Distribution and Room 

Air Mixing Risks to 

Retrofitted Homes

A. Burdick 

IBACOS, Inc.

December 2014
NOTICE

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Distribution and Room Air Mixing Risks to Retrofitted Homes

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: 

A. Burdick 

IBACOS, Inc.

2214 Liberty Avenue

Pittsburgh, PA 15222

NREL Technical Monitor: Stacey Rothgeb

Prepared under Subcontract No. KNDJ-0-40341-04

December 2014
iv

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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

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Contents

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

List of Tables .............................................................................................................................................. vi

Definitions.................................................................................................................................................. vii

Executive Summary................................................................................................................................. viii

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

1.1 Research Questions.......................................................................................................................... 3

2 Mathematical and Modeling Methods................................................................................................. 4

2.1 Load Calculation.............................................................................................................................. 4

2.2 Duct Sizing and Layout ................................................................................................................... 5

3 Experimental Methods ......................................................................................................................... 6

3.1 Measurement of Face Velocity, Throw, and Spread........................................................................ 6

3.2 Measurements to Identify Potential Issues....................................................................................... 9

4 Results and Discussion..................................................................................................................... 10

4.1 Reduced Load Airflow Volumes................................................................................................... 10

4.2 Reduced Airflows Within Ducts.................................................................................................... 10

4.3 Terminal Conditions...................................................................................................................... 11

4.4 Leakage Versus Pressure ............................................................................................................... 14

5 Conclusions ........................................................................................................................................ 17

References................................................................................................................................................. 19
vi

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List of Figures

Figure 1. Two-story floor plan.................................................................................................................... 4

Figure 2. Mockup of the testing duct runs ............................................................................................... 6

Figure 3. 4G boot configuration ................................................................................................................ 7

Figure 4. 4H boot configuration................................................................................................................. 7

Figure 5. Measurement grid ....................................................................................................................... 7

Figure 6. 4G boot configurations with the 4 × 10 floor register: (A) 100% open; (B) farthest 

from the duct, 50% open; (C) nearest to the duct, 50% open; (D) middle, 50% open ................... 8

Figure 7. 4H boot configurations with the 4 × 10 floor register: (A) 100% open; (B) farthest 

from the duct, 50% open; (C) nearest to the duct, 50% open; (D) middle, 50% open ................... 8

Figure 8. Air profiles of an unrestricted 4 × 10 floor register in two boot configurations 

(4G and 4H).......................................................................................................................................... 12

Figure 9. Reduced flow profiles through an open and restricted 4 × 10 register with different 

restriction strategies for the 4G boot configuration: (A) 100% open; (B) farthest from 

the duct, 50% open; (C) nearest to the duct, 50% open; (D) middle, 50% open .......................... 13

Figure 10. Reduced flow profiles through an open and restricted 4 × 10 register with different 

restriction strategies for the 4H boot configuration: (A) 100% open; (B) farthest from 

the duct, 50% open; (C) nearest to the duct, 50% open; (D) middle, 50% open .......................... 14

Figure 11. Duct leakage versus pressure............................................................................................... 15

Figure 12. Duct static pressure versus duct airflow rate for different register restrictions and 

duct leakage........................................................................................................................................ 15

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

List of Tables

Table 1. Number of Homes With Furnaces in Cold or Very Cold Climates, by Fuel Type* ................ 3

Table 2. Pre- and Post-Retrofit Specifications......................................................................................... 4

Table 3. Loads and Airflows for Pre- and Post-Retrofit ........................................................................ 10

Table 4. Airflows and Velocities for Pre- and Post-Retrofit.................................................................. 11

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

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Definitions

ACCA Air Conditioning Contractors of America

ACH Air changes per hour

cfm Cubic feet per minute

ft/min Feet per minute

HVAC Heating, ventilation, and air conditioning

NYSERDA New York State Energy Research and Development Authority

Pa Pascals

RECS Residential Energy Consumption Survey

SHGC Solar heat gain coefficient
viii

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Executive Summary

An energy efficiency upgrade reduces a home’s heating and cooling load. If the load reduction is 

great enough and the heating, ventilation, and air conditioning system warrants replacement, that

system is often upgraded with a more efficient, lower capacity system that meets the load of the 

upgraded house. For a single-story house with floor supply air diffusers, the ducts often are 

removed and upgraded. For houses with ducts that are embedded in walls, the cost of demolition 

precludes the replacement of ducts. The challenge with the use of existing ducts is that the 

reduced airflow creates a decreased throw at the supply registers, and the supply air and room air 

do not mix well, leading to potential thermal comfort complaints. This project investigates this 

retrofit scenario. The issues and solutions discussed here are relevant to all climate zones, with 

emphasis on climates that require cooling. 

