Simplified Space Conditioning 

in Low-Load Homes: Results 

from the Fresno, California, 

Retrofit Unoccupied Test House

Dave Stecher and Andrew Poerschke

IBACOS, Inc.

February 2014
NOTICE

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iii

Simplified Space Conditioning in Low-Load Homes: Results from the 

Fresno, California, Retrofit Unoccupied Test House

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: 

Dave Stecher and Andrew Poerschke

IBACOS, Inc.

2214 Liberty Avenue

Pittsburgh, PA 15222

NREL Technical Monitor: Michael Gestwick

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

February 2014
iv

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v

Contents

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

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

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

Acknowledgments ................................................................................................................................... viii

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

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

1.1 Typical Airflow Volume Ducted Distribution to the Bedrooms .........................................3

1.2 Low Airflow Volume Ducted Distribution to the Bedrooms ..............................................3

1.3 No Ducted Distribution to the Bedrooms ............................................................................4

2 Field Test Methods ............................................................................................................................... 7

2.1 Comfort Criteria...................................................................................................................7

2.2 Instrumentation Setup ..........................................................................................................8

3 Field Test Results............................................................................................................................... 10

3.1 ASHRAE Standard 55-2010 Analysis...............................................................................10

3.2 ACCA Manual RS Analysis ..............................................................................................16

4 Discussion........................................................................................................................................... 21

5 Calibration of a Computational Fluid Dynamics Model for Transfer Grille Airflow..................... 23

5.1 Computational Fluid Dynamics Model Description..........................................................23

5.2 Methods..............................................................................................................................25

5.3 Results................................................................................................................................25

5.4 Discussion..........................................................................................................................27

6 Conclusions ........................................................................................................................................ 29

References................................................................................................................................................. 31
vi

List of Figures

Figure 1. Front view of the Fresno test house ......................................................................................... 2

Figure 2. Typical airflow volume ducted distribution to the bedrooms (numbers correspond 

to zone damper numbers in Table 1).................................................................................................. 3

Figure 3. Low airflow volume ducted distribution to the bedrooms (numbers correspond to 

zone damper numbers in Table 1) ...................................................................................................... 4

Figure 4. Implementation of the Emery (1969) study (left) in the Fresno test house (right) ............... 5

Figure 5. No ducted distribution to the bedrooms (numbers correspond to zone damper 

numbers in Table 1).............................................................................................................................. 6

Figure 6. Sensor locations ......................................................................................................................... 9

Figure 7. Plan view of the CFD model, showing the upper and lower transfer grilles and fan 

locations .............................................................................................................................................. 23

Figure 8. Close-up of the CFD model mesh, showing the upper transfer grille (pink) 

connecting the bedroom (light blue) and hallway (tan).................................................................. 24

Figure 9. Plot comparing temperatures of the three runs .................................................................... 26

Figure 10. Vertical section view of the bedroom through the fan centerline, showing faninfluenced air movement (scale is air temperature in Kelvin) ....................................................... 28

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

List of Tables

Table 1. Distribution System Control Dampers ....................................................................................... 3

Table 2. Failure Mode Cooling Typical Airflow—2 Days ....................................................................... 10

Table 3. Failure Mode Cooling Low Airflow —8 Days ........................................................................... 11

Table 4. Failure Mode Cooling No Ducted Distribution—25 Days. ...................................................... 11

Table 5. Failure Mode Heating Typical Airflow—19 Days ..................................................................... 12

Table 6. Failure Mode Heating Low Airflow—28 Days .......................................................................... 12

Table 7. Failure Mode Heating No Ducted Distribution—20 Days........................................................ 12

Table 8. Failure Mode Cooling Typical Airflow—Non-ASHRAE Conditions—19 Days...................... 13

Table 9. Failure Mode Cooling Low Airflow—Non-ASHRAE Conditions—27 Days ........................... 13

Table 10. Failure Mode Cooling No Ducted Distribution—Non-ASHRAE Conditions—83 Days ...... 14

Table 11. Failure Mode Heating Typical Airflow—Non-ASHRAE Conditions—26 Days .................... 14

Table 12. Failure Mode Heating Low Airflow —Non-ASHRAE Conditions—39 Days ........................ 15

Table 13. Failure Mode Heating No Ducted Distribution—Non-ASHRAE Conditions—37 Days ...... 15

Table 14. Passing Rate for All Systems and Modes—ASHRAE Conditions ....................................... 16

Table 15. Passing Rate for All Systems and Modes—Non-ASHRAE Conditions............................... 16

Table 16. Typical Airflow Volume System During Cooling Mode ........................................................ 17

Table 17. Low Airflow System During Cooling Mode............................................................................ 18

Table 18. No Ducted Distribution System During Cooling Mode......................................................... 18

Table 19. Typical Airflow Volume System During Heating Mode......................................................... 19

Table 20. Low Airflow System During Heating Mode ............................................................................ 20

Table 21. No Ducted Distribution System During Heating Mode ......................................................... 20

Table 22. Exterior Boundary R-Values and Temperatures ................................................................... 24

Table 23. RMS Error Values for Modeled Temperatures at Each Location......................................... 27

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

Definitions

ACCA Air Conditioning Contractors of America

BEopt Building Energy Optimization (software)

Btu British thermal units

Btu/h British thermal units per hour

CFD Computational fluid dynamics

CFM Cubic feet per minute

HVAC Heating, ventilation, and air conditioning

RMS Root mean square
viii

Acknowledgments

This report was prepared for the U.S. Department of Energy Building America program. Funding 

for this work was provided by the National Renewable Energy Laboratory under Subcontract 

No. KNDJ-0-40341-03.

