy modeler? You can see my
project list if you want to. If you listen closely you will hear whatever
opinion you want to hear about me and my work. Thank god for VFDs, VRVs and
EC motors, eh?
--
Carol Gardner PE
My thought process-
I have had a few designs which had meeting rooms which could hold a large
number of people. If I were to size the equipment for the max number of
people on the hottest day the equipment size would be a few tons larger.
Example. A high school youth room in a church on the third (top) floor with
a max occupancy of 30 people.
My conclusion. If 30 high school kids try to meet in that room at 4:00 p.m.
in the middle of summer, that room will get warm.... They should find another
room. The sanctuary isn't likely going to be in use at 4:00.
There have been other meeting rooms I have used this line of thought. So far
I have only applied it to meeting rooms. Simple, if you try to pack a
meeting room with people on a hottest day at the hottest part of the day, it
may get hot. It isn't a poor design, it is using the 'art' of engineering.
Best advice. Talk to the owner. There is a lot of equipment sizing
reduction opportunity by finding the 'worse practical' design conditions as
opposed to the typical 'worst possible' conditions.
Two other examples of this which another designer shared with me.
1. Schools don't usual hold classes in the summer. Find the peak during
school months, not in July. The design will typically be large enough to
handle summer school. And this condition can be modeled to check.
2. Christmas and Easter are not in the summer. And services are usually in
the morning. A church typically has no need for a system to be designed for
100% at 4 p.m. in the summer.
(This doesn't help for LEED since you must use the same schedules as the
baseline, but it helps in actual energy savings. This could actually hurt
you in LEED with UNMET hours if you don't modify your schedule to match your
assumptions.)
*I am really enjoying this sharing of information. Thank you all.*
John Eurek
LEED AP
I think this is a wonderful time to be in the HVAC industry. New tools
are changing the outdated, over simplified tools and methods used in the
past (and present). I would like to make one comment on the below
message. The sentence "Building occupants are intolerant of too hot and
too cold." This is one good example of the changing attitudes in the
building industry. Resent research suggest that this sentence should
really read " Building occupants with no control over their environment
are intolerant to too hot or too cold". When occupants know they can
control their environment (maybe with a window) they will dress
accordingly and expand their comfort zone.
On a "right sizing" note, I do believe if owners would like to get
aggressive and truly right size equipment they will also need to
shoulder the possible risks, i.e. 10 or 20 hours out of the year you may
not be able to keep your cooling set point. Its energy modeling that
will quantity these risks and size these systems, not a side ruler and a
design day (hour).
This exchange has been a nice change of pace from locating weather
files. It's very interesting to learn other current points of view in
our industry,
Best Regards,
Brad Acker, P.E.
My thought process-
I have had a few designs which had meeting rooms which could hold a large
number of people. If I were to size the equipment for the max number of
people on the hottest day the equipment size would be a few tons larger.
Example. A high school youth room in a church on the third (top) floor with
a max occupancy of 30 people.
My conclusion. If 30 high school kids try to meet in that room at 4:00 p.m.
in the middle of summer, that room will get warm.... They should find another
room. The sanctuary isn't likely going to be in use at 4:00.
There have been other meeting rooms I have used this line of thought. So far
I have only applied it to meeting rooms. Simple, if you try to pack a
meeting room with people on a hottest day at the hottest part of the day, it
may get hot. It isn't a poor design, it is using the 'art' of engineering.
Best advice. Talk to the owner. There is a lot of equipment sizing
reduction opportunity by finding the 'worse practical' design conditions as
opposed to the typical 'worst possible' conditions.
Two other examples of this which another designer shared with me.
1. Schools don't usual hold classes in the summer. Find the peak during
school months, not in July. The design will typically be large enough to
handle summer school. And this condition can be modeled to check.
2. Christmas and Easter are not in the summer. And services are usually in
the morning. A church typically has no need for a system to be designed for
100% at 4 p.m. in the summer.
(This doesn't help for LEED since you must use the same schedules as the
baseline, but it helps in actual energy savings. This could actually hurt
you in LEED with UNMET hours if you don't modify your schedule to match your
assumptions.)
*I am really enjoying this sharing of information. Thank you all.*
John Eurek
LEED AP
Taking this discussion back to energy simulation and its impact on
design. Over-sizing systematically occurs but from an energy
perspective it is not always bad. One of our jobs as energy modelers is
to identify the impacts of over-sizing and to drive the design to
maximum annual efficiency. So if the designer is going to oversize the
pump, we need to try and quantify the savings potential of including a
VFD for balancing instead of using throttling valves. If the designer
is going to oversize the boiler, we need to justify to the owner the
incremental costs for purchasing the most efficient boiler setup, which
could be a boiler with a high turndown ratio and oxygen trim controls or
multiple smaller boilers.
Also, we may want equipment/systems to be oversized in some situations.
Our job as modelers is to quantify the energy impacts of design
decisions, which may include over-sizing. And what does over-sizing
always mean? Should pipes be sized for 1, 2 or 3ft loss/100 ft, should
ducts be sized for , .1, .2, or .3 ft loss/100 ft, should air handler
coils be sized for 200 fpm or 400 fpm?
Yes over-sizing costs money to the owner up front, but when designed,
installed, and operated correctly - in some situations over-sizing can
be the most efficient design with the most attractive life cycle cost.
Glen R Anderson, PE