In this project, IBACOS performed load calculations for a two-story 1960s house and characterized duct sizes and layouts based on industry “rules of thumb” (Herk et al. 2014). The team 

performed duct-sizing calculations for unaltered ducts and post-retrofit airflows and examined 

airflow velocities and pressure changes with respect to various factors. The team then used a 

mocked-up duct and register setup to measure the characteristics of isothermal air—to reduce the 

effects of buoyancy from the observations—passing through the duct and leaving the register. 

The reduced airflows through the original large registers significantly reduced the throw and 

spread of the delivered air. By reducing the net free area of the register by 50%, the team 

observed increases in throws and spreads. With airflow of 50 cfm, reducing the net free area by 

50% increased the face velocity to an acceptable range. The throws and spreads of the restricted 

register configuration thus improved.

The impact to duct leakage by restricting the register to increase throw and spread also was 

investigated with the mocked-up duct system to simulate normal leakage and a duct with a 2-in. 

hole. The increased leakage due to the 2-in. hole is approximately 20 cfm at 25 Pa. The duct 

system was operated over a range of flow rates, and the duct static pressure was recorded for the 

duct without a hole and for the duct with a 2-in. hole. The ducts with a 2-in. hole had less 

pressure than those without. Also, at 100 cfm, the 50% restriction resulted in an increase of 13 Pa 

for the ducts with no hole; for the ducts with a 2-in. hole, the restriction increased the static 

pressure 7 Pa.
1 

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1 Introduction and Background

Occupant comfort is the overall objective of the forced-air heating, ventilation, and air 

conditioning (HVAC) system, and air distribution is the means of delivering conditioned air to 

provide occupant comfort in a room. Several current standards offer guidance and criteria on air 

distribution and occupant comfort in buildings and describe how those standards relate to homes. 

ASHRAE Standard 55 provides criteria for overall comfort and localized discomfort, including 

stratification1 (ASHRAE 2010). Air Conditioning Contractors of America (ACCA) Manual T 

offers guidance on air distribution basics (Rutkowski 2009b). ACCA Manual RS uses a wholehouse approach of designing for occupant comfort (Rutkowski 1997), and ACCA Manual D 

provides specific guidelines for residential duct design (Rutkowski 2009a). The main comfort 

criteria related to the air distribution system are temperature mixing and uniformity, avoidance of 

drafts that can be created by the supply outlets, and system noise (Rutkowski 1997, 2009b). 

Energy upgrades that reduce heating and cooling loads may reduce airflows in a home when the 

HVAC system is upgraded with a more energy-efficient, lower capacity system. IBACOS found 

that the basic retrofit efforts performed on retrofit unoccupied test houses in Fresno, California 

(Stecher and Poerschke 2014), and Round Hill, Virginia (Kampert et al. 2012), reduced cooling 

loads by approximately 50%. This also was the case for heating and cooling loads with deep 

energy retrofits on houses IBACOS studied in conjunction with the New York State Energy 

Research and Development Authority (NYSERDA) (Herk et al. 2014). 

Given that a home energy upgrade to the shell of a house is completed and that the load is 

reduced 50%, the new HVAC system airflow requirement will be reduced by 50%. This 

reduction in system airflow could cause comfort problems in the home from inadequate terminal 

conditions for airflow, throw,2

 and supply with poor room air mixing. 

The primary reason that ducts and registers are not replaced is cost—duct replacement is 

expensive. Likewise, utility rebate modeling does not show significant energy savings from duct 

replacement. Return on investment is the driving factor of many home energy upgrade programs, 

and the expense of total duct replacement is difficult to justify (GreenHomes America 2013). If 

the existing ductwork and registers are retained in the retrofitted house, the volumetric flow rate 

correspondingly would be reduced by 50%, with corresponding velocity reductions. 

