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

Daniel Knapp danielk at arborus.ca
Wed Jan 26 11:50:14 PST 2011


Hi Nick,

An air wall is treated as a heat transfer surface in DOE, with a certain thermal conductivity (or U-value).  It is my understanding that DOE assumes that everything is steady state; that is, the temperature in the spaces on either side of the air wall are held constant at their set points.  The LOADS program will calculate the heat transfer to be Q = UA (T_warm - T1_cool) from the warmer to the cooler space.  This heat transfer will show up as a constant heating load in the warmer space and a constant cooling load in the cooler space.  Depending on the size of this heating/cooling load relative to the capacities of the systems, this could lead to unmet heating/cooling hours.  

The size of this load will depend only on the area of the interior wall separating the two spaces, the thermal conductivity of the wall, and the difference in temperatures.  The size of this load will not depend on the geometry or volume of the spaces in question, or on the internal loads in each space, etc.  

If you are using an air wall to force a space to accept two systems, then I would be very careful about where the wall goes to ensure that the operation of the two systems best reflects the design intention.  For example, you might want to consider how the placement of the wall separating the spaces affect the external loads that each space experiences, etc.  

Cheers,
Dan

—
Daniel Knapp, PhD, LEED® AP O+M
danielk at arborus.ca

Arborus Consulting
Energy Strategies for the Built Environment
www.arborus.ca
76 Chamberlain Avenue 
Ottawa, ON, K1S 1V9 
Phone: (613) 234-7178 ext. 113
Fax: (613) 234-0740



On 2011-01-26, at 12:48 PM, Nick Caton wrote:

> 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
>  
> <image001.jpg>
>  
> 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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