<font face="tahoma,sans-serif">Thanks Bruce. </font><span class="Apple-style-span" style="font-family: tahoma, sans-serif; ">Great reply. </span><div><font class="Apple-style-span" face="tahoma, sans-serif">I completely agree that modelers need to have a good technical knowledge.</font></div>
<div><font class="Apple-style-span" face="tahoma, sans-serif">Oversizing AHU is justified now. I appreciate your help.</font></div><div><font class="Apple-style-span" face="tahoma, sans-serif"><br></font></div><div><font class="Apple-style-span" face="tahoma, sans-serif"><br>
</font></div><div><font class="Apple-style-span" face="tahoma, sans-serif">Much thanks,</font></div><div><font class="Apple-style-span" face="tahoma, sans-serif">Aru</font></div><div><font class="Apple-style-span" face="tahoma, sans-serif"><br>
</font></div><div><font class="Apple-style-span" face="tahoma, sans-serif"><br></font><div class="gmail_quote">On Tue, Jul 26, 2011 at 7:28 AM, Bruce Easterbrook <span dir="ltr"><<a href="mailto:bruce5@bellnet.ca">bruce5@bellnet.ca</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
<div bgcolor="#FFFFFF" text="#000000">
<font size="+1"> The intent of 90.1 and using a modelling program
like eQuest is to size the equipment closer to the actual demand.
The 15% and 25% variance factors were used in the past before the
computerized modelling we are using today. Doing load
calculations buy hand or using a spreadsheet made for much more
deviation between the calculations and the building. Less things
were accounted for as well. By providing more certainty using a
modelling program we should be reducing these safety factors.
These factors are still a best guess and the designer must keep
all this in mind. The next step in the process is specifying the
equipment for the building. So if the modeller has a 15% SF on
the equipment and because most equipment is built with discrete
sizes, the specifier adds another 20% by picking the next size up
you can see how things start to chain together and get out of
control. Next step is bidding, the specifier uses Trane as base
bid but Carrier wins the process and submits shop drawings. They
are normally required to provide a superior bid, there is another
8%. Remember as well the second and third increases are now
compounding, we are not 43% over sized but probably more like
50%+. My point is the original design and SF should be used from
start to finish.<br>
Next question is where to apply these factors. They should be
on the air handlers. If your fan is too small and your coils are
too small what you get supplied from the plant is irrelevant. You
physically do not have the equipment to handle the diversity. So
the method is to account for the diversity in the end units, and
have the plant at 1.0 or less. Your SF on the units flows through
to the plant. Note I mentioned "or less". Consider a building
with a north south split. An AHU on the north side will only see
diversity due to weather. A south side AHU will see diversity due
to weather and sun. You may consider different diversity factors
for each. Throw in 10 buildings of different sizes and
orientations you can see the effect in the deviation on the plant
using old methods. eQuest due to the through nature of the model
can account for all this. Can is a critical word, it all depends
on the model. There is still diversity in the model. The amount
depends on what the money people want to spend modelling. LEED is
the attempt to control these factors, to set a minimum quality
level for the model. It is now realized this is not enough.
Original LEED buildings on the majority do not perform as the
model suggested they would. Now commissioning is the new buzz
word. As if that is the only reason. If your units are too small
commissioning won't fix the problem either. If your units are too
big the system will work but not efficiently. Part of the problem
is the process, hence the proposal that modellers be PE's. That
may help but it wouldn't solve many of the problems. The model is
important but is only effective if used as a reference start to
finish. The safety factors come from using eQuest and doing
project reviews. Comparing the actual building performance to the
model and taking a look at the allowances for variance. There is
only one group responsible for this whole process, it is the
mechanical engineer who has assumed responsibility for the
building project.<br>
The short answer is there is no rule of thumb for the safety
factors. More experience allows you to guess better but it is
still a guess because every project is different. Model feedback
is spotty. The safety factors should be applied at the terminal
equipment not at the plant. Mostly, 15% for cooling and 25% for
heating are too big. In most cases .5 cfm/SF is too small. But
that is where I start. Get the model built and running, let it
run on defaults, let it determine the initial sizing, thoroughly
review the sim output, then start tuning it.<br>
Bruce Easterbrook P.Eng.<br>
Abode Engineering<br>
</font><div><div></div><div class="h5"><br>
On 25/07/2011 07:47 PM, Arunabha Sau wrote:
</div></div><blockquote type="cite"><div><div></div><div class="h5"><font face="tahoma, sans-serif">Howdy!</font>
<div><font face="tahoma, sans-serif"><br>
</font></div>
<div><font face="tahoma, sans-serif"><br>
</font></div>
<div><font face="tahoma, sans-serif">So,
I just saw a project which<span style="border-collapse:collapse"> has applied its oversizing factors (1.15 for
cooling and 1.25 for heating) to the plant rather than the
air handlers. ASHRAE 90.1 Appendix G </span><span style="border-collapse:collapse">page
180, G3.1.2.2 Equipment capacities</span><span style="border-collapse:collapse">
does not specify where the oversizing should occur. Have
you seen this method used before? Is it an acceptable and
accurate way of meeting the baseline modeling requirements?</span></font></div>
<span style="border-collapse:collapse"><font face="tahoma, sans-serif">
<div class="gmail_quote"><br>
</div>
<div class="gmail_quote">I appreciate your help.</div>
<div class="gmail_quote"><br>
</div>
<div class="gmail_quote"><br>
</div>
<div class="gmail_quote"><br>
</div>
<div class="gmail_quote"><br>
</div>
<div class="gmail_quote">Much thanks,</div>
<div class="gmail_quote">Aru</div>
</font></span>
<br>
<fieldset></fieldset>
<br>
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