[Equest-users] What's in an Air Wall?

David Eldridge dse at grummanbutkus.com
Wed Jan 26 12:19:34 PST 2011


Although “highly-conductive” you wouldn’t necessarily assume that the space
temperatures end up being identical – there is still some resistance in your
example, even if very small, and the area of interface is not infinite
either.



Your last example with area/volume – the heat transfer will be limited by
the size and thermal conductivity of this air wall.  There are also radiant
and storage effects from the other surfaces in the zone that might keep the
two from being in equilibrium – that said your approach may be fine as you
may not have widely differing temperatures/loads.  One possible tweak might
be to allocate your internal gains in these two modeled spaces to load the
separately modeled HVAC systems along how you think they would actually
perform in the real “two-system-one-zone” space.



David



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David S. Eldridge, Jr., P.E., LEED AP BD+C, BEMP, HBDP

*Grumman/Butkus Associates*

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*From:* equest-users-bounces at lists.onebuilding.org
[mailto:equest-users-bounces at lists.onebuilding.org] *On Behalf Of *Nick
Caton
*Sent:* Wednesday, January 26, 2011 11:49 AM
*To:* equest-users at lists.onebuilding.org
*Subject:* [Equest-users] What's in an Air Wall?



Hi everyone,



A discussion on [bldg-sim] prompted me to bring up a topic that’s been
bugging me in the “eQuest fundamentals” department…



I have a general understanding that eQuest does not fundamentally model
airflow (specifically, convection of internal loads) between zones.

-          The DOE-2 entry for INT-WALL-TYPE says an internal “air”
partition “ …designates a non-physical interior surface with no mass (i.e.,
an opening between spaces) across which convection can take place.”

-          A wizard-generated “air” internal partition has a construction
with U-factor of 2.7… very conductive.

-           To draw a conclusion – two zones connected with an “air”
partition are “connected” thermally.  In practice, the internal loads in one
are “combined” with the other.

-          This means heat in one zone should travel to the other in a rapid
fashion during the hourly simulation, until the space temperatures are
identical between the two.



I hope my understanding thus far is correct, because from here I have some
questions that dig at what’s going on under the hood:

1.       Imagine an air partition “connects” zones A and B.  These zones
have separate systems and separate thermostats with different setpoints.  If
zone A’s thermostat wants to be much warmer than zone B, is it possible the
systems will “fight” each other and cause mutual unmet hours?

2.       In the same setup, if Zone A is identical in geometry to Zone B,
but has 2x the internal/external loads, does it follow that the system for
System A will handle 2x the internal loads as System B, or are they summed
and applied equally to the two systems on an hourly basis?

3.       Is the “distribution of loads behavior” affected if Systems A & B
are specified with different capacities and/or airflows?

4.       If one space is larger in area/volume than the other, does that
affect how the collective loads are distributed to the corresponding
systems?



I have “exploited” air partition behavior in the past to get around the “one
system per zone” rule (need two RTU’s serving that space?  Just make an
imaginary air wall!).  However I want to be sure before I continue this
practice or advise others to do the same that there aren’t any major
potential pitfalls in how the loads/systems are distributed/affected…



~Nick



[image: cid:489575314 at 22072009-0ABB]**

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*NICK CATON, E.I.T.***

PROJECT ENGINEER

Smith & Boucher Engineers

25501 west valley parkway

olathe ks 66061

direct 913 344.0036

fax 913 345.0617

www.smithboucher.com* *
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