The authors acknowledge GreenEarthEquities and GTK Flow Analysis for their contributions to

this project.
ix

Executive Summary

IBACOS anticipates that houses achieving 50% whole-house source energy savings with respect 

to the Building America 2010 Benchmark (Hendron and Engebrecht 2010) will be “low load.” 

Low load is defined by IBACOS as a house with a thermal enclosure that yields a maximum 

space heating and cooling load of less than 10 Btu/h/ft2 of conditioned floor area (31.5 W/m2). 

These small loads can be met by systems other than today’s typical ducted forced-air systems. 

For example, distributed fan coils with minimized ducts, terminal fan coil units, or point source 

units with buoyant force or ventilation driven distribution may provide sufficient occupant 

comfort in a low-load home. These systems, which can have lower total installed costs than 

traditional ducted forced-air systems (Stecher 2011), allow the thermal enclosure characteristics 

of low-load houses to provide first-cost savings in addition to operational cost savings.

The purpose of this study is to help determine cost-effective solutions for heating and cooling 

houses that are designed to be energy efficient. This is done by testing the occupant comfort 

performance of some concepts that may already exist on the market but are not in use by 

production homebuilders. In some cases, the products are market available, but their use in 

housing may be a new application. The standards used to assess the performance of the systems 

in this study are Air Conditioning Contractors of America (ACCA) Manual RS (Rutkowski 

1997) and ASHRAE Standard 55-2010 (ASHRAE 2010a).

This report addresses the following research questions:

• To what extent do alternative space conditioning distribution strategies meet ACCA and 

ASHRAE guidelines for room-to-room temperature uniformity and stability, 

respectively? Under what weather conditions do indoor temperatures rise above or fall 

below the guidelines?

• How substantially do the alternative strategies differ from typical ducted forced-air 

systems in their ability to meet the ACCA and ASHRAE guidelines? 

• Where applicable, in what ways and by how much does the measured performance of the 

distribution system differ from the computer-modeled performance?

To address these questions, IBACOS performed energy modeling and created two low-load test 

facilities with instrumentation to enable the testing of several experimental alternatives to 

traditional forced-air distribution designs. One facility is a retrofit unoccupied test house in 

Fresno, California; the other is a new construction unoccupied test house in Pittsburgh, 

Pennsylvania. Several systems were tested in each house. This report outlines the results of the 

three systems tested in the Fresno, California, house:

• Typical airflow volume ducted distribution system to the bedrooms and a single point of 

delivery into the main living space 

• Low airflow volume ducted distribution system to the bedrooms and a single point of 

delivery into the main living space

• No ducted distribution to the bedrooms but over-door and bottom-of-door transfer grilles 

used to facilitate free movement of air when the bedroom doors are closed and a single 

point of delivery into the main living space.
x

The measured performance of the three systems in the Fresno test house with respect to 

ASHRAE Standard 55-2010 Section 5.2.5 (Temperature Variations with Time) (ASHRAE 

2010a) showed that failures occurring over a time period greater than 15 min were rare for all 

three distribution systems. Failures tended to be cyclic in nature, with a temperature change 

greater than ± 2°F over a 15-min interval. Of the three systems, the system with no distribution 

ductwork to the bedrooms most frequently met these requirements in the bedrooms, whereas the 

typical airflow volume with ducted distribution to the bedrooms system was most successful in 

the kitchen, hallway, and hallway bathroom. The performance of the low airflow system ranked 

between the performance of the typical system and the non-ducted system for these rooms but 

had the best performance of the three systems in the living room.

Conversely, the system of typical airflow volume with ducted distribution to the bedrooms was 

the best at meeting ACCA Manual RS guidelines (Rutkowski 1997) for room temperature 

variation from the thermostat set point, with only the living room showing failures in cooling 

mode and with all rooms showing the fewest and least extreme failures in heating mode. The 

system with no distribution ductwork to the bedrooms performed the worst, with a high 

percentage of failures in the non-actively conditioned bedrooms. In the bedrooms, the nonducted system failed approximately 50% of the time in cooling mode and almost 100% of the 

time in heating mode. By contrast, the typical system never failed in cooling mode and failed 

approximately 10% of the time in heating mode.

