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

[Nick Caton]

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

 

 

 

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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