The purpose of this investigation is to determine under what conditions a supply air delivery 

problem could potentially occur and to evaluate the feasibility of modifying the duct systems 

using minimally invasive strategies to improve air distribution effectiveness. Research was 

needed to evaluate two concerns: (1) at what level of space conditioning airflow reduction does 

the supply outlet throw become a potential thermal comfort issue, and (2) in what conditions is 

an engineering solution needed. Although an adequate return strategy also is important to system 

performance, this study assumes the existing return and the upgraded system with reduced 

airflow are adequately sized. 

 

1 Stratification is the layering of differing air temperatures in a room, where the air is cooler toward the floor and 

warmer near the ceiling.

2 Throw is the distance that an air stream travels from an outlet at a specific terminal velocity.
2

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ASHRAE Fundamentals, Section 33.16 (ASHRAE 2009), states, “Higher supply velocities 

produce better room air diffusion than lower velocities, but velocity is not critical in selecting 

these units for heating.” The second half of that statement—that velocity is not critical—is 

sometimes used in defense of ignoring supply outlet selection. Section 33.16 of ASHRAE 

Fundamentals is based on Straub (1956), along with the work of Straub and Chen (1957), using 

relatively large volumes of air and only for heating mode. Even in cold climates, houses have a 

mixed load where the cooling load airflows dominate the duct design sizing (Herk et al. 2014). 

According to ACCA Manual D, residential duct systems have a direct and significant impact on 

occupant comfort and homeowner satisfaction (Rutkowski 2009a). To maintain a uniform 

temperature and to avoid stratification, the supply air also must mix adequately with the air in the 

room. Mixing of air within the room is a function of both the supply outlet and the shape of the 

airstream entering the room. As velocities increase in duct systems, so does the noise level. 

Ducts are sized to maintain the maximum velocity of air without adding noise to the room. 

ACCA Manual D further cautions that while most practitioners appreciate supply air paths and 

flows, some underestimate the importance of supply air outlets (Rutkowski 2009a).

Furthermore, according to ACCA Manual T, the selection and sizing of supply air outlets are as 

important to a successful design as making load calculations, selecting equipment, or sizing 

ducts. Supply air outlets must mix the raw supply air with the room air without creating drafts or 

stagnant zones (Rutkowski 2009b). For example, if a 4 × 8 supply outlet in the system was 

properly selected for sufficient throw and noise criteria at 500–700 ft/min face velocity when 

delivering 110 cfm, that same supply outlet delivering half the airflow (i.e., 55 cfm) falls below 

the manufacturer’s recommended velocity range necessary for sufficient throw and spread3 to 

deliver room air mixing (Hart & Cooley, Inc. 2010). 

The information provided in this report can be applied to all retrofitted houses that have reduced 

airflow with unaltered supply air outlets, although it will be most useful in homes with floor 

perimeter outlets served through sheet metal ductwork that is already located within conditioned 

space where cooling is required. Systems using flex duct in unconditioned space are more likely 

to have ductwork and supply air outlets replaced during a retrofit because the ease of access and 

lower material costs reduce the expense of duct replacement (GreenHomes America 2013). 

Therefore, houses with all-metal ducts in conditioned space with floor supply air outlets are a 

prime target for this study. Previous work that IBACOS completed from the 2000 U.S. Census 

characterized house types through a set of specifications based on the code requirements during 

time periods identified in cold and very cold climate zones: 31.4 million homes built prior to 

1970, 14.4 million homes built from 1970 through 1989, and 7.1 million homes built from 1990 

through 1999. According to the Residential Energy Consumption Survey (RECS) (EIA 2011), 

the respective number of homes with forced-air furnaces in cold and very cold climates is 

25.3 million, as shown in Table 1.

 3 Spread is the width of the air stream leaving the supply outlet.
3

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Table 1. Number of Homes With Furnaces in Cold or 

Very Cold Climates, by Fuel Type*

Fuel Type Number of Homes 

(millions)

Natural Gas 20.1

Electricity 1.8

Fuel Oil 1.7

Propane 1.7

Total 25.3

* EIA (2011).

