Performance Analysis of a 

Modular Small-Diameter 

Air Distribution System 

Andrew Poerschke

IBACOS, Inc.

Armin Rudd

ABT Systems, LLC

March 2016
NOTICE

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iii

Performance Analysis of a Modular Small-Diameter 

Air Distribution System

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: 

Andrew Poerschke

IBACOS, Inc.

2214 Liberty Avenue

Pittsburgh, PA 15222

Armin Rudd

ABT Systems, LLC

726 E. Maple Street

Annville, PA 17003

NREL Technical Monitor: Stacey Rothgeb

Prepared under Subcontract No. KNDJ-0-40341-05

March 2016
iv

The work presented in this report does not represent 

performance of any product relative to regulated 

minimum efficiency requirements.

The laboratory and/or field sites used for this work are 

not certified rating test facilities. The conditions and 

methods under which products were characterized for 

this work differ from standard rating conditions, as 

described.

Because the methods and conditions differ, the reported 

results are not comparable to rated product performance 

and should only be used to estimate performance under 

the measured conditions.
v

Contents

List of Figures ............................................................................................................................................ vi

List of Tables ............................................................................................................................................. vii

Definitions................................................................................................................................................. viii

Executive Summary................................................................................................................................... ix

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

1.1 Background..........................................................................................................................2

2 Mathematical and Modeling Methods................................................................................................. 5

2.1 Mockup System and Load Sizing ........................................................................................5

2.2 Empirical Airflow Characterization.....................................................................................5

3 Research and Experimental Methods ................................................................................................ 8

3.1 Research Questions..............................................................................................................8

3.2 Technical Approach.............................................................................................................8

3.3 Air Handling Unit, Plenum, and Duct Runout Configurations and Construction ...............8

3.3.1 Air Handling Unit ....................................................................................................8

3.3.2 Supply Air Plenums.................................................................................................9

3.3.3 Duct Runouts .........................................................................................................10

3.4 Measurements....................................................................................................................11

3.4.1 Measurement of Differential Pressure ...................................................................12

3.4.2 Measurement of Total System Airflow..................................................................12

3.4.3 Measurement and Calculation of Runout Duct Airflow........................................12

3.4.4 Airflow Visualization.............................................................................................12

3.4.5 Measurement of Sound Level ................................................................................12

3.4.6 Measurement of Air Handling Unit Power............................................................13

3.5 Equipment..........................................................................................................................13

4 Results................................................................................................................................................. 14

4.1 Baseline System.................................................................................................................14

4.2 As-Installed Example Mockup ..........................................................................................20

4.3 Measured Results and Air Conditioning Contractors of America Manual D....................24

4.4 Airflow Visualization.........................................................................................................25

5 Discussion........................................................................................................................................... 30

6 Conclusions ........................................................................................................................................ 32

References................................................................................................................................................. 35

Appendix A: Flow Sensor Calibration..................................................................................................... 37

Appendix B: Test Volumetric Flow Rate and Velocity Data.................................................................. 39
vi

List of Figures

Figure 1. A small-diameter manifold (left) versus a traditional, centrally ducted system (right) ....... 2

Figure 2. Velocity profile for round duct................................................................................................... 7

Figure 3. Using an adjustable-hole saw to cut the manifold outlets ..................................................... 9

Figure 4. A CNC router carving plenum boxes ...................................................................................... 10

Figure 5. Testing a side outlet manifold configuration......................................................................... 11

Figure 6. Duct testing mockup for an example case, showing the duct numbers............................. 11

Figure 7. Diagram of duct runout modifications.................................................................................... 14

Figure 8. Duct airflow deviation for 2-in. manifold boxes—Box 1 (top) and Box 2 (bottom) ............ 15

Figure 9. Duct airflow deviation for 2-in. manifold boxes—Box 3 (top) and Box 4 (bottom) ............ 16

Figure 10. Duct airflow deviation for 2.5-in. manifold boxes—Box 5 (top) and Box 6 (bottom) ....... 17

Figure 11. Duct airflow deviation for 2.5-in. manifold boxes—Box 7 (top) and Box 8 (bottom) ....... 18

Figure 12. Duct airflow deviation for MSHP manifold boxes—Box 9 (top) and Box 10 (bottom) ..... 19

Figure 13. Example house floor plan for mockup (second floor of the home)................................... 20

Figure 14. Airflow deviation of the mockup duct system relative to duct length............................... 21

Figure 15. Example system delivered energy ........................................................................................ 22

Figure 16. Measured duct pressure versus airflow relationship and common fan curves............... 23

Figure 17. Mockup duct system showing elbows ................................................................................. 25

Figure 18. Flow in top-down configuration after 1 s (top series) and 4 s (bottom series) ................ 26

Figure 19. Airflow exiting a 2-in. smooth outlet (20 CFM)..................................................................... 27

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

List of Tables

Table 1. Laminar and Turbulent Flow Limits............................................................................................ 6

Table 2. Measurement Equipment Required .......................................................................................... 13

Table 3. Results from the As-Installed Mockup ..................................................................................... 21

Table 4. Manifold and Duct System Flow Measurement....................................................................... 24

Table 5. Measured Results....................................................................................................................... 25

Table 6. Summary of Plenum Box Testing Results ............................................................................... 28

Table 7. Sensor Correction Factors ........................................................................................................ 37

Table 8. Velocity Measurement and Calculation Comparison.............................................................. 37

Table 9. Sensitivity of Flow Measurement to Sensor Angle................................................................. 38

Table 10. Box 1 Raw Data......................................................................................................................... 39

Table 11. Box 2 Raw Data......................................................................................................................... 39

Table 12. Box 3 Raw Data......................................................................................................................... 39

Table 13. Box 4 Raw Data......................................................................................................................... 40

Table 14. Box 5 Raw Data......................................................................................................................... 40

Table 15. Box 6 Raw Data......................................................................................................................... 40

Table 16. Box 7 Raw Data......................................................................................................................... 41

Table 17. Box 8 Raw Data......................................................................................................................... 41

Table 18. Box 9 Raw Data......................................................................................................................... 41

Table 19. Box 10 Raw Data....................................................................................................................... 42

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

Definitions

ACCA Air Conditioning Contractors of America

AHU Air handling unit

Btu/h British thermal units per hour

CFM Cubic feet per minute

HVAC Heating, ventilating, and air conditioning

ICC International Code Council

MSHP Mini-split heat pump

NC Noise criteria

PVC Polyvinyl chloride

SP Static pressure
ix

Executive Summary

Previous work by the U.S. Department of Energy’s Building America research team IBACOS 

and other Building America teams has shown that for all but the most simple house geometries 

conditioned air must be supplied to each thermal zone. Standard methods that use traditional duct 

geometries have several drawbacks, which include difficulty in bringing ductwork into 

conditioned space because of the necessary bulkheads, long duct runs, large duct sizes, and a 

larger surface area to seal. The homebuilding industry is interested in alternative spaceconditioning systems that can address these concerns and provide thermal comfort and energy 

efficiency.

This report investigates the feasibility of using a home-run manifold small-diameter duct system 

to provide space-conditioning air to individual thermal zones in a low-load home. This compact 

layout allows duct systems to easily be brought within conditioned space via interior partition 

walls. Centrally locating the air handling unit in the house significantly reduces duct lengths. The 

plenum box is designed so that each connected duct receives a similar amount of airflow—

regardless of its position on the box. Furthermore, within a reasonable set of length restrictions 

each duct continues to receive similar airflow. For the plenum box investigated in this report, 

duct runs longer than 10 ft and shorter than 25 ft had airflows of 20–16 cubic feet per minute 

(CFM).

The design method uses an additive approach to reach the total required zonal airflow. Once the 

airflow rate needed to satisfy the thermal load of a zone has been determined, the total number of 

duct runs to a zone can be calculated by dividing the required airflow by the standard airflow 

from each duct. For example, if a zone requires 60 CFM of conditioned air to meet the load and 

the base airflow from each duct is 20 CFM, three ducts would be specified for that zone. This 

method differs from that of traditional duct system design, which relies on varying the diameter 

of the trunk and branch ductwork to manage static pressure (SP) and to supply the desired 

airflow to each thermal zone. The additive approach greatly simplifies the design effort and 

reduces the potential for duct design mistakes. 