The typical airflow volume ducted distribution system to the bedrooms and a single point of 

delivery into the main living space was the most effective at meeting the ACCA guidelines in the 

bedrooms, although it did this at the expense of meeting the ASHRAE Standard 55-2010 Section 

5.2.5 (Temperature Variations with Time) requirements (ASHRAE 2010a). It is notable that the 

system had difficulty in meeting both guidelines in the room that contained the thermostat—the 

living room. Based on its failure to meet ACCA Manual RS guidelines (Rutkowski 1997), the 

transfer grilles used with the non-ducted distribution system appeared to be ineffective at 

enabling heat transfer with the bedrooms via natural convection. 

To allow confident use in other houses of the model developed for this study, the root mean 

square (RMS) error should be approximately 0.5°F to allow confident application of the ACCA 

Manual RS (Rutkowski 1997) and ASHRAE Standard 55-2010 (ASHRAE 2010a) criteria to the 

model for use in predicting the performance of over-door and bottom-of-door transfer grilles. 

The measured performance differs significantly from the modeled performance, with a bedroom 

RMS error ranging from 1.18°F to 4.51°F, depending on the iteration of the model and the 

measurement type. Therefore, the model should not be used in other studies without further 

refinement. 

Cutting-edge builders are installing single point space conditioning systems with no means of 

providing conditioned air to bedrooms except via open doors. This report offers insight into a 

strategy for providing conditioned air to those rooms when the doors are closed and reports 

heretofore unmeasured data for the range and frequency of potential thermal discomfort that 

occupants may experience when this strategy is used. Builders can now use this information to 

discuss space conditioning options with their clients to determine the level of potential 

discomfort the occupants are willing to accept to have a cost-optimized, cutting-edge, energy 

efficient house.
1

1 Introduction and Background

IBACOS anticipates that houses achieving 50% whole-house source energy savings with respect 

to the Building America 2010 Benchmark (Hendron and Engebrecht 2010) will be “low load.” 

Low load is defined by IBACOS as a house with a thermal enclosure that yields a maximum 

space heating and cooling load of less than 10 Btu/h/ft2 of conditioned floor area (31.5 W/m2). 

These small loads can be met by systems other than today’s typical ducted forced-air systems. 

For example, distributed fan coils with minimized ducts, terminal fan coil units, or point source 

units with buoyant force or ventilation driven distribution may provide sufficient occupant 

comfort in a low-load home. These systems, which can have lower total installed costs than 

traditional ducted forced-air systems (Stecher 2011), allow the thermal enclosure characteristics 

of low-load houses to provide first-cost savings in addition to operational cost savings.

The purpose of this study is to help determine cost-effective solutions for heating and cooling 

houses that are designed to be energy efficient. This is done by testing the occupant comfort 

performance of some concepts that may already exist on the market but are not in use by 

production homebuilders. In some cases, the products are market available, but their use in 

housing may be a new application. The standards used to assess the performance of the systems

in this study are ACCA Manual RS (Rutkowski 1997) and ASHRAE Standard 55-2010

(ASHRAE 2010a). These two standards, or portions thereof, are also what traditional ducted 

forced-air systems can be required to meet (Masco Home Services 2013). Although some may 

question if most field-installed traditional ducted forced-air systems actually do meet these 

standards, a large-scale study of the field-installed performance of traditional ducted forced-air 

systems is beyond the scope of this research. However, such a study should be considered by 

researchers in the future.

This report addresses the following research questions:

• To what extent do alternative space conditioning distribution strategies meet ACCA and 

ASHRAE guidelines for room-to-room temperature uniformity and stability, 

respectively? Under what weather conditions do indoor temperatures rise above or fall 

below the guidelines?

• How substantially do the alternative strategies differ from typical ducted forced-air 

systems in their ability to meet ACCA and ASHRAE guidelines? 

• Where applicable, in what ways and by how much does the measured performance of the 

distribution system differ from the computer-modeled performance?

To answer these questions, IBACOS performed energy modeling and created two low-load test 

facilities with instrumentation to enable the testing of several experimental alternatives to 

traditional forced-air distribution designs. One facility is a retrofit unoccupied test house located 

in Fresno, California; the other is a new construction unoccupied test house located in Pittsburgh, 

Pennsylvania. 

This report focuses on the results of the systems tested in the Fresno, California, retrofit 

unoccupied test house. Figure 1 shows a front view of the Fresno test house. The west-facing, 

slab-on-grade, 1,621-ft2 house includes 2 × 4 wall construction with R-13 cellulose, a vented 

attic with R-60 blown-in cellulose, windows with a 0.30 U-value and 0.30 solar heat gain 
2

coefficient, and rigorous air-sealing measures. The house has well-shaded windows on the east 

and west sides due to large overhangs and adjacent fences, trees, and buildings. 