1.1 Research Questions

The following research questions are answered by this project:

1. What are the heating and cooling loads and the corresponding total airflow volume and 

room-by-room airflow volume reductions in both the pre-retrofit and post-retrofit 

conditions? What are the conditions of post-retrofit airflows within the pre-retrofit ducts 

(i.e., velocity, pressure loss, friction) based on ACCA and ASHRAE calculations?

2. In an isothermal condition, what are the face velocities, corresponding throws, and 

spreads of the post-retrofit airflows through the pre-retrofit size ducts and supply outlets 

that correspond to the reduced flows in Question 1? Do the post-retrofit airflows through 

the pre-retrofit size ducts and supply outlets improve terminal conditions at the supply 

outlets? 

3. What are the potential issues with methods for a strategy to increase throw by decreasing 

the net free area of the diffuser?
4

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2 Mathematical and Modeling Methods

IBACOS used modeling with ACCA Manual J (Rutkowski 2006) to answer the research 

questions related to the reduced heating and cooling loads and the corresponding reduced total 

airflow volume and room-by-room airflow volume. The answers to the research question related 

to the conditions of post-retrofit airflows within the pre-retrofit ducts (i.e., velocity, pressure loss, 

friction) were based on calculations from ASHRAE Fundamentals (ASHRAE 2009). 

2.1 Load Calculation

To determine the total loads and room-by-room airflows, IBACOS used ACCA-approved 

Manual J (Rutkowski 2006) load calculation software. The load calculations were performed for 

a two-story 1960s vintage house. Figure 1 shows the floor plan for this two-story house. 

Figure 1. Two-story floor plan

Table 2 lists the energy efficiency upgrade measures for the post-retrofit condition. These are 

relevant to the thermal enclosure and are based on previous IBACOS work completed in a 

NYSERDA deep energy retrofit study (Herk et al. 2014). 

Table 2. Pre- and Post-Retrofit Specifications

Component Pre-Retrofit Post-Retrofit

Basement Walls Uninsulated R-4 rigid

Windows

Single pane, 

U-value = 0.869, 

SHGC = 0.619

Double pane, 

U-value = 0.285, 

SHGC = 0.35

Ducts Leaky, uninsulated, 

in conditioned space

Leaky, uninsulated, 

in conditioned space

Crawl Space Walls Uninsulated R-4 rigid

Attic R-19 R-38

Infiltration 19 ACH50 15 ACH50

Exterior Walls Uninsulated R-13

ACH is air changes per hour. SHGC is solar heat gain coefficient.
5

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2.2 Duct Sizing and Layout

To characterize the duct sizes and layout in this project, IBACOS used industry “rules of thumb” 

and the house characteristics recorded in the NYSERDA deep energy retrofit study by Herk et al.

(2014). IBACOS performed ASHRAE Fundamentals (ASHRAE 2009) duct-sizing calculations 

for the unaltered ducts and the post-retrofit airflows. The team input the post-retrofit airflows 

into a duct design spreadsheet that used ASHRAE duct design calculations to look at airflow 

velocities and pressure changes with respect to friction factor, duct size, duct length, and fittings. 

IBACOS quantified the post-retrofit room-by-room airflows for the pre-retrofit ducts by 

calculating the new equipment flows through the existing ducts sizes against the ASHRAE 2009 

duct-sizing calculations. This analysis was used to evaluate whether airflow delivered to each 

room meets the room-by-room load. 
6

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3 Experimental Methods

IBACOS used a mocked-up duct and register setup to measure the characteristics of isothermal 

air—to reduce the effects of the buoyancy on heated or cooled air from the observations—

passing through the duct and leaving the register.4

3.1 Measurement of Face Velocity, Throw, and Spread

Two 10-ft lengths of 6-in. round sheet metal duct with two configurations of boot were used to 

mock up the testing duct runs, as shown in Figure 2. Based on the industry “rules of thumb” used 

to characterize the duct layout for the modeling effort, the duct runs were not sealed with mastic 

along the seams. 

