NISTIR 6967

LoopDA – Natural Ventilation Design and 

Analysis Software

W. Stuart Dols 

Steven J. Emmerich 
 
NISTIR 6967

LoopDA – Natural Ventilation Design and 

Analysis Software

W. Stuart Dols 

Steven J. Emmerich 

Building Environment Division 

Building and Fire Research Laboratory 

National Institute of Standards and Technology 

Gaithersburg, MD 20899-8633 

Prepared for: 

Architectural Energy Corporation 

Boulder, Colorado 

and 

U.S. Department of Energy

April 2003 

U.S. Department of Commerce

Donald L. Evans, Secretary

Technology Administration

Phillip J. Bond, Undersecretary of Commerce for Technology

National Institute of Standards and Technology

Arden L. Bement, Jr., Director 

ABSTRACT

The Loop Equation Design Method has been proposed for sizing ventilation airflow components of 

natural and hybrid ventilation systems. While the approach has been demonstrated on a limited basis, 

the method has been automated in order to better evaluate its reliability under a more controlled, i.e., 

less error-prone, environment. This report describes a computer program developed by NIST that 

implements the Loop Equation Design Method of sizing the openings of naturally ventilated 

buildings. The tool, referred to as LoopDA for Loop Design and Analysis, is integrated with an 

existing multi-zone analysis tool CONTAMW. LoopDA provides the designer of natural ventilation 

systems with an environment in which to perform and document the process of designing the 

opening sizes of natural ventilation systems and analyzing the system behavior under a variety of 

operating conditions. This report describes the first version of the LoopDA program, provides an 

example of its application to the design of a naturally ventilated building and describes needs for 

future enhancements to the tool to increase its usefulness within the design community. 

Key Words: airflow analysis; building technology; computer program; design tool; indoor air 

quality; multizone analysis; natural ventilation 

iii
SOFTWARE DISCLAIMER

This software was developed at the National Institute of Standards and Technology by employees of 

the Federal Government in the course of their official duties. Pursuant to title 17 Section 105 of the 

United States Code this software is not subject to copyright protection and is in the public domain. 

LoopDA is an experimental system. NIST assumes no responsibility whatsoever for its use by other 

parties, and makes no guarantees, expressed or implied, about its quality, reliability, or any other 

characteristic. We would appreciate acknowledgement if the software is used. 

This software can be redistributed and/or modified freely provided that any derivative works bear 

some notice that they are derived from it, and any modified versions bear some notice that they have 

been modified. 

Certain trade names or company products are mentioned in the text to specify adequately the 

experimental procedure and equipment used. In no case does such identification imply 

recommendation or endorsement by the National Institute of Standards and Technology, nor does it 

imply that the equipment is the best available for the purpose.

iv
Table of Contents

Abstract.......................................................................................................................................... iii 

Software Disclaimer....................................................................................................................... iv 

1 Introduction................................................................................................................................1 

2 Loop Equation Design Method..................................................................................................2 

3 The LoopDA Software Tool ......................................................................................................3 

3.1 Installing and Running LoopDA....................................................................................4 

3.2 Basic Approach to Design Using LoopDA....................................................................5 

3.3 Working with LoopDA..................................................................................................6 

4 Design Example using LoopDA..............................................................................................18 

4.1 Sample Building...........................................................................................................18 

4.2 Design Conditions and Criteria....................................................................................18 

4.3 Pressure Loops.............................................................................................................20 

5 Conclusion ...............................................................................................................................29 

6 References................................................................................................................................30 

7 Acknowledgments....................................................................................................................31 

v
vi
1 INTRODUCTION

NIST is investigating the application of state-of-the-art natural ventilation concepts in small 

commercial buildings in California and developing design methods for natural ventilation in new 

and retrofit applications. An earlier report [1] reviewed the application of natural ventilation in 

commercial buildings including the associated technology, potential advantages of natural 

ventilation and issues that need to be addressed. The report also addressed opportunities and issues 

specific to the application of natural ventilation to commercial buildings in California including 

analysis of climate suitability via a new ventilative cooling metric, consideration of ambient air 

quality, and discussion of relevant codes and standards. It also reviewed current design and analysis 

processes and tools and described a plan for the development of new design and analysis guidance 

and tools. 

The key recommendation of the earlier report was to develop a software tool to implement the Loop 

Equation Design Method within the existing multi-zone modeling environment of CONTAM [2] to 

gain the advantages of the existing user interface, airflow element base, and capability to perform

detailed analysis after completing the natural ventilation system design. The Loop Equation Design 

Method (presented in detail by Emmerich et al. [1]) is a method for sizing of natural ventilation 

components that is general and practical and allows direct consideration of stack and wind-driven 

airflow that is critical in addressing natural ventilation airflow. The tool should assist the designer in 

performing the needed steps of developing the natural ventilation system design including: 

establishing the global geometry and topology of zones and interconnecting flow paths, establishing 

design conditions, setting up and solving the representative loop equations based on the “design” 

form of the airflow components, and analyzing operation under a variety of conditions. 

This report describes the first version of the LoopDA program (for Loop Design and Analysis tool), 

which was developed as a version of CONTAM. The report is organized into three main sections – 

Loop Equation Design Method (a brief review of the method implemented in LoopDA), the 

LoopDA Software Tool (a description of the program and how to use it), and Design Example Using 

LoopDA (an example application of LoopDA to the design of a natural ventilation system for a 

small commercial building). 

1 
2 LOOP EQUATION DESIGN METHOD

The Loop Equation Design Method that is proposed for the Design Development stage of the overall 

design process of a natural ventilation system is described in detail by Emmerich, Dols, and 

Axley[1]. (Note: The other design stages – Conceptual Design and Design Performance Evaluation – 

both precede and follow the Design Development Stage and are also described in [1]. Detailed 

theory of the method is presented in [3] and [4].). The Loop Equation Design Method consists of the 

following eight steps: 

1. Lay out the global geometry and multi-zone topology of the natural ventilation flow loops for 

each zone of the building. 

2. Identify an ambient pressure node and additional pressure nodes at entries and exits of each 

flow component along the loops. 

3. Establish design conditions: wind pressure coefficients for envelope flow components, 

ambient temperature, wind speed and direction, and interior temperatures; evaluate ambient 

and interior air densities. 

4. Establish first-order design criteria (i.e., a ventilation objective) and apply continuity to 

determine the objective design airflow rates required for each natural ventilation flow 

component. 

5. Form the forward loop equations for each loop established in step 1 above by systematically 

accounting for all pressure changes while traversing the loop. 

6. Determine the minimum feasible sizes for each of the flow components by evaluating 

asymptotic limits of the loop equation for the design conditions. 

7. Develop and apply a sufficient number of technical or non-technical design rules or 

constraints to transform the under-determined design problem defined by each loop equation 

into a determined problem. 

8. Develop an appropriate operational strategy to accommodate the regulation of the natural 

ventilation system for variations in design conditions. 

As detailed in the following section, the user will perform some of these steps explicitly (e.g., 

Step1). Other steps are performed implicitly by LoopDA without user interaction (e.g., Step 5). Still 

others are accomplished by a combination of explicit and implicit actions (e.g., Step 3). 

2 
3 THE LOOPDA SOFTWARE TOOL

LoopDA was developed as a means to perform the eight steps of the Loop Equation Design Method

presented in the previous section. While it does not fully automate all eight steps, it greatly 

simplifies and provides a means to manage the entire process. 

Specifically, the software accommodates each of the steps as follows: 

1. LoopDA provides a SketchPad interface that enables you to draw a schematic representation 

of the global geometry and multizone topology of the building and to draw the natural 

ventilation flow loops through the relevant airflow paths of the building. 

2. The SketchPad provides the ambient pressure node and keeps track of the pressure nodes 

associated with each of the airflow paths that you identify on the SkethPad. The direction in 

which you draw the loops establishes the intended direction of natural ventilation airflow for 

the purposes of design. 

3. LoopDA provides for the establishment of design conditions by allowing you full control in 

setting ambient conditions of temperature, wind speed and direction. It also enables you to 

set the design temperatures of all airflow paths and automatically calculates the air densities 

of each. The program also provides a means to input the wind pressure coefficient of all 

exterior openings. 

4. LoopDA provides a means for you to define the first-order design criteria for each airflow 

path to be sized, however, it is up to you to select the design criteria and to ensure that 

continuity is not violated in the event that an opening serves multiple flow loops. 

5. Once you have established the geometry, design conditions and criteria and drawn the 

flow/pressure loops, LoopDA will form the forward loop equations for each loop by 

traversing the loop in the established direction and accounting for pressure changes due to the 

pressure/flow relationships of the various flow components, wind and stack effects. 

6. LoopDA calculates the minimum feasible sizes of each unsized flow component in a loop by 

evaluating asymptotic limits of the loop equation for the design conditions. 

7. LoopDA provides the ability to export loop information to a spreadsheet template (provided 

with the program) that displays all the data associated with a given loop, generates 

asymptotic plots and thus provides a means to view relationships between the flow 

components of a loop. This aids the application of design constraints, selection of component 

sizes and documentation of the steps in designing the natural ventilation airflow paths. 

8. Having sized the natural ventilation airflow, you can then utilize LoopDA to analyze the 

building performance under varying conditions. LoopDA implements the established 

multizone building simulation capabilities of CONTAMW 2.0. You perform analysis to 

investigate the effects of unintentional air infiltration, non-design weather conditions, and 

forced-flow elements to simulate hybrid ventilation systems. 

3 
3.1 Installing and Running LoopDA 

LoopDA runs on Windows 95/98/NT/XP/2000. There are three basic components to LoopDA 

software system: the main Windows executable program LoopDA.EXE, the spreadsheet template 

LOOPDATEMPLATE.XLS, and the analysis or simulation engine CONTAMX2L.EXE. 

The setup files are contained in the self-extracting archive file "LoopDAz.exe." Simply execute the 

self-extracter to decompress the setup files. Extract the files to the subdirectory of your choice, or 

simply select the default location. Once you have extracted the setup files, you can run the setup 

program, "setup.exe." This will prompt you to perform an automatic installation of the program. 

Read the instructions to complete the installation. 

 Files Installed 

The following table lists the files installed by the setup program. For each file, the directory to which 

it is installed, the name and a brief description are given. The <program> directory is that selected by 

you when you install the program. The default is C:\Program Files\LoopDA. The <font> directory is 

that of the operating system fonts. The default is <windows>/FONTS, where <windows> depends on 

the operating system you are using, e.g., Windows 2000 <windows> = WINNT and Windows 95/98 

<windows> = WINDOWS. 

Directory File Name Description

<program> LoopDA.exe User interface

Contamx2l.exe Solver

cwhelp2.hlp

cwhelp2.cnt 

CONTAMW help files 

olch2d32.dll

roboex32.dll 

Charting and help display 

dynamic link libraries 

LoopDA.doc LoopDA documentation to 

be used in conjunction with 

CONTAMW help files 

<program>\samples *.prj Sample project files 

*.xls Spreadsheet files

<font> walton##.fnt SketchPad fonts where ## 

ranges from 01 to 16 for 

different SketchPad 

resolutions 

 Uninstalling LoopDA 

The LoopDA setup program will also provide you with an uninstall feature. You uninstall LoopDA 

much as you would a typical Windows program. Access the Control Panel from the Settings

selection of the Start menu. Select Add/Remove Programs from the Control Panel. Select LoopDA 

from the list of installed programs and click the "Add/Remove..." button to uninstall LoopDA. 

 Running LoopDA 

After you install LoopDA, you can run it by selecting LoopDA from the NIST program group of the 

Start menu. 

4 
3.2 Basic Approach to Design Using LoopDA 

This section present a brief over of the basic steps to take to implement the Loop Equation Design 

Method using LoopDA to size the airflow components of a natural ventilation system. Ideally you 

would perform steps a – d once for a given model of a building and iterate through steps e and f. The 

details of each of these steps are presented in the following section Working with LoopDA. Step g is 

the process of performing simulations based on your design. 

a. Define ambient design conditions (see Working with Weather and Wind) 

Set temperature, wind speed and wind direction 

b. Draw the elevation view of the building (see Working with Walls and Zones) 

Draw the walls 

Define the zones 

c. Draw ventilation airflow and duct flow paths to be sized (see Drawing Airflow Paths and 

Drawing Ducts) 

d. Define airflow and duct flow elements (see Working with Inverse Airflow Elements and 

Working with Inverse Duct Flow Elements) 

Select inverse element type 

Set design airflow rate Qdes 

Set design temperature Tdes 

Set wind pressure coefficient Cp(θ) 

e. Draw loops (see Working with Pressure Loops) 

View loop data 

Export loop data 

f. Import into spreadsheet (see Working with the LoopDA Spreadsheet) 

g. Perform analysis 

Set the forward element properties based on design values. 

Switch to analysis mode and run simulation as with CONTAMW.

5 
3.3 Working with LoopDA 

This section serves as a user guide for LoopDA. Using LoopDA is very similar to using 

CONTAMW, so the information contained in this section is presented as a supplement to the 

CONTAMW 2.0 User Manual [2] that is available while running LoopDA as an online help system. 

References to sections of the CONTAMW User Manual / online help will be given in the following 

sections as {CW: Section} and the areas in which the two programs differ will be presented in detail. 

3.3.1 Working with the SketchPad 

When working with LoopDA, a major portion of your interaction will be with the SketchPad or 

drawing region of the interface to develop a schematic representation of your building. The basic use 

of the SketchPad is very similar to CONTAM, i.e., drawing, selecting and editing building 

component data. {CW: Working with the SketchPad} The fundamental difference is that LoopDA 

presents an elevation view of a vertical section of a building, whereas CONTAMW presents plan 

views of individual levels of a building. In CONTAMW there are level commands and toolbar 

buttons that provide access to different building levels. These commands are not available in 

LoopDA. 

Because LoopDA is meant to be both a “true” design tool as well as an analysis tool, it has an 

additional mode of operation that is not in CONTAMW, i.e., the Design mode. You size ventilation 

components in the design mode and therefore an analysis would not be appropriate due to undefined 

components. The analysis mode is intended to enable the evaluation of a given design once all of the 

components have been defined or sized. While in the analysis mode, the drawing tools will be 

deactivated so that you can’t modify your design. You set the mode of operation via the analysis and 

design toolbar buttons shown below in Figure 1. 

Figure 1 – LoopDA toolbar highlighting Design/Analyses mode buttons 

3.3.2 Working with Weather and Wind 

The weather and wind features are used to establish the design ambient conditions for both design 

and analysis modes of operation. However, only the Steady State weather data is relevant to the 

design mode of operation. Set the ambient temperature, wind speed and direction of the Steady State 

weather data of the Weather and Wind Properties prior to working with your airflow loops as this 

data is used in forming the pressure loop equations. Note that because LoopDA presents an elevation 

view, the wind direction should be from either the left or right side of your drawing. This 

corresponds to either a direction of 0o or 180o respectively, based on the default relative north value 

of -90o (CONTAMW defaults to 0o) as also established in the Weather and Wind Properties. Also, 

set the reference wind speed data to obtain the desired wind speed modifier for your building 

location {CW: Defining Steady State Weather and Wind} 

The CONTAMW User Manual contains a detailed presentation of how weather and wind are 

handled by CONTAMW and LoopDA in the analysis mode. {CW: Working with Weather and 

Wind} As presented in that section, both steady state and transient weather data can be implemented 

depending on the type of simulation you are performing. 

3.3.3 Working with Walls and Zones 

The mechanics of drawing walls to form zones are exactly the same as in CONTAMW. The main 

difference between LoopDA and CONTAMW is that you are drawing elevation views of a building 

6 
as opposed to plan views. Each drawing that you create should represent a vertical section of the 

building in question. To represent multiple sections, you could either draw them individually on a 

single SketchPad, or you can create individual LoopDA project files for each building section. {CW:

Drawing Walls, Ducts and Controls; CW: Working with Walls} 

Walls must be drawn so that fully bounded regions are created that make up the zones of your 

building representation. The properties of each zone must then be defined by a zone icon as typical 

for building components. {CW: Working with Zones} The zone properties are not needed in the 

design process, but are required during the analysis phase. 

3.3.4 Working with Airflow Paths 

In LoopDA, airflow paths are the openings between zones, including the ambient, that form direct 

connections between the zones (as opposed to ducted connections discussed below). In the design 

mode, you first lay out (draw) the location of the natural ventilation airflow paths that will make up 

the ventilation flow loops and then define their design properties. 

3.3.4.1 Drawing Airflow Paths 

You draw airflow paths in LoopDA exactly as done in CONTAMW. However, in LoopDA, you can 

only place airflow paths directly on walls. {CW: Creating Airflow Paths} 

3.3.4.2 Defining Airflow Paths 

Once a flow path is drawn you must define its design properties in order to use the path in a 

ventilation airflow loop. The following items are relevant to the design mode of operation, i.e., to 

form Loop Design Equations. 

Airflow Element – Airflow elements describe the mathematical relationship between the flow 

through an airflow path and the pressure drop across the path. You must select an airflow 

element type prior to setting the other parameters. As presented in the following subsection, the 

design mode is only concerned with Inverse Airflow Element types. LoopDA currently supports 

only two types of inverse airflow elements, but all types of CONTAMW airflow elements are 

supported in the analysis mode. You should create an airflow element for each flow path you 

want to design in order to maintain a unique set of parameters for each path. 

Elevation – Set the elevation of each airflow path relative to the reference height of zero for the 

bottom of the building. This value will be used in calculating stack pressures in both the design 

and analysis modes. 

Wind Pressure – Set the wind pressure to be variable and define a wind pressure coefficient 

profile. You can simply set this to be a constant profile or to vary according to the angle of 

incidence of the wind on the wall or flow path. {CW: Airflow Path – Wind Properties} Care 

should be taken when defining flow paths on horizontal walls, as they will default to have 

azimuth angles that might not make sense. Be sure to set the reference wind speed modifier to 

the desire value either individually for each flow path or globally via the Weather data that 

defaults to provide a modifier of 1.0. 

Detailed information on the analysis parameters are provided in {CW: Working with Airflow 

Paths}, and details of working with the Inverse Airflow Element data are provided below. 

7 
3.3.5 Working with Inverse Airflow Elements 

Basically, an inverse airflow element is one that can be sized using the loop equation design method. 

In CONTAM, airflow component equations are formulated in the so-called “forward” form while the 

loop method uses these same equations in “inverse form” – a form relating the pressure drop across 

components to the volumetric (or mass) flow rate and characteristic design parameters (CDP), e.g., 

an area for an orifice element. A detailed discussion of inverse airflow elements is presented in 

Axley [3] and [4]. 

This version of LoopDA supports two types of inverse airflow elements: orifice and general power 

law. The mathematical relationships for each are presented here in forward and inverse form. 

 Orifice Element 

Forward form: 2 / V C & = d Aorfc ∆p ρ

Inverse form:

2

2 2 2 d orfc

V

p C A

ρ ∆ = &

Characteristic Design Parameter: Aorfc

Corresponding Analysis Parameter: Cross sectional area

 General Power Law Element 

Note: This element can be used to represent a range of flow paths, including a self-regulating vent 

when a relatively large value of the exponent n is specified [2]. 

Forward form: ( )

n

V C= ∆p &

Inverse form:

1

V n

p C

  ∆ =    

&

Characteristic Design Parameter: C

Corresponding Analysis Parameter: Flow coefficient (C)

3.3.5.1 Creating Inverse Airflow Elements 

To create an inverse airflow element you select the “New Element” button on the Flow Element 

page of the Airflow Paths Property Sheet that is displayed when defining an airflow path. This will 

display a list of all available types of forward airflow elements. Select either the general power law 

element, represented in the list as Q = C(dP)^n, or the orifice element from the list of elements on 

the Airflow Element Models dialog box. This will reveal the Airflow Element Properties sheet that 

contains two pages of parameters: one that contains the analysis parameters specific to the type of 

model selected and one containing the design parameters for the inverse element. 

3.3.5.2 Defining Inverse Airflow Elements 

Figure 2 shows the Airflow Element Properties sheet for an orifice element and the design 

parameters associated with the element. The parameters on the Design page are those used in 

formulating the pressure loop equations along with the previously mentioned airflow path properties. 

8 
Figure 2 – Airflow Element Properties sheet displaying design data for 

an inverse orifice element 

For each of these inverse elements the design page displays information that is specific to the 

element for which the properties are being displayed. The following is a description of each of the 

properties displayed. 

 Name: This is the airflow element name you provide for this element. This name is used for both 

the forward and inverse flow element. 

 Flow Rate: Specify the design flow rate for this element. It is your responsibility to ensure that 

design flows do not violate continuity when multiple loops pass through common flow paths, as 

is the case for Loops 2 and 3 in Figure 9 of the example presented later.

 CDP Design: Once you have selected a value for the CDP (characteristic design parameter), enter 

it here and select Defined as described below. The type of parameter this value represents will 

depend on the type of inverse element you are sizing.

 Temperature: Set the design air temperature for this element.

 Defined/Undefined: Select Undefined when the element has not yet been sized. As you select 

sizes for elements, you set their CDP Design values and set them to be Defined.

 CDP: This is the name given by LoopDA for this particular element. This will be either Aorfc or 

Ce corresponding to the orifice and power law elements respectively. When you view and export 

loop data, a distinct area or diameter identifier will be provided for the element. This identifier 

will include the path number assigned by LoopDA.

 CDP Order: This is the order of the exponent of the design parameter as it would appear in the 

loop design equation as detailed by Axley [3] and [4]. This will be either 2.0 or 1/n 

corresponding to an area (orifice) or flow exponent (general power law) respectively.

9 
3.3.6 Working with Ducts 

You can use LoopDA to size duct segments and inlet and outlet terminals that represent stacks or 

connections between zones. In the design mode, you first draw the individual duct segments then 

define their design properties. LoopDA implements the Darcy-Colebrook duct flow element. This 

element provides for the sizing of both duct flow segments based on a friction factor and fittings

(inlets and outlets) based on a loss coefficient. 

3.3.6.1 Drawing Ducts 

You draw ducts in LoopDA exactly as done in CONTAMW. However, in LoopDA, you are working 

in the elevation view. {CW: Working with Ducts} You should draw duct segments for each inlet, 

airflow segment and outlet of a duct as shown in the figure below. This will provide a separate term

in the loop equation for each component of the duct system. The details of defining duct segments 

and terminals will be presented in the following sections. 

Figure 3 – Detail for drawing ducts for loop analysis 

10 
3.3.6.2 Defining Ducts 

Once a duct is drawn you must define its design properties in order to use it in a ventilation airflow 

loop. The following items are relevant to the design mode of operation, i.e., to form Loop Design 

Equations. 

Duct Flow Element – Duct flow elements describe the mathematical relationship between the 

flow through a duct segment and the pressure drop across the path. You must select a duct flow 

element type prior to setting the other parameters. As presented in the following subsection, the 

design mode is only concerned with Inverse Duct Flow Element types. LoopDA currently 

supports only one type of inverse duct flow element, but all types of CONTAMW duct flow 

elements are supported in the analysis mode. 

Select whether you want to create an Airflow Segment or Terminal/Fitting (use this for creating 

inlets and outlets that you want to size based on a loss coefficient). This will enable/disable and 

zero-out various input parameters for the duct to help insure that you define only those that are 

relevant to the type of segment as described below. 

Figure 4 – Duct segment/fittings properties sheet 

Segment Length – If you are defining a segment icon to be a duct segment (as opposed to a 

terminal), provide a value for the Duct segment length and make sure the Sum of loss coefficients

is set to zero. 

Friction Factor – If you are defining a segment icon to be an airflow segment, provide a value for 

the Friction factor. You can use the friction factor calculator provided on the Duct Airflow 

Element Properties input dialog box to help determine this value and the Roughness value for 

analysis. 

Segment Sum of Loss Coefficients – If you are defining a segment icon to be a duct 

Terminal/Fitting (inlet or outlet), then provide a value for the Sum of loss coefficients and make 

sure the Duct segment length is set to zero. 

Elevation – Set the elevation of each duct junction and terminal icon relative to the reference 

height of zero for the bottom of the building. This value will be used in calculating stack 

pressures in both the design and analysis modes. Note that the difference in elevations of 

adjacent junctions is not necessarily the same as the length of the segment between them. 

Wind Pressure – See previous Defining Airflow Paths section. 

11 
3.3.6.3 Working with Inverse Duct Flow Elements 

Inverse duct flow elements are similar in nature to airflow elements. This version of LoopDA 

supports only the Darcy-Colebrook type of inverse duct flow element. However, this element can be 

used to size either a Duct component (Airflow segment) or Duct fitting component (Fitting/Terminal) 

as described by Axley [2]. The forward form shown below is that implemented in CONTAM. This 

form can be split into two terms: one containing the loss due to the friction factor f (and the duct 

dimensions) and one containing the losses due to fittings ΣCd. 

 Darcy-Colebrook Element 

Forward form: ( )

2 2

/ h l

A p V

ρ fL D C

∆ = + Σ

&

Inverse forms:

 Duct component 

2 2

2 2 2 h e

V fL V fL

p A D D

ρ ρ ∆ = = & &

5

Characteristic Design Parameter: Equivalent duct diameter De which simplifies to the 

actual diameter D for circular ducts (See Chapter 34 of 

[5] for a discussion of hydraulic, actual and equivalent 

duct diameters.) 

Corresponding Analysis Parameter: Duct shape and dimensions 

Relevant Parameters: Set segment Sum of loss coefficients Cl to 0.0. Design 

value for friction factor f and Duct Shape. 

 Duct fitting component 

2

2 2 l

V

p C

A

ρ ∆ = Σ &

Characteristic Design Parameter: Opening area A

Corresponding Analysis Parameter: Duct shape and dimensions 

Relevant Analysis Parameter: Set Duct segment length L to 0.0 

Note: friction factor and duct shape and dimensions are 

not relevant to fittings 

3.3.6.4 Creating Inverse Duct Flow Elements 

To create an inverse duct flow element you select the “New Element” button on the Flow Element 

page of the Duct Segment/Fittings Properties sheet that is displayed when defining a duct flow path. 

This will reveal the Duct Airflow Element Properties sheet that contains three pages of parameters: 

one that contains the analysis parameters specific to the Darcy-Colebrook duct model, one 

containing Shape, Size and Leakage data and one containing the Design parameters for the inverse 

element. 

12 
3.3.6.5 Defining Inverse Duct Flow Elements 

Figure 5 shows the Duct Airflow Element Properties sheet for a Darcy-Colebrook element and the 

design parameters associated with the element. The parameters on the Design page are those used in 

formulating the pressure loop equations along with the previously mentioned duct flow path 

properties. 

Figure 5 – Duct Airflow Element Properties sheet displaying design data 

for a Darcy-Colebrook element (Fitting/Terminal)

For each of these inverse elements the design page displays information that is specific to the 

element for which the properties are being displayed. The following is a description of each of the 

properties displayed. 

The Name, Flow Rate, Temperature and Defined/Undefined items are the same as described in the 

case of inverse airflow elements. 

 Segment Type: This indicated whether the duct segment icon represents an Airflow segment or a 

Fitting/Terminal segment as selected when defining the Duct Segment/Fitting Properties (see 

Figure 4). 

 CDP Design: Once you have selected a value for the CDP (characteristic design parameter), enter 

it here and set the element to be “defined.” The type of parameter this value represents will 

depend on the type of inverse element you are sizing. This will be either an equivalent duct 

diameter De or opening area A depending on whether you are sizing a duct or terminal, 

respectively. Further, De is the actual diameter when dealing with circular ducts as described in 

Chapter 34 of the ASHRAE Fundamentals [5].

 Friction Factor: This value is only required for airflow segments and is ignored for 

fittings/terminals. It is dependant on the roughness and Reynolds number. Therefore, it is 

indeterminable at design time, because the flow, and hence the velocity, in the duct is not known. 

13 
You can use the right side of the page to estimate a design friction factor. Details on friction 

losses are presented in Chapter 34 of the ASHRAE Fundamentals [5].

 CDP: This is the name given by LoopDA for this particular element. This will display “A” or 

“De” to indicate either area or diameter for fitting/terminal segment or airflow segment

respectively. However, when you view and export loop data, a distinct area or diameter identifier 

will be provided for the element. This identifier will include the duct segment number assigned 

by LoopDA.

 CDP Order: This is the order of the exponent of the design parameter as it would appear in the 

loop design equation as detailed by Axley [3] and [4]. This will be either 2.0 or 5.0 

corresponding to an area (fitting/terminal) or diameter (airflow segment) respectively.

3.3.7 Working with Pressure Loops 

Once you have defined the paths and ducts that you want to size, you can now form pressure loops 

that define the desired airflow paths of ventilation air through the building. The basic method 

consists of drawing loops by connecting design flow paths and ducts on the SketchPad in the 

direction that air should flow through them; selecting the loop to generate a loop equation; reviewing 

loop properties within LoopDA; exporting loop data to a spreadsheet template to view loop equation 

details, review asymptotic plots and apply design constraints to select component sizes. Once you 

decide on the size of an element in a loop, you return to LoopDA and set that element’s CDP and 

regenerate the loop equation so that the defined element is “moved to the right hand side” of the loop 

equation. Continue this process until all elements in a loop are sized and for each loop under 

consideration. 

3.3.7.1 Drawing Loops 

Drawing loops is very similar to drawing walls with the wall drawing tool {CW: Drawing Walls}. 

Select the loop drawing tool from the toolbar as shown highlighted in Figure 6. 

Figure 6 – LoopDA toolbar highlighting the Loop Drawing Tool button 

The following rules apply when drawing loops: 

• 

• 

• 

• 

• 

• 

You must begin drawing loops in the ambient zone. 

Draw loops in the direction you want air to flow through the building. 

Each loop must end where it began (LoopDA will automatically complete the loop when you 

reach the beginning cell on the SketchPad). 

Each loop must contain exactly two flow paths that are connected to the ambient, i.e., an inlet 

into the building and an outlet from the building. 

Loops are allowed to cross over and share common sections with the exception of the loop 

direction/selection arrow. 

Loops may not cross over themselves. 

3.3.7.2 Selecting Loops 

To select a loop once it is drawn click on its direction arrow. This will highlight the loop in orange 

as shown later in the description of the pressure loops of the design example (see Figure 9). 

14 
3.3.7.3 Deleting Loops and Loop Elements 

To delete a loop, select it to then click the Delete key on the keyboard or select Edit→Delete from

the menu. You should always delete loops that pass through any airflow or duct flow paths before 

you move or delete the paths or ducts through which a loop passes. 

3.3.7.4 Viewing and Exporting Loop Data 

To access loop properties either select the loop then press the Enter key on the keyboard or doubleclick on the loop selection arrow. This will display the Pressure Loop Properties dialog box (see 

Figure 11 presented later in the Pressure Loops section of the design example). This dialog box 

displays the following information: 

 Loop Number: This is an identification number assigned by LoopDA.

 Name: You can enter an optional name for this loop to help you identify it when you export it to 

a spreadsheet. 

 Description: You can enter an optional detailed description for this loop to help you further 

identify it when you export it to a spreadsheet. This can be very useful as you progress through 

the iterative stage of sizing elements in that same loop.

 Number of Loop Nodes: This is the number of airflow paths, duct junctions, terminals and 

segments through which the loop passes. 

 Number of Equation Terms: This is the number of terms that would appear on the left hand side 

of the pressure loop equation. There will be an equation term for each airflow path and duct 

segment through which the loop passes, i.e., the sizable components, whether defined or 

undefined. 

 Driving Pressures: These are the pressure differences induced by the wind and buoyancy-driven 

airflows for the selected loop. In terms of the loop equation, these are the values that appear on 

the right-hand-side of the equation. 

Wind: This is the sum of the wind pressures acting on the two external openings of the loop in 

consideration. 

2

2

o h met

w p

C U P C ρ ∆ = ∆

∆Cp is the algebraic sum of pressure coefficients. It is summed positively when traversing 

the loop from the exterior to a wall surface and negatively otherwise. 

ρo is the density of the outdoor air calculated by LoopDA based on the ambient 

temperature as entered in LoopDA (and CONTAMW) under Weather and Wind 

Parameters. 

Umet is the design wind speed at the meteorological station as entered in LoopDA (and 

CONTAMW) under Weather and Wind Parameters. 

Ch is the wind speed modifier determined based on terrain and elevation effects in 

LoopDA (and CONTAMW) under Weather and Wind Parameters. 

15 
Stack: This is the sum of the buoyancy-induced stack pressure differences determined by 

traversing the loop systematically in the order of airflow accounting for differences in density 

and elevation along the way. 

P g s ij ij ∆ = ∑ρ ∆z

g is the acceleration of gravity (9.81 m/s

2

). 

ρij is the density of air within the flow element based on the design air temperature of the 

inverse element. 

∆zij is the difference in elevation between two nodes in the loop. It is positive for drops and 

negative for rises in elevation along the loop. 

Total Pressure Drop (RHS): This is the total pressure drop available to drive airflow through the 

undefined airflow components. This accounts for any defined Flow Element Terms within the 

loop in question and is the value that would appear on the right-hand-side (RHS) of the loop 

equation. 

∆ = P P Total ∆ s + ∆Pw −∑∆Pdefined

 Flow Element Terms: This is a detailed accounting of each term that would appear on the lefthand-side of the loop equation. It includes the Type of component (path or duct), the component 

number # (as assigned by LoopDA), the Name of the characteristic design parameter including 

the appended component number, the Order of the exponent of the CDP as it would appear in the 

loop equation, the value of the element-specific Numerator that would be associated with this 

component in the loop equation, the Asymptote or minimum value of the CDP that would provide 

the total pressure drop equivalent to the RHS value above, whether or not the item is Defined or 

has been sized and should be accounted for on the RHS of the loop equation, and the Value of 

the CDP if it has been defined. 

The Element Term that is highlighted in this list will be that against which all others will be 

plotted when the asymptotic plots are generated by the spreadsheet once the loop data is 

imported into the LoopDA spreadsheet template. You can select the component against which 

you would like all others to be plotted. By default the CDP with the highest order is highlighted 

unless it is already defined. If all undefined elements have the same order, then the first element 

in the list is highlighted. 

 Export Loop Button: Click this button to export loop data. You will be prompted to give the file a 

name (having a default extension of .txt). This will create a tab-delimited ASCII file that can be 

easily imported into the LoopDA spreadsheet template. Make note of the name and location of

the file you save, so you can locate it for importing into a spreadsheet program. 

16 
3.3.8 Working with the LoopDA Spreadsheet 

This version of LoopDA takes advantage of the powerful features of spreadsheet software by 

exporting loop data in a format compatible with typical spreadsheet programs. A spreadsheet 

template is provided with the software that simplifies the presentation of the loop equation data 

generated by LoopDA. Along with the LoopDA project file, the spreadsheet also serves as a means 

of documenting the steps you go through in performing your design analysis. 

The sections immediately following provide the mechanics of using the LoopDA spreadsheet, 

whereas the practical use of the spreadsheet in the Pressure Loop Sizing Method is presented further 

along in the Design Example. 

3.3.8.1 LoopDA Spreadsheet Template 

The LoopDA spreadsheet template is a simple spreadsheet that contains headings for the loop data, a 

section that calculates plot ranges for the CDPs, and a chart for each set of loop data you import that 

automatically plots the imported data. 

The exported data provide much more detailed information than is presented in the Pressure Loop 

Properties dialog box. The data is subdivided into the following sections: 

• 

• 

• 

• 

• 

• 

• 

• 

File Information 

Loop Information 

Outdoors 

Wind Pressures 

Stack Pressures 

Loop Flow Elements 

CDP Asymptotic Values 

Chart Data 

3.3.8.2 Saving Loop Spreadsheets 

Before you begin working with the LoopDA spreadsheet template, you should save it under a new 

name specific to your current project. It’s up to you as to how you manage your loop spreadsheets. 

One way is to maintain a separate spreadsheet for each loop that contains the various stages of 

undefined elements. The template is preset with four sets of data pages and asymptotic plot pages. 

You can easily increase the number of pages by simply copying the existing ones. 

3.3.8.3 Importing Loop Data 

To import a LoopDA-generated loop file, open the file using your spreadsheet software and parse it 

as a tab-delimited file. Then simply select the top left cell of the desired sheet in the template 

(typically A1) and paste the data (values only using the Paste Special... command for example). 

3.3.8.4 Plotting Loop Data 

Data will be plotted automatically for up to five elements (four terms vs. one). You can expand this 

by simply copying the cells located in the bottom of column E to the column(s) on the right of E. 

3.3.9 Performing Analysis 

Be sure to set the forward element parameters based on your design values. 

17 
4 DESIGN EXAMPLE USING LOOPDA 

4.1 Sample Building 

The use of LoopDA to design a natural ventilation system is demonstrated using an example from

Axley [3]. The sample building is based conceptually on the Inland Revenue Building in England 

(see [6] for more detail on this building). It is worthwhile to note that by using this sample building 

the Conceptual Building phase mentioned above is already completed – allowing us to focus on the 

Design Development phase. 

Figure 7 shows an elevation section view of this building model. As seen in the figure, the building 

is three stories tall with the 1st and 2nd floors being ventilated by air flowing in through inlet orifices 

a and c and out through a common exhaust stack represented by zone s and opening h. The 3rd floor 

is ventilated by air flowing in through two self-regulating inlet vents e and f (see [3] for a detailed 

description of these vents) and out through another exhaust stack represented by opening g. Each 

zone of the building has a volume of 800 m3

. 

1 2 3

4 5

6

7

8 9 10

11 12

13 14 15 16

17

18

19

0.5 m

0.5 m

a

b

c

d

e f

g

h

zone 1

zone 2

zone 3

vol = 800 m 3

20

21

zone

“s”

T1

T2

T3 

To = 20 oC

Uref = 2 m/s

Ts

22

self-reg.

inlet

stack terminal Cp-20 = -0.5

Cp-14 = +0.3

Figure 7 – Global geometry, topology, and pressure nodes for the ventilation flow loops of a 

building model based on the Inland Revenue Building, England 

(repeated from Figure 3.16 of Axley 2001)

4.2 Design Conditions and Criteria 

In addition to the global geometry presented in Figure 7, the designer must select indoor 

temperatures, outdoor temperature, wind speed, wind pressure coefficients, and ventilation flow 

rates to perform the design analysis. It is not obvious what values to select for many of these 

parameters. For example, it may not be sensible to select extremes (e.g., very low wind speed) for 

the design ambient conditions. Such extremes may rarely occur or may prove too severe a challenge 

for a pure natural ventilation system. For many U.S. climates, it may be necessary to either tolerate 

limited periods during which indoor environmental conditions will be outside design conditions 

considered typical for conventional, mechanically conditioned buildings or to provide some

supplemental mechanical systems (i.e., a hybrid ventilation system) for such periods. That said, the 

procedure followed below provides a realistic example problem only and is not being proposed as 

18 
formal guidance for determining design conditions for natural ventilation systems. Emmerich, Dols, 

and Axley [1] described a simple method to evaluate the suitability of a given climate to cool a 

commercial building with natural ventilation and went on to apply that method to ten California 

locations. That analysis indicated that the coastal climates of California are well suited to natural 

ventilation with respect to climatic considerations. Therefore, the building described above will be 

situated in San Francisco for the purposes of this sample application of LoopDA. Per the earlier 

climate suitability analysis, a natural ventilation system could potentially cool a commercial building 

with combined solar and internal heat gains of 10 W/m2

 to 80 W/m2 greater than 90 % of the hours 

of the year with a supplemental night cooling system expected to be effective for any days that 

would be overheated based on direct ventilative cooling alone. Assuming a heat gain rate of

20 W/m2

 (a low but achievable design target), the required average ventilation rate is 2.2 h-1 with a 

standard deviation of 2.6 h-1. Therefore, a design ventilation rate of 5 h-1 (or about one standard 

deviation above the average) is a reasonable design objective for summer cooling. The coincident 

design indoor temperature for this summer condition may be selected as 26 oC (based on the high 

end of ASHRAE’s summer thermal comfort zone [5]). 

Figure 8 shows the hourly outdoor temperatures and coincident wind speeds for the San Francisco 

TMY2 weather file [7]. From Figure 8, it appears conservative to choose an outdoor temperature of 

20 oC with a wind speed of 2 m/s for the design conditions as hours with both higher temperatures 

and lower wind speeds are rare as shown by the dashed box. 

Although it is typical to use a wind pressure profile that is dependent on wind direction (and possibly 

relative location of an opening on a wall), a single design value will be chosen for this example 

assuming the building will be oriented to take advantage of prevailing wind direction. Per Figure 

16.7 of the ASHRAE Handbook of Fundamentals [5], a surface average wind pressure coefficient of 

0.3 will be selected for the ventilation openings in the walls. Per Figure 16.9 of the ASHRAE 

Handbook of Fundamentals [5], a single value of -0.5 will be used for the wind pressure coefficient 

on the exhaust stacks. 

0

2

4

6

8

10

12

14

16

18

0 5 10 15 20 25 30 35 40

Temperature (C)

Wind speed (m/s)

Figure 8 – Wind speeds vs. outdoor temperatures for San Francisco TMY2 weather file 

19 
4.3 Pressure Loops 

With the building geometry and design conditions established, one may use LoopDA to form the 

loop equations and size the various natural ventilation system components. Figure 9 shows the 

LoopDA Sketchpad representation of the building elevation with the four pressure loops that must be 

analyzed. Loops 1 and 4 are equivalent loops following nodes 13-14-15-18-20-19-13 and nodes 21-

17-16-18-20-21 serving zone 3 (per Figure 7). Loop 2 follows nodes 8-9-10-11-12-6-7-22-8 serving 

zone 2 and Loop 3 follows nodes 1-2-3-4-5-6-7-22-1 serving zone 1. 

Loop4

Loop3 

Loop2 

Loop1 

Figure 9 – LoopDA Sketchpad representation of natural ventilation design sample problem 

20 
4.3.1 Loops 1 & 4 

Loops 1 and 4 are equivalent loops each providing 0.55 m3/s (half of the design 5 h-1) to zone 3 via 

self-regulating inlet vents. Both loops exhaust through the stack terminal g on Figure 7 for a total 

design flow of 1.1 m3

/s through the stack terminal. The self-regulating inlet vents are modeled in 

LoopDA as power law flow elements with an exponent of 0.1 (see Axley 2001 for more 

information) and an undetermined characteristic design parameter (CDP) Ce (see Figure 10). The 

stack terminal is modeled as an orifice with the area Aorfc,g (referred to as Aorfc1 by LoopDA) as the 

CDP. The complete pressure loop properties from LoopDA are shown in Figure 11. 

 

 Figure 10 – Airflow element design window Figure 11 – Pressure loop properties window for 

 for the self-regulating inlet vent of Loop 1 Loop 1 

After entering the information for Loop 1, the loop data are exported to a tab-delimited file that is 

pasted into the LoopDA spreadsheet template (click the icon below to view the Loop 1 information 

as it appears after being pasted into the LoopDA template). 

LOOPDA Loop1 

Worksheet

21 
Since Loop 1 has only two elements with undetermined CDPs, the LoopDA template plot is simple 

and shows the sizing relationship between Ce and Aorfc,g (see Figure 12). From Figure 12, the CDP 

for either element may be chosen (based on other considerations such as available component sizes 

from catalog data) and the minimum required size for the other CDP is fixed. For this example, 

selecting a value of 0.8 for Ce dictates a minimum orifice area of 0.85 m2

 which might be rounded up 

to 1.0 m2

 based on availability. 

Loop_1

0.1

1.0

10.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0

Ce2

Other Elements

Aorfc1

Figure 12 – Asymptotic relationship between CDP of self-regulating inlet vent Ce (Ce2) and 

CDP of stack terminal Aorfc,g (Aorfc1) for Loop 1 

4.3.2 Loop 2 

Loop 2 supplies 1.1 m3/s (i.e., 5 h-1) to zone 2 via inlet c on Figure 7. The flow exhausts through 

outlet d into the stack zone s and out the stack terminal h on Figure 7. Note that the stack must also 

exhaust the ventilation flow from zone 1 for a total design flow of 2.2 m3

/s through the stack. The 

inlet and outlet vents are modeled in LoopDA as orifice flow elements with Aorfc,c (Aorfc5) and Aorfc,d

(Aorfc4) as the undetermined CDPs. The stack is modeled using a Darcy-Colebrook duct model with 

an orifice as the inlet (the CDP is the area A3), an orifice as the outlet (the CDP is A1), and a 1m

long round duct segment (the CDP is the duct diameter D3). The complete pressure loop properties 

from LoopDA are shown in Figure 13. Note that, if desired, the user must impose the condition that 

the stack orifice areas be based on the duct diameter during the sizing process as LoopDA treats 

them as independent CDPs. 

22 
Figure 13 – Pressure loop properties window for Loop 2 

After entering the information for Loop 2, the loop data are exported to a tab-delimited file which is 

pasted into the LoopDA spreadsheet template (click the icon below to view the Loop 2 information 

as it appears after being pasted into the LoopDA template). 

LoopDA Loop 

Worksheet1

Loop 2 has only five elements with undetermined CDPs (although D2, A1, and A3 are related). 

Figure 14 from the LoopDA template plot shows the sizing relationship between D2 and all other 

CDPs. From Figure 14, D2 will be set at 1.2 m. Basing the stack inlet and outlet areas (A1 and A3) 

on this diameter sets these areas at 1.1 m2

 (which is larger than the minimum requirements for these 

areas per the Loop 2 worksheet and Figure 14). 

23 
Loop_2

0.1

1.0

10.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0

D2

Other Elements

Aorfc5

Aorfc4

A3

A1

Figure 14 – Asymptotic relationships for Loop 2 

Unlike Loop 1, Loop 2 still has undetermined CDPs. Returning to the LoopDA sketchpad, entering 

the parameters established for the stack duct and terminals (D2, A1 and A3), and exporting Loop 2 

(see Figure 15 for the pressure loop properties) for a second time yields a second worksheet for this 

loop (click the icon below Figure 15 to view). 

Figure 15 – Loop 2 pressure loop properties after setting the sizes of D2, A1 and A3 

24 
LoopDA Loop 2 

Worksheet2

As seen in the worksheet and Figure 15, Aorfc,c (Aorfc5) and Aorfc,d (Aorfc4) remain to be selected. 

Figure 16 shows the asymptotic relationship between these remaining undetermined CDPs. From

Figure 16, the inlet Aorfc5 will be selected as 2.0 m2

, which sets the minimum outlet Aorfc4 also at 

approximately 2.0 m2

. 

Loop_2

1.0

10.0

100.0

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0

Aorfc5

Other Elements

Aorfc4

Figure 16 – Asymptotic relationship of remaining undetermined CDPs for Loop 2 

25 
4.3.3 Loop 3 

Loop 3 consists of the same type of elements as Loop 2 – inlet orifice a (Aorfc7), outlet orifice b 

(Aorfc6), and the shared stack exhaust g. As seen in Figure 17, the previously defined stack CDPs 

are retained leaving only the inlet and outlet of zone 1 to be defined. The LoopDA spreadsheet is 

similar to the second Loop 2 worksheet (click below to view). The asymptotic relationship between 

the undetermined CDPs Aorfc6 and Aorfc7 is shown in Figure 18. Selecting the outlet to be the same

as for zone 2 at 2.0 m2

 sets the minimum size for the inlet Aorfc7 at approximately 1.3 m2. 

LoopDA Loop 3 

Worksheet

Figure 17 – Pressure loop properties window for Loop 3 

26 
Loop_3

1.0

10.0

0.0 2.0 4.0 6.0 8.0 10.0 12.0

Aorfc7

Other Elements

Aorfc6

Figure 18 – Asymptotic relationships for Loop 3 

4.3.4 Design Summary and Analysis 

The completed design of the four ventilation loops is summarized below in Table 1. 

CDP Loop Element Figure 7 LoopDA Size

1 Self-regulating inlet vent Ce Ce2 0.80

1 Stack terminal Aorfc,g Aorfc1 1.00

2 Inlet Aorfc,c Aorfc5 2.00

2 Outlet Aorfc,d Aorfc4 2.00

2 Stack inlet Astack in A3 1.13

2 Stack diameter Dstack duct D2 1.20

2 Stack outlet Astack term A1 1.13

3 Inlet Aorfc,a Aorfc7 1.30

3 Outlet Aorfc,b Aorfc6 2.00

3 Stack inlet Astack in A3 1.13

3 Stack diameter Dstack duct D2 1.20

3 Stack outlet Astack term A1 1.13

4 Self-regulating inlet vent Ce Ce3 0.80

4 Stack terminal Aorfc,g Aorfc1 1.00

Table 1 – Design values of sized components 

LoopDA may now be used to analyze the completed design by performing simulations ranging from

steady state under non-design conditions to a seasonal or annual analysis using TMY2 or other 

weather data for the location. One condition that could be analyzed is the potential for excess 

outdoor airflow under a winter condition. Referring back to Figure 8, an outdoor temperature of 5 oC 

and a wind speed of 8 m/s will be used. Also, the wall opening wind pressure coefficients will be 

increased to 0.6 to increase the impact of wind for this case. The indoor temperature is set to 20 oC 

for this case. 

27 
Figure 19 – Simulation results for winter condition analysis. Blue lines indicate direction and 

relative magnitude of airflow (note: status bar indicates a flow of 1.21 sm3

/s 

for the self-regulating inlet vent).. 

The results of the design analysis of a winter condition are shown in Figure 19. This analysis shows 

the risk of greatly over ventilating the space if no provision is made for altering the ventilation 

openings under a winter operating condition as the flow into zones 1 and 2 increases to 3 to 4 times 

the design flow. The self-regulating inlets perform better but the outdoor airflow still doubles from

the design case. For a real building, the designer would likely specify a desired minimum winter 

ventilation flow and then use LoopDA in design mode to determine desired vent opening sizes in a 

winter mode. An operational strategy could then be established whereby, for example, half of the 

openings are used during winter operation. 

28 
5 CONCLUSION

This version of LoopDA was developed to demonstrate the implementation of the Loop Equation 

Design Method within an existing multi-zone modeling environment. It will serve to provide 

designers with a framework for implementing the design method in order to become familiar with 

the method, to explore the practicality of using this method within the overall natural ventilation 

system design process, and to provide feedback to the developers as to the potential for this tool for 

use by the design and analysis community. 

As this is a preliminary version, it is reasonable that the potential for enhancements exists. Some

potential enhancements have already been identified including: the implementation of a more diverse 

set of inverse flow component types, more robust user interface features and consideration for the 

design of mechanical components of hybrid ventilation systems. In general, inverse airflow elements 

are relatively easy to add to the existing environment. However, depending on the element type, the 

inverse solution to sizing some elements could prove to be more difficult than others, e.g., a 

polynomial fan curve. The user interface could perhaps be improved by eliminating the need for the 

external spreadsheet; providing automated population of analysis components based on design 

component sizes; verifying continuity of flow through components serving multiple loops; 

generation of reports summarizing design conditions and system design, enabling the application of 

a more diverse set of design constraints, e.g., requiring duct inlet, outlet and segments of the same

size or providing a direct means to consider infiltration. 

29 
6 REFERENCES

1. Emmerich, S.J., W.S. Dols, and J.W. Axley. Natural Ventilation Review and Plan for Design 

and Analysis Tools. NISTIR 6781, National Institute of Standards and Technology. 2001. 

2. Dols, W.S. and G.N. Walton. CONTAMW 2.0 User Manual. National Institute of Standards and 

Technology. DRAFT 2002. 

3. Axley, J.W. Application of Natural Ventilation for U.S. Commercial Buildings. GCR-01-820 

NISTIR 6781, National Institute of Standards and Technology. 2001. 

4. Axley, J.W. Residential Passive Ventilation Systems: Evaluation and Design. AIVC Technical 

Note 54. Coventry, AIVC. 

5. ASHRAE 2001. ASHRAE Handbook - 2001 Fundamentals, Atlanta, GA. 

6. Irving, S. and E. Uys. CIBSE Applications Manual: Natural Ventilation in Non-domestic 

Buildings. 1997, CIBSE: London. 

7. Marion, W. and K. Urban. User’s Manual for TMY2s. National Renewable Energy Laboratory, 

1995. 

30 
7 ACKNOWLEDGMENTS

The California Energy Commission through the Architectural Energy Corporation under Interagency 

Agreement #00-OA-045 and the U.S. Department of Energy under Interagency Agreement #DEAI01-01EE27615 supported this effort. The authors would also like to acknowledge the contribution 

of Vern Smith of the Architectural Energy Corporation and Andrew Persily of NIST as well as 

James W. Axley for his work in the area of the design of natural ventilation systems and George N. 

Walton for the development of the multi-zone modeling analysis tool CONTAM upon which the tool 

developed herein is based. 

31