Figure 1. Front view of the Fresno test house

Based on the specifications implemented during the retrofit of the Fresno test house, the resultant 

calculated design loads are 16,680 Btu/h in heating and 15,934 Btu/h in cooling. This 

corresponds to 10.3 Btu/ft2

-h and 9.8 Btu/ft2-h on a per-unit area basis, respectively. Building 

Energy Optimization (BEopt) modeling shows energy savings of 50.1%. Full specifications for 

this house are documented by Stecher and Imm (2013).

The following systems were tested in the Fresno house:

• Typical airflow volume ducted distribution system to the bedrooms and a single point of 

delivery into the main living space 

• Low airflow volume ducted distribution system to the bedrooms and a single point of 

delivery into the main living space

• No ducted distribution to the bedrooms but over-door and bottom-of-door transfer grilles 

used to facilitate free movement of air when the bedroom doors are closed and a single 

point of delivery into the main living space 

Table 1 provides details about the three distribution systems outlined above for the Fresno test 

house. These three distribution systems were implemented in the Fresno test house by using zone 

dampers controlled by a central data logger. By opening and closing specific dampers, as 

detailed in Table 1, each distribution system could be operated for a specific duration of time 

(e.g., 1 week). The zone damper numbers in Table 1 correspond with the labels shown in Figure 
3

2 and Figure 3. The detailed house and system design and commissioning were documented by 

Stecher and Imm (2013).

Table 1. Distribution System Control Dampers

Zone Dampers 1 2 3 4

System 1 –

Typical Airflow Open Closed Open Closed

System 2 –

Low Airflow Open Open 5/6 cycles;

closed 1/6 cycles

Closed 5/6 cycles; 

open 1/6 cycles Closed

System 3 –

No Ducted 

Distribution

Open Open Closed Closed

1.1 Typical Airflow Volume Ducted Distribution to the Bedrooms

Distribution system 1—typical airflow volume ducted distribution system to the bedrooms and a 

single point of delivery into the main living space—used ductwork sized according to ACCA 

Manual D (Rutkowski 2009) to provide airflows to meet ACCA Manual J loads (Rutkowski 

2006, Stecher and Imm 2013). Figure 2 shows the distribution system layout.

Figure 2. Typical airflow volume ducted distribution to the bedrooms

(numbers correspond to zone damper numbers in Table 1)

1.2 Low Airflow Volume Ducted Distribution to the Bedrooms

Distribution system 2—low airflow volume ducted distribution system to the bedrooms and a 

single point of distribution delivery into the main living space—was inspired by the Passivhaus 

ventilation systems in Europe, whereby the entire peak heating load of a residence is delivered 
4

via the volume of ventilation air required for good indoor air quality (Feist et al. 2005). In the 

case of the Fresno house, the amount of ventilation air required for good indoor air quality was 

40 CFM per ASRHAE Standard 62.2 (ASHRAE 2010b). Assuming a temperature difference 

between the supply air and room air of 25°F, the total delivered energy that 40 CFM can deliver 

is 1,084 Btu/h. This value is approximately 7% of the total peak load of the house (Stecher and 

Imm 2013), most likely inadequate for sufficient occupant comfort if used as the sole source of 

space conditioning. However, if the conditioned ventilation supply air were delivered to only the 

three bedrooms of the house, the difference between the delivered load and the required load 

would be halved. The remaining portion of the house, which has an open floor plan, could be 

conditioned by a single supply register. The total ACCA Manual D (Rutkowski 2009) specified 

airflow for the three bedrooms is 235 CFM. Although still approximately six times greater than 

what the test system would deliver, it was anticipated that the substantial difference would reveal

a clear success-to-failure transition point at certain load conditions on the house. 

This concept was developed after the test air handler unit and ductwork were already installed; 

thus, installing a reduced-capacity set of ductwork was not an option. Instead, to deliver the 

appropriate amount of airflow, the damper controlling the ductwork to the bedrooms was opened 

only once every six air handler unit cycles (#3 in Figure 3). This operation schedule—shown 

fractionally in Table 1 and graphically in Figure 3—was intended to ensure that the equivalent of 

40 CFM would be delivered to the bedrooms for a given day of operation.

Figure 3. Low airflow volume ducted distribution to the bedrooms

(numbers correspond to zone damper numbers in Table 1)

1.3 No Ducted Distribution to the Bedrooms

Distribution system 3—no ducted distribution to the bedrooms but over-door and bottom-of-door 

transfer grilles used to facilitate free movement of air when the bedroom doors are closed and a 

single point of delivery into the main living space—was based on research performed by Barakat 
5

(1985). Barakat’s research indicated that the steady-state heat transfer through an open doorway 

at a 2.7°F temperature difference was 1,050 Btu/h via natural convection, with 48 Btu/h via 

radiation. Building on this work, multizone airflow calculations and computational fluid 

dynamics (CFD) simulations performed by Feist et al. (2005) of dwellings meeting the 

Passivhaus energy standard indicate that, at a temperature difference of 1.8°F between spaces, 

expected heat transfer rates are 300–600 Btu/h through open doors and 0.3–0.6 Btu/h/ft2 of 

internal partition wall area. In a low-load home, these values may provide a substantial 

contribution to satisfying an individual room load before providing active conditioning. Field test 

data obtained by IBACOS (2008, 2010a, 2010b) from a Passivhaus in climate zone 5 also 

support this hypothesis. 