Figure 2. Mockup of the testing duct runs

The supply air outlets were 4 × 10 stamped steel, rectangular floor registers, indicative of those 

found in the NYSERDA study house (Herk et al. 2014). Two boot configurations were used: the 

ACCA Manual D fitting 4G boot with the long side of the register perpendicular to the duct run 

(Figure 3), and the ACCA Manual D fitting 4H boot with the long side of the register parallel 

with the duct run (Figure 4) (Rutkowski 2009a). The boots were installed in a square of plywood 

elevated in a simulated floor surface with floor joists.

 4 In an isothermal process, the temperature is constant.
7

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Figure 3. 4G boot configuration Figure 4. 4H boot configuration

IBACOS used a black backdrop with white lines drawn at 6-in. intervals to create a grid for 

measurements. The grid guided the measurements by placing the datum at the midpoint of the 

register, as shown in Figure 5. 

Figure 5. Measurement grid

Hot wire anemometer
8

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The team used an Energy Conservatory Duct Blaster5 to induce the isothermal airflow through 

the duct runs. With a manometer, it was possible to measure airflow through the Duct Blaster. 

Also, the team used an Energy Conservatory FlowBlaster6 capture flow hood accessory to 

measure airflow leaving the register. Both devices employ an Energy Conservatory DG-700 

digital pressure and flow gauge.7 The accuracy of the Duct Blaster and FlowBlaster are both 3% 

of the flow reading or 1 cfm, and 1% of the pressure reading or 0.15 Pa, whichever is greater. 

Using 100 cfm as the original design airflow, the flows then were reduced to 50 cfm, 25 cfm, and 

12 cfm to represent 50%, 75%, and 88% reductions in airflow, respectively. 

For each flow rate, IBACOS took airflow velocity measurements at various locations for the 

register at 100% open area. Then 50% of the register was restricted in three configurations: 50% 

blocked on the half nearest to the duct, 50% blocked on the half farthest from the duct, and only 

the middle 50% open. Figure 6 and Figure 7 show the 4G and 4H boot configurations, 

respectively, with the 4 × 10 register net free area reduced by 50%. 

Figure 6. 4G boot configurations with the 4 × 10 floor register: (A) 100% open; (B) farthest from the 

duct, 50% open; (C) nearest to the duct, 50% open; (D) middle, 50% open

Figure 7. 4H boot configurations with the 4 × 10 floor register: (A) 100% open; (B) farthest from the 

duct, 50% open; (C) nearest to the duct, 50% open; (D) middle, 50% open

The team initially placed a fog machine directly behind the Duct Blaster fan to visualize the 

changes in throw and spread when the airflow volume and blockage of the register were varied. 

Following the pattern identified by the fog, the areas were located on the grid where the air 

 5 Duct Blaster. Minneapolis, MN: The Energy Conservatory. www.energyconservatory.com/products/ductblaster%C2%AE-systems-and-accessories. 6 FlowBlaster. Minneapolis, MN: The Energy Conservatory. www.energyconservatory.com/products/ductblaster%C2%AE-systems-and-accessories/flowblaster%C2%AE-capture-hood-accessory. 7 DG-700 digital pressure and flow gauge. Minneapolis, MN: The Energy Conservatory. 

www.energyconservatory.com/products/digital-pressure-gauges.
9

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reached a terminal velocity of 50 ft/min using a TSI 9555 VelociCalc hot wire anemometer.

8 The 

device has an accuracy of 3% of the reading or 3 ft/min, whichever is greater, and the accuracy 

statement ranges from 30 ft/min to 9,999 ft/min. Because the research team was seeking 

50 ft/min, the team was well within the accuracy range. The velocity readings were taken at the 

center of the register approximately 10 in. from the grid, attempting to be in the plane with the 

highest velocity, or directly above the register. The test was considered complete when a 

perimeter defining where the design terminal velocity condition reached 50 ft/min was obtained 

for each airflow volume and blockage configuration.

3.2 Measurements to Identify Potential Issues

Decreasing the net free area of the register will increase the duct static pressure. Because the 

flow is reduced in the retrofit scenario due to the reduced load on the house and the new furnace, 

the increased static pressure may not increase duct leakage. 

First, the research team measured the duct leakage (with multipoint tests) for a 10-ft-long, 6-in.-

diameter metal duct with an unsealed longitudinal seam. Then a 2-in.-diameter hole was drilled 

2.5 ft away from the diffuser, and the leakage was measured again. 

The team completed all leakage tests using the Energy Conservatory Duct Blaster, FlowBlaster, 

and DG-700 manometer. Leakage rates were calculated using ASHRAE Fundamentals (2005), 

Section 35.14, Equation 37 for duct leakage:

* ,

n Q = C ∆Ps

where Q is the leakage flow, ∆Ps is the difference in static pressure between the interior and 

exterior of the duct, and C and n are calculated coefficients.

Second, the research team recorded the pressure in the duct for flow rates up to 100 cfm for both 

scenarios with and without the 2-in.-diameter hole in the duct. The team used a Duct Blaster to 

produce the airflow and a manometer to record the static pressure at the midpoint of the duct. 

 8 TSI VelociCalc hot wire anemometer. TSI Incorporated. Shoreview, MN. www.tsi.com/velocicalc-multi-functionventilation-meter-9555/.
10

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4 Results and Discussion

4.1 Reduced Load Airflow Volumes

The results of the load calculations for the pre- and post-retrofit conditions show approximately a 

65% reduction in total airflow volume. Table 3 shows the relationship between the pre- and postretrofit loads versus airflows as calculated per ACCA Manual J (Rutkowski 2006) for each space 

served by the forced air system. The thermal enclosure measures applied to the model reduced 

the overall heating and cooling loads by more than half. The heating load was reduced by 

54,709 Btu/h, and the cooling load was reduced by 18,719 Btu/h (69% and 61% reductions, 

respectively). Subsequently, the overall system airflow was reduced by as much as 65%. 

Table 3. Loads and Airflows for Pre- and Post-Retrofit

Room

Pre-Retrofit

(Btu/h)

Post-Retrofit

(Btu/h)

Pre-Retrofit

(cfm)

Post-Retrofit

(cfm)

Heating Cooling Heating Cooling Heating Cooling Heating Cooling

Den 11,203 7,119 4,166 2,664 187 308 99 138

Living Room 6,953 3,308 2,522 1,471 116 143 60 76

Kitchen 4,029 2,729 1,465 1,678 67 118 35 87

Dining 5,563 2,122 2,020 684 93 92 48 35

Bedroom 2 8,324 3,596 2,653 1,127 139 156 63 58

Master Walkin Closet 3,492 1,104 1,241 280 58 48 29 15

Bath 1,306 628 419 185 22 28 10 10

Master 

Bedroom 6,536 3,177 2,133 1,205 109 138 51 62

Bedroom 1 8,713 2,814 3,693 982 145 122 88 51

Basement 23,586 4,079 4,685 1,187 393 177 111 61

Total 79,705 30,588 24,996 11,870 1,580* 712*

*Totals are based on the higher of the two flows for heating and cooling.

4.2 Reduced Airflows Within Ducts

The pre-retrofit duct sizes assumed were 6 × 10-in. rectangular sheet metal for the supply trunk, 

and all supply branches were assumed to be 6-in. round sheet metal. When the post-retrofit 

airflows were applied to the 6-in. metal ducts used in the duct layout per industry “rules of 

thumb,” the following characteristics of velocity and pressure were calculated.

Table 4 details the change in airflow and corresponding change in velocity for each duct run in 

the house. The overall reduction in airflow was 63%, and reductions in diffuser face velocities 

ranged from 41% to 78%. 
11

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Table 4. Airflows and Velocities for Pre- and Post-Retrofit

Room

Pre-Retrofit Post-Retrofit Difference Difference

Flow 

Rate 

(cfm)

Velocity 

(ft/min)

Flow 

Rate 

(cfm)

Velocity 

(ft/min)

Flow 

Rate 

(cfm)

Velocity 

(ft/min) %

Den A 103 525 37 188 66 337 64%

Den B 103 525 37 188 66 337 64%

Den C 103 525 37 188 66 337 64%

Basement 1 197 1,003 43 219 154 784 78%

Living Room A 72 367 33 168 39 199 54%

Living Room B 72 367 33 168 39 199 54%

Dining 93 474 43 219 50 255 54%

Basement 2 197 1,003 43 219 154 784 78%

Kitchen A 59 300 35 178 24 122 41%

Kitchen B 59 300 35 178 24 122 41%

Bedroom 1 145 738 58 295 87 443 60%

Master Walk-in 

Closet 58 295 24 122 34 173 59%

Bath 27 138 9 46 18 92 67%

Bedroom 2 156 795 58 295 98 500 63%

Master Bedroom 138 703 54 275 84 428 61%

Total 1,582 – 579 – 1003 – 63%

4.3 Terminal Conditions

Figure 8 shows the baseline air profiles at 50 ft/min terminal velocity of the four measured 

airflow volumes through an unrestricted 4 × 10 floor register for the 4G and 4H boot 

configurations. As the airflows are reduced through an unrestricted register, the throws and 

spreads also are reduced. For 50 cfm, the throw for both boot configurations drops to about 5 ft. 

For 25 cfm, the air was thrown roughly 3 ft, and at 12 cfm, there was virtually no throw. The 

lack of throw, spread, and air velocity at the register poses a risk to air mixing and occupant 

comfort. Additionally, the throw of the 4H boot configuration went only to one side because of

the momentum of the air. 
12

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Figure 8. Air profiles of an unrestricted 4 × 10 floor register in

two boot configurations (4G and 4H)

Figure 9 displays the air profiles at 50 ft/min terminal velocity of the reduced airflow volumes 

through a 50% restricted 4 × 10 floor register in the 4G boot configuration. When the net free 

area of the register was reduced by 50%, the throw increased. In the 4G boot configuration with 

the half nearest to the duct restricted (B), the air was thrown toward the wall. With the 50% 

farthest from the duct restricted (C), the air was thrown away from the wall. 

Figure 10 displays the air profiles at 50 ft/min terminal velocity of the reduced air volumes 

through a 50% restricted 4 × 10 floor register in the 4H boot configuration. In all measured 

cases, the throw increased; however, drastic differences occurred in the direction of the throw,

depending on which side was restricted. In the configuration where the half nearest to the duct 

was restricted (B), air was thrown farther in the same direction as in the unrestricted scenario. In 

the configuration where the half farthest from the duct (C) was restricted, the air was forced in 

the opposite direction.
13

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 9. Reduced flow profiles through an open and restricted 4 × 10 register with different 

restriction strategies for the 4G boot configuration: (A) 100% open; (B) farthest from the duct, 50% 

open; (C) nearest to the duct, 50% open; (D) middle, 50% open
14

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 10. Reduced flow profiles through an open and restricted 4 × 10 register with different 

restriction strategies for the 4H boot configuration: (A) 100% open; (B) farthest from the duct, 50% 

open; (C) nearest to the duct, 50% open; (D) middle, 50% open

4.4 Leakage Versus Pressure

To investigate the impact of duct leakage for the pre- and post-retrofit scenarios where the 

register was restricted but the airflow was reduced, IBACOS completed two duct leakage tests as 

previously described. Figure 11 presents the test results.

Reading from Figure 11, the 6-in. round duct leaked 10 cfm at 25 Pa, whereas the same duct with 

a 2-in. hole leaked 30 cfm. The increased leakage due to the 2-in. hole is approximately 20 cfm 

at 25 Pa.

The duct system was operated over a range of flow rates, and the duct static pressure was 

recorded for the duct without a hole and for the duct with a 2-in. hole. Figure 12 presents the 

results of these measurements. 
15

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 11. Duct leakage versus pressure

Figure 12. Duct static pressure versus duct airflow rate for

different register restrictions and duct leakage

y = 1.24x0.6366

R2 = 0.996

y = 3.8763x0.6357

R2 = 0.998

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20

0

5

10

15

20

25

30

35

40

45

50

0 10 20 30 40 50

Pressure (inwg)

Duct Leakage (CFM)

Pressure (Pa)

No Holes 2" diameter hole

0.00

0.02

0.04

0.06

0.08

0.10

0

5

10

15

20

25

0 20 40 60 80 100

Duct Static Pressure (inwg)

Duct Static Pressure (Pa)

Duct Airflow Measured at Duct Blaster (CFM)

No Hole / 50%

Restriction

2" Hole / 50%

Restriction

No Hole /

Unrestricted

2" Hole /

Unrestricted
16

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 12 indicates that the ducts with a 2-in. hole had less pressure than the ducts without a 2-in. 

hole. The figure also indicates that at 100 cfm, the 50% restriction resulted in an increase of 

13 Pa for the duct with no hole; for the duct with a 2-in. hole, the restriction increased the static 

pressure 7 Pa. An inference from the figure is an approximate 6 Pa reduction due to the 2-in. hole 

at 100 cfm. 

The potential leakage resulting from restricting the supply register can be found using both 

Figure 11 and Figure 12. For example, reading from Figure 12 at 100 cfm, the duct with a 2-in. 

hole and no restriction has a static pressure of approximately 9 Pa. Reading from Figure 11, the 

duct leakage at 9 Pa is 16 cfm. When the airflow in the duct system is reduced by 50% and the 

register opening is restricted 50%, the airflow becomes 50 cfm, and the duct static pressure is 

4.5 Pa. Figure 11 indicates that there is less than 5 cfm duct leakage at this pressure. The purple 

arrows added to the chart in Figure 12 illustrate the end result of this analysis.

If the airflow is reduced more than 35%, the static pressure will be reduced enough to reduce the 

duct leakage, and a larger reduction in airflow will be needed for a tighter duct system to prevent 

the same leakage from occurring.

The assumptions used in this analysis are specific to the duct system (duct material, hole 

location, register restriction, and static pressure measurement location), although the results 

indicate that the system leakage should be evaluated prior to restricting the registers, and the 

system airflow should be reduced. 
17

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

5 Conclusions

When a home is significantly improved with a variety of thermal enclosure measures, the peak 

heating and cooling loads are reduced. If the HVAC system is upgraded to better meet the loads 

and the duct system is not upgraded, the throw from the diffusers may not be adequate for room 

mixing. 

This project addressed three research questions. The first question follows:

1. What are the heating and cooling loads and the corresponding total airflow volume and 

room-by-room airflow volume reductions in both the pre-retrofit and post-retrofit 

conditions? What are the conditions of post-retrofit airflows within the pre-retrofit ducts 

(i.e., velocity, pressure loss, friction) based on ACCA and ASHRAE calculations?

The team performed load calculations based on ACCA and ASHRAE formulas for the pre- and 

post-retrofit conditions, and those load calculations showed that the heating load decreased by 

69%, and the cooling load decreased by 61%. The overall system airflow was reduced by as 

much as 65%. 

The second research question follows:

2. In an isothermal condition, what are the face velocities, corresponding throws, and 

spreads of the post-retrofit airflows through the pre-retrofit size ducts and supply outlets 

that correspond to the reduced flows in Question 1? Do the post-retrofit airflows through 

the pre-retrofit size ducts and supply outlets improve terminal conditions at the supply 

outlets? 

Reduced flows through the original (now oversized) registers result in unacceptable performance 

of throw and spread of delivered air. By reducing the net free area of the register by 50%, the 

team observed increases in both throws and spreads. With airflow of 50 cfm, reducing the net 

free area by 50% increased the face velocity to an acceptable range. The throws and spreads of 

the restricted register configuration improved terminal conditions at the supply outlets. 

Finally, the third research question follows:

3. What are potential issues with methods for a strategy to increase throw by decreasing the 

net free area of the diffuser? 

By adding duct restriction to the end of the supply runs, the overall static pressure of the duct is 

increased. The result is that all holes in the existing duct will be exposed to a higher static 

pressure and will leak more. Increasing the register throw by decreasing the net free area results 

in an increase in duct static pressure and could contribute to increased duct leakage. If the 

airflow is reduced by more than 35%, the static pressure will be reduced enough to reduce the 

duct leakage, and a larger reduction in airflow is needed for a tighter duct system to prevent the 

same leakage from occurring.
18

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

The assumptions used in this analysis are specific to the duct system (duct material, hole 

location, register restriction, and static pressure measurement location), although the results do 

indicate that the system leakage should be evaluated prior to restricting the registers, and the 

system airflow should be reduced. Installers should use their best judgment when attempting to 

increase the throw of conditioned air into a space. If the static pressure (and consequently the 

duct leakage) becomes too high, the installer should upgrade the duct system to reduce leakage. 
19

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

References

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DOE/GO-102014-4544 ▪ December 2014

buildingamerica.gov