By setting up equivalent branch runouts and measuring the air velocity in a representative sample 

of ducts, the research team found two plenum box designs that maintained satisfactory uniform 

airflow from each branch duct (±5%).

After choosing the ideal box geometry, a full mockup duct design was then laid out for 

laboratory testing of airflow, SP, and power consumption of a ducted mini-split heat pump 

(MSHP) blower. The duct system was designed such that it could be installed in—and meet the 

airflow requirements of—the second floor of an example 3,600-ft2 production home.

Measured results indicate that this plenum design can satisfy the heating load (8,280 Btu/h). 

However, the total airflow falls short of satisfying the cooling load. The SP inside the plenum 

box, which is 51.5 Pascals (0.21 in. water column), limited the total airflow of the MSHP blower, 

which limits the total thermal capacity. A slightly oversized MSHP ducted unit could overcome 

the elevated SP. Fan energy consumption is kept to 0.22 watts per cubic feet per minute or lower 

by using short duct runs and smooth duct material. 
1 

1 Introduction and Background

Previous work by the U.S. Department of Energy’s Building America research team IBACOS 

and other Building America teams has shown that for all but the simplest house geometries, 

conditioned air must be supplied to each room to maintain comfort. Homebuilders and heating, 

ventilating, and air-conditioning (HVAC) system installers need an inexpensive way to reliably 

supply the correct amount of conditioned air to individual rooms without performing complex 

duct system design calculations. Current practices for designing and installing air delivery 

systems have many possible failure modes: 

Errors in calculating load

Incorrect equipment selection

Errors in duct system design

Incorrect in-field duct and register/grille component selection

Poor installation of ductwork, which causes airflow restriction

Poor sealing of ductwork and connections

Inefficient operation due to duct runs in unconditioned space

Failure to commission and properly balance the installed system.

This report looks at the performance of a small-diameter modular air distribution system with 

home-run ductwork, which can reduce the risk of these failure modes that lead to comfort 

problems. Home-run ductwork refers to individual branch ducts that leave a central distribution 

manifold and have a predictable airflow associated with only the diameter of the duct. Figure 1

compares the proposed manifold duct layout to a traditional, centrally ducted air handling unit 

(AHU). In theory, with a mature home-run system design, an inspector could simply count the 

number of home-run ducts and sum the total airflow to a given room. This number then could be

compared to an airflow specification based on the calculated load.

Low-load homes built to 2012 International Energy Conservation Code (ICC 2012) energy 

standards are appropriate targets for this technology, because their small envelope loads greatly 

reduce the magnitude of airflow needed and provide opportunities to bring the small-diameter 

ductwork into conditioned space. Mini-split heat pump (MSHP) manufacturers are rapidly 

introducing high-efficiency, ducted units that can be used to condition one or more rooms as a

zonal distributed space-conditioning strategy instead of a full central system. Zonal systems are 

intended to serve only a few rooms that are close in physical proximity to significantly reduce

the required length of duct runs and the pressure drop associated with long duct runs. These 

systems increase the feasibility of using small-diameter duct runs. 
2

Figure 1. A small-diameter manifold (left) versus a traditional, centrally ducted system (right)

1.1 Background

The difficulties with creating central space-conditioning systems (i.e., design, balancing, 

operation, supply outlet locations, fan operation strategies) for low- and very-low-load homes 

have been well documented through modeling and field measurements (IBACOS 2006a, 2006b, 

2007; Rittelmann 2008; Broniek 2008). Aldrich (2009) documented the significant impact 

internal gains of occupants and simple electronics can have in the heating season to offset loads

in houses with single-point supply heating systems and minimal distribution systems. Results 

from studies conducted at the IBACOS unoccupied test houses in Pittsburgh, Pennsylvania

(Poerschke and Stecher 2014), and Fresno, California (Stecher and Poerschke 2013) have shown 

that in well-insulated homes with open floor plans, sufficient temperature uniformity meeting Air 

Conditioning Contractors of America (ACCA) Manual RS standards (Rutkowski 1997) can be 

achieved in the open space by providing space conditioning from a single outlet location. 

Although central, open spaces can be straightforward to condition with simple systems, 

individual rooms that are isolated from the main open space present a challenge for providing 

comfort. The effect of solar heat gain can be pronounced in these instances. Sun-tempered 

houses on Martha’s Vineyard, Massachusetts, showed significant fluctuations in temperatures at 

the first-floor main spaces and second-floor bedrooms with only east- or west-facing windows 

(Stecher et al. 2012). Results from the IBACOS unoccupied test house in Pittsburgh, 

Pennsylvania (Poerschke and Stecher 2014), also showed the expected result that bedrooms with 

significant southern and western glazing tend to overheat when the rest of the house is close to 

the set point temperature.

IBACOS studied passive air transfer from the central spaces to bedrooms with doors open and 

doors closed in its two unoccupied test houses: one in Fresno, California (Stecher and Poerschke 

2013), and one in Pittsburgh, Pennsylvania (Poerschke and Stecher 2014). With interior partition 

doors open, rooms maintained a uniform temperature. However, when interior partition doors 
3 

were closed, passive air inlets provided poor communication between the room and the space 

containing the thermostat, which resulted in room-to-room temperature differences beyond 

ACCA Manual RS recommendations (Rutkowski 1997). Measured data (Aldrich 2010) indicated 

that limited fan-forced supply from the first-floor main space to the second-floor bedrooms with 

doors closed generally can maintain comfort conditions from a single source of heating in the 

main living space in a cold climate during the heating season. Aldrich (2010) and Townsend et 

al. (2009) indicated that some level of active air mixing throughout the house also is necessary to 

ensure fresh ventilation air reaches all spaces. 

One possible solution is to provide individual room space conditioning (e.g., individual room fan 

coils or heat pump units) that can maintain desired temperatures independently in each room. 

Although design research has been performed on such systems (Drost and Wegeng 1996), 

IBACOS has not found market-acceptable residential units in the capacity (1,000–3,000 Btu/h) 

that is necessary to provide individual room conditioning while integrating with a central control 

system.

The performance of ductless MSHP units has been documented by several U.S. Department of 

Energy Building America teams. Findings by Ueno and Loomis (2014) indicate that even small 

homes required at least two indoor head units to mitigate stratification between floors. The 

largest house in the Ueno and Loomis (2014) study (2,300 ft2), which is closer in size to typical 

production homes, required three additional indoor head units after comfort problems were 

reported in the most geometrically remote rooms. 

As mentioned, IBACOS has conducted alternative HVAC system research in houses that meet 

the Building America 50% energy savings targets through 2012 at unoccupied test houses in 

Fresno, California (Stecher and Poerschke 2013), and Pittsburgh, Pennsylvania (Poerschke and 

Stecher 2014). In light of that research and other past IBACOS research (IBACOS 2006a, 2006b, 

2007; Rittelmann 2008; Broniek 2008), as well as the research by others as cited above, 

IBACOS has concluded that for all but the simplest house plans, some form of conditioned air 

distribution to each room is required to provide occupant comfort. More specifically, IBACOS 

reached the following conclusions:

Because of uneven external and internal gains, individual rooms with closed doors may 

not achieve temperature uniformity (per ACCA Manual RS [Rutkowski 1997]) with only 

one thermostat, even with rightsized central systems. Production builders have 

communicated this concern to IBACOS, even with homes built at the 2012 International 

Energy Conservation Code levels of performance (ICC 2012). 

Variable output systems that can modulate capacity (Btu/h) and airflow rate (CFM) can 

provide superior occupant comfort and improved energy efficiency throughout the year. 

In homes with open floor plans, fewer points of supplied conditioned air can maintain 

temperature uniformity in the central living spaces to satisfy recommendations in ACCA 

Manual RS (Rutkowski 1997). 

Toe-to-head air temperature stratification, averaged throughout a low-load house, is 

rarely greater than 3°F.
4 

Bringing conventionally sized ductwork into conditioned space represents a significant 

challenge to production homebuilders. 

Passive heat transfer through open doors is a function of the door opening size, the size of 

any transitional spaces (e.g., a hallway), or other openings between the actively 

conditioned space and the room with no active conditioning, and the temperature 

difference of the room with no active conditioning. 

Based on occupant preferences, interior doors may remain open all day. However, from a 

passive air transfer standpoint, the worst-case scenario is a house with interior partition 

doors that are closed 24 hours per day. 