However, if interior partition doors are not always open, the potential for heat transfer to occur 

via natural convection can be reduced. Fortunately, Emery (1969) found that when a small,

rectangular opening of height h is placed in a wall at the ceiling plane and one is placed at the 

floor plane on the order of h/H ≥ 0.05—where h = h1 = h2, h1 is measured down from the ceiling 

for the upper opening, h2 is measured up from the floor for the lower opening, and H is the floorto-ceiling height—no loss in flow occurs compared to the baseline case of a single clear opening 

of height H in the wall spanning from the floor to the ceiling (see Figure 4).

Figure 4. Implementation of the Emery (1969) study (left) in the Fresno test house (right)

This assumes the width of all openings is the same in both cases. Based on the findings from this 

work, conditioned air was delivered to a single location in the Fresno test house—the main living 

space—and a market-available, acoustically dampened, and light-shielding through-wall transfer 

grille was strategically located above and in the bottom of each bedroom door to enable airflow 

via natural convection from the main living space to the bedrooms (shown in Figure 4 and as 

H

Door Actuator 

h1

h2

Hallway Bedroom

H

h2

h1
6

green boxes with arrows in Figure 5), even when the bedroom doors are closed. Key changes 

from the Emery study as a result of implementation of off-the-shelf components include h1 does 

not equal h2, the opening widths are not equal, and the openings are not clear due to light and 

noise baffles and grilles. (The numbers on Figure 5 correspond to the zone damper numbers in 

Table 1.) Full specifications were documented by Stecher and Imm (2013).

Figure 5. No ducted distribution to the bedrooms

(numbers correspond to zone damper numbers in Table 1)
7

2 Field Test Methods

The research team performed field tests for 1 year in the Fresno retrofit unoccupied test house to 

determine the extent to which each of the systems did or did not meet established guidelines. To 

ensure the collection of data for each system throughout the entire year, each system was 

operated for 1 week at a time before switching to the next system. IBACOS then used the 

collected data in an analysis incorporating the relevant comfort criteria. In this case, the criteria 

were limited to temperature—specifically ACCA Manual RS (Rutkowski 1997) and ASHRAE 

Standard 55-2010, Sections 5.2.5 (Temperature Variations with Time), 7.3.2 (Temperature 

Cycles and Drifts), and 7.4 (Measuring Conditions) (ASHRAE 2010a)—because maintaining an 

acceptable indoor temperature represents the bare minimum requirement that a heating, 

ventilation, and air conditioning (HVAC) system must meet. Assuming some systems in this 

study are shown to be non-viable, future studies can focus on assessing the ability of the 

remaining systems to meet other relevant comfort criteria. Bedroom doors were closed during all 

system tests unless otherwise mentioned.

2.1 Comfort Criteria

Occupant comfort is based on a combination of factors, including occupant clothing and activity 

level, room air temperature and humidity, mean radiant temperature, and room air velocities

(ASHRAE 2010a). In this study, dry bulb temperature was determined to be the primary factor 

of consideration because most residential HVAC systems turn on and off based solely on the dry 

bulb temperature measured by the thermostat. To fail in this area indicates a fundamental failure 

of the system; to succeed in controlling temperature prompts follow-up questions of performance 

in other areas (e.g., humidity control) that require different experiment setups. The relevant

standards for room air temperature are defined by ACCA Manual RS (Rutkowski 1997) and

ASHRAE Standard 55-2010 Sections 5.2.5 (Temperature Variations with Time), 7.3.2 

(Temperature Cycles and Drifts), and 7.4 (Measuring Conditions) (ASHRAE 2010a).

ACCA Manual RS (Rutkowski 1997) requires the dry bulb temperature measured within any 

room of the house to be within ± 3°F of the thermostat setting during the cooling season.

Similarly, the temperature during the heating season in any room must be within ± 2°F of the 

thermostat set point temperature. The temperature difference measured between any two rooms 

in the house (also known as the room-to-room temperature difference) should be no greater than 

4°F in the heating season and no greater than 6°F in the cooling season. Although ASHRAE 

Standard 55-2010 also provides guidance on room-to-room temperature differences, the ACCA 

standard was chosen because it was more straightforward to use for the data collected in this 

study.

ASHRAE Standard 55-2010 Section 5.2.5 (Temperature Variations with Time) outlines three 

types of fluctuations that are unsatisfactory to occupants: cyclic variations, drifts, and ramps. 

Cyclic variations involve temperature changes of greater than ± 2°F over a 15-min interval. 

Drifts are temperature changes that are due to internal or external loads acting on the building, 

whereas ramps are caused by the space conditioning system. The temperature change during a 

drift or ramp can be positive or negative, and the allowable change in temperature is based on the 

duration of time over which it occurs. Specifically, for any 15-min period, no more than 2°F

change is allowed; for any 30-min period, no more than 3°F is allowed. For any 60-min period,

no more than 4°F is allowed. For any 120-min period, no more than 5°F is allowed, and for any 

240-min period, no more than 6°F is allowed (ASHRAE 2010a). 
8

To assess the criteria outlined in ASHRAE Standard 55-2010 Section 5.2.5, Section 7.3.2 

(Temperature Cycles and Drifts) and Section 7.4 (Measuring Conditions) are provided in the 

standard (ASHRAE 2010a). Section 7.3.2 (Temperature Cycles and Drifts) provides the 

equations to follow to determine the difference in temperature over a given period of time.

Section 7.4 (Measuring Conditions) provides the conditions in which to perform the 

measurements. Specifically, the indoor-to-outdoor temperature difference during the 

measurement period must be greater than 50% of the indoor-to-outdoor temperature difference 

used for the heating or cooling design calculation. Additionally, for the heating season, the 

weather should be cloudy; for the cooling season, the weather should be sunny (ASHRAE 

2010a). In this case, the climate of Fresno, California, is predominantly sunny throughout the 

year, even in winter; therefore, the exception allowing for conditions representative of the 

climate was followed. 

Section 7.4 (Measuring Conditions) of ASHRAE Standard 55-2010 (ASHRAE 2010a) requires 

only two hours of data collection to assess system performance. In this study, the analysis was 

performed for every day in which the average outdoor and indoor temperatures met Section 7.4 

requirements (e.g., for all the minutes of all the days with conditions at or above 50% of the 

design conditions). For contrast, the analysis also was performed using the data from the time 

periods not meeting the conditions required for Section 7.4 (e.g., for all the minutes of all the 

days with conditions less than 50% of the design conditions).

2.2 Instrumentation Setup

To assess the performance of the three distribution systems with respect to the ACCA

Manual RS (Rutkowski 1997) and ASHRAE Standard 55-2010 (ASHRAE 2010a) guidelines, 

instrumentation was installed throughout the Fresno test house. The research team installed 

shielded, aspirated thermocouples in each room of the house to measure air temperature 

(Figure 6). This style of aspirated thermocouple is commonly used in Building America field 

tests and has been shown to accurately measure dry bulb temperature. The construction of the 

shield consists of an inner piece and outer piece of PVC pipe, with foil adhered to the adjacent 

faces acting as a shield. A small fan positioned above the pipe draws air upward and past the 

thermocouple, further reducing the error associated with direct sunlight on the shield. Supply

temperature measurements were made using unshielded thermocouples. Although no analysis 

was conducted to ensure the air velocity is adequate to mitigate any radiation exchange with duct 

surfaces, this effect is assumed to have minimal impact on the results of the research.

Measurement of the outdoor conditions comprised temperature, humidity, and incident solar 

radiation. Measurements from 20-s scans were averaged over 1 min and recorded (Stecher and 

Imm 2013). The data used for this analysis were collected from the systems installed in the 

Fresno retrofit unoccupied test house from June 1, 2012, to March 1, 2013.
9

Figure 6. Sensor locations
10

3 Field Test Results

Each system of the Fresno test house was operated for 1 week at a time throughout the course of 

a year to capture system operation in each season. Measured temperature data from each room 

were analyzed to determine the capability of the system in meeting ASHRAE Standard 55-2010 

Section 5.2.5 (Temperature Variations with Time), Section 7.3.2 (Temperature Cycles and 

Drifts), and Section 7.4 (Measuring Conditions) (ASHRAE 2010a) and by ACCA Manual RS 

(Rutkowski 1997) as described in Section 2.1 of this report.

3.1 ASHRAE Standard 55-2010 Analysis

Results from the ASHRAE Standard 55-2010 analysis are shown in Table 2 through Table 7. For 

each design condition—ASHRAE heating, ASHRAE cooling, and all other conditions—there is 

a table for each operational mode (typical airflow, low airflow, and no ducted distribution). The 

percentage values in each table are based on the total amount of time—measured in days—used 

for the assessment. 

In cooling mode, the typical airflow system had a high passing rate for the kitchen, hallway, 

master bathroom, and hallway bathroom. The primary mode of failure for the other rooms was

cyclic, with fluctuations of greater than 2°F occurring every 15 minutes (Table 2). The low 

airflow system had a high passing rate for the master bathroom, east bedroom, and northeast 

bedroom. All other rooms had predominantly cyclic failure, although the kitchen showed a 

greater than 10% failure rate under the 30-minute ramp or drift mode (Table 3). During the 

operation of the no ducted distribution system, the master bedroom, master bath, east bedroom,

and northeast bedroom had zero failures in any of the modes defined by ASHRAE Standard 55-

2010 Section 5.2.5 (Temperature Variations with Time). However, the living room failed due to 

cyclic variation 100% of the time, and the kitchen, hallway, and hallway bathroom also had 

frequent failure rates due to cycles induced by the operation of the HVAC system (Table 4).

Failures via 60-, 120-, or 240-min drifts or ramps were rare to non-existent.

Table 2. Failure Mode Cooling Typical Airflow—2 Days Kitchen Family/ Dining Hallway Bath 2 Bedroom 2 (NE) Bedroom 3 (E)

Master 

Bedroom

Master Bath

Pass % 89.5 0 98.7 100 36.7 61 1.1 100

Fail 

%

Mode

Cyc. 10.5 93.5 1.3 0 58.9 39 98.1 0

30 0 6.2 0 0 4.4 0 0.7 0

60 0 0.3 0 0 0 0 0 0

120 0 0 0 0 0 0 0 0

240 0 0 0 0 0 0 0 0
11

Table 3. Failure Mode Cooling Low Airflow—8 Days Kitchen Family/ Dining Hallway Bath 2 Bedroom 2 (NE) Bedroom 3 (E)

Master 

Bedroom

Master Bath

Pass % 10 5.3 49.7 39 83.4 92.1 55.1 96.8

Fail 

%

Mode

Cyc. 77.8 93 49.2 60.3 11 7.3 41.6 3.2

30 12.1 1.6 0.8 0.7 2.9 0 3 0

60 0.1 0.1 0.3 0 1.8 0.3 0.1 0

120 0 0 0 0 0.5 0.1 0.2 0

240 0 0 0 0 0 0 0 0

Table 4. Failure Mode Cooling No Ducted Distribution—25 Days Kitchen Family/ Dining Hallway Bath 2 Bedroom 2 (NE) Bedroom 3 (E) Master Bedroom

Master Bath

Pass % 0.5 0 30.6 25.1 100 100 100 100

Fail 

%

Mode

Cyc. 88.6 100 68.4 74.7 0 0 0 0

30 10.9 0 1.1 0.2 0 0 0 0

60 0.1 0 0 0 0 0 0 0

120 0 0 0 0 0 0 0 0

240 0 0 0 0 0 0 0 0

In heating mode, the typical airflow system had a high passing rate for the master bedroom, 

master bathroom, and hallway bathroom. The primary mode of failure for the other rooms was 

cyclic (Table 5). The low airflow system had a high passing rate for the master bedroom, master 

bathroom, east bedroom, and northeast bedroom. All other rooms had predominantly cyclic 

failure (Table 6). During the operation of the no ducted distribution system, the master bedroom, 

master bath, east bedroom, and northeast bedroom had zero failures in any of the modes defined 

by ASHRAE Standard 55-2010 Section 5.2.5 (Temperature Variations with Time) (ASHRAE 

2010a). Similar to the case of the cooling mode, the living room, kitchen, hallway, and hallway 

bathroom had frequent failure rates due to cyclic variation in temperature (Table 7). 
12

Table 5. Failure Mode Heating Typical Airflow—19 Days Kitchen Family/ Dining Hallway Bath 2 Bedroom 2 (NE) Bedroom 3 (E) Master 

Bedroom

Master Bath

Pass % 0 0.1 13.5 93.8 45.9 19 86.1 99.6

Fail 

%

Mode

Cyc. 86 90.7 76.9 6.2 48.9 76.4 13.8 0.4

30 10.8 9 9.2 0 5.1 4.6 0.1 0

60 3.3 0.2 0.4 0 0.1 0 0 0

120 0 0 0 0 0 0 0 0

240 0 0 0 0 0 0 0 0

Table 6. Failure Mode Heating Low Airflow—28 Days Kitchen Family/ Dining Hallway Bath 2 Bedroom 2 (NE) Bedroom 3 (E)

Master 

Bedroom

Master Bath

Pass % 12.2 12.2 16 33.4 88.5 91.5 92.4 94.5

Fail 

%

Mode

Cyc. 85.8 86.6 78.7 63.8 6.9 7.2 7.4 5.3

30 1.6 1.2 4.9 2.8 2.1 0.8 0.2 0.2

60 0.4 0 0.4 0 2 0.2 0 0

120 0 0 0 0 0.5 0.3 0 0

240 0 0 0 0 0 0 0 0

Table 7. Failure Mode Heating No Ducted Distribution—20 Days Kitchen Famil

y/

Dining

Hallway

Bath 2

Bedroom 2 

(NE)

Bedroom 3 

(E)

Master 

Bedroom

Master Bath

Pass % 1 1 2.6 35.6 100 100 100 100

Fail 

%

Mode

Cyc. 98.7 98.9 92.1 61.6 0 0 0 0

30 0.2 0 5 2.8 0 0 0 0

60 0 0 0.3 0 0 0 0 0

120 0 0 0 0 0 0 0 0

240 0 0 0 0 0 0 0 0

In addition to the analysis performed during the conditions called out by ASHRAE Standard 55-

2012 Section 7.4 (Measuring Conditions) (ASHRAE 2010a), the analysis also was performed 

during weather conditions that did not meet the requirements of Section 7.4. This occurred when 
13

the outdoor temperature was lower than the 50% design cooling temperature and greater than the 

50% design heating temperature (e.g., non-peak and midseason conditions). These data are 

tabulated for each system for each mode: cooling or heating, H1. 

As shown in Table 8 through Table 10 for the cooling mode, the typical airflow system had a 

greater than 80% passing rate for the kitchen, hallway, master bathroom, and hallway bathroom. 

The primary mode of failure for the other rooms was cyclic (Table 8). The low airflow system 

had a greater than 80% passing rate for the master bathroom, east bedroom, and northeast 

bedroom. The hallway and master bedroom were slightly less than 80% passing, and most of 

their failures were due to cyclic variation. The kitchen and living room failed most of time due to 

cyclic variation (Table 9). During the operation of the no ducted distribution system, the master 

bedroom, master bath, east bedroom, and northeast bedroom had almost zero failures in any of 

the modes defined by ASHRAE Standard 55-2010 Section 5.2.5 (Temperature Variations with 

Time). The living room, kitchen, hallway, and hallway bathroom had frequent failure rates due to 

cyclic variation in temperature (Table 10).

Table 8. Failure Mode Cooling Typical Airflow—Non-ASHRAE Conditions—19 Days Kitchen Family/ Dining Hallway Bath 2 Bedroom 2 (NE) Bedroom 3 (E) Master Bedroom Master Bath

Pass % 91.7 16.3 98.6 100 59.6 77.6 29.1 100

Fail 

%

Mode

Cyc. 8.3 76.7 1.4 0 38.1 21.8 69.9 0

30 0 6.8 0 0 2.4 0.5 1 0

60 0 0.1 0 0 0 0 0 0

120 0 0 0 0 0 0 0 0

240 0 0 0 0 0 0 0 0

Table 9. Failure Mode Cooling Low Airflow—Non-ASHRAE Conditions—27 Days Kitchen Family/ Dining Hallway Bath 2 Bedroom 2 (NE) Bedroom 3 (E) Master Bedroom Master Bath

Pass % 35.8 32.8 73.8 55.7 88.5 92.3 78.6 91.2

Fail 

%

Mode

Cyc. 56.4 66 25.6 43.8 10.5 7.5 19.9 8.3

30 7.7 1.2 0.6 0.6 0.7 0.2 1.4 0.5

60 0.1 0 0 0 0.3 0 0 0

120 0 0 0 0 0 0 0 0

240 0 0 0 0 0 0 0 0
14

Table 10. Failure Mode Cooling No Ducted Distribution—Non-ASHRAE Conditions—83 Days Kitchen Family/ Dining Hallway Bath 2 Bedroom 2 (NE) Bedroom 3 (E) Master Bedroom Master Bath

Pass % 33.2 31.7 51 48.6 100 99.9 100 100

Fail 

%

Mode

Cyc. 59.4 68.3 47 50.2 0 0.1 0 0

30 7.4 0 2 1.2 0 0 0 0

60 0 0 0 0 0 0 0 0

120 0 0 0 0 0 0 0 0

240 0 0 0 0 0 0 0 0

As shown in Table 11 through Table 13 for the heating mode, the typical airflow system had a

greater than 90% passing rate for the master bedroom, master bathroom, and hallway bathroom. 

The primary mode of failure for the other rooms was cyclic (Table 11). The low airflow system 

had a greater than 90% passing rate for the master bedroom, master bathroom, east bedroom, and 

northeast bedroom. All other rooms were closely divided between passing and cyclic failure

(Table 12). During the operation of the no ducted distribution system, the master bedroom, 

master bath, east bedroom, and northeast bedroom had zero failures in any of the modes defined 

by ASHRAE Standard 55-2010 Section 5.2.5 (Temperature Variations with Time). The living 

room, kitchen, hallway, and hallway bathroom had frequent failure rates due to cyclic variation 

in temperature (Table 13). In all system operation modes, the rate of failure was lower than that 

observed during ASHRAE Standard 55-2010 Section 7.4 (Measuring Conditions) guidelines.

Table 11. Failure Mode Heating Typical Airflow—Non-ASHRAE Conditions—26 Days Kitchen Family/ Dining Hallway Bath 2 Bedroom 2 (NE) Bedroom 3 (E) Master Bedroom Master Bath

Pass % 15.6 16.8 38.6 99.7 66.6 54.8 98.3 99.6

Fail 

%

Mode

Cyc. 74.4 74.6 55.1 0.3 31.9 43 1.7 0.4

30 9.6 8.6 6.3 0 1.5 2.2 0 0

60 0.5 0 0 0 0 0 0 0

120 0 0 0 0 0 0 0 0

240 0 0 0 0 0 0 0 0
15

Table 12. Failure Mode Heating Low Airflow—Non-ASHRAE Conditions—39 Days Kitchen Family/ Dining Hallway Bