Computational Fluid 

Dynamics Analysis of Flexible 

Duct Junction Box Design

Robert Beach and Duncan Prahl

IBACOS, Inc.

Rich Lange

Applied Science Consultants, LLC

December 2013
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iii

Computational Fluid Dynamics Analysis of 

Flexible Duct Junction Box Design

Prepared for: 

The National Renewable Energy Laboratory

On behalf of the U.S. Department of Energy’s Building America Program

Office of Energy Efficiency and Renewable Energy

15013 Denver West Parkway

Golden, CO 80401

NREL Contract No. DE-AC36-08GO28308

Prepared by: 

Robert Beach and Duncan Prahl

IBACOS, Inc.

2214 Liberty Avenue

Pittsburgh, PA 15222

Rich Lange

Applied Science Consultants, LLC

615 Washington Road, Suite 505

Pittsburgh, PA 15228

NREL Technical Monitor: Michael Gestwick

Prepared under Subcontract No. KNDJ-0-40341-03

December 2013
iv

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v 

Contents

List of Figures ........................................................................................................................................... vii

List of Tables ............................................................................................................................................ viii

Definitions................................................................................................................................................... ix

Executive Summary.................................................................................................................................... x

1 Introduction and Background ............................................................................................................. 1

1.1 Introduction ..........................................................................................................................1

1.2 Background ..........................................................................................................................1

1.2.1 Existing Standards for Junction Box Design ...........................................................1

1.2.2 ASHRAE and ACCA Recommendations ................................................................4

1.2.3 Low-Load Homes, Box-Plenums, and Small-Diameter Ducts ................................5

1.2.4 Computational Fluid Dynamics and Duct Fittings ..................................................6

1.3 Research Questions ..............................................................................................................7

2 Mathematical and Modeling Methods................................................................................................. 8

2.1 General Approach ................................................................................................................8

2.2 Computational Fluid Dynamics Modeling...........................................................................8

2.3 Inputs ................................................................................................................................11

2.3.1 Configurations, Sets, and Cases .............................................................................11

2.3.2 Ranges ....................................................................................................................11

2.4 Calculation of Pressure Losses ..........................................................................................13

2.4.1 Losses Within the Junction Box ............................................................................14

2.4.2 Losses at Transitions: From Inlet into Box and Box into Outlet ...........................15

2.4.3 Losses Associated with the Ducts ..........................................................................15

2.4.4 Balancing Losses ...................................................................................................16

2.5 Comparison to ACCA Manual D .......................................................................................16

3 Results................................................................................................................................................. 19

3.1 Configuration A—Four Outlets .........................................................................................19

3.1.1 Set 1: Four Outlets with an Entrance Diffuser .......................................................19

3.1.2 Set 2: Four Outlets, Equal Flows ...........................................................................21

3.1.3 Set 3: Four Outlets, Unequal Flows .......................................................................22

3.1.4 Set 4: Four Outlets, Equal Flows, High Velocities ................................................24

3.2 Configuration B—Three Outlets........................................................................................25

3.2.1 Set 5: Three Outlets, Unequal Flows, Equilateral Triangle Box ...........................26

3.2.2 Set 6: Three Outlets, Equal Flows, Isosceles Triangle Box ...................................27

3.3 Configuration C—Two Outlets..........................................................................................28

3.3.1 Set 7: Two Outlets, Unequal Flows, Equilateral Triangle Box .............................29

4 Discussion........................................................................................................................................... 31

4.1 Configuration A—Four Outlets .........................................................................................31

4.1.1 Set 1: Four Outlets with an Entrance Diffuser .......................................................31

4.1.2 Set 2: Four Outlets, Equal Flows ...........................................................................31

4.1.3 Set 3: Four Outlets, Unequal Flows .......................................................................32

4.1.4 Set 4: Four Outlets, Equal Flows, High Velocities ................................................32

4.2 Configuration B—Three Outlets........................................................................................33

4.2.1 Set 5: Three Outlets, Unequal Flows, Equilateral Triangle Box ...........................33

4.2.2 Set 6: Three Outlets, Equal Flows, Isosceles Triangle Box ...................................33

4.3 Configuration C—Two Outlets..........................................................................................33

4.3.1 Set 7: Two Outlets, Unequal Flows, Equilateral Triangle Box .............................33
vi 

5 Conclusions ........................................................................................................................................ 34

5.1 How Can Current Junction Box Design Standards Be Augmented? .................................34

5.2 How Do Individual Geometric Parameters Affect the Proportions and Losses of 

Airflow in Rectilinear and Triangular Junction Boxes Serving Two to Four Discrete 

Rooms? ..............................................................................................................................34

5.3 How Conservative Are Current ACCA Manual D, Appendix 3, Group 11 Guidelines 

for Designing Flexible Duct Junction Boxes? ..................................................................36

5.4 How Do Junction Boxes Perform When Outlet Duct Diameters Correspond to the 

Lower Room Loads of High Performance Homes? ...........................................................36

5.5 Future Research .................................................................................................................37

References................................................................................................................................................. 38

Appendix A: Detailed Results.................................................................................................................. 39

Appendix B: Code Barriers on the Use of Flammable Materials in Space Conditioning Systems, 

by Duncan Prahl ................................................................................................................................. 53
vii 

List of Figures

Figure 1. Typical junction box installation and assumed branch diameters (in inches)..................... 1

Figure 2. Experimental plenum configuration and swirl phenomena ................................................... 2

Figure 3. Detail of entrance section .......................................................................................................... 3

Figure 4. Current ACCA recommended construction of flexible duct junction boxes........................ 5

Figure 5. Schematic modular system ....................................................................................................... 6

Figure 6. Validation test configuration ..................................................................................................... 8

Figure 7. Typical mesh densities............................................................................................................. 10

Figure 8. Configurations A, B, and C as number of outlets and box shape ....................................... 11

Figure 9. Plan and section elevation of a typical system ..................................................................... 14

Figure 10. Midplane total pressure gradient .......................................................................................... 14

Figure 11. Two entrance diffuser designs.............................................................................................. 19

Figure 12. Box and duct geometry to evaluate four outlets with an entrance diffuser ..................... 20

Figure 13. Midplane pressure gradients for four outlets with an entrance diffuser .......................... 20

Figure 14. Box and duct geometry to evaluate four outlets, equal flows ........................................... 21

Figure 15. Midplane pressure gradients for four outlets, equal flows ................................................ 22

Figure 16. Box and duct geometry to evaluate four outlets, unequal flows ....................................... 23

Figure 17. Midplane pressure gradients for four outlets, unequal flows ............................................ 24

Figure 18. Box and duct geometry to evaluate four outlets, equal flows, high velocities ................ 24

Figure 19. Midplane velocity gradients................................................................................................... 25

Figure 20. Box and duct geometry to evaluate three outlets, unequal flows, equilateral triangle 

box ....................................................................................................................................................... 26

Figure 21. Midplane pressure gradients for three outlets, unequal flows, equilateral triangle 

box ....................................................................................................................................................... 27

Figure 22. Box and duct geometry to evaluate three outlets, equal flows, isosceles triangle 

box ....................................................................................................................................................... 27

Figure 23. Midplane pressure gradients, three outlets, equal flows, isosceles triangle box............ 28

Figure 24. Box and duct geometry to evaluate two outlets, unequal flows, equilateral triangle 

box ....................................................................................................................................................... 29

Figure 25. Midplane pressure gradient for two outlets, unequal flows, equilateral triangle box ..... 30

Figure 26. Rectangular box...................................................................................................................... 35

Figure 27. Triangular box, three outlets ................................................................................................. 35

Figure 28. Triangular box, two airflows .................................................................................................. 36

Unless otherwise noted, all figures and photos were created by IBACOS. 
viii

List of Tables

Table 1. Flow Rates for Small Duct Diameters......................................................................................... 6

Table 2. Mesh Grid Densities ..................................................................................................................... 9

Table 3. Normalized Mass Flow Rates ...................................................................................................... 9

Table 4. Outline of Configurations Studied............................................................................................ 13

Table 5. Four Outlets with an Entrance Diffuser, Results Summary ................................................... 21

Table 6. Four Outlets, Equal Flows, Results Summary......................................................................... 22

Table 7. Four Outlets, Unequal Flows, Results Summary .................................................................... 23

Table 8. Four Outlets, Equal Flows, High Velocities, Results Summary............................................. 25

Table 9. Three Outlets, Unequal Flows, Equilateral Triangle Box, Results Summary....................... 26

Table 10. Three Outlets, Equal Flows, Isosceles Triangle Box, Results Summary............................ 28

Table 11. Two Outlets, Unequal Flows, Equilateral Triangle Box, Results Summary........................ 29

Table 12. Set 1: Four and Five Outlets with an Entrance Diffuser, Fully Developed Losses............ 39

Table 13. Set 1: Four and Five Outlets with an Entrance Diffuser, Detailed Results......................... 40

Table 14. Set 2: Four Outlets, Equal Flows, Fully Developed Losses ................................................. 41

Table 15. Set 2: Four Outlets, Equal Flows, Detailed Results .............................................................. 42

Table 16. Set 3: Four Outlets, Unequal Flows, Fully Developed Losses............................................. 43

Table 17. Set 3: Four Outlets, Unequal Flows, Detailed Results.......................................................... 44

Table 18. Set 4: Four Outlets, Equal Flows, High Velocities, Fully Developed Losses ..................... 45

Table 19. Set 4: Four Outlets, Equal Flows, High Velocities, Detailed Results .................................. 46

Table 20. Set 5: Three Outlets, Unequal Flows, Equilateral Triangle Box, Fully Developed 

Losses ................................................................................................................................................. 47

Table 21. Set 5: Three Outlets, Unequal Flows, Equilateral Triangle Box, Detailed Results ............ 48

Table 22. Set 6: Three Outlets, Equal Flows, Isosceles Triangle Box, Fully Developed Losses ...... 49

Table 23. Set 6: Three Outlets, Equal Flows, Isosceles Triangle Box, Detailed Results ................... 50

Table 24. Set 7: Two Outlets, Unequal Flows, Equilateral Triangle Box, Fully Developed Losses .. 51

Table 25. Set 7: Two Outlets, Unequal Flows, Equilateral Triangle Box, Detailed Results ............... 52

Table 26. Approximate Flow Rates for Duct Diameters ........................................................................ 54

Table 27. Flame Spread and Smoke Developed Ratings From the 2012 IRC ..................................... 56

Table 28. Flame Spread and Smoke Developed of Common Materials............................................... 56

Table 29. Requirements in Various Sections of the 2012 IRC to Limit Loss of Life and Fire 

Promulgation....................................................................................................................................... 57

Unless otherwise noted, all tables were created by IBACOS. 
ix

Definitions

ACCA Air Conditioning Contractors of America

ACCA Manual D Air Conditioning Contractors of America Manual D 

Btuh British thermal units per hour

CFD Computational fluid dynamics

cfm Cubic feet per minute

DOE U.S. Department of Energy

EL Equivalent length

HVAC Heating, ventilation, and air conditioning

IECC International Energy Conservation Code

IRC International Residential Code

IWC Inches of water column

NBFU National Board of Fire Underwriters
x

Executive Summary

This research explores the relationships between pressure and physical configurations of flexible 

duct junction boxes. Through a suite of computational fluid dynamics (CFD) simulations, 

relationships between individual box parameters and total system pressure have been predicted. 

The current Air Conditioning Contractors of America (ACCA) guidance found in Group 11 of 

Appendix 3 in ACCA Manual D (Rutkowski 2009) allows for unconstrained variation in the 

number of takeoffs, box sizes, and takeoff locations. The only variables currently used in 

selecting an equivalent length (EL) are the velocity of the air in the duct and the friction rate,

given the first takeoff is located at least twice its diameter away from the inlet. This condition 

does not account for other factors having an impact on pressure loss across these types of 

fittings. 

Due to this loosely constrained model for box design, contractors are using the fitting as a lowcost, “one-size-fits-all” solution and are circumventing the principle of right-sizing heating, 

ventilation, and air conditioning (HVAC) systems. Rather than completely eliminating the 

fitting—cost and flexibility are necessary considerations when choosing components—IBACOS 

believes better guidance can improve the performance of duct systems using this fitting.

To analyze the individual effects of the acting parameters, the IBACOS team used a series of 

CFD simulations to quantify the effects of different box designs. Appendix A documents the 

detailed simulation inputs and results. For each simulation, the team converted the pressure loss 

within the box to an EL to compare the variation in the ACCA Manual D guidance to the 

simulated variation. IBACOS chose cases that correlate reasonably to flows typically 

encountered in the field as determined by a survey of ACCA Manual J (Rutkowski 2006) load 

calculations of typical house plans from across the country. The team analyzed differences in 

total pressure due to increases in the number and location of takeoffs, the box dimensions, the 

velocity of the air (currently the only parameter accounted for with ACCA Manual D Group 11 

fittings), and whether the entrance fitting is included (currently optional). Furthermore, and 

perhaps most important, the team calculated the additional balancing losses for all cases due to 

discrepancies between the intended outlet flows and the natural flow splits created by the fitting. 

IBACOS developed general recommendations intended to supplement current guidance. Most 

importantly, this report reiterates the need for balancing dampers on all duct runs. ACCA 

Manual D predicates its entire design process on this condition. The outlet flows of a specific 

junction box are highly sensitive to its geometry, and it is unrealistic to build a box that will 

predictably divide the airflow into target rates. Also, balancing losses are significantly higher 

when there is an outlet directly opposite the inlet; therefore, outlets should be placed only on 

the sides.

An important goal for a designer is to implement symmetry in the junction box wherever 

possible. In certain asymmetrical cases, balancing losses were found to be significantly higher 

than in other symmetrical cases where the unbalanced splits were reasonably close to the targets.

Junction box size is another important factor to consider. To conserve material, junction boxes 

should be made as small as possible. However, outlets should be placed at the rear of the 
xi

junction box to minimize pressure losses. Likewise, junction box width will help to minimize 

oscillations and instabilities in the airflow. The research team proposes to limit the allowable 

number of takeoffs to three and four; a trunk and branch system is far more efficient when more 

than four takeoffs are needed, and a metal wye fitting is better when splitting only in two. 

In parallel with this technical report, IBACOS is developing a related Measure Guideline that 

will document a constrained approach to designing junction boxes that utilizes the trends 

observed in these simulations. 
1

1 Introduction and Background

1.1 Introduction

Currently, the production home-building industry prefers junction boxes as the primary means to 

split airflow in flexible duct HVAC systems. This is largely due to the flexibility of design, 

compact size, and low cost of junction boxes. However, implementation in the field is 

substandard, characterized by poorly performing HVAC systems and occupant complaints.

ACCA Manual D outlines the standard procedure for designing residential duct systems. 

However, the “recommended” geometric parameters of the boxes do not account for all factors 

affecting the pressure drop across a balanced junction box and therefore offer no guarantee to the 

designer that the EL values are accurate. Better guidance is needed for right-sized duct system 

design.

Although low in cost, junction boxes are poorly implemented by trades in production 

environments. Figure 1 depicts a common installation configuration. Although the flexible duct 

installation shown in this figure is good (i.e., well supported, smooth radius turns, duct pulled to 

full length), the junction box does not adhere to ACCA design parameters, and no dampers are 

installed to balance the flow after installation. This box probably does not perform as intended by 

the HVAC designer.

Figure 1. Typical junction box installation and assumed branch diameters (in inches)

1.2 Background

1.2.1 Existing Standards for Junction Box Design

All existing standards relating to the design of junction boxes to pressure loss apparently follow 

from experiments done by Gilman et al. (1951). Their study was “a laboratory investigation of 

the pressure characteristics and air distribution in a type of plenum chamber designated as a boxplenum.” Their research found that rotational flow in the plenum could occur with a number of 

different entrance conditions and that the rotational flow could change direction, resulting in 

What is WUFI?

16

6

6

6

6

8 6

8

8

24x24x40 Box

Assumed sizes
2

unstable branch duct airflows. Figure 2 recreates the results of one of the tested plenums; it

illustrates the general configuration of the experimental setup and streamlines of the observed 

swirling phenomena. Note the flow favoring the right side of the plenum.

Figure 2. Experimental plenum configuration and swirl phenomena

Measurements also indicated that the turbulence within the box-plenum represented up to five

times the losses related to the calculated losses associated with the entrance condition. The 

Gilman experiments explored a limited number of box-plenum sizes and outlet flows and

determined that the presence of the entrance fitting—shown here as Figure 3—produced the 

lowest pressure drop and the most stable flow among all the entrance fittings tested. Its 

performance was measured as a combination of its natural loss and the loss associated with 

dampening the outlets to achieve the desired flows. The Gilman study goes so far as to say the 

entrance fitting should be used in any practical applications to ensure stable flow and efficient 

flow splitting.
3

Figure 3. Detail of entrance section

(Courtesy of the University of Illinois at Urbana-Champaign Archives)

A general model (see Equation 1) for total loss was determined for use in practical HVAC 

system design utilizing box-plenums. This model applied only within the limited geometries 

tested, if the preferred entrance fitting was used and the first outlet was at least 12 in. from 

the inlet:

( ) ( ) Lt Ko VP o Kn VP n = ⋅ + ⋅ , (1)

where

Lt = total loss of any junction box, in inches of water column (IWC)

Ko = a constant, the value of which depends on factors such as the design of 

the entrance fitting, the amount of turbulence within the junction box, 

and the physical dimensions of the junction box

Kn = a constant, the value of which depends on the performance of the 

plenum takeoff fitting

( ) VP o = velocity head based on the velocity in the trunk duct, IWC

( ) VP n = velocity head based on the average branch duct velocity, IWC

The constant, Ko, is driven by the physical parameters of the box and fittings and is derived from 

measured data. The Gilman team determined that, practically speaking, Equation 2 is an 

appropriate model for junction boxes between 3 and 9 ft long, with the centerlines of all takeoffs 

at least 12 in. from the entrance, and that air is introduced at one end and through the preferred 

entrance section.

( ) ( ) Lt VP o VP n =1.00 ⋅ + 0.45⋅ (2)
4 

Note that the model coefficients in Equation 2 are specific to the physical parameters of the box 

and would change if, for example, a different entrance fitting were used. 

1.2.2 ASHRAE and ACCA Recommendations

ASHRAE (2012) directly references the Gilman study (Gilman et al. 1951) and states that 

0.05 in. of water value is appropriate to account for pressure loss through the plenum, provided a 

stability-inducing entrance fitting is included, the blower is moving less than 2,250 cfm, and the 

box proportions are limited as follows: 

• The minimum clearance between the first outlet and the inlet is two-thirds times the 

inlet width.

• The box width is 2.5 times the entrance width. 

• The length of the box is twice its width.

ACCA Manual D describes a comprehensive set of recognized duct sizing principles and 

calculations for optimizing the design of residential duct systems. Appendix 3 of ACCA Manual 

D provides information about the airflow resistance produced 

by various types of supply and return fittings. For residential 

duct systems, and as a matter of convenience, this resistance 

has been quantified by assigning an EL value to each type of 

fitting. EL values allow for the determination of the longest 

run, which is used to select the right-sized blower. Junction 

box EL values are provided in ACCA Manual D Group 11 

and are assumed to be valid if the box construction adheres to 

the following recommendations and the accompanying 

diagram shown here as Figure 4 (Rutkowski 2009): 

1. “The entrance has a diffuser fitting that recovers velocity pressures and prevents swirl 

(optional). 

2. There should be a straight approach and straight exits. 

3. Exit openings are on only the side (no top or bottom exits). 

4. There should be an exit opening at least two diameters from the entrance: 

2×= DL . 

5. Make the box as small as possible, but comply with rule 4.” 

Equivalent length: 

For duct fittings, the airflow 

resistance produced by a 

fitting is equivalent to the 

quantity of feet of straight 

duct that produces the 

same airflow resistance 

(Rutkowski 2009).
5

Figure 4. Current ACCA recommended construction of flexible duct junction boxes 

(Rutkowski 2009)

1.2.3 Low-Load Homes, Box-Plenums, and Small-Diameter Ducts

Codes for new construction have significantly improved, and houses built to the 2012 

International Energy Conservation Code (IECC 2012) can have load densities of 900–

1,200 ft2

/12,000 Btu/h of nominal cooling. This translates to roughly 0.33–0.44 cfm of 

conditioned air/ft2 of living area at peak conditions. This leads to bedroom airflows of 40–

100 cfm and aggregate living space airflows of 150–250 cfm. Appendix B includes more 

technical details relating to this topic.

These realities of new construction also are finding their way into existing homes that are 

undertaking moderate to deep energy retrofits. Although the load densities in these existing 

homes are somewhat lower, the problem remains for downsized air handling equipment being 

installed in oversized leaky ducts in attics. Likewise, it is difficult to retrofit a duct system below 

the conditioned ceiling, with the associated loss of floor space for the air handler and extensive 

soffits needed to accommodate ducts from a central HVAC unit. 

Research by Ridouane and Gawlik (2011) has shown that high sidewall interior supply registers 

can provide good comfort for occupants. Ridouane (2010) shows that 500 and 700 fpm for 

heating and cooling, respectively, provide enough momentum for air to mix in the room and that

lower temperature air at the outlets in heating mode is desirable to minimize stratification. 

Residential space conditioning equipment typically consists of one unit for the entire house. 

Historically, high-end systems were split into two systems to zone the house but still relied on a 

central air handler with a duct system that distributes the air throughout the zone. Proper design 

of duct systems becomes increasingly difficult as the room airflow requirements drop, especially 

when attempting to keep the system in reasonable balance and with higher air velocity supply 

outlets to facilitate mixing in the room.

One solution to this problem is not to use a central heating and cooling unit with ducts running

throughout the house. If the heating and cooling system (air handling units and associated 

ductwork) is broken down into smaller discrete parts, multiple systems can serve different 

spaces. Locating systems in close proximity to the loads served enables significantly shorter duct 
6

runs, lower static pressures in the system, and potentially greater use of temperature 

setup/setback in unoccupied spaces (e.g., bedrooms). 

To make this strategy feasible in the United States, two major hurdles must be overcome. The 

first challenge is the availability and cost of equipment, which is less of a technical challenge and 

more of a market challenge. The other factor is low-cost, simple, leak-free duct systems that can 

be modularized to accommodate the necessary airflows for each room in increments of 

approximately 10–15 cfm. Table 1 gives approximate flow rates for various duct diameters.

Table 1. Flow Rates for Small Duct Diameters

Duct Diameter, 

in.

cfm @ 

500 fpm

cfm @ 

700 fpm

1.5 6 9

2 11 15

3 25 34

4 44 61

5 68 95

6 98 137

Small modular systems (small fan coil units), as shown schematically in Figure 5, will have 

configurations similar to those shown in ACCA Manual D, Group 11 of Appendix 3.

Figure 5. Schematic modular system

1.2.4 Computational Fluid Dynamics and Duct Fittings

Contemporary research is looking into methods for producing pressure drop data for HVAC 

designers without the need for physical testing. This effort is driven by the high costs associated 

with physical testing, and CFD is viewed as one possible solution that can provide rapid 
7

turnaround for new duct fittings without the need for physical testing of every configuration 

(Shao and Riffat 1995).

Shao and Riffat (1995) detailed the factors when using CFD that affect the accuracy of pressure 

loss estimations in duct fittings. They analyzed the effects of grid density and distribution, choice

of turbulence models, interpolation schemes, and the length of downstream ducts. They 

determined that, for any dissimilar fitting, CFD simulation parameters should be calibrated to 

experimental data.

Mumma et al. (1998) performed a series of CFD simulations and parallel experiments, 

concluding that CFD could effectively determine ductwork loss coefficients. However, in very 

low loss cases, the simulations disagreed with the experimental data, which could be attributed to 

incorrect surface roughness.

Liu et al. (2012) submitted an entry to a competition for determining pressure loss coefficients 

for duct fittings with no experimental data against which to calibrate, organized by the ASHRAE 

Technical Committee on Duct Design. Mesh sizes of about 10 mm were sufficient to minimize 

grid density–induced errors. Pressure loss was determined to be highly sensitive to surface 

roughness used to calculate friction factors. The standard k-ε (k-epsilon) turbulence model was 

employed, and the estimated pressure drop matched the measured data well. Generally, the CFD 

predictions were within 20% of the measured data.

1.3 Research Questions

Based on this past work and the anticipated continuation of the use of the junction box, IBACOS 

believes certain configurations will minimize the EL and simplify the required balancing 

necessary to achieve specific flow rates. Conversely, there are likely times when a junction box 

should not be used. This project seeks to characterize the junction box geometric configurations 

and location of ducts in those boxes that optimize the EL and need very little balancing.

The following research questions were asked as part of this project:

• How can current junction box design standards be augmented?

• How do individual geometric parameters affect the proportions and losses of airflow in 

rectilinear and triangular junction boxes serving two to four discrete rooms?

• How conservative are current ACCA Manual D, Appendix 3, Group 11 guidelines for 

designing flexible duct junction boxes?

• How do junction boxes perform when outlet duct diameters correspond to the lower room 

loads of high performance homes?
8

2 Mathematical and Modeling Methods

2.1 General Approach

IBACOS used the background research to inform the construction of three-dimensional CFD 

models that represent a small subset of possible junction box configurations. The team used these 

models to analyze various parameters over which a designer would have control. The team 

calibrated the CFD model against the limited experimental data and the calculation methods

documented in the background research. 

2.2 Computational Fluid Dynamics Modeling

Figure 6 illustrates a configuration replicated from the Gilman study for which there was 

documentation of the measurements. Outlet numbers are noted. The measurements were used to 

validate the simulation parameters and appropriate mesh grid densities. Although not a rigorous 

validation, the results confirm the simulations are realistic. All outlets in this configuration are 

7 in. The box dimensions are 3 ft × 3 ft and 1 ft tall. 

Figure 6. Validation test configuration

Table 2 shows the mesh grid densities of Model 1 and Model 2. Table 3 documents the data 

taken from the Gilman study and the corresponding results of two grid densities. The density 

roughly doubled between the two, yet there was very little change between the normalized mass 

flow rates. Therefore, meshes in subsequent cases used grid densities within the Table 2 range. 

There is qualitative agreement between the Gilman study and the simulation presented here, but 

rigorous calibration was not done because of uncertainties in the Gilman measurements.

1

2

5

4

6

3
9

Table 2. Mesh Grid Densities

Model 

Number

Number of 

Elements

Number of 

Nodes

1 194,520 218,449

2 393,982 432,228

Table 3. Normalized Mass Flow Rates

Outlet 

Number

Gilman 

Study

Model 1 Model 2

1 0.148 0.110 0.113

2 0.175 0.207 0.210

3 0.206 0.236 0.236

4 0.175 0.207 0.203

5 0.148 0.110 0.111

6 0.148 0.131 0.128

The IBACOS team used the ANSYS CFX computer code.1 They used the standard k-ε

turbulence model based on findings from Liu et al. (2012). The team tested other turbulence 

models in CFX, and for the phenomena studied, those models did not show any advantages.

The IBACOS team used structured meshes for the boxes where possible. The models generally 

ranged between 250,000 and 500,000 elements. For ease of modeling, CFX dissimilar mesh 

interfaces (Generalized Grid Interfaces) were used, with CFX calculating the interfaces where 

the inlet and outlet pipe faces met the plenum. Some cases employed imprinted faces and 

continuous meshes, and the results were similar. The outlet sections had swept meshes with 

density ratios in the 1:12 range. Outside the boundary layer in the outlet sections, which were 

five elements deep, the mesh densities were kept similar to the mesh densities in the plenum. 

Inside the plenum, the mesh density was uniform and fine, without further refinement at the 

boundary layer or coarseness in the center. This avoids bias of flow due to higher mesh densities 

in any region within the box. That is, meshes were sufficiently fine to allow for sharp pressure 

gradients anywhere within the box, rather than predicting where sharp gradients would be with 

finer meshes only in those areas. Figure 7 documents a typical mesh cross section.

 1 ANSYS, Inc. Canonsburg, PA: 

www.ansys.com/Products/Simulation+Technology/Fluid+Dynamics/Fluid+Dynamics+Products/ANSYS+CFX.
10

Figure 7. Typical mesh densities

The research team applied turbulent, fully developed velocity profiles calculated by CFX to the 

inlet pipe entrances. They did not simulate cases where a bend occurred near the entrance to the 

box. Outlet ducts were sufficiently long—in most cases, 200 in.—to allow the flow to fully 

develop after exiting the box.

The mass fractions exiting each of the modeled outlets were tracked with monitor points. Some 

models were stable, with mass and momentum residuals as defined by CFX achieving levels as 

low as 1.E-6. Some cases were oscillatory in nature, and once a repeatable pattern of monitor 

point oscillation was achieved, the calculations were halted at the local extremes. The team used 

symmetry conditions in many cases; however, because it is known that, in some cases, symmetry 

conditions suppress oscillations, full models also were employed.

In the outlets, surface roughnesses were applied to the duct boundaries to induce pressure losses 

corresponding to the implied roughness values from a duct sizing slide rule. The slide rule 

specifies loss rates (IWC/100 ft) for given volumetric flow rates within flex duct of specific 

diameters. Simulation surface roughness values were tuned until pressure drops in straight ducts 

precisely matched the loss rates for duct diameters specified by the slide rule. 

Within the box, roughnesses were specified to match duct board material. Full calibration of the 

surface characteristics was not performed because the focus was on relative differences due to 

configurations. To precisely analyze the EL tables, surface roughnesses would need to be 

calibrated to measured data as discussed in the Background section.

Automatic time stepping was used. Results where flow oscillations occurred were checked by 

lowering the time step. The flow oscillations persisted, indicating they likely exist in real-world 

duct systems. The team found that some of the situations modeled had fundamental instabilities, 

replicating the results of Gilman’s study (Gilman et al. 1951).

In some studies, flow balancing was required to make outlet flows match the design flow. When 

balancing was needed, small regions at the ends of the outlets were made into sub-domains with 

applied momentum sources terms. The resistance regions were placed sufficiently far from the 

box to allow the flow to fully develop in the outlets, thus isolating the effect of the added 

resistance. How, specifically, a physical balancing damper would add resistance was not 

simulated. Only the quantity of back pressure applied to achieve a balanced state was necessary 

for this study. In some instances, these were “k-factors”; more often, a permeability-type loss 
11

was applied. The team varied the values until the target flow splits were achieved with at least 

one outlet with zero additional resistance.

2.3 Inputs

The total number of possible variations in junction boxes is impractical to exhaustively 

characterize. Currently, the ACCA Manual D Group 11 fittings constrain the design and assume 

that the same pressure values apply for all possible configurations. ACCA provides little 

guidance on the millions of configurations possible based on box dimensions, number of outlets, 

diameters, and airflows. Additionally, designers seeking to minimize the amount of balancing 

and to maximize system performance are not served by ACCA Manual D.

2.3.1 Configurations, Sets, and Cases

For this project, the team developed three different, general configurations and within those 

configurations developed sets to enable varying of different parameters. Within each set, a 

specific case was developed that represents a single model with all the parameters defined. A 

“reference case” was developed to simulate one configuration similar to that used by Gilman et 

al. (1951). In all, 53 individual cases were simulated for this project, documenting variations in 

pressure loss across a small sample of designs. A detailed description of the sets and cases can be 

found in Appendix A.

2.3.2 Ranges

For this project, the team designed the cases to represent a reasonable range of parameters, based 

on field experience of what likely conditions would be across a range of airflows. 

2.3.2.1 Number of Outlets

IBACOS chose to bound the problem by using typical configurations seen in the field that would 

serve from two to four rooms in a home. Figure 8 diagrams these three configurations

(Configurations A, B, and C) in terms of the number of outlets tied to a box shape. The team 

suspected that this limitation in itself would lead to better correlation between ACCA Manual D 

pressure drops and the simulations and ultimately the installed systems.

Figure 8. Configurations A, B, and C as number of outlets and box shape

2.3.2.2 Flow Rates

The IBACOS team chose flow rates to represent those commonly encountered in the field. For 

the purposes of this study, the research team used flow rates of 10, 100, and 250 cfm as 
12

commonly occurring flow rates bounding a typical range. The team chose experimental cases to 

contain combinations of these flows. Because most rooms with higher loads would contain at 

least two registers to supply the air, the maximum flow rate for the experiments was 250 cfm. 

For HVAC systems in high performance homes, 10 cfm is seen as a reasonable modular value. 

This also was a flow rate associated with ventilation, which, in low-load homes, may be the 

dominant requirement for sizing the ducts.

The team also studied asymmetrical cases to determine if a single EL value was appropriate

when the flow split was non-uniform. Many duct fittings that have a highly asymmetrical 

configuration are given different EL values for the outlets, whereas junction boxes receive only 

one value. This hypothesis is based on ACCA Manual D fittings with this condition, such as 

Group 9A fittings, which are given two EL values.

2.3.2.3 Duct Sizing

The IBACOS team sized ducts to the nearest nominal diameter using a flex duct calculator with 

the desired cfm flow rate and a design friction rate as close to, but not exceeding, 0.08 IWC 

(20 Pa)/100 ft of EL. The duct diameters were controlled by the flow rates due to this constraint. 

In one set, velocities were roughly doubled for the same duct diameters, which resulted in 

significantly higher pressures.

2.3.2.4 Box Dimensions, Outlet Locations, and Outlet Spacing

To study the effects of box dimensions, the IBACOS team simulated a variety of sizes. Every set 

included the minimum-sized box based on minimum 2-in. spacing between outlets and between 

ducts and box edges. This encompassed what a builder would prefer, namely, material 

efficiency. It called into question and tested the requirement of two times the outlet diameter 

spacing between the inlet and the first outlet. The goal was to provide more specific insight into 

how box dimensions may be leveraged to achieve better performing fittings. Within reason, the 

team also tested larger box sizes to see if they provide an advantage (or disadvantage) in pressure 

loss. 

2.3.2.5 Summary of Configurations and Sets

Table 4 shows a general breakdown of the configurations and sets, with comments on the 

parameters varied for the cases and the reasons for the variations.
13

Table 4. Outline of Configurations Studied

Configuration 

(see Figure 8) Set Name Remarks

A 1 Four outlets with an 

entrance diffuser

Investigate the benefit of an entrance diffuser 

similar to the type 5 design presented by 

Gilman et al. (1951).

A 2 Four outlets, equal 

flows

Box dimensions and outlet spacing were 

varied. Inlet flow remained constant.

A 3 Four outlets, unequal 

flows

Outlet locations were swapped to determine 

the best way to lay out outlets of different 

flows. Box dimensions, flow rates, and outlet 

centerlines were held constant.

B 4 Four outlets, equal 

flows, high velocities

Three inlet cfm values and, for each of these, 

two outlet velocities were simulated to gauge 

the effect of increased velocity.

B 5 Three outlets, unequal 

flows, equilateral

triangle box

An asymmetrical case with two different box 

sizes was considered. The boxes were 

equilateral triangles to achieve a wider profile. 

The effect of box size and outlet locations 

along the sides was simulated.

B 6 Three outlets, unequal 

flows, isosceles triangle 

box

Three outlet cfm values were simulated in 

minimum-sized, isosceles-triangle-shaped

boxes. Locations of the outlet on the left side 

were varied.

C 7 Two outlets, unequal 

flows, equilateral 

triangle box

A simple two-outlet box with unequal outlet 

flows was considered. Two box sizes were 

simulated, and the locations of the outlets 

were varied.

2.4 Calculation of Pressure Losses

This study simulated the pressure losses associated with a total system consisting of an inlet duct, 

an inlet duct to junction box connection, a triangular or rectangular junction box, a box to outlet 

duct connection, two to four outlet ducts, and any balancing that was necessary to achieve design 

airflows. The IBACOS team simulated the full system to prevent negating effects occurring

outside the box confines yet due to the box’s inefficiencies in turning/splitting the flow. Figure 9

shows an example system configuration. Figure 10 shows an example resultant total pressure 

gradient of two cases from Set 3.
14

Figure 9. Plan and section elevation of a typical system

Figure 10. Midplane total pressure gradient

2.4.1 Losses Within the Junction Box

The primary output from the simulations is a pressure loss value attributed to the junction box. 

For each simulated case, the team calculated the pressure loss in the box by subtracting the 

normal, fully developed losses associated with the outlet duct and straight inlet from the total 

pressure of the system measured at the inlet, as shown in Equation 3:
15

( ) ∆Pbox = ∆Ptotal − ∆Pinlet + ∆Poutlet , (3)

where

∆Pbox = pressure drop attributed to the junction box

∆Ptotal = total pressure drop from the beginning of the inlet to the end of the 

outlet

∆Pinlet = pressure drop of a fully developed flow profile across the inlet’s 

length, from Equation 5

∆Poutlet = pressure drop of a fully developed flow profile across the outlet’s

length, from Equation 5

2.4.2 Losses at Transitions: From Inlet into Box and Box into Outlet

The junction box as a fitting connects the two ducts together and changes the characteristics of 

the flow leaving the inlet duct and entering the outlet ducts. These are characterized as abrupt 

expansion and contraction geometries. The research team made a comparison between a veined 

entrance fitting and an abrupt expansion fitting, and the outlet fitting geometry was held constant 

throughout the experiments.

2.4.3 Losses Associated with the Ducts

Taken individually, the inlet and outlet ducts have losses associated with them equal to their loss

rate (IWC/unit length) times their length. Outlets were extended by the rule of thumb 

10 diameters to allow for a fully developed flow to occur at the end of the outlet. This enabled

the team to calculate the losses inside the outlet duct that were created by the outlet condition 

between the box and duct.

The relationship of the roughness values within the CFD simulations was used to calculate the 

friction factor in Equation 4. The Moody diagram (Moody 1944) gives the friction factor for a 

Reynolds number and relative roughness. The relative roughness is equal to the surface 

roughness (from the simulation) divided by the diameter. Equation 4 was shown to estimate the 

same pressure drop for a straight section of pipe as a simple CFD simulation.

The research team calculated a head loss rate (IWC/100 ft) using the Darcy–Weisbach formula 

shown in Equation 4:

water c

air

h

w g

V

D

f L H ⋅ ⋅

⋅ ⋅ ⋅ ⋅ = 2144 2

ρ

ρ , (4)

where

Hw = head loss rate, IWC/100 ft

f = friction factor, taken from the Moody diagram
16

L = length of the duct, 100 ft

Dh = hydraulic diameter of the duct, ft

ρ air = constant, density of air, lb/ft3

ρ water = constant, density of water, lb/ft3

V = velocity of the air, ft/s

c g = conversion factor, 32.174 lbm ∙ ft/lbf ∙ s

2

For each case, the team multiplied the head loss by the lengths of the inlet and outlet ducts to 

obtain the normal, fully developed losses associated with them, as shown in Equation 5. Because 

the simulation results were reported in Pascals, the team converted the loss in IWC to Pascals by 

multiplying by 248.84.

100

⋅ ⋅248.84 ∆ = H L P w

duct , (5)

where

∆Pduct = inlet or outlet duct pressure drop, Pa

Hw = head loss rate, IWC/100 ft

L = length of the duct, ft

2.4.4 Balancing Losses

The flow splits have direct implications on the amount of balancing required. The natural flow 

split is equal to the ratio of the outlet’s area to the total area of all the outlets. The target flow 

split is the flow split determined by the designer. Balancing is needed when a design does not

achieve an ideal natural split due to limitations in standard duct sizes and the layout of the 

fittings. In junction boxes, this design failure must be made up with additional balancing. The 

intent of a high performance system designer is to minimize the amount of balancing necessary 

to maximize efficiency.

In any given case, at least one outlet will not need additional resistance. In cases where the 

layout is symmetrical, more than one outlet may not need additional resistance. Balanced 

conditions are compared to ACCA Manual D values because given EL values assume balanced 

flow values. Natural flows that were within 10% of the target were deemed to be acceptable and 

were not further balanced because ACCA Manual D is a conservative approximation. In prebalanced conditions, each outlet will have a different EL. 

2.5 Comparison to ACCA Manual D

ACCA Manual D provides EL values for duct fittings, which are used to determine the amount 

of pressure the blower must overcome in an HVAC system. These values are supplied with 
17

reference velocities and friction rates. In ACCA Manual D Group 11 fittings, a range of outlet 

duct velocities is provided, with corresponding EL values for designing systems that use these

fittings. 

ACCA Manual D Section A3-3, Equivalent Length Values of Other Scenarios (Rutkowski 2009)

provides an equation to modify an EL to match the specific system velocities and friction rates. 

Most EL values in ACCA Manual D are for airspeeds of 900 or 700 fpm. However, designers 

technically should be modifying EL values based on actual system airspeeds because the actual 

airspeed changes when standard duct diameters are chosen. The design friction rate will rarely 

compute to exactly 0.08 IWC. Designers are instructed to adjust the EL values supplied for the 

various duct fittings according to their specific velocity and friction rates. This analysis assumes 

a designer would, after roughly determining the design friction rate of the system based on the 

longest effective length, adjust the initial EL values from the reference velocity (400–900 fpm) 

and friction rate (0.08 IWC) to the values corresponding to ducts sized to the design friction rate. 

It is unclear the extent to which practitioners follow this requirement, but not doing so can yield 

grossly inaccurate EL selection. Equation 6, which is from ACCA Manual D, is used to adjust 

EL values:

















⋅ 













 = ⋅

x

r

r

x

x r FR

FR

V

V EL EL

2

,

(6)

where

ELx = adjusted equivalent length, ft

ELr = equivalent length at the reference velocity, ft

Vx = actual velocity in the duct, ft/min

Vr = reference velocity in the duct, ft/min

FRx = calculated friction rate at the actual air velocity, IWC/100 ft

FRr = friction rate at the reference velocity, IWC/100 ft

The output from the simulation is a pressure drop, and to compare a corresponding ACCA 

Manual D EL value, the research team converted the pressure drop to an EL using Equation 7. 

The friction rate used to calculate the pressure drop is taken from the same duct from which the 

velocity was used to choose the corresponding EL from ACCA Manual D. This is perhaps more 

precise than necessary because the design friction rate would probably be used by most 

practitioners. However, to make “apples-to-apples” comparisons of values, this level of precision 

was used.

FR

P ELbox ⋅∆ = 2.49 , (7)
18

where

ELbox = calculated equivalent length of the box from CFD simulation, ft

∆P = box pressure drop, Pa

FR = friction rate, IWC/100 ft

2.49 = conversion from Pascals to IWC
19

3 Results

This section provides an overview of the simulation results, focusing on the extents of variations 

from each set. A diagram is supplied for each set, illustrating how cases vary within the set. 

Appendix A contains detailed documentation of simulation inputs, flow, and pressure values.

The IBACOS team completed simulations for seven sets, as described in Table 4, exploring how 

pressure loss varies with changing flow targets and configurations. As much as possible, within 

each set the goal was to create “apples-to-apples” comparisons to attribute differences to single 

parameters. Each configuration shown in Figure 8 was isolated for analysis. 

The results of representative cases are presented for each set analyzed. The worst-case flow 

noted in the tables is the outlet that had an unbalanced flow rate farthest from the target. The 

target flow of an outlet duct is its cross-sectional area divided by the sum of all the outlets’ areas. 

Detailed results can be found in Appendix A. In all tables, the “ACCA ELbox” is the ELbox value 

calculated using Equation 7 and should not be confused with the ACCA Manual D EL value 

given in Group 11.

3.1 Configuration A—Four Outlets

Configuration A simulated four-outlet configurations with differing box geometries and differing 

outlet flow splits. 

3.1.1 Set 1: Four Outlets with an Entrance Diffuser

The team evaluated the impact of an entrance diffuser, which is currently optional under ACCA 

Manual D. Gilman et al. (1951) showed the diffuser suppresses flow instabilities and oscillations 

in the outlets, as shown previously in Figure 2, and increases system efficiency. The IBACOS 

team modeled two diffuser designs, as shown in Figure 11, on a highly constrained box, as 

shown in Figure 12. (The dashed outlet at the back of the box is the location of the additional 

outlet in Case 6 and Case 9.) Small box sizes were used to replicate what a production builder 

would likely build in an HVAC system where the minimal amount of material was used to 

construct the box.

Figure 11. Two entrance diffuser designs
20

Figure 12. Box and duct geometry to evaluate four outlets with an entrance diffuser

The team found substantial differences in EL for the four-outlet configuration with and without 

the two entrance diffuser designs. Figure 13 shows pressure gradients calculated in the box for 

two cases.

Figure 13. Midplane pressure gradients for four outlets with an entrance diffuser

Table 5 summarizes the illustrative cases. The IBACOS research team ran additional cases with 

an outlet at the back, hypothesizing that the benefit found by Gilman et al. (1951) (i.e., an 

increase in efficiency of approximately 50%) may have been due, in part, to the deflection of air 

by the diffuser into the side outlets rather than the air moving straight through the box into the 
21

back outlet. Smooth-wall outlet ducts were used to more closely resemble the Gilman study duct 

characteristics. 

Table 5. Four Outlets with an Entrance Diffuser, Results Summary

Case

Outlet 

Opposite 

the Inlet? 

Yes/No

Diffuser 

Type

Worst-Case 

Flow, cfm

(Target 

Flow, cfm)

Unbalanced Box Loss, 

Pa Difference, 

Pa Front Back End

1 No None 294.3*

(225) 8.6* 6.3* – 2.3*

2 No Spread 218.7

(225) 11.4 11.7 – 0.3

3 No Straight 225.0

(225) 8.4 8.4 – 0.0

6 Yes None 299.3

(225) 12.7 9.7 8.8 3.9

9 Yes Straight 277.6

(225) 11.0 9.6 8.6 2.4

*Oscillatory result.

3.1.2 Set 2: Four Outlets, Equal Flows

The set represented in Figure 14 was used to evaluate the impact of duct spacing in a rectangular 

box where all outlet airflows were desired to be the same. The box size was somewhat larger 

than that of the four outlets with an entrance diffuser (Set 1); however, the dimension to the first 

duct from the inlet end (L1) of the box was less than d × 2 as recommended in the ACCA 

Manual D Group 11 fittings. Most simulations showed flow oscillation, indicating fundamental 

instabilities in the flow. 

Figure 14. Box and duct geometry to evaluate four outlets, equal flows
22

The team found that the simulated unbalanced outlet airflows generally were close to 10% of the 

target airflows. In the one case where balancing was undertaken, the total system pressure 

increased by more than 17%. Table 6 shows the best- and worst-case results of this set of 

simulations, and Figure 15 shows the midplane pressure gradients. The best (wider) case is 

represented on the left; the worst case is on the right. (Note that the image represents only onehalf of the box.)

Table 6. Four Outlets, Equal Flows, Results Summary

Case

Box 

Width, 

in.

L1, 

in.

L2, 

in.

L3, 

in.

Worst-Case 

Flow, cfm

(Target 

Flow, cfm)

Simulated 

EL, 

ft

ACCA 

ELbox, 

ft

∆EL, 

ft

Best

(Case 2) 20 8 2 2 252*

(250) 55* 51 4

Worst

(Case 7) 20 2 2 8 296*

(250) 64* 51 13

*Oscillatory result.

Figure 15. Midplane pressure gradients for four outlets, equal flows

3.1.3 Set 3: Four Outlets, Unequal Flows

The set represented in Figure 16 was configured to analyze different outlet flows and the impact 

of duct position on balancing and overall box pressure.
23 

Figure 16. Box and duct geometry to evaluate four outlets, unequal flows 

The simulation results shown in Table 7 indicate that ACCA Manual D EL values are 10–20 ft

lower than what would likely be found in a real box of this configuration. Had there been a larger 

distance between the inlet and first outlet, pressures may have been lower but probably not to a 

level below the ACCA Manual D EL numbers. Figure 17 shows the midplane pressure gradients, 

with a symmetrical case on the left and an asymmetrical one on the right. 

Table 7. Four Outlets, Unequal Flows, Results Summary

Case Description

Worst Case: 

Unbalanced 

Flow, cfm

(Target Flow, 

cfm)

Simulated 

Balanced 

Loss, 

Pa

Simulated 

Balanced 

EL, 

ft

ACCA 

ELbox, 

ft

∆EL, 

ft

7 in. 10 in. 7 in. 10 in. 7 in. 10 in. 7 in. 10 in.

2 Symmetrical 124.6

(100)

225.4

(250) 11.2 12.3 61 64 51 10 13 

4 Symmetrical 105.0

(100)

245.0

(250) 13.8 13.0 67 71 51 16 20 

1 Asymmetrical 118.3

(100)

268.1

(250) 12.6 11.8 63 62 51 12 11 
24

Figure 17. Midplane pressure gradients for four outlets, unequal flows

3.1.4 Set 4: Four Outlets, Equal Flows, High Velocities

By studying this configuration, the research team sought to confirm the velocity relationship 

expressed in Equation 1 from ACCA Manual D as the primary driver of junction box pressure 

loss and to evaluate low airflows at differing velocities. The team simulated the impact of using 

the box as a pressure restriction to increase the velocity in the outlet duct compared to the inlet 

duct. Figure 18 shows two of the configurations represented by Set 4.

Figure 18. Box and duct geometry to evaluate four outlets, equal flows, high velocities
25

Table 8 shows higher pressure losses than predicted by ACCA Manual D EL values as overall 

velocities increased. Higher velocity and friction rates aligned better to ACCA Manual D.

Figure 19 shows the midplane velocity gradients for Set 4. The case on the left represents the low 

velocity case; the right is the high velocity case. (Note that the image represents only one-half of 

the box.)

Table 8. Four Outlets, Equal Flows, High Velocities, Results Summary.

Case

Outlet 

Flow, 

cfm

Outlet 

Velocity, 

cfm

Outlet 

Friction 

Rate, 

IWC/100 ft

Simulated 

Balanced 

Box Loss, 

Pa

Simulated 

Box EL, 

ft

ACCA 

ELbox, 

ft

∆EL, 

ft

Low 

Velocity

(Case 3)

100 374 0.07 8.7 48 35 13

High 

Velocity

(Case 4)

100 733 0.39 13.7 14 14 0

Figure 19. Midplane velocity gradients

3.2 Configuration B—Three Outlets

Configuration B simulated three-outlet configurations with differing triangular box geometries 

and differing outlet flow splits. 
26

3.2.1 Set 5: Three Outlets, Unequal Flows, Equilateral Triangle Box

The box shown in Figure 20 was shaped as an equilateral triangle, with three outlet flows: two at 

100 cfm and one at 250 cfm. The team found ACCA Manual D EL values to be acceptable to 

conservative, depending on the outlet position. Two box sizes were simulated. One was 

constrained by the minimum dimension of the side of the duct with two outlets, and the other 

added 6 in. to the box side to study the impact of moving the single outlet duct along the side.

Figure 20. Box and duct geometry to evaluate three outlets, unequal flows, equilateral triangle box

Table 9 provides best-and worst-case results, and Figure 21 shows the difference in pressures 

between locating the single outlet closer to the inlet (left) and pushing it toward the back (right)

of the box. 

Table 9. Three Outlets, Unequal Flows, Equilateral Triangle Box, Results Summary

Result 

(Case)

Side 

Length, 

in.

Single 

Outlet 

Position

Simulated 

Box Loss, 

Pa

Simulated 

Box EL, 

ft

ACCA 

ELbox, 

ft

∆EL, 

ft

7 in. 10 in. 7 in. 10 in. 7 in. 10 in.

Worst

(Case 1) 20 Front 11.5 10.4 49 44 31 18 13

Best 

(Case 2) 20 Centered 7.1 6.0 30 26 31 –1 –5

Bigger Box 

(Case 6) 20 Back 8.2 7.3 35 32 13 4 1
27

Figure 21. Midplane pressure gradients for three outlets, unequal flows, equilateral triangle box

3.2.2 Set 6: Three Outlets, Equal Flows, Isosceles Triangle Box

Set 6 was similar to Set 5; however, in Set 6, the box was shaped as an isosceles triangle, as 

shown in Figure 22. The dashed outlet ducts represent the alternate outlet location’s limit. The 

team sought to achieve three equal flows, totaling 30 cfm (3 × 10 sfm), 300 cfm (3 × 100 cfm), 

and 750 cfm (3 × 250 cfm). Duct diameters increased to account for the higher flow, and the box 

dimensions were modified slightly to accommodate the larger inlet and outlet duct sizes.

Figure 22. Box and duct geometry to evaluate three outlets, equal flows, isosceles triangle box
28

Table 10 provides the best- and worst-case results, and Figure 23 shows pressures in the boxes

for the best case (left) and worst case (right).

Table 10. Three Outlets, Equal Flows, Isosceles Triangle Box, Results Summary

Result 

(Case)

Outlet 

Flow, 

cfm

Single 

Outlet 

Position

Simulated 

Box Loss, 

Pa

Simulated 

Box EL, 

ft

ACCA 

ELbox, ft

∆EL, 

ft

Worst A

(Case 2) 10 Centered 1.5 12 8 4

Best A

(Case 1) 10 Back 1.4 10 8 2

Worst B

(Case 4) 100 Front 9.3 46 27 19

Best B

(Case 6) 100 Back 6.1 35 27 8

Worst C

(Case 7) 250 Front 15.8 55 42 13

Best C

(Case 9) 250 Back 12.1 42 42 0

Figure 23. Midplane pressure gradients, three outlets, equal flows, isosceles triangle box

3.3 Configuration C—Two Outlets

Configuration C simulated a two-outlet configuration with differing triangular box geometries 

and unequal outlet airflows. 
29

3.3.1 Set 7: Two Outlets, Unequal Flows, Equilateral Triangle Box

Figure 24 shows the configuration with two outlets receiving 100 and 250 cfm, respectively. The 

dashed ducts represent alternate locations of outlet ducts. The team simulated one 16-in. and one 

24-in. equilateral triangular box to evaluate the relative impact of asymmetrical flows and outlet 

position in the box. The depth was 16 in. for both boxes. Table 11 shows that generally locating 

outlets as far from the inlet as possible yielded lower pressure losses. Abushakra et al. (2002) 

found that ACCA Manual D EL values were reasonable for boxes that were sized to exactly fit 

the inlet and outlet duct sizes. The losses were found to be higher than the ACCA Manual D 

values and the EL of a traditional sheet metal wye fitting. Despite an asymmetrical flow target, 

the need for balancing was minimal. Figure 25 shows pressure gradients at the midplane of 

two Set 7 cases, highlighting the negative effect of locating outlets near the inlet (right) versus 

far (left).

Figure 24. Box and duct geometry to evaluate two outlets, unequal flows, equilateral triangle box

Table 11. Two Outlets, Unequal Flows, Equilateral Triangle Box, Results Summary

Result 

(Case)

Side 

Length, 

in.

L1, 

in.

L2, 

in.

Simulated 

Box Loss, 

Pa

Simulated 

Box EL, 

ft

ACCA 

ELbox, 

ft

∆EL, 

ft

7 in. 10 in. 7 in. 10 in. 7 in. 10 in.

Worst

(Case 1) 24 15 12 4.9 3.9 34 27 35 –1 –8

Best 

(Case 2) 24 2 2 8.1 7.1 57 50 35 22 15

Smaller Box 

(Case 6) 24 7 – 5.9 4.9 41 34 35 6 –1
30

Figure 25. Midplane pressure gradient for two outlets, unequal flows, equilateral triangle box

10” 7” 10” 7”
31 

4 Discussion

This report frames its discussion around how the pressure drops calculated in CFD simulations 

compare to ACCA Manual D EL values and how different configurations perform better or 

worse than each other. The hypothesis was that in junction boxes, their flexible constraints create 

a wider range of EL than presented in ACCA Manual D, and better insight into the effects of 

other parameters, such as box size, will result in better performance of systems using these 

fittings. One goal was to determine whether variations found in the simulations are significant 

enough to warrant adjustment to the current model employed by ACCA Manual D for junction 

boxes or could lead to generalized best practice for the use of junction boxes. 

It is important for the ACCA Manual D EL numbers to be conservative to favor oversizing the 

blower to meet the overall airflow requirements of the system. ACCA further recommends that 

balancing dampers should be installed in all runs to add the resistance needed to match the fan 

power with the available pressure and desired airflow to each room. 

Given the simulation time necessary, the team selected a limited range of representative airflows 

as discussed in Section 2.3.2. Discussion of each configuration is presented below, and general 

recommendations can be found in the Conclusions section. 

4.1 Configuration A—Four Outlets 

4.1.1 Set 1: Four Outlets with an Entrance Diffuser

In small boxes, the use of the “straight” entrance diffuser lowered calculated pressure losses

between 10 and 20% compared to other cases. However, this reduction was not to the degree 

found by Gilman et al. (1951). This indicates the specificity of entrance diffuser design is 

important and must be adjusted to specific box configurations. As it stands, if a junction box is 

not built within a 3-ft wide and 3-ft to 8-ft long range, as studied by the Gilman team, it will not 

perform as effectively. As shown by the differences in results between the “spread” and 

“straight” fittings simulated in this study, the performance is highly sensitive to fitting geometry, 

which is difficult to fabricate precisely on site by a contractor.

Also of note are the differences between cases with and without an outlet directly opposite the 

inlet. Placing an outlet directly opposite the inlet contributes to significantly higher balancing 

and thus higher box pressure drop. Indicative of performance differences, the magnitude of 

variation among individual outlet pressures indicates the amount of balancing needed to make all 

outlets the same. With an outlet opposite the inlet, this pressure difference was consistently 

greater than in cases with outlets only on the sides, indicating much higher balancing pressures. 

This result is the justification of the three recommended configurations tested, none of which has 

an outlet opposite the entrance. 

4.1.2 Set 2: Four Outlets, Equal Flows 

The research team found that, although varying the outlet locations on a box with four outlets 

with the intent to achieve equal airflow to each outlet did significantly impact the calculated EL 

of the box, an 8-in. space between the inlet and outlet, with the outlets placed close together, 

yielded EL values within 5 ft of ACCA Manual D values. Placing the first outlets close to the 

inlet yielded significantly higher pressures upwards of 15 additional feet of EL, marking a 

downward pressure trend as the distance from the inlet to the first outlet increases. This is 
32

parallel to the ACCA Manual D requirement for a distance of two times the outlet duct diameter 

between the inlet and the first outlet. Twice the diameter may be a conservative estimate. Also, 

the optimal distance and spacing will differ for every configuration, and a twice-the-diameter 

rule should account for all configurations as a conservative value. The simulations marked a 

significant drop in EL with less than the ACCA Manual D requirement for distance, and any 

additional distance would likely have yielded little or no additional benefit; however, this is only 

for limited configurations. Further tests would need to be done to precisely confirm the rule for 

all possible boxes but would likely be reasonably close to or less than twice the diameter. 

Pressure losses varied little with an increased space between the two outlets.

Making the box wider induced stability in the system and eliminated balancing. Currently, there 

are no width recommendations for ACCA Manual D Group 11 fittings. It appears that widening 

the box to at least twice the inlet diameter could be a recommended practice for more stable flow 

rates.

4.1.3 Set 3: Four Outlets, Unequal Flows

The team found that in boxes where the desired airflows are asymmetrical in a minimally sized 

box, calculated EL values are above ACCA Manual D EL values. Ducts near the end of the box

will receive proportionally more air than any near the inlet, as would be expected. The lowest 

calculated EL was found in Case 2, where the outlet with a larger friction rate was taken closest 

to the rear of the box. Case 4, with higher flow outlets at the back, did not require balancing.

All cases were within a reasonable range of pressure, despite differing outlet layouts. Case 2 and 

Case 4 employed left-to-right symmetry; front to back, they were asymmetrical. This created 

more predictable flows, although not necessarily lower pressure drops. Although the total 

pressures were not lower with left-to-right symmetry, balancing was lower. Left-to-right 

symmetry will yield more predictable results and produce a more self-balancing system. Novel 

asymmetrical layouts, although possibly optimal, are difficult to predict and therefore are not 

recommended.

4.1.4 Set 4: Four Outlets, Equal Flows, High Velocities

All cases—except Case 2, which performed well below the ACCA Manual D estimate—were 

found to have higher calculated EL values compared to the ACCA Manual D EL values. The 

difference was less pronounced in the lower volume, lower velocity cases (Case 1 and Case 2).

ACCA Manual D seems to predict EL better for higher velocity and higher resistance junction 

boxes. This could be due to the higher flow inducing more back pressure into the box and thus 

more efficiently splitting the air.

The results of Set 4 indicate further analysis should be undertaken for small distributed systems 

using “pancake” fan coil units, as are common with mini-split or hydronic systems. ACCA 

Manual D provides little guidance for very small plenum configurations with multiple branch 

ducts. The fact that these duct runs may be very short also may impact the EL values.

In high friction rate systems such as small-diameter homerun systems, box loss is a much smaller 

percentage of the total system pressure. For a large increase in total system pressure, such as a 

3.3 times increase between Case 1 and Case 2, the box loss increased by only 1.6. This indicates 

that for high velocity systems, junction boxes can be reasonable choices for fittings. 
33

4.2 Configuration B—Three Outlets

The ACCA Manual D EL values of triangular boxes configured to split flows three ways were 

generally reasonable or conservative compared to the calculated EL values. Box size, triangle 

shape, and the distance of the single duct from the back of the box create variations from the 

ACCA Manual D EL values. The lowest calculated EL values are highly dependent on all three 

variables.

4.2.1 Set 5: Three Outlets, Unequal Flows, Equilateral Triangle Box

The team found that locating the single duct either in the center or as close to the back of the 

larger box as possible yielded almost identical calculated EL values, approximately equal to the 

ACCA Manual D EL values. On the smaller box, the lowest calculated EL value was found to 

occur when the single duct was centered with a calculated EL value lower than the ACCA 

Manual D value. The smaller box performed better than the larger one with the outlets farther 

from the inlet, suggesting that in triangular boxes, larger boxes do not necessarily benefit 

performance. Balancing was relatively high in all cases—10%–20% additional pressure. Case 2 

performed best because it required the least amount of balancing.

4.2.2 Set 6: Three Outlets, Equal Flows, Isosceles Triangle Box

Set 6 studied variations in velocity and airflow in a box shaped like an isosceles triangle. As was 

found in Set 4, small-diameter ducts with lower velocities and lower airflows yielded poorer 

agreement with ACCA Manual D. Similar to all other sets, placing inlets (Case 3, Case 6, and 

Case 9) toward the back of the box yielded the lowest pressures. 

4.3 Configuration C—Two Outlets

The final configuration studied two unequal flows in a box shaped like an equilateral triangle. 

The team studied this configuration because it is one that is prevalent in the industry.

4.3.1 Set 7: Two Outlets, Unequal Flows, Equilateral Triangle Box

The team found that the ELs of the small boxes with outlets placed at the back were reasonably

represented by ACCA Manual D. Other configurations were found to have EL values 17%–63% 

higher (5–27 ft of additional EL) than ACCA Manual D. The larger box size in this configuration 

had significant benefit in performance, which was not the case in the three-outlet configuration. 

Note that a metal wye fitting (ACCA Manual D fitting “9O”) has an EL value of 15 at 900 fpm, 

0.08 IWC, compared to the 95 EL value for the splitter box (fitting Group 11 at the same 

reference airspeed and friction rate). For simple splitting of airflow, the wye fitting is a much 

better choice. 
34

5 Conclusions

Designers should recognize that although junction boxes are economical from a first-cost 

perspective, they add more pressure to systems compared to trunk and branch designs. This 

pressure drop must be accounted for in the design of the duct system and may result in the need 

for larger duct sizes compared to those in a trunk and branch system.

In general, this research proposed that through further constraining current design limits 

provided in ACCA Manual D, Appendix 3, Group 11, better airflow control, stable airflow, and 

minimized pressure losses could be more consistently achieved by contractors who install flex 

duct junction box fittings. 

The following research questions were asked as part of this project:

• How can current junction box design standards be augmented?

• How do individual geometric parameters affect the proportions and losses of airflow in 

rectilinear and triangular junction boxes serving two to four discrete rooms?

• How conservative are current ACCA Manual D, Appendix 3, Group 11 guidelines for 

designing flexible duct junction boxes?

• How do junction boxes perform when outlet duct diameters correspond to the lower room 

loads of high performance homes?

5.1 How Can Current Junction Box Design Standards Be Augmented?

Group 11 fittings could be further elaborated upon to discuss various configurations and flow 

splits with corresponding EL values. This would provide practitioners with better guidance and 

the ability to compare EL values with other specific ACCA Manual D, Appendix 3 fittings. In 

Section 5.2, IBACOS has provided initial guidance based on results of this research. 

5.2 How Do Individual Geometric Parameters Affect the Proportions and Losses 

of Airflow in Rectilinear and Triangular Junction Boxes Serving Two to Four 

Discrete Rooms?

The “two-diameter to first outlet” rule comes into question. The IBACOS team found that 

absolute pressure losses attributable to the box do trend downward as the distance increases 

between the inlet and the first outlet. The relative difference in pressure appears to be overly 

conservative, given the actual level of precision in the ACCA Manual D process. This causes 

junction boxes to be sized larger than necessary. Smaller boxes not fully implementing a “twicethe-diameter” rule will save on materials. Width seems to be more important to reduce overall 

pressure losses in the box than the distance from the inlet to the first outlet. Furthermore, it is

easier to use width as the driving dimensional parameter in triangle boxes.

In rectangular boxes, it is clear that simply not having an outlet directly opposite the inlet is more 

beneficial to reducing overall system pressure than the presence of the entrance diffuser fitting, 

especially when cost is considered. The flow into the back outlet was proportionally higher than 

any other case that the team ran with outlets only on the sides. The entrance fittings are not 

available prefabricated, and it is expected they would have to be made by the contractors. The 

configuration of the entrance fitting also was found to have a significant impact on the overall 
35

pressures, which likewise appear to interact closely with the dimensions of the box. The 

suggestion of the use of entrance fittings should remain optional and perhaps be removed from 

ACCA Manual D until a better design process for an entrance diffuser that is tied to box 

configuration is developed. 

This study found that the pressure loss and associated calculated EL values of junction boxes are

highly variable based on the configuration, the number of flow splits desired, the location of the 

outlets in the box, the size of box, the airflow through the box, and the incoming and outgoing 

velocities. This study was limited in terms of the number of configurations and flow rates that 

were able to be studied. Any conclusions presented should be treated as general and should be 

used in conjunction with practitioners’ actual field experience, rather than as modifications to 

any guidance or information provided in ACCA Manual D. The following general 

recommendations will ensure minimized pressure losses and more predictable balancing:

1. General recommendations

• All runs should have balancing dampers.

• Lay out the junction boxes to maximize the geometric symmetry (both side to side 

and front to back) and symmetrical allocation of airflow. A symmetrical layout better 

controls flow splits and minimizes the need for balancing.

2. Configuration A—Rectangular box (Figure 26)

• Place outlets only on the sides of boxes. Outlets opposite the 

inlet should be avoided due to the large additional balancing 

resistance to force air into side outlets.

• Outlets generally should be located as far from the inlet as 

possible in rectangular boxes, with a preference to reduced 

material. A minimum distance of twice the outlet diameter is a 

conservative rule of thumb.

• Make the box width approximately three times the inlet width. Nearly all simulated 

rectangular boxes had oscillating flows in the outlets. A wider box seems to minimize 

flow oscillation. 

• The entrance diffuser (which, in practice, is rarely, if ever, used) does not improve 

performance for inlet flows below 1,000 cfm in small rectangular boxes. The use of 

an entrance fitting does minimize or eliminate oscillations in rectangular junction 

boxes.

3. Configuration B—Triangular box with three outlet airflows 

(Figure 27)

• A minimum distance of approximately one to two outlet 

diameters between the inlet and the single outlet will yield lower 

pressure drops.

• Make the box as small as possible to conserve material while 

maintaining requisite widths.

Figure 27. 

Triangular box, 

three outlets

Figure 26. 

Rectangular 

box
36

4. Configuration C—Triangular box with two outlet airflows 

(Figure 28)

• This fitting is not recommended. A metal wye fitting is 

significantly better for this purpose. 

5.3 How Conservative Are Current ACCA Manual D, 

Appendix 3, Group 11 Guidelines for Designing 

Flexible Duct Junction Boxes?

This research did not conclusively quantify how conservative ACCA 

Manual D EL values are. However, among the cases simulated, the relationship between ACCA 

Manual D and simulated EL values varied significantly. Factors such as design airflow rates, the 

nominal duct diameters, and the associated airspeed, as well as the number of ducts, the location 

of ducts, and the box geometry, result in some simulations showing EL values 50% above 

adjusted ACCA Manual D EL values; others are very close to ACCA Manual D EL values, and 

others are overly conservative.

It is important to note that the issues around making the comparison between the simulation 

results and ACCA Manual D, Group 11 EL values are provided for a single friction rate at a 

number of different air velocities. In reality, once a nominal duct size is selected for a given air 

volume, the actual airspeed and friction rate will vary within the inlet duct, the box itself, and 

each outlet duct. The team chose the highest friction rate and corresponding velocity from the 

simulation to modify the ACCA Manual D, Group 11 EL values using an equation given in 

ACCA Manual D, Appendix 3 (p. 146). ACCA Manual D states that “equivalent length values 

are sensitive to the friction rate and very sensitive to velocity” (p. 146). The team found that once 

nominal duct diameters had been selected based on a single friction rate, the velocities in the 

inlet and outlet ducts could vary by up to 300 fpm. This is why the team chose to reiterate the 

importance of adjusting EL values to specific designed velocities and friction rates.

The ACCA Manual D process adds the effect of balancing dampers in a separate section from 

calculating total EL; a flat 0.03 IWC (7.47 Pa) is added for each damper (Step 2 of the manual, 

where component pressure losses are added together). The total component losses are subtracted 

from the available static pressure of the blower, which is then divided by the total EL to yield a 

design friction rate. This rate is used to size the ducts to approximate the correct pressure drop 

across each duct run. 

The balancing pressures simulated throughout this study ranged between 0.0 and 0.016 IWC 

(0–4 Pa). The best configurations had a balancing loss of 0 to approximately 0.006 IWC 

(0–1.5 Pa). This indicates that if the current ACCA Manual D EL values include some assumed 

balancing losses, the EL values may be able to be reduced if the designer chooses better 

configurations.

5.4 How Do Junction Boxes Perform When Outlet Duct Diameters Correspond to 

the Lower Room Loads of High Performance Homes?

It was observed in low airflow conditions (10–100 cfm, 2–5-in. diameter ducts) that balancing 

losses were eliminated in all cases. The four-outlet configuration required no balancing to 

achieve target airflows, with unbalanced outlet flows within 2% of targets. Two of the three 

cases in the three-outlet configuration required some balancing, but one case was within 10% of 

Figure 28. 

Triangular box, 

two airflows
37

the design targets. Balancing losses generally increased with airflow rate, even when outlet 

velocity was the same. The limited number of configurations evaluated with smaller duct 

diameters found some to have a fairly large discrepancy as a percentage difference from ACCA 

Manual D EL values adjusted for flow and velocity; however, the absolute differences are not 

large enough to make much difference in the duct design process, given the overall level of 

precision in the ACCA Manual D process.

5.5 Future Research

The IBACOS team will seek to validate the CFD simulations against new experimental data to 

confirm the trend that ACCA Manual D effectively represents actual pressures of these systems. 

Generally, the CFD simulations trended to having higher pressures and therefore higher adjusted 

EL values using the equations given in ACCA Manual D, especially with lower airflows. This 

indicates a potential need for an additional adjustment to EL based on flow volume as well as 

velocity and friction rate.

More precisely, prefabricated junction boxes could be developed as low cost but high quality 

fittings. For example, a manufacturer could preconfigure and cut optimal designs that can be 

folded and quickly taped together on site. This would eliminate discrepancies between the time 

needed to ensure quality fabrication and well-performing layouts.

Additionally, future research may look into new models for designing junction boxes that may 

more specifically account for the individual parameters and allow for a wider variety of 

configurations. From an efficiency standpoint, conservative values lead to higher energy 

consumption. A more precise EL calculation will result in a lower fan power or speed, resulting 

in saved energy while still meeting the required flow rates. This is especially important in lowload homes where optimized performance is a goal.
38

References

Abushakra, B.; Dickerhoff, D.; Walker, I.; Sherman, M. (2002). Laboratory Study of Pressure 

Losses in Residential Air Distribution Systems. Berkeley, CA: Lawrence Berkeley National 

Laboratory. Report LBNL-49293. 

ASHRAE. (2012). 2012 ASHRAE Handbook: HVAC Applications. Atlanta, GA: ASHRAE.

Gilman, S.F.; Martin, R.J.; Konzo, S. (1951). Investigation of the Pressure Characteristics and 

Air Distribution in Box-Type Plenums for Air Conditioning Duct Systems. University of Illinois 

Engineering Experiment Station Bulletin, No. 393, 1951 University of Illinois Archives, Record 

Series Number 11/2/801. Urbana, IL: University of Illinois at Urbana-Champaign.

https://www.ideals.illinois.edu/handle/2142/4424.

IECC. (2012). 2012 International Energy Conservation Code. Washington, DC: International 

Code Council.

Liu, W.; Long, Z.; Chen, Q; (2012). “A Procedure for Predicting Pressure Loss Coefficients of 

Duct Fittings Using CFD (RP-1493).” HVAC&R Research 18(6): 1168–1181(14).

Moody, L.F. (1944). “Friction Factors for Pipe Flow.” Transactions of the ASME 66(8): 671–

684. www.chem.mtu.edu/~fmorriso/cm310/MoodyLFpaper1944.pdf.

Mumma, S.A.; Mahank, T.A.; Ke, Y-P. (1998). “Analytical Determination of Duct Fitting Loss 

Coefficients.” Applied Energy 61(4): 229–247. 

Ridouane, E.H. (2010). “Numerical Evaluation of Indoor Air Distribution in High Performance 

Homes.” Presented at Residential Building Energy Efficiency Meeting 2010, July 21, 2010.

http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/ns/b23_indoor_air_di

str.pdf. 

Ridouane. E.H.; Gawlik, K. ( 2011). “Prediction of Air Mixing from High Sidewall Diffusers in 

Cooling Mode.” Preprint. Golden, CO: National Renewable Energy Laboratory. NREL/CP5500-49010.

Rutkowski, H. (2006). Manual J—Residential Load Calculation, 8th edition, Version 2. 

Arlington, VA: Air Conditioning Contractors of America.

Rutkowski, H. (2009). Manual D—Residential Duct Systems, 3rd edition, Version 1.00. 

Arlington, VA: Air Conditioning Contractors of America.

Shao, L.; Riffat, S.B. (1995). “Accuracy of CFD for Predicting Pressure Losses in HVAC Duct 

Fittings.” Applied Energy 51: 233–248.
39

Appendix A: Detailed Results

This appendix documents the simulation inputs and results for each set. Each set is documented 

similarly, consisting of two tables. 

The first table in each set includes the fully developed losses of the inlet and outlet ducts present 

in all cases from the set. This includes simulated flow values and duct diameters and variables 

for calculating the friction factor per Equation 3 and Equation 4.

The second table in each set contains simulation results and the box loss calculations per 

Equation 5, the conversion to EL from pressure using Equation 7, and the comparable EL values 

from ACCA Manual D adjusted using Equation 6 to the maximum friction rate and velocity in 

the simulation from the reference values in the manual. The first columns in the second table 

identify each case number and any parameters that are changing between cases. The next 

columns document the measurements taken from the CFD simulations through to the calculated 

box losses and EL values. Finally, the ACCA Manual D comparable values are noted, along with 

the difference between the simulations and ACCA Manual D. Positive values in the last two 

columns indicate potential additional pressure not accounted for by ACCA Manual D that could 

be present in systems installed with the given configuration, dimensions, and flows.

Set 1: Four and Five Outlets with an Entrance Diffuser

Gilman et al. (1951) described an entrance fitting as diagrammed in Figure 3 as greatly 

increasing performance in junction boxes. Comparisons were made between boxes with and 

without the fitting. In addition, two “spreads” were used to test if either performed better. Initial 

results in a box similar to that in Set 1 with four outlets—two on each side of a rectangle—did 

not yield a significant reduction in pressure, and a hypothesis was that the results in the Gilman 

study were so dramatic because there was an outlet out the back of the box. Thus, with the same 

outlet flows, an additional outlet was placed at the back, and the inlet flow increased by the 

additional outlet’s flow. Rectangular and circular inlet ducts were used because the design of the 

fitting aligned better with a rectilinear profile. The outer edges of the circular duct caused the 

turning to be less pronounced because most of the flow was concentrated in the center vanes.

The detailed, fully developed loss calculation inputs and results for Set 1 are presented in Table 

12. Table 13 details the inputs and results from the simulations of Set 1.

Table 12. Set 1: Four and Five Outlets with an Entrance Diffuser, Fully Developed Losses

Inlet B 16 1,125 806 108,984 0 0.000 0.018 0.0537 16 0.0007 0.2

InletA 16 900 645 87,188 0 0.000 0.019 0.0359 16 0.0005 0.1

Outlet 10 225 413 34,875 0 0.000 0.023 0.0289 200 0.0048 1.2 Flow, cfm Diameter, in.

ID

Loss Rate, IWC/ 

100 ft

Friction Factor

k/d

Roughness

Reynolds

Velocity, fpm

Loss, Pa

Length, in.Loss, IWC
40

Table 13. Set 1: Four and Five Outlets with an Entrance Diffuser, Detailed Results

Front Back End Front Back End Front Back End Front Back End

1 None Pipe 155.7* 294.3* – 12.3* 10.7* 8.1* – 8.6* 6.3* – 96* 70* –

(225) (225) –

2 Spread Pipe 231.3 218.7 – 15.6 12.7 13.0 – 11.4 11.7 – 128 131 –

(225) (225) –

3 Straight Pipe 225.0 225.0 – 12.4 9.7 9.7 – 8.4 8.4 – 94 94 –

(225) (225) –

4 Spread Square 248.9 201.2 – 16.9 13.5 14.7 – 12.2 13.4 – 137 150 –

(225) (225) –

5 Straight Square 234.5 215.6 – 12.9 9.9 10.4 – 8.6 9.1 – 96 102 –

(225) (225) –

6 None Pipe 140.6 272.3 299.3 15.2 14.1 11.0 10.2 12.7 9.7 8.8 95 72 66

(225) (225) (225)

7 None Square 149.5 264.1 297.8 15.8 14.5 11.9 10.8 13.1 10.5 9.5 98 79 71

(225) (225) (225)

8 Spread Pipe 173.1 218.0 342.8 17.6 16.0 15.1 11.5 14.6 13.7 10.2 109 102 76

(225) (225) (225)

9 Straight Pipe 182.1 241.6 277.6 14.1 12.3 11.0 10.0 11.0 9.6 8.6 82 72 64

(225) (225) (225)

10 Spread Square 172.3 215.1 350.2 19.4 17.7 16.8 12.7 16.4 15.5 11.3 122 116 85

(225) (225) (225)

11 Straight Square 180.0 222.8 319.5 13.8 12.0 11.1 8.2 10.7 9.7 6.9 80 73 51

(225) (225) (225)

Total Pressure, Pa Unbalanced Box Loss, Pa Unbalanced Box EL, ft

(Target Flow, cfm)

Unbalanced Flow, cfm

Pressure at 

Inlet, Pa

 Inlet Profile 

 Fitting 

Case

 *Oscillatory result. 
41

Set 2: Four Outlets, Equal Flows

Set 2 investigates the relationship between box size and spacing between outlets. The team used 

2-in. and 8-in. values as possible values for each of the three varying parameters: L1, L2, and L3. 

All combinations of values were simulated. Only these values were fluctuated from case to case 

to isolate the effect of changing geometry. The resulting box lengths ranged from 26 to 44 in. In 

addition, the case with flow split closest to the target—Case 20B—was made wider to test if 

additional width caused it to perform poorly. Similarly, the case with the unbalanced split that 

was farthest from the target was balanced to measure the maximum balancing loss. The detailed,

fully developed loss calculation inputs and results for Set 2 are presented in Table 14. Table 15

details the inputs and results from the simulations of Set 2.

Table 14. Set 2: Four Outlets, Equal Flows, Fully Developed Losses

Inlet 16 1,000 716 96,875 0.2 0.013 0.042 0.0994 16 0.0013 0.3

Outlet 10 250 458 38,750 0.15 0.015 0.045 0.0705 200 0.0117 2.9 Roughness

ID

Diameter, in.

Flow, cfm

Velocity, fpm

Reynolds

k/d

Friction Factor

Loss Rate, IWC/ 

100 ft

Length, in.Loss, IWC

Loss, Pa
42

Table 15. Set 2: Four Outlets, Equal Flows, Detailed Results

W L1 L2 L3 Front Back Front Back Front Back Front Back Front Back Back Front

1 20 2 2 2 222.5* 277.5* 22.7* – 3.2 3.2 – – 19.5* 19.5* – 16.3* 16.3* – 66* 66* 66 51 15 31%

(250) (250)

2 20 8 2 2 248.5* 251.5* 20.2* – 3.6 3.4 – – 16.6* 16.8* – 13.3* 13.5* – 54* 55* 55 51 4 9%

(250) (250)

3 36 8 2 2 248.5 251.5 19.7 – 3.5 3.6 – – 16.2 16.1* – 12.97 12.9* – 52 52 52 51 1 3%

(250) (250)

4 20 8 8 2 240* 260* 19.2* – 3.3 3.8 – – 16.4* 15.4* – 13.1* 12.1* – 53* 49* 53 51 2 5%

(250) (250)

5 20 8 8 8 223.5* 276.5* 18* – 2.8 4.3 – – 15.5* 13.7* – 12.3* 10.4* – 50* 42* – 51 – –

(250) (250)

6 20 2 8 8 210* 290* 19* – 2.5 4.8 – – 16.6* 14.2* – 13.4* 11* – 54* 44* – 51 – –

(250) (250)

7 20 2 2 8 204.5* 295.5* 19.3* 22.7 2.4 4.9 3.5 3.2 15.8* 14.4* 15.9 12.5* 11.1* 15.9 51* 45* 64 51 14 27%

(250) (250)

8 20 8 2 8 224.5* 275.5* 18.3* – 2.9 4.2 – – 15.4* 14.1* – 12.1* 10.8* – 49* 44* – 51 – –

(250) (250)

9 20 2 8 2 228.5* 271.5* 21.1* – 3.0 4.2 – – 18.1* 16.9* – 14.9* 13.7* – 60* 55* 60 51 9 19%

(250) (250)

 Balanced Box Loss, Pa 

Balanced

ΔEL, ft

ΔEL, %

Case

Inlet Outlet Pressure, Pa

Pressure, 

Pa

Balanced Box EL, ft

ACCA EL, ft

Unbalanced

Balanced

Unbalanced Balanced

Total Pressure, Pa Unbalanced 

Box EL, ft

Unbalanced 

Box Loss, Pa

Unbalanced

Unbalanced 

Flow, cfm

(Target Flow, 

cfm)

Dimensions, in.

*Oscillatory result. 
43

Set 3: Four Outlets, Unequal Flows

Set 3 examines the effects of pressure at the inlet when the relative outlet locations of an 

asymmetrical condition are varied. This experiment is driven by the likelihood of balancing 

losses increasing when higher flow outlets are located before lower flow outlets, due to the 

preference of the air to flow into the rear outlets. The centerlines of the outlet ducts, the inlet 

flow rate, and the box dimensions were held constant between cases. The detailed, fully 

developed loss calculation inputs and results for Set 3 are presented in Table 16. Table 17 details 

the inputs and results from the simulations of Set 3.

Table 16. Set 3: Four Outlets, Unequal Flows, Fully Developed Losses

Inlet 16 1,000 716 96,875 0.2 0.013 0.042 0.0994 16 0.0013 0.3

Outlet A 7 100 374 22,143 0.2 0.029 0.058 0.0859 200 0.0143 3.6

Outlet B 10 250 458 38,750 0.2 0.020 0.050 0.0779 200 0.0130 3.2 Loss, IWC Loss, Pa

k/d

Friction Factor

ID

Diameter, in.

Flow, cfm

Velocity, fpm

Reynolds

Loss Rate, IWC/ 

100 ft

Roughness

Length, in.
44

Table 17. Set 3: Four Outlets, Unequal Flows, Detailed Results

FL BL BR FR FL BL BR FR FL BL BR FR 7 10 7 10 7 10 7 10

1 7 7 10 10 101.5 118.3 268.1 212.1 16.8 18.8 16.5 16.5 15.6 15.3 12.6 11.8 63 62 51 12 11 24% 22%

(112) (112) (238) (238) (7) (7) (10) (10)

(100) (100) (250) (250)

2 10 7 7 10 225.4 124.6 124.6 225.4 17.5 18.7 15.9 15.1 15.1 15.9 11.2 12.3 61 64 51 10 14 21% 27%

(238) (112) (112) (238) (10) (7) (7) (10)

(250) (100) (100) (250)

3 10 7 10 7 197.4 139.3 266.7 91.7 15.8 18.6 15.2 16.4 15.0 16.3 12.4 11.4 61 66 51 10 15 20% 30%

(238) (112) (238) (112) (10) (7) (10) (7)

(250) (100) (250) (100)

4 7 10 10 7 105.0 245.0 245.0 105.0 19.5 20.0 17.6 16.6 16.6 17.7 13.8 13.0 67 71 51 16 21 33% 41%

(112) (238) (238) (112) (7) (10) (10) (7)

(100) (250) (250) (100)

Balanced

Unbalanced

ACCA EL, ft

ΔEL, ft

ΔEL, %

Balanced Box EL, ft

Balanced Total Pressure, Pa

(Diameter, in.)

Balanced Box Loss, Pa

Inlet 

Pressure, Pa

Case

Outlet Diameters, in. Unbalanced Flow, cfm

(Natural Flow, cfm)

(Target Flow, cfm)
45

Set 4: Four Outlets, Equal Flows, High Velocities

Set 4 was designed to compare the differences in pressure loss through a junction box when the 

outlet diameters decrease, resulting in higher velocities and friction rates. The inlet diameter, the 

flow rate, and the box dimensions remained constant. Three flow rates were tested with two 

outlet diameters each. Flow was split into four outlets and balanced to achieve equal flow into 

each outlet, and the losses attributed to the box were calculated. The detailed, fully developed 

loss calculation inputs and results for Set 4 are presented in Table 18. Table 19 details the inputs 

and results from the simulations of Set 4.

Table 18. Set 4: Four Outlets, Equal Flows, High Velocities, Fully Developed Losses

Inlet 1 5 40 293 12,400 0.09 0.018 0.050 0.0642 60 0.0032 0.8

Outlet A 2 10 458 7,750 0 0.000 0.033 0.2598 200 0.0433 10.8

Outlet B 3 10 204 5,167 0 0.000 0.037 0.0382 200 0.0064 1.6

Inlet 2 12 400 509 51,667 0.18 0.015 0.045 0.0720 60 0.0036 0.9

Outlet 5 100 733 31,000 0.09 0.018 0.048 0.3858 200 0.0643 16.0

Outlet C 7 100 374 22,143 0.12 0.017 0.048 0.0713 200 0.0119 3.0

Inlet 3 16 1,000 716 96,875 0.2 0.013 0.042 0.0994 60 0.0050 1.2

Outlet D 7 250 935 55,357 0.2 0.029 0.057 0.5286 200 0.0881 21.9

Outlet E 10 250 458 38,750 0.15 0.015 0.045 0.0705 200 0.0117 2.9

k/d

Friction Factor

Loss Rate, IWC/ 

100 ft

Length, in.Loss, IWC

Loss, Pa

ID

Diameter, in.

Flow, cfm

Velocity, fpm

Reynolds

Roughness
46

Table 19. Set 4: Four Outlets, Equal Flows, High Velocities, Detailed Results

Front Back Front Back

1 40 293 3 204 9.9 10.1 5.2 5.3 4.6 4.6 2.3 14 13 1 5%

(10) (10)

2 40 293 2 458 10.0 10.0 18.5 18.5 15.1 15.1 3.6 6 8 –2 –29%

(10) (10)

3 400 509 7 374 94.4* 105.6* 14.04* 14.8 12.4 12.5 8.7 48 35 14 40%

(100) (100)

4 400 509 5 733 98.4 101.6 38.6 39.6 30.4 30.6 13.7 14 14 1 5%

(100) (100)

5 1,000 716 10 458 216* 284* 21.3* 24.1 20.6 20.9 16.7 68 51 17 34%

(250) (250)

6 1,000 716 7 935 237.5* 262.5* 56* 60.4 45.8 45.5 22.7 17 16 2 11%

(250) (250)

 Balanced Box Loss, Pa 

 Balanced Box EL, ft 

 ACCA EL, ft 

 ΔEL, ft ΔEL, %

Unbalanced

Balanced

Case

 Inlet Flow, cfm 

 Inlet Velocity, fpm 

 Outlet Diameters, in. 

 Outlet Veloctiy, fpm 

(Target Flow, 

cfm)

Unbalanced 

Flow, cfm

Inlet Pressure, 

Pa

Balanced Total 

Pressure, Pa

 *Oscillatory result. 
47

Set 5: Three Outlets, Unequal Flows, Equilateral Triangle Box

Similar to Set 3, Set 5 explores the effect of greater asymmetry on balancing and total pressure. 

It also seeks to determine if box performance better aligns with ACCA Manual D data when 

splitting into three as opposed to four. It is expected that balancing losses will be greater when 

the split is the most asymmetrical. Two box sizes also were simulated to garner whether box size 

plays a part in the box loss. Finally, the larger left outlet was shifted along the side of the box to 

examine the effect of outlet location. The detailed, fully developed loss calculation inputs and 

results for Set 5 are presented in Table 20. Table 21 details the inputs and results from the 

simulations of Set 5.

Table 20. Set 5: Three Outlets, Unequal Flows, Equilateral Triangle Box, Fully Developed Losses

Inlet 12 450 573 58,125 0.2 0.017 0.046 0.0944 16 0.0013 0.3

Outlet 10 250 458 38,750 0.2 0.020 0.050 0.0779 200 0.0130 3.2

Outlet 7 100 374 22,143 0.2 0.029 0.058 0.0859 200 0.0143 3.6 Roughness

ID

Diameter, in.

Flow, cfm

Velocity, fpm

Reynolds

k/d

Friction Factor

Loss Rate, IWC/ 

100 ft

Length, in.Loss, IWC

Loss, Pa
48

Table 21. Set 5: Three Outlets, Unequal Flows, Equilateral Triangle Box, Detailed Results

RF RB L RF RB L 7 10 7 10 7 10 7 10

1 20 2 101.7* 138.6* 209.3* 13.1* 17.5 15.3 15.4 13.9 11.5 10.4 49 44 31 18 13 58% 43%

(111.5) (111.5) (227)

(100) (100) (250)

2 20 4 88.7* 127.8* 213.8* 11.5* 13.1 10.9 11.0 9.6 7.1 6.0 30 26 31 –1 –5 –2 –17%

(111.5) (111.5) (227)

(100) (100) (250)

3 20 8 91.8* 121.5* 236.7* 11.9* 16.3 13.7 14.1 12.9 10.2 9.4 43 40 31 12 9 40% 29%

(111.5) (111.5) (227)

(100) (100) (250)

4 26 2 88.7 135.0 226.4 11.7 15.7 13.4 13.5 12.1 9.7 8.6 41 37 31 10 6 33% 18%

(111.5) (111.5) (227)

(100) (100) (250)

5 26 8 74.7 119.3 256.1 11.4 14.4 12.2 12.3 10.9 8.5 7.4 36 31 31 5 0 16% 2%

(111.5) (111.5) (227)

(100) (100) (250)

6 26 14 76.5 124.7 248.9 12.8 14.2 12.0 12.0 10.9 8.2 7.3 35 31 31 4 0 12% 1%

(111.5) (111.5) (227)

(100) (100) (250)

ΔEL, ft

ΔEL, %

Balanced Box Loss, PaBalanced Box Loss, EL

ACCA EL, ft

Case

 L1, in. 

 Width, in. 

Balanced

Unbalanced

(Target Flow, cfm)

(Natural Flow, cfm)

Balanced Total 

Pressure, Pa

Inlet Pressure, 

Pa

Unbalanced Flow, cfm

*Oscillatory result. 
49

Set 6: Three Outlets, Equal Flows, Isosceles Triangle Box

Set 6 compares pressure variations as the location of an outlet duct varies. Three flow rates were 

tested, with each case using the smallest possible junction box while maintaining an isosceles 

profile. Each flow rate was simulated with two outlets on the right and one on the left. The left 

outlet was moved within its side to three positions: at the front, centered, and at the back. The 

detailed, fully developed loss calculation inputs and results for Set 6 are presented in Table 22. 

Table 23 details the inputs and results from the simulations of Set 6.

Table 22. Set 6: Three Outlets, Equal Flows, Isosceles Triangle Box, Fully Developed Losses

Inlet A 4 30 344 11,625 0.15 0.038 0.063 0.1391 6 0.0007 0.2

Outlet A 3 10 204 5,167 0.15 0.050 0.073 0.0755 60 0.0038 0.9

Inlet B 10 300 550 46,500 0.15 0.015 0.045 0.1010 7 0.0006 0.1

Outlet B 7 100 374 22,143 0.15 0.021 0.052 0.0771 160 0.0103 2.6

Inlet C 14 750 702 83,036 0.2 0.014 0.044 0.1143 14 0.0013 0.3

Outlet C 10 250 458 38,750 0.2 0.020 0.050 0.0779 160 0.0104 2.6 Roughness

k/d

Friction Factor

Loss Rate, IWC/ 

100 ft

Length, in.

ID

Diameter, in.

Flow, cfm

Velocity, fpm

Reynolds

Loss, Pa

Loss, IWC
50

Table 23. Set 6: Three Outlets, Equal Flows, Isosceles Triangle Box, Detailed Results

RF RB L RF RB L

1 10 2 8.1 13.3 8.6 4.0 4.7 4.1 4.1 4.1 1.4 12 8 3 38%

(10) (10) (10)

2 10 5 9.4 12.0 8.7 4.3 4.8 4.2 4.2 4.1 1.5 12 8 3 40%

(10) (10) (10)

3 10 7 8.76* 10.32* 10.92* 4.1 4.3 3.7 3.7 3.7 1.0 11 8 2 25%

(10) (10) (10)

4 100 2 72.3 134.4 93.3 10.7 13.9 12.0 11.4 11.5 9.3 46 27 19 71%

(100) (100) (100)

5 100 7 90.9 118.8 90.3 10.5 12.1 9.8 9.9 9.7 7.2 39 27 12 45%

(100) (100) (100)

6 100 11 94.8 103.2 102.0 10.7 11.1 8.8 8.8 8.8 6.1 35 27 8 32%

(100) (100) (100)

7 250 2 240* 286.5* 223.5* 14.2 18.7 14.3 15.8 15.7 15.8 55 42 13 31%

(250) (250) (250)

8 250 8 217.5 297.0 235.5 14.5 16.1 12.6 12.9 12.7 12.9 45 42 3 6%

(250) (250) (250)

9 250 14 238.5 258.0 253.5 15.1 15.5 12.1 12.1 12.1 12.1 42 42 0 1%

(250) (250) (250)

 ΔEL, ft 

Balanced Total 

Pressure, Pa

Inlet 

Pressure, PaBalanced Unbalanced

(Target Flow, cfm)

ΔEL, %

Unbalanced Flow, 

cfm

Case

 Outlet Flow 

 L1, in. 

 Balanced Box Loss, Pa 

 Balanced Box Loss, EL 

 ACCA Equivalent 

 *Oscillatory result. 
51

Set 7: Two Outlets, Unequal Flows, Equilateral Triangle Box

Aside from a simple symmetrical split into two, Set 7 represents the simplest scenario for a 

junction box. The locations of the outlets were shifted along the sides of two box sizes to 

determine if “at the back makes things better” is indeed a universal trend. Box size also was 

increased to indicate if the larger size and the distance between the inlet and the first outlet 

improve performance. The detailed, fully developed loss calculation inputs and results for Set 7

are presented in Table 24. Table 25 details the inputs and results from the simulations of Set 7.

Table 24. Set 7: Two Outlets, Unequal Flows, Equilateral Triangle Box, Fully Developed Losses

Inlet 12 350 446 45,208 0.2 0.017 0.046 0.0574 16 0.0008 0.2

Outlet A 10 250 458 38,750 0.2 0.020 0.050 0.0779 200 0.0130 3.2

Outlet B 7 100 374 22,143 0.2 0.029 0.058 0.0859 200 0.0143 3.6Loss, Pa

ID

Diameter, in.

Flow, cfm

Velocity, fpm

Reynolds

Roughness

k/d

Friction Factor

Loss Rate, IWC/ 

100 ft

Length, in.Loss, IWC
52

Table 25. Set 7: Two Outlets, Unequal Flows, Equilateral Triangle Box, Detailed Results

7 10 7 10 7 10 7 10 7 10 7 10

1 16 2 – 124.6 225.4 12.0 12.8 10.7 9.3 6.9 5.9 49 41 35 14 7 40% 19%

(115.1) (234.9)

(100) (250)

2 16 4.5 – 119.7 230.3 11.3 12.4 10.2 8.9 6.4 5.4 45 38 35 10 3 30% 10%

(115.1) (234.9)

(100) (250)

3 16 7 – 116.9 233.1 10.8 11.8 9.7 8.3 5.9 4.9 41 34 35 7 0 19% –1%

(115.1) (234.9)

(100) (250)

4 24 2 2 112.35 237.65 12.9 14.0 11.8 10.5 8.1 7.1 57 50 35 22 15 63% 43%

(115.1) (234.9)

(100) (250)

5 24 2 7 110.6 239.4 12.9 13.7 11.5 10.2 7.8 6.8 54 47 35 20 13 57% 37%

(115.1) (234.9)

(100) (250)

6 24 8.5 7 115.5 234.5 11.9 12.8 10.7 9.3 6.9 5.9 48 41 35 14 7 40% 20%

(115.1) (234.9)

(100) (250)

7 24 15 7 118.65 231.35 10.8 12.7 10.6 9.2 6.8 5.8 48 40 35 13 6 38% 17%

(115.1) (234.9)

(100) (250)

8 24 15 12 109.55 240.45 10.3 10.8 8.6 7.3 4.9 3.9 34 27 35 0 -7 –1% –21%

(115.1) (234.9)

(100) (250)

Case

 Width, in. 

 L1, in.  L2, in. 

(Natural Flow, cfm)

(Target Flow, cfm)

Balanced 

Total 

Pressure, Pa

Pressure at 

Inlet, Pa

Unbalanced Flow, cfm

Unbalanced

Balanced

ΔEL, ft

ΔEL, %

Balanced Box Loss, PaBalanced Box Loss, EL

ACCA Equivalent
53

Appendix B: Code Barriers on the Use of Flammable Materials in 

Space Conditioning Systems, by Duncan Prahl

Background

Research cited by Walker (2007) indicates that current fan blower efficiency is roughly 10%–

15%, indicating opportunities for more efficient air movement through aerodynamically 

designed fan blowers. This will become increasingly important if air handler fans also are used 

for air circulation and ventilation during non-heating or cooling periods. In addition, duct 

systems could be constructed using lightweight plastic components, which would allow for small 

diameters that are appropriately sized for lower airflows. 

Two ways to achieve energy efficiency that are well documented are bringing ducts inside 

conditioned space and sealing the duct system (NREL 2005). New and existing homes 

commonly have duct systems that are in unconditioned attics and are poorly sealed. 

Conventional solutions to bring ducts inside conditioned space include redesign to integrate 

ducts in floor spaces, insulating at the underside of the roof deck to create a conditioned attic and 

leaving the ducts in the attic, creating soffits dropped below the ceiling line, or some 

combination of these strategies. In addition, space must be found inside the conditioned space for 

the air handling unit, typically requiring approximately 6 ft2 of floor area. 

Codes for new construction have significantly improved, and houses built to the 2012 IECC 

often have load densities of 900–1,200 ft2

/12,000 Btu/h of nominal cooling. This translates to 

roughly 0.33–0.44 cfm of conditioned air per square foot of living area at peak conditions. This 

leads to bedroom airflows of 40–100 cfm and aggregate living space airflows of 150–250 cfm. 

These realities of new construction are also finding their way into existing homes that are 

undertaking moderate to deep energy retrofits. Although the load densities are somewhat lower, 

there remains the problem of downsized air handling equipment being installed in oversized 

leaky ducts in attics or the difficulty of retrofitting a duct system below the conditioned ceiling,

with the associated loss of floor space for the air handler and extensive soffits needed to 

accommodate ducts from a central HVAC unit. 

Past research by Ridouane and Gawlik (2011) has shown that high sidewall interior supply 

registers can provide good comfort for occupants. Ridouane (2010) showed that 500 and 

700 fpm for heating and cooling provides enough momentum for the air to mix in the room. This 

research also showed that lower temperature air at the outlets in the heating mode is desirable to 

minimize stratification. 

Residential space conditioning equipment typically is one unit for the entire house. Historically, 

higher-end systems were split into two systems to zone the house but still relied on a central air 

handler with a duct system that distributes the air throughout the zone. Proper design of duct 

systems becomes increasingly difficult as the room cfm drops, especially when attempting to 

keep the system in reasonable balance and have higher supply outlet air velocities to facilitate 

mixing in the room.

One solution to this problem is to not locate the heating and cooling unit centrally and force the 

ducts to go throughout the house. Instead, the solution is to break down the heating and cooling 
54

system into smaller discrete parts and to allow multiple systems to serve different spaces. This 

allows for significantly shorter duct runs, low static pressures in the system, and potentially

greater use of temperature setup/setback in unoccupied spaces (e.g., bedrooms). 

To make this strategy feasible in the United States, two major hurdles must be overcome. The 

first hurdle is equipment availability and cost, which is less of a technical challenge and more of 

a market challenge. The second hurdle is finding low-cost, simple, leak-free duct systems that 

can be modularized to accommodate the necessary flows for each room in increments of 

approximately 10–15 cfm. Table 26 gives approximate flow rates for various duct diameters.

Table 26. Approximate Flow Rates for Duct Diameters

Duct Diameter

(in.)

cfm @ 

500 fpm

cfm @ 

700 fpm

1.5 6 9

2 11 15

3 25 34

4 44 61

5 68 95

6 98 137

One solution that would achieve the desired ducts is to use readily available plastic plumbing 

piping. Pipe diameters could be mixed and matched to provide the appropriate airflow for a 

room, and the solvent welding of joints is inherently airtight. These duct systems have low static 

pressure and a straight duct roughness. However, polyvinyl chloride plastic pipe currently is 

approved for use in plumbing systems but not for use in above-grade duct systems. 

Code Research

A literature search was undertaken to determine the genesis of the duct flammability 

requirements in the 2012 International Residential Code (IRC 2012). The first codes in the 

United States were developed by the National Board of Fire Underwriters (NBFU) as a means to 

encourage the construction of buildings that would not catch fire, and if those buildings did catch 

fire, it would not spread throughout the building or to other buildings. The history of the first 50 

years of the NBFU is well documented by Brearley (1916), and the underlying purpose of the 

codes was offered by the NBFU (1945). Relative to fire safety in ducted systems, the NBFU 

published a guide (NBFU 1915) that requires the ducts to be “made of galvanized iron or other 

approved non combustible material” and recommends that fans be interconnected to fire and 

smoke alarm systems so that the fans shut down in the event of a fire. Another NBFU publication 

(NBFU 1935) indicates “recent fires” in metal ducts with flammable linings and that the fire 

department had a difficult time fighting the fire that was inside the duct system. NBFU (1935) 

also indicates that “only fire resistive linings acceptable to the inspection department having 

jurisdiction may be used inside of ducts.”

Brearley (1916) states that electricity was seen as a major new contributor to fires in buildings in 

the early 1900s, which gave rise to the development of the National Electric Code by the NBFU. 

Specific recommendations in several NBFU pamphlets imply that either direct sparking or sparks 
55

from static electricity generated by fans and belts in ventilation and space conditioning systems 

is a specific concern that should be avoided, presumably to limit the possibility of fires.

Plastics began to be introduced into the building industry in the 1930s and more aggressively in 

the 1950s. In 1954, the U.S. Chamber of Commerce held a meeting on the use of plastics in 

buildings and published a proceedings (Building Research Institute 1955) that contained one 

section specifically related to plastic duct systems. Skiest (1966) documents the Society of the 

Plastics Industry’s Code Advisory Committee activities that resulted in the development of the 

first “Model Chapter on Plastics for Inclusion in Building Codes.” This first Model Chapter 

language references material classes A (flame spread less than 25), B (flame spread 25 to greater 

than or equal to 75), C (flame spread 75 to 250), and D (flame spread 250 to 500).

Research on fires within ducts was conducted for the U.S. Department of Mines related to the 

propagation of fires in wood-lined mine shafts or the propagation of fires within ventilation duct 

systems for fresh air to mines (Perzak et al. 1987; Lee et al. 1980). Several key findings from this 

research related to the performance of ducts in residential buildings state that fire did not enter 

air supply or exhaust outlets in duct systems, provided the opening was an aperture as opposed to 

a large boot with a supply outlet diffuser. In addition, fires from flammable materials on the 

inside of the ducts consume the available oxygen in the ducts and create a zone of combustible 

gases in front of the flame. In addition, Lee et al. (1980) found that the fire creates a pressure 

regime that can actually overwhelm any fan-induced airflows, even at high velocities. This 

causes fire to spread toward the fan. 

Testing by the Building Research Establishment (2005) suggests that steel return grilles or face 

dampers help to limit the entry of flame into a duct from a fire source below it. It further suggests

that when fans are turned off 2 min after smoke is detected at the air handler, the duct 

temperatures remained below 70°C (158°F).

Analysis and Interpretation

Table 27 summarizes the 2012 IRC (IRC 2012) allowable use of materials with relatively high 

flame spread and smoke developed rating as surface materials, including up to 10% of the 

exposed surface area of rooms to use materials with a flame spread less than 75 and unlimited 

smoke developed. Foam plastic insulation is allowed with a thermal barrier. The interior surface 

of a duct must meet a flame spread of less than 25 and a smoke developed of less than 50. Table 

28 provides a summary of flame spread ratings for several different materials.
56 

Table 27. Flame Spread and Smoke Developed Ratings From the 2012 IRC 

(IRC 2012)

Code Section 

(IRC 2012)

Flame Spread 

(ASTM E84)

Smoke Developed

(ASTM E84)

R302.9 (Interior Finishes)

200

(unlimited for “trim,” doors 

and windows and finished 

1/28-in. thick adhered to 

surface no worse than paper)

450

R302.10 (Insulation) 25 450

R316.3 (Foam Plastic) 75 450

R316.5.9 Plastic Trim (<10% Wall 

+ Ceiling Area) 75 Unlimited

R302.9.4, R316.5.10 Foam Plastic 

Interior Finish 200 (or pass NFPA 286) 450 (or pass NFPA 286)

M1601.1.1.2 (Factory Made Ducts) 0/25 Not specified

M1601.1.1.6 (Duct Systems) 200 Not specified

M1601.1.2 (Underground Ducts, 

Max. 150°F SAT)

25

(inferred from M1601.3)

50

(inferred from M1601.3)

M1601.3.1 and 2 (Duct 

Lining/Covering, and Shall Not 

Flame, Glow, Smolder, Smoke 

under ASTM C411)

25 50

Table 28. Flame Spread and Smoke Developed of Common Materials 

Material Flame Spread 

(ASTM E84)

Smoke Developed 

(ASTM E84)

Red Oak 100 100

Polyvinyl Chloride 10–15 > 300

Gypsum Board 10–15 0

It appears that all of these various requirements have been developed based on specific industry 

interests (i.e., flexible duct manufacturers, plastic duct manufacturers, foam plastic 

manufacturers, HVAC equipment manufacturers, gypsum manufacturers). This fragmentation of 

the code substantially limits the possibility for innovative solutions to be developed and 

implemented in the industry. 

Engineering and fire protection principles indicate the basic assumptions shown in Table 29 that 

should be followed to limit loss of life (occupants and firefighters) in the event of a house fire. 
57

Table 29. Requirements in Various Sections of the 2012 IRC

to Limit Loss of Life and Fire Promulgation

(IRC 2012)

Characteristics Least Restrictive 

Code Limit

Use materials and systems that limit the spread of fire 

from the location of origin—from the burning object 

in the room to the rest of room, from a burning room 

to adjacent rooms, and from a burning building to an 

adjacent building.

200 Flame Spread 

(ASTM E84)

Use materials and systems that limit the development 

of smoke to enable occupant escape and firefighting 

efforts.

450 Smoke Developed 

(ASTM E84)

Use automatic systems to warn occupants in the event 

of fire and to help suppress fires before the fires grow 

out of control.

Sprinklers and smoke/ 

carbon monoxide alarms 

required

Electrical and fuel burning equipment in buildings 

should not be the originating source of fire (should not 

create sparks, excessive heat, etc.).

UL tests

Systems in buildings (e.g., HVAC, structure, 

plumbing, electrical, thermal, water management, 

finishes) should not substantially contribute to the 

spread of fire, hot gases, or smoke within the building 

or from building to building.

UL tests, fire blocking, 

ASTM E84

Recommended Guidance

Currently, builders have no choice but to follow code requirements. The IRC could be modified 

to accommodate various duct materials, as follows:

M1601.1.1 (Revised), M1601.3 (Revised), M1603 (New)

Revise as follows:

M1601.1.1 Above-ground duct systems. Above-ground duct systems shall conform to the 

following:

1. Equipment connected to duct systems shall be designed to limit discharge air temperature to a 

maximum of 250°F (121°C). 

2. Factory-made air ducts shall be constructed of Class 0 or Class 1 materials as designated in 

Table M1601.1.1(1).

Exception: Factory-made air ducts with a flame spread of 25 and a smoke developed of 450 

are allowed provided the duct system is provided with smoke detection system control that 

meets the requirements of Section M1603.

3. Fibrous duct construction shall conform to the SMACNA Fibrous Glass Duct Construction 

Standards or NAIMA Fibrous Glass Duct Construction Standards.
58

4. Minimum thickness of metal duct material shall be as listed in Table M1601.1.1(2). 

Galvanized steel shall conform to ASTM A653. Metallic ducts shall be fabricated in accordance 

with SMACNA Duct Construction Standards Metal and Flexible.

5. Use of gypsum products to construct return air ducts or plenums is permitted, provided that the 

air temperature does not exceed 125°F (52°C) and exposed surfaces are not subject to 

condensation.

6. Duct systems shall be constructed of materials having a flame spread index not greater than 

200.

M1601.3 Duct insulation materials. Duct insulation materials shall conform to the following 

requirements:

1. Duct coverings and linings, including adhesives where used, shall have a flame spread index 

not higher than 25, and a smoke-developed index not over 50 when tested in accordance with 

ASTM E 84 or UL 723, using the specimen preparation and mounting procedures of ASTM E 

2231.

Exceptions: Spray application of polyurethane foam to the exterior of ducts in attics and crawl 

spaces shall be permitted subject to all of the following:

1. The flame spread index is not greater than 25 and the smoke-developed index is not greater 

than 450 at the specified installed thickness. 

2. The foam plastic is protected in accordance with the ignition barrier requirements of 

Sections R316.5.3 and R316.5.4.

3. The foam plastic complies with the requirements of Section R316.

1. Spray application of polyurethane foam to the exterior of ducts in attics and crawl spaces 

shall be permitted subject to all of the following:

1. The flame spread index is not greater than 25 and the smoke-developed index is not 

greater than 450 at the specified installed thickness. 

2. The foam plastic is protected in accordance with the ignition barrier requirements of 

Sections R316.5.3 and R316.5.4.

3. The foam plastic complies with the requirements of Section R316.

2. Duct lining with a flame spread of 25 and a smoke developed of 450 is allowed, provided 

the duct system is provided with smoke detection system control that meets the requirements 

of Section M1603.

2. Duct coverings and linings shall not flame, glow, smolder or smoke when tested in accordance 

with ASTM C411 at the temperature to which they are exposed in service. The test temperature 

shall not fall below 250°F (121°C). Coverings and linings shall be listed and labeled.

Exceptions: Duct lining with a flame spread of 25 and a smoke developed of 450 is allowed,

provided the duct system is provided with smoke detection system control that meets the 

requirements of Section M1603.

3. External duct insulation and factory-insulated flexible ducts shall be legibly printed or 

identified at intervals not longer than 36 inches (914 mm) with the name of the manufacturer, the 
59

thermal resistance R-value at the specified installed thickness and the flame spread and smokedeveloped indexes of the composite materials. Spray polyurethane foam manufacturers shall 

provide the same product information and properties, at the nominal installed thickness, to the 

customer in writing at the time of foam application. All duct insulation product R-values shall be 

based on insulation only, excluding air films, vapor retarders or other duct components, and shall 

be based on tested C-values at 75°F (24°C) mean temperature at the installed thickness, in 

accordance with recognized industry procedures. The installed thickness of duct insulation used 

to determine its R-value shall be determined as follows:

3.1. For duct board, duct liner and factory-made rigid ducts not normally subjected to 

compression, the nominal insulation thickness shall be used.

3.2. For ductwrap, the installed thickness shall be assumed to be 75 percent (25-percent 

compression) of nominal thickness.

3.3. For factory-made flexible air ducts, the installed thickness shall be determined by 

dividing the difference between the actual outside diameter and nominal inside diameter by 

two.

3.4. For spray polyurethane foam, the aged R-value per inch measured in accordance with 

recognized industry standards shall be provided to the customer in writing at the time of foam 

application. In addition, the total R-value for the nominal application thickness shall be 

provided.
60

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61

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Performance. Berkeley, CA: Lawrence Berkeley National Laboratory.

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