IBACOS believes the following issues must be resolved for production builders to begin 

to adopt new strategies and methodologies for space-conditioning systems in low-load 

houses:

o Appropriate HVAC system responses (e.g., equipment, air distribution, controls) 

for new low-load homes are needed across peak load and nonpeak load conditions 

in all climate zones. 

o Whole-house performance tradeoffs (e.g., thermal uniformity, efficiency, and 

cost) must be quantified when choosing between competing space-conditioning 

and ventilation system designs. 

o The technical merits of variable airflow output, small-duct space-conditioning 

systems must be compared to readily available alternatives from the perspectives 

of cost, performance (e.g., energy and system noise), and occupant comfort. 

Alternative system types include traditional centrally ducted systems, limited 

ducted distribution systems with fewer supply points, and point-source systems.
5 

2 Mathematical and Modeling Methods

The majority of the results from this project are based on laboratory measurements in two 

phases: (1) testing of various plenum box designs with similar duct runouts, and (2) testing of the 

best plenum box with a mockup duct runout. The research team performed an uncertainty 

analysis on individual measurements. The team also calibrated the laboratory measurements 

according to individual sensors and compared those measurements to expected fluid behavior.

2.1 Mockup System and Load Sizing

To design the duct layout for the sample mockup system, the team first performed a load sizing 

using Wrightsoft Version 13.0.10 on a two-story, slab-on-grade house design. The house design 

has many rooms that are distant from the main living space and is a challenge to condition with a 

single ducted system. A large house such as this ideally would have two HVAC systems—one 

on each floor. Based on this load sizing, the team determined the required airflow rates to meet 

the load. To best align with concurrent IBACOS work efforts, the team chose the house design 

based on the production house in Herk and Rapport (2015). The team then determined the 

number of duct runouts needed to satisfy this load based on the laboratory airflow testing results. 

A floor plan for this house can be found in Section 4 as Figure 13.

2.2 Empirical Airflow Characterization

The team wished to compare the measured airflow to the established fundamentals of fluid 

dynamics. A basic understanding of flow characteristics can be described by the Reynolds 

number. This number can predict whether a flow will be laminar or turbulent. In a round pipe, 

for Reynolds numbers less than 2,100, flows typically are laminar, whereas Reynolds numbers 

greater than 4,000 typically are turbulent (Munson et al. 2009). Reynolds numbers between 

2,100 and 4,000 suggest that the flow is in an ill-defined “transition” region, and as such, 

empirical measurements must be made to characterize the flow. The team used Equation 1 to 

calculate the Reynolds number:

VD Re , (1)

where

Re = Reynolds number

 = Density of fluid 

 = Dynamic viscosity

V = Velocity

D = Hydraulic diameter

For the 2-in. and 2.5-in. ducts considered in this project, the velocities shown in Table 1 dictate 

the transition between laminar and turbulent flows. For all but the lowest airflow rates, the flow 
6 

was turbulent. Flow rates between the maximum laminar value and the minimum turbulent value 

are in a transition region where the exact state cannot be characterized with existing equations. 

The flows studied in this project are rarely in this region.

Table 1. Laminar and Turbulent Flow Limits

Unit Maximum 

Laminar Flow

Minimum 

Turbulent Flow

2.5-in. Duct ft/min 99 189

CFM 3.4 6.4

2-in. Duct

ft/min 124 236

CFM 2.7 5.2

Because the air velocity was sampled at the centerline of each duct, an understanding of the 

airflow velocity profile was necessary to correlate measured air velocities to total duct flow rates. 

From Eq. 2, which is the equation for the empirical power-law velocity profile for turbulent 

flows (Munson et al. 2009), the team determined that the total average velocity was 85% of the 

centerline velocity. At the pipe centerline, the velocity is equal to the measured centerline value. 

As the distance from the centerline approaches the radius of the pipe, the velocity approaches 

zero. Figure 2 shows the profile for the 2-in.-diameter and 2.5-in.-diameter ducts. 

n

R

r

V

V

c

x

1

1 , (2)

where

Vx = Velocity at a distance from the centerline

Vc = Velocity at the centerline

r = Distance from the centerline

R = Radius of the pipe

n = Power law value, function of Reynolds number (typical value of 7 can be used)
7

Figure 2. Velocity profile for round duct

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Vx/Vc

Distance from Centerline (in.) 2.5 in. 2.0 in.
8 

3 Research and Experimental Methods

3.1 Research Questions

The following research questions will be answered by this project: 

Can a strategy of additive, individual, small-diameter ducts (2-in. and 2.5-in. diameter) 

provide adequate airflow to give comfort in a low-load house?

What are the fan energy tradeoffs and maximum recommended run lengths for smalldiameter ducts connected to small plenums?

What load characteristics could each duct serve, and what other parameters (velocity, 

throw) are associated with such a system?

What are the ideal configuration and geometry of a small plenum for a home-run smalldiameter duct system that would be adaptable to MSHP systems or small distributed 

central systems and that would supply equal airflow through each duct without 

balancing?

What is the energy impact of locating ducts inside conditioned space relative to the

higher fan energy needed to move the airflow under higher flow resistance? 

3.2 Technical Approach

IBACOS assessed the performance of a small-diameter (2-in.-diameter and 2.5-in.-diameter) 

ducted air-distribution system. The research team constructed 10 mockup plenum boxes with 

small-duct runouts. Measurements were collected on each duct system configuration to establish 

airflow, velocity, and pressure drop. Then the team constructed the mockup duct system to 

mimic a system that could be installed in the house floor plan (shown later in Figure 13). The 

measured results for airflow and delivered energy were compared to calculated room loads.

IBACOS designed an ideal system layout for the house plan to provide the least complex 

installation. The duct layout was set up in the test facility (shown later in Figure 6). Duct 

performance characteristics were measured, including airflow, air velocity, pressure drop, and 

AHU fan power. 

3.3 Air Handling Unit, Plenum, and Duct Runout Configurations and 

Construction

This section describes the configurations of the AHUs, supply plenums, and small-diameter duct 

runouts that extend from the supply plenum of the ideal duct system. Construction of the 

plenums with runout takeoffs is detailed. Technical data are provided in tables related to total 

system airflow, runout air velocity, runout airflow, number of runouts required to carry the total 

system airflow, and runout load capacity.

3.3.1 Air Handling Unit

Two types of AHUs made up the basic driver for the air-distribution systems: (1) a commercially 

available AHU made for ducting air from a 1-ton variable-capacity, variable-airflow MSHP

system (Samsung EH035CAV, 265 to 370 CFM), and (2) a commercially available, usercontrolled, variable-speed blower that was suitable for testing purposes.
9

The team also considered a third scenario in which a boost fan was installed in series with the 

main ducted MSHP AHU. The boost fan consisted of a Duct Blaster connected to the return 

plenum area of the ducted mini-split. Because the added duct system impeded the MSHP AHU 

from achieving its rated airflow, the boost fan was activated and ramped up so the total system 

airflow reached the rated airflow of the MSHP. The final mockup duct configuration was the 

only one tested with the additional boost fan.

Each AHU was mounted on a test wall setup with various plenum and duct configurations 

attached for testing. The test wall was designed to allow for efficient substitution of plenum 

boxes and duct systems.

3.3.2 Supply Air Plenums

Supply air plenum boxes were designed to be straightforward to construct and test. All plenum 

designs were simple rectangular boxes. Future testing may optimize the shape of the most 

promising box layouts.

The team constructed supply air plenums from 0.5-in.-thick cabinet-grade plywood. An 

adjustable-diameter hole saw was used to cut tight-fitting holes for connecting the polyvinyl 

chloride (PVC) ductwork (Figure 3), so no additional sealant would be needed.

Figure 3. Using an adjustable-hole saw to cut the manifold outlets

The team used 0.25-in. polycarbonate sheet on four sides to construct an additional set of duct 

boxes. The final two sides were constructed from plywood and painted a flat black. The team

used computer numeric control (CNC) equipment to make the cutouts of plastic and plywood to 

provide tight dimensional tolerances (Figure 4). Clear panels in the boxes enabled the team to

use theatrical smoke to observe the airflow. A planar laser was shone through the bottom of the

box to highlight a two-dimensional slice of the flow.
10

Figure 4. A CNC router carving plenum boxes

3.3.3 Duct Runouts

The team constructed the duct runouts from 2-in.-diameter and 2.5-in-diameter Schedule 40 PVC

pipe. This material was chosen because of its low airflow resistance, ease of making leak-free 

joints, availability, and rigidity; the rigidity allows for easy setup of the mockup systems.1 The 

team installed each of the duct runouts to match the hole configuration of the respective duct 

box.

Then the team tested each plenum box in one of four configurations. First, to gather baseline 

data, the team connected ducts of equal length to each outlet. Next, the team considered three 

variations that were intended to change the resistance of each duct:

1. One duct run length was increased by a factor of 1.5 or 2 (increased resistance).

2. Several 90° bends were added to some of the duct runs (increased resistance).

3. One duct length was reduced by a factor of 2 (halved) (decreased resistance).

Figure 5 shows testing of a side outlet manifold configuration. Figure 6 shows the example duct 

system testing setup for the chosen house. In testing the example duct layout, ductwork was bent 

in such a way that it would lie flat; however, in a real house, the ductwork would take a threedimensional shape.

 1 Another ongoing IBACOS effort is coordinating with industry stakeholders and the codes community to make 

PVC ductwork code-approved for air delivery.
11

Figure 5. Testing a side outlet manifold configuration

Figure 6. Duct testing mockup for an example case, showing the duct numbers

3.4 Measurements

This section describes measurements IBACOS took in the laboratory for this project.
12

3.4.1 Measurement of Differential Pressure

The research team measured the static pressure (SP) differential from the supply plenum to the 

ambient room environment and recorded the values for each airflow level and duct configuration 

tested. These pressure measurements established the pressures against which the air-moving 

equipment worked in each test case. A project goal was to design systems that minimize the SP

to minimize fan energy. 

3.4.2 Measurement of Total System Airflow

IBACOS sealed the AHUs and plenums so essentially no air leaked from those components. 

Then the team calculated the total airflow of the system by summing the individual duct airflows. 

Some cases had too few airflow sensors to have a dedicated sensor in each duct runout. For these 

cases, the team used the average duct airflow rate to extrapolate the total airflow. This latter 

approach was used only for the case in which all duct runs were of equal length. As an example 

of this method, say ten duct runouts and eight sensors were available. If the average reading on 

each sensor showed 20 CFM, the total system airflow was assumed to be 200 CFM. This 

assumption may introduce some error if one of the duct runouts that was not sampled had a 

significantly different airflow.

3.4.3 Measurement and Calculation of Runout Duct Airflow

IBACOS used a hot-wire anemometer installed in the center of each duct to measure the air 

velocity in each small-diameter runout. The probe was installed through the side wall of the duct, 

with 20 duct diameters straight distance upstream and a minimum of 5 duct diameters 

downstream of the probe. The team then calculated the volumetric airflow rate by multiplying 

the area of the duct by the velocity and a correction factor of 0.85 to account for the turbulent 

velocity profile.

Each velocity sensor had a consistent bias error that was calculated and removed from each 

sensor reading. Appendix A includes a discussion of this process.

3.4.4 Airflow Visualization

The research team used flow visualization to qualitatively analyze the airflow pattern in the 

plenums and into the small-diameter duct runouts. To do this, the team released theatrical smoke 

into the center of the supply plenum 18 in. before the manifold box. Then the team used a highdefinition video camera to record the smoke as it moved through the clear-top plenum against a 

flat black background. 

3.4.5 Measurement of Sound Level

As the air-moving equipment became more loaded against the airflow resistance and as the air 

velocity increased in the ducts, the sound level increased, as expected. The noise level rating of 

an air outlet is directly proportional to the air volume supplied through the outlet; the sound 

increases as more air is supplied. Typically, the noise level of an air outlet is rated with a noise 

criteria (NC) sound pressure value. The NC level depends on the air volume and neck velocity 

(Price 2011; AHRI 2008). The NC value assumes a 10-decibel default for average room 

absorption and an approximate distance of 5 ft from a single source. Large outlets generally are 

quieter at the same airflow rate than smaller sizes of the same model because of higher free area 

or lower inlet velocity, or both. Outlets may be selected so the NC level does not exceed the 

ASHRAE recommended values for the space being considered (ASHRAE 2011). An NC level of 
13

17 can be considered barely perceptible, whereas an NC level of 37 may be marginally 

acceptable. 

Background noise in the laboratory posed a measurement uncertainty issue that limited detailed 

comparisons. More accurate sound level testing may be of greater importance when the systems 

are field applied and are tested in future work. 

3.4.6 Measurement of Air Handling Unit Power

Power draw of the AHU was measured with a true root-mean-square power meter for only the 

MSHP mockup configuration. Because of the nature of the experimental setup, measuring the 

power draw during the tests using the commercial blower would not have been representative of 

a system as installed in a house. This measurement would be useful for comparing the relative 

power required for each airflow value; however, the test setup was not identical to the complete 

set of duct system components used in a real house.

3.5 Equipment

Table 2 shows the measurement equipment used for this project.

Table 2. Measurement Equipment Required

Measurement Equipment

Static Pressure Differential from the 

Supply Plenum to the Laboratory 

Room

Pressure transducer, auto-zeroing, accuracy < ±1.0 Pa*, 

minimum range 0–250 Pa (The Energy Conservatory DG-700)

Airflow of the Total System The Energy Conservatory Duct Blaster system with a digital 

pressure gauge as a powered flow hood

Air Velocity/Air Speed in SmallDiameter Duct Outlets/Runouts

Hot-wire anemometer transducer, accuracy ±5% (Elektronik 

EE575)

Airflow Visualization Theatrical smoke emitter, planar laser (from laser level), highresolution video camera

Temperature and Relative Humidity 

of the Laboratory Room Standard hygrometer

Sound Level Sound meter (EXTECH 407780A)

AHU Power True root-mean-square power meter (Watt Node WNC-3Y208-MB)

*Pascal
14

4 Results

This section presents the data from the ten tested duct box designs, followed by the mockup

example. Of the ten boxes, the first four were connected to 2-in. ducts; the second four used

similar designs and were connected to 2.5-in. ducts. The final two duct boxes were connected to 

the ducted MSHP using 2-in. ducts. The boxes were designed as examples that would be easy to 

fit into interior wall cavities or small bulkheads. 

4.1 Baseline System

Data collected from the box testing are summarized in this section. The emphasis of this data is 

to provide an understanding of the airflow distribution across all the individual duct runs. For 

each tested box, a drawing of the box is shown, followed by four charts. These charts show the

percentage deviation of each duct run from the median value airflow rate. As perturbations were 

made to the duct runout arrangement, the median duct airflow value was held constant from the 

baseline test so the plots can be compared horizontally. Each perturbation is shown on the same 

y-axis scale as the baseline graphic. Line colors correspond to the outline of the hole in each box. 

For duct runs with no measurement, the drawing has no colorized line. Whenever a perturbation 

was made, the line was tripled in weight for identification. Figure 7 shows a rendering of each 

modification to the duct runout. These plots are shown in Figure 8 through Figure 11. The results 

from the MSHP duct box are shown in Figure 12. Some of the measurements show random 

deviation in individual measurements. Box 3 in Figure 9, for example, shows a sudden change in 

outlet 3 at 100 CFM. This is likely due to recording error and should be disregarded. Appendix B 

includes data tables that contain the air velocity and flow rate measured from each duct box. 

Figure 7. Diagram of duct runout modifications
15

Figure 8. Duct airflow deviation for 2-in. manifold boxes—Box 1 (top) and Box 2 (bottom)

0 100 200 300 0 100 200 300

-50%

-30%

-10%

10%

30%

50%

0 100 200 300 0 100 200 300

-50%

-30%

-10%

10%

30%

50%

0 100 200 300 0 100 200 300 0 100 200 300 0 100 200 300

 Baseline 90° 1.5x 1/2x 

 Baseline 90° 1.5x 1/2x 
16

Figure 9. Duct airflow deviation for 2-in. manifold boxes—Box 3 (top) and Box 4 (bottom)

-50%

-30%

-10%

10%

30%

50%

0 100 200 300 400 0 100 200 300 400 0 100 200 300 400 0 100 200 300 400

-50%

-30%

-10%

10%

30%

50%

0 100 200 300 400 0 100 200 300 400 0 100 200 300 400 0 100 200 300 400

 Baseline 90° 1.5x 1/2x 

 Baseline 90° 2x 2x 
17

Figure 10. Duct airflow deviation for 2.5-in. manifold boxes—Box 5 (top) and Box 6 (bottom)

-50%

-30%

-10%

10%

30%

50%

0 200 400 0 200 400 0 200 400 0 200 400

-50%

-30%

-10%

10%

30%

50%

0 100 200 300 0 100 200 300 0 100 200 300 0 100 200 300

 Baseline 90° 1.5x 1/2x 

 Baseline 90° 1.5x 1/2x 
18

-50%

-30%

-10%

10%

30%

50%

0 200 400 0 200 400 0 200 400 0 200 400

-50%

-30%

-10%

10%

30%

50%

0 200 400 0 200 400 0 200 400 0 200 400

Figure 11. Duct airflow deviation for 2.5-in. manifold boxes—Box 7 (top) and Box 8 (bottom)

 Baseline 90° 1.5x 1/2x 

 Baseline 90° 2x 2x 
19

Figure 12. Duct airflow deviation for MSHP manifold boxes—Box 9 (top) and Box 10 (bottom)

-30%

-20%

-10%

0%

10%

20%

30%

40%

155 160 165 170 150 155 160 165 170 150 155 160 165 175 180 185 190 195

190 200 210 220 230 175 185 195 205

-30%

-20%

-10%

0%

10%

20%

30%

40%

190 200 210 220 230 190 200 210 220 230

 Baseline 90° 2x 2x 

 Baseline 90° 1.5x 1/2x 
20

4.2 As-Installed Example Mockup

Figure 13 shows the second-floor plan of the sample home used as a basis for the mockup. The 

ductwork design was such that each run could fit into the required space of the test house. Table 

3 shows the results and airflow balance from the example mockup. For reference, the duct 

numbers listed in the table match the ducts labeled in Figure 6. Figure 14 presents the airflow 

difference of the mockup duct system relative to duct length.

Figure 13. Example house floor plan for mockup (second floor of the home)
21

Table 3. Results from the As-Installed Mockup

Duct 

Number

Airflow

(CFM)

Length 

(ft)

Number of 

Bends

Fan Speed

Low Medium High

1 17 19 19 12.7 2

2 15 16 16 19.1 2

3 15 16 16 21.58 2

4 12 13 14 36.41 3

5 13 15 15 31.02 3

6 14 15 15 27.45 3

7 18 20 20 14.7 1

8 16 17 18 21.2 1

9 18 19 20 9 1

10 21 23 24 5.1 1

Power (Watts) 26.3 32.6 39.2

Total CFM 160 172 176

Watt/CFM 0.16 0.19 0.22

Static Pressure (Pa) 43.0 48.4 51.5

Figure 14. Airflow deviation of the mockup duct system relative to duct length

The key measure of success for the example mockup is the ability of the duct system to meet the 

sample home’s calculated heating and cooling loads from ACCA Manual J (Rutkowski 2006). 

Equation 3 and Equation 4 were used to calculate delivered energy. Heating mode assumed only 

sensible energy change through the coil with a 50°F temperature rise. Cooling mode assumed a 

25°F temperature reduction and 22 grains of water removed per pound of dry air. Figure 15

shows the delivered energy of the example system compared to the calculated loads.

-30%

-20%

-10%

0%

10%

20%

30%

40%

0 5 10 15 20 25 30 35 40

Percent Difference from Median

Duct Length (ft)

176 172 160 (CFM) Fan Setting
22 

Sensible Heat = 1.1 ( ) (3)

Latent Heat = 0.69 (

grains

lb ) (4)

Figure 15. Example system delivered energy

Once the team measured the airflow for which the MSHP blower was capable, additional 

measurements were taken with a Duct Blaster boost fan in series to increase the SP and flow rate. 

Figure 16 presents the resulting duct curve along with the AHU manufacturer–supplied airflow 

rate versus SP curves. The intersection of these curves represents possible system operation 

points.

0

500

1000

1500

2000

2500

3000

3500

Bed 3 Media Room Guest Suite Bed 5 Bed 4

BTU/h

Delivered Heating Calculated Heating Delivered Cooling Calculated Cooling
23

Figure 16. Measured duct pressure versus airflow relationship and common fan curves

Data collected when the Duct Blaster was used as a boost fan are presented in Table 4. These

data are included to provide an idea of possible flow rates given a more powerful MSHP fan. The 

first three test measurements used standard Duct Blaster airflow measurement techniques to 

target zero external SP in the return plenum. The second three tests aimed to recreate the supply 

plenum pressure measured with only the MSHP fan connected. Finally, the Duct Blaster was 

ramped up to determine the SP needed for additional airflow through the duct system. As the SP

was boosted, the team noticed significant air leaking from the MSHP cowling. A subsequent test 

of the duct leakage revealed that when the outlet of each duct was sealed with duct mask, total 

leakage at 24 Pascals (Pa) was 20 CFM. The team then sealed the MSHP cowling with duct 

mask and retested the duct leakage to find it had reduced to 11 CFM. These results led the team 

to believe significant leakage had occurred through the MSHP cowling, which is evident when 

the Duct Blaster measured cubic feet per minute is compared with the total system airflow 

calculated in Table 3 (166 versus 176 CFM). Taking this leakage into consideration, the method 

of calculating total system airflow by summing the individual duct airflow rates provides an 

accurate number.

y = 6E-05x1.5459

R2 = 0.9985

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 200 400 600 800

Pressure (inwc)

Airflow (CFM)

Traditional Fan Curve 1

Traditional Fan Curve 2

Traditional Fan Curve 3

High Velocity Fan Curve

Mock-Up Duct System Curve

Mock-Up Trendline Curve
24

Table 4. Manifold and Duct System Flow Measurement

Duct 

Blaster

Ring

MSHP 

Fan 

Setting

Supply 

SP

(Pa)

Duct 

Blaster

(CFM)

Return 

SP

(Pa)

MSHP 

Power

(W)

2 1 37 154 0 28.8

Aiming for 0 Duct Blaster

SP 2 2 40.6 161 0 34.9

2 3 42.5 166 0 41.9

2 1 43 172 10.4 29.1 Aiming for same plenum 

SP as test without the 

Duct Blaster

2 2 48.7 186 13.8 35.4

2 3 51.5 193 14.4 42.2

2 3 59.4 211 26.2 42.6

Using a Duct Blaster as a 

boost fan to push beyond 

AHU flow

2 3 72.8 238 46.4 43.1

2 3 83.2 260 63.5 43.6

1 3 89.6 271 72 44.0

1 3 103.4 301 95.2 44.1

1 3 118.9 326 121.8 44.5

4.3 Measured Results and Air Conditioning Contractors of America Manual D

The MSHP mockup layout used multiple 90° fittings in the design. Because pressure drop 

through the small-diameter duct system can be significant, the research team calculated the 

pressure drop of the system using values provided in ACCA Manual D (Appendix 3, Group 8 

Elbows and Offsets) (Rutkowski 2009). The analysis revealed a difference between some of the

measured pressure drops associated with 90° elbows and the ACCA-specified pressure drop. 

Specifically, ACCA Manual D indicates that for a radius/diameter ratio of 1.0 of a smooth 

elbow, the equivalent length is 15 ft per 90° elbow; the empirical measurements indicated that 

the equivalent length of the fitting was 5 ft. To further investigate this discrepancy, the team used 

a Duct Blaster to provide airflow and used a manometer to measure the pressure drop due to a 2-

in.-diameter 90° PVC elbow. A number of configurations were evaluated (Table 5). For 

reference, a close-up of the elbows used in testing is presented in Figure 17.
25

Figure 17. Mockup duct system showing elbows

Table 5. Measured Results

Number 

of Number 

of 

Elbows

Pressure Drop

(Pa) [in. water column]

10-Ft 

Sections

10 CFM 15 CFM 20 CFM 25 CFM 30 CFM 40 CFM

0 1 3.9 [0.02] 7.4 [0.03] 11.4 [0.05] 16.9 [0.07] 23.3 [0.09] 37.9 [0.15]

1 0 8.7 [0.03] 17 [0.07] 27.3 [0.11] 39.5 [0.16] 53 [0.21] 86.4 [0.35]

2 0 14.1 [0.06] 28.3 [0.11] 45.8 [0.18] 68.2 [0.27] 90.2 [0.36] 145.3 [0.58]

2 1 16.1 [0.06] 31.7 [0.13] 50.1 [0.20] 74.4 [0.30] 101.3 [0.41] 158.9 [0.64]

2 2 18.7 [0.08] 36 [0.14] 58.8 [0.24] 80.5 [0.32] 115.7 [0.46] 178.3 [0.72]

The team used the results shown in Table 5 to determine the equivalent length of the 90° elbow. 

The results indicate that the equivalent length of a 90° elbow is 4 ft, based on the equivalent 

pressure drop of straight duct. This is 73% lower than the data provided in ACCA Manual D

(Rutkowski 2009). The reason for the significant difference between the ACCA values and the 

measured data is ACCA’s higher friction rate assumption for the duct material. 

4.4 Airflow Visualization

The flow visualizations shown in Figure 18 are from the test case with all side outlets similar to 

those of Box 1. Still images extracted from video clips at 1 s and 4 s from injecting a 1-s pulse of 

smoke into the supply plenum 12 in. before the plenum box are shown. Images from higher 

airflow rates than those presented were too obscured to be usable because of the high velocity of 

the smoke front. A brighter laser would have allowed for faster shutter speeds and the capture of 

higher airflow rates. The camera and lens setup was ISO 5000, 0.01-s shutter speed, 85-mm focal 

length, and f/1.2 aperture. 
26

Figure 18. Flow in top-down configuration after 1 s (top series) and 4 s (bottom series)

Video footage shows a tendency for the airflow to form a rotational vortex in the bottom of the 

box. A better box design would include a taper toward the bottom of the box to accommodate the 

loss in air pressure.

In addition to visualizing the airflow inside the plenum box, the team used a similar setup to 

image the airflow leaving the duct. For this test, the team connected a Duct Blaster to a single 

10-ft section of 2-in.-diameter pipe and supplied a constant 20 CFM of air through the pipe. 

Theatrical smoke was introduced near the inlet of the Duct Blaster. A horizontal planar laser was 

shone through the smoke from a vantage point immediately above and parallel with the duct 

outlet. Video footage was captured from a vantage point 12.5 ft below the duct outlet. Figure 19

represents a single frame taken from the video footage several seconds after the smoke was 

introduced, when the shape of the smoke had taken a steady state. The luminance value was 

inverted for ease of viewing the image.

A qualitative analysis of the throw indicates the throw reaches nearly 10 ft from the outlet.

Poerschke (2015) provided further discussion about throw from small-diameter duct systems.
27

Figure 19. Airflow exiting a 2-in. smooth outlet (20 CFM)

The results of the duct box testing are summarized in Table 6. “Min. Flow” represents the 

minimum total system airflow rate tested. “Max. Flow” represents the maximum tested airflow 

rate. The “Median Port” is the plenum outlet that had the statistical median airflow. These values 

are presented for the lowest “low flow” and the highest “high flow” tested.
28

Table 6. Summary of Plenum Box Testing Results

Box Diameter

(in.) Ducts

Min.

Flow 

(CFM)

Median 

Port(s) at 

Low Flow

Max.

Flow 

(CFM)

Max.

Flow 

Standard

Deviation

Median 

Port(s) at 

High 

Flow

Min.

SP

(Pa)

Max.

SP

(Pa)

Blower

1 2.0 7 35 Not 

consistent 246 0.08 Not consistent – – Com. Blower

2 2.0 10 37 3 250 0.31 3 – – Com. 

Blower

3 2.0 10 42 4 300 0.27 4 2.1 110.7 Com. 

Blower

4 2.0 10 45 1,3,7 324 0.31 1,3,7 1.5 71.1 Com. 

Blower

5 2.5 8 49 5,6,7 389 0.23 4,5 2.1 113.8 Com. 

Blower

6 2.5 10 48 1 280 0.47 2 0.7 27.4 Com. 

Blower
29

Box Diameter

(in.) Ducts

Min.

Flow 

(CFM)

Median 

Port(s) at 

Low Flow

Max.

Flow 

(CFM)

Max.

Flow 

Standard

Deviation

Median 

Port(s) at 

High 

Flow

Min.

SP

(Pa)

Max.

SP

(Pa)

Blower

7 2.5 10 55 4,7 395 0.30 4,6,7 1.4 70.2 Com. 

Blower

8 2.5 10 54 5 380 0.47 5 0.7 24.3 Com. 

Blower

9 2.0 8 157 1,2 169 0.29 1,2 – – MSHP

10 2.0 10 199 1,5 230 1.10 1,5 20.2 23.2 MSHP

11 2.0 10 160 2,8 180 3.20 2,5 43.0 51.5

MSHP 

+ Duct 

Blaster
30

5 Discussion

Several features describe the most successful box geometries. Initially, some plenum boxes 

showed excellent balance; all duct runs had equal length.

Each duct box showed some unique behavior because of geometry and hole location. The tall 

and narrow Box 1 and Box 5 both showed relatively equal exiting airflow; the bottom outlets 

received slightly more airflow than the top outlets. 

Box 2 and Box 6 showed the least uniform behavior because of the relatively small inlet aimed 

directly at a few outlets. In this case, the bottom outlets also were favored because of the inlet 

design. Airflow was coming from above the box, into a 90° elbow, with little distance to 

straighten the flow. This caused pressure to build in the bottom of the box and thus greater 

airflow out of the bottom-most outlets. A better design for this box would incorporate a flowdiffusing device into the box but at the expense of creating more SP. The box design showing the 

most uniform baseline airflow, Box 9, had an inlet geometry that closely matched the layout of 

the outlet holes.

Box 4 and Box 8 showed a preference for the bottom outlets receiving more airflow; adding a 

90° elbow at the end of some outlets had a small but noticeable impact on the outlet airflow. 

Typically, a reduction in airflow of 5% or less was measured. This is to be expected with such a 

small restriction added. 

Increasing one duct length by a factor of 1.5 resulted in a roughly 30% airflow reduction to that 

duct. Only Box 9 showed a measurably better behavior to keep the airflow in balance with this 

restriction. A 20% reduction in airflow was measured in Box 9. This test had a cooling coil 

directly before the box inlet at the exit of the AHU. The coil may have helped to balance any SP

gradients that may have formed within the box.

Halving or doubling several duct runs had a mixed effect; the longer duct runs received less 

airflow, and the shorter duct runs received more airflow. Because several duct lengths were 

adjusted, any individual duct airflow was not impacted as significantly as the case with only one 

duct run modified.

Box 9 and Box 10 showed excellent airflow stability. In these cases, the fan was blowing the air 

across a cooling coil, which provided a more even inlet condition into the box. Because Box 9 

showed the most balanced airflow, it was chosen to be connected to the mockup duct system.

Increasing the total airflow through the duct box did not cause a significant change in airflow 

balance between duct runouts except in Box 6, where the inlet conditions were laminar below 80 

CFM and turbulent above 140 CFM. This was apparent in the measured data, as the relative 

airflow balance between each duct run varied significantly at the lower airflow rates when the 

flow transitioned from laminar to turbulent. This effect was not pronounced in the other boxes,

because the air had to turn before exiting.

The results from the as-installed mockup indicate that relatively balanced airflow can be 

achieved under some reasonable set of design restrictions. These results suggest that a design 
31

guideline with simple strategies could be created if many permutations of duct lengths and bends

are tested. In the tested duct layout, the minimum duct length was 9 ft and the maximum duct 

length was 25 ft with one or two elbows, and the airflow values ranged from 16 to 20 CFM. This 

is a reasonable expectation for the variations in branch duct airflows across a duct system. As 

shown in Figure 15, the total delivered energy was adequate for heating mode; in cooling mode,

airflow was apparently insufficient to provide adequate cooling. However, the blower fan was 

not able to supply the correct maximum airflow for the cooling coil. A more powerful blower 

motor would be able to supply the necessary airflow to condition the space.

The measured results of this mockup suggest that one ducted unit can condition the entire second 

floor of this house, whereas a system using ductless MSHP head units would require a 

combination of three wall-mounted units to condition the space. Short duct runs and a compact 

layout result in an efficient system, despite the small duct diameter. The measured electric power 

needed to move the air in the mockup case ranged from 26 to 39 watts, for a fan watt draw of 

0.16 to 0.22 watt/CFM. A fan watt draw of 0.5 watt/CFM is considered the cutoff for an efficient 

system. California’s Title 24 specifies a minimum fan efficiency of 0.56 watt/CFM. The 

manifold proved to be efficient, and even with a boost in airflow, the watt draw would be 

expected to remain below 0.5 watt/CFM. Static pressure also remained low at 51.5 Pa (0.20 in.

water column). 

A plot showing the duct curve for the mockup system, as well as several fan curves, also is 

shown in Figure 16 for comparison. The intersection of a fan curve and the duct curve is where a 

hypothetical system would operate. The mockup duct system would work with either a 

traditional or a high-velocity blower.
32 

6 Conclusions

The purpose of this project was to use laboratory measurements to determine if a small-diameter 

duct, home-run manifold air distribution system with additive zonal airflow could be feasible. 

Measurements show that under a reasonable set of system parameters, such a home-run system is 

feasible.

The research team tested ten manifold designs. The best-performing design, Box 9 with a wide 

inlet and outlet geometry, was chosen and used for testing with the mockup duct runout. The 

measured results showed a range of +30% to –20% airflow compared to the median airflow 

value for the mockup. If a reasonable set of conditions is applied to the data, such that a duct run 

should be longer than 10 ft and shorter than 25 ft, the range of airflow values would fall within 

+20% and –10%, which is an acceptable range for modern duct systems. Furthermore, Box 9 was 

designed to fit to existing ducted MSHP geometries and offers an easy method to bring this 

design to market.

The calculated thermal energy delivery for the mockup system compares favorably with the 

calculated load in heating mode. In cooling mode, the cooling capacity falls short, based on the 

airflow delivered by the test blower. However, the measured airflow was not up to the total rated 

airflow for the MSHP system. Based on measured results using a boost fan, if a higher-capacity 

MSHP blower was used, the cooling load could probably be met.

Total duct resistance is more important than outlet position on the box for a well-designed box. 

In the case of Box 9, outlet position had no measurable effect on the individual duct airflow.

Ultimately, results from this study indicate that when one of the ideal manifold box designs is 

used, airflow from individual duct runs in this manifold system can be predictable, based on duct 

length and the number of elbows. 

Future work could look at the following areas:

Expand upon the mockup test case, and identify the complete range of duct runout 

configurations that provide adequately balanced airflow without additional balancing 

measures.

Install the duct system in a test house, and measure the efficiency and ability of the 

system to provide comfort.

Further consider the design and aesthetic implications of bringing small-diameter 

ductwork into conditioned space. An ideal design would require minimal use of obtrusive 

bulkheads and centrally locating the AHU and ductwork.

The goal of this project was to answer the following research questions. A brief summary answer 

is included after each research question.

Can a strategy of additive, individual, small-diameter ducts (2-in. and 2.5-in. diameter) 

provide adequate airflow to give comfort in a low-load house?
33 

o The measured results indicate that the duct system can provide adequate airflow 

for heating mode. If the MSHP blower is slightly oversized, the total system can 

probably meet the cooling demand.

What are the fan energy tradeoffs and maximum recommended run lengths for smalldiameter ducts connected to small plenums?

o The increased fan energy consumption expected from using smaller-diameter 

ductwork is minimized with smooth duct material and a compact duct design. The 

watt draw of the mockup duct system ranged from 0.16 to 0.22 watt/CFM.

Measured results also indicate that relatively balanced airflow can be achieved by

using duct runouts longer than 9 ft and shorter than 35 ft. 

What load characteristics could each duct serve, and what other parameters (velocity, 

throw) are associated with such a system? 

o The duct system can deliver energy for a variety of load conditions, depending on 

the blower and connected coil. The mockup system can deliver 960 to 1,500 Btu/h 

per duct in heating mode and 740 to 1,170 Btu/h per duct in cooling mode, based 

on measured airflow rates. Qualitative visual analysis indicates the throw is

similar to other high-velocity duct systems. Air velocities measured immediately 

before the duct outlet were 125 and –2,000 feet per minute for the tested airflow 

rates. Duct velocities for the mockup test case were 1,000 feet per minute (±150 

feet per minute) and represent expected system operation. 

What are the ideal configuration and geometry of a small plenum for a home-run smalldiameter duct system that would be adaptable to MSHP split systems or small distributed 

central systems and that would supply equal airflow through each duct without 

balancing?

o Two geometries ultimately showed the most airflow balance: (1) the geometry of 

Box 1 and Box 5 with a vertical row of outlets and a perpendicular inlet, and 

(2) the geometry of Box 9 with a wide and a narrow inlet and corresponding wide 

and narrow rows of outlets. Ultimately, the geometry proved to be less significant 

than the location of the box inlet relative to the duct outlets. The worst-performing 

designs had a small air inlet blowing directly into several outlets. 

What is the energy impact of locating ducts inside conditioned space relative to the

higher fan energy needed to move the airflow under higher flow resistance? 

o Throughout the project, the team determined that because of the shorter duct runs 

and smooth duct material, the manifold system does not result in duct resistances 

high enough to exclude the system from further investigation. The measured fan 

watt draw of 0.16 to 0.22 watt/CFM is on par with today’s efficient systems. 

California’s Title 24 requires forced-air systems to do better than 0.58 watt/CFM. 

Even with a more powerful blower, the fan energy consumption will probably not 

exceed the 0.58 watt/CFM threshold.

o Many studies have looked at the impact of bringing ductwork into conditioned 

space. Vineyard et al. (2003) measured an energy savings of 31% in heating and 

36% in cooling for a traditional duct system. Further, the study found, for a high-
34

velocity system, a 46% savings in heating and 35% savings in cooling by bringing 

ductwork from the attic into conditioned space. Because IBACOS found no 

energy penalty associated with the manifold duct system, no further investigation 

is warranted.
35

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the Application of Air Terminals and Air Outlets. Arlington, VA: Air-Conditioning, Heating, and 

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Aldrich, R. 2009. “Efficient Houses with Minimal HVAC.” Presented at the Northeast 

Sustainable Energy Association’s Building Energy 2011 Conference, Boston, MA, March 

2011. www.nesea.org/buildingenergy/bepresentations/.

Aldrich, R. 2010. Point-Source Heating Systems in Cold-Climate Homes: Wisdom Way Solar 

Village. Norwalk, CT: Steven Winter Associates, Inc.

ASHRAE. 2011. Handbook, HVAC Applications, Noise and Vibration Control. Atlanta, GA: 

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37

Appendix A: Flow Sensor Calibration

The research team performed a comparative analysis to verify the consistency among the airvelocity sensors used in the laboratory test measurements. Table 7 presents the results of this 

analysis.

Table 7. Sensor Correction Factors

Sensor Correction (%)

10 2.2343

11 –3.2512

12 –1.2128

13 0.3681

14 –1.1798

15 0.0466

16 12.7841

To arrive at these values, all data from each sensor were considered. The team then used various 

statistical tests to determine extreme outliers that were causing a skew in the readings. The 

values that proved to be outliers were omitted from further mean bias calculations. A mean

percent deviation from the median value for each sensor was calculated and is shown in Table 7. 

To understand the relationship between the air velocity reading and volumetric flow rate, the 

team conducted the following test. A single air velocity sensor was inserted into a 2-in.-diameter

duct run and connected to a Duct Blaster. The Duct Blaster was set to a volumetric flow rate, and 

the corresponding air velocity was measured. The measured air velocity values were then 

multiplied by the area of the duct to determine the volumetric flow rate. The calculated flow rate 

was then multiplied by 0.85 to compensate for the shape of the velocity profile. These three flow 

rate values are compared in Table 8. Measured velocities were taken in the center of the duct, 

thus overestimating the volumetric flow rate before the correction factor was applied.

Table 8. Velocity Measurement and Calculation Comparison

Duct Blaster CFM 30 25 20 15 10

Velocity Sensor Measurement (FPM) 1,580 1,360 1,120 870 650

Calculated CFM 35 29 25 19 14

Calculated CFM with Velocity Profile 

Correction (0.85) 29 25 21 16 12

Percent Difference (1 and 4) –2% 0% 4% 7% 20%

In addition to insertion depth, the angle of the probe could have an impact on the measured 

velocity. The minimal impact of rotating the velocity probe by 15° clockwise and 15°

counterclockwise is shown in Table 9.
38

Table 9. Sensitivity of Flow Measurement to Sensor Angle

Insertion Angle 

(deg)

Measured Velocity 

(m/s)

% Difference 

from 0°

0 7.1 –

5 7.11 0.14

10 7.1 0.00

15 7.11 0.14

–5 7.06 –0.56

–10 7.02 –1.13

–15 6.93 –2.39
39

Appendix B: Test Volumetric Flow Rate and Velocity Data

Table 10 through Table 19 present the corrected velocity and calculated flow rate data for the 

10 mockup plenum boxes tested. The value listed in the “Control” setting represents the fan 

controller value. This value is arbitrary and is listed only for reference.

Table 10. Box 1 Raw Data

CFM 246 216 182 147 107 70 35

Control 35 30 25 20 15 10 5

Duct CFM FPM CFM FPM CFM FPM CFM FPM CFM FPM CFM FPM CFM FPM

1 36 1,630 31 1,430 26 1,197 21 980 16 712 10 459 5 230

2 35 1,612 30 1,388 26 1,176 21 941 15 690 10 454 5 230

3 36 1,651 31 1,415 26 1,190 21 958 15 691 10 455 5 228

4 35 1,603 30 1,371 25 1,157 21 956 15 684 10 446 5 227

5 37 1,674 31 1,434 26 1,210 21 966 15 704 10 458 5 229

6 36 1,662 31 1,420 26 1,207 21 976 15 709 10 463 5 225

7 32 1,454 31 1,441 26 1,191 21 965 16 716 10 461 5 219

Table 11. Box 2 Raw Data

CFM 175 152 126 98 73 48 26

Control 35 30 25 20 15 10 5

Duct CFM FPM CFM FPM CFM FPM CFM FPM CFM FPM CFM FPM CFM FPM

1 21 972 18 834 15 694 12 542 9 407 6 275 3 147

2 23 1,059 20 912 16 753 13 600 10 452 7 303 4 165

3 24 1,108 21 963 17 792 14 622 10 462 7 309 4 164

4 27 1,225 23 1,056 19 873 15 680 11 501 7 330 4 174

5 29 1,315 25 1,143 21 947 16 742 12 553 8 354 4 189

6 28 1,272 24 1,096 20 917 15 709 11 521 7 335 4 185

7 24 1,085 21 950 17 799 13 617 10 455 6 292 4 167

Table 12. Box 3 Raw Data

CFM 210 182 153 121 88 57 30

Control 35 30 25 20 15 10 5

Duct CFM FPM CFM FPM CFM FPM CFM FPM CFM FPM CFM FPM CFM FPM

1 28 1,280 24 1,117 21 958 16 746 12 541 8 355 4 195

2 28 1,271 24 1,104 20 936 16 736 12 549 8 356 4 172

3 28 1,286 25 1,130 21 944 16 732 12 543 7 337 4 189

4 30 1,377 26 1,188 22 1,004 17 792 13 576 8 375 4 193

5 32 1,466 28 1,283 23 1,057 18 836 13 608 9 391 5 209

6 33 1,500 28 1,304 24 1,092 19 861 14 629 9 400 4 205

7 32 1,447 27 1,233 22 1,030 18 841 13 601 9 392 4 200
40

Table 13. Box 4 Raw Data

CFM 227 200 168 134 100 65 32

Control 35 30 25 20 15 10 5

Duct CFM FPM CFM FPM CFM FPM CFM FPM CFM FPM CFM FPM CFM FPM

1 33 1,524 29 1,319 24 1,103 20 901 14 647 9 428 5 210

2 29 1,314 25 1,140 21 955 17 765 12 569 8 378 4 191

3 34 1,566 30 1,364 25 1,146 19 892 15 688 10 442 5 223

4 36 1,665 32 1,460 27 1,235 22 997 16 740 10 471 5 226

5 28 1,262 24 1,107 20 927 16 735 12 546 8 357 4 190

6 35 1,617 30 1,387 26 1,170 20 917 15 696 10 440 5 215

7 32 1,456 31 1,410 26 1,177 21 953 15 702 10 445 4 205

Table 14. Box 5 Raw Data

CFM 340 295 247 195 142 92 43

Control 35 30 25 20 15 10 5

Duct CFM FPM CFM FPM CFM FPM CFM FPM CFM FPM CFM FPM CFM FPM

1 45 1,334 39 1,155 33 974 26 761 19 557 12 365 6 171

2 46 1,352 40 1,166 33 980 26 769 19 562 13 370 6 174

3 51 1,482 43 1,273 37 1,072 29 840 21 607 13 393 6 182

4 49 1,432 42 1,242 35 1,039 28 818 20 598 13 385 6 179

5 50 1,468 43 1,262 36 1,047 28 824 20 593 13 384 6 177

6 50 1,457 43 1,267 36 1,061 28 829 21 604 14 400 6 186

7 50 1,454 44 1,286 37 1,085 30 885 22 639 14 405 6 190

Table 15. Box 6 Raw Data

CFM 196 169 140 112 83 56 33

Control 35 30 25 20 15 10 5

Duct CFM FPM CFM FPM CFM FPM CFM FPM CFM FPM CFM FPM CFM FPM

1 25 733 21 627 18 523 14 425 11 322 8 222 5 134

2 23 685 20 593 17 495 14 409 10 304 7 215 5 136

3 23 669 20 577 16 481 13 393 10 292 7 201 4 125

4 24 707 21 603 17 496 14 407 10 304 7 214 5 133

5 27 782 23 662 19 546 15 435 11 320 7 214 5 137

6 39 1,142 34 989 28 831 22 643 17 488 11 320 6 167

7 35 1,027 31 902 25 726 20 574 14 407 9 269 5 148
41

Table 16. Box 7 Raw Data.

CFM 276 238 197 154 116 75 38

Control 35 30 25 20 15 10 5

Duct CFM FPM CFM FPM CFM FPM CFM FPM CFM FPM CFM FPM CFM FPM

1 35 1,023 30 894 25 741 20 573 15 430 10 283 5 147

2 36 1,058 31 919 26 762 20 593 15 449 10 294 6 167

3 38 1,118 33 971 27 803 21 629 16 467 10 307 5 150

4 41 1,205 35 1,037 29 848 23 671 17 493 11 329 5 159

5 44 1,279 37 1,097 31 907 24 710 18 529 11 330 6 169

6 42 1,229 36 1,059 29 853 23 674 18 520 11 328 6 165

7 41 1,193 35 1,016 29 851 23 671 17 509 11 315 6 167

Table 17. Box 8 Raw Data

Table 18. Box 9 Raw Data

CFM 138 145 148

Control Low Medium High

Duct CFM FPM CFM FPM CFM FPM

1 20 904 21 954 21 982

2 20 908 21 949 21 972

3 19 890 21 942 21 959

4 20 908 21 966 21 981

5 19 888 20 935 21 962

6 20 911 21 964 21 981

7 19 893 20 934 21 947

CFM 266 233 194 153 111 74 38

Control 35 30 25 20 15 10 5

Duct CFM FPM CFM FPM CFM FPM CFM FPM CFM FPM CFM FPM CFM FPM

1 34 1,565 30 1,382 25 1,127 20 896 15 687 10 463 5 237

2 40 1,811 35 1,604 29 1,325 23 1,040 18 806 12 548 6 266

3 38 1,731 31 1,428 27 1,257 21 946 14 640 9 411 5 242

4 42 1,932 37 1,680 31 1,431 24 1,092 17 768 11 519 6 272

5 38 1,741 33 1,504 28 1,261 22 1,017 16 745 11 482 5 241

6 30 1,384 28 1,262 24 1,093 19 858 12 541 8 372 5 214

7 44 2,018 39 1,807 31 1,419 25 1,149 20 897 13 590 6 270
42

Table 19. Box 10 Raw Data

CFM 139 154 161

Control Low Medium High

Duct CFM FPM CFM FPM CFM FPM

1 20 927 22 1,019 23 1,065

2 20 922 22 1,025 24 1,078

3 20 904 22 1,000 23 1,042

4 19 870 21 959 22 991

5 20 919 22 1,020 23 1,070

6 19 854 21 962 22 996

7 21 985 24 1,082 25 1,133
DOE/GO-102016-4751 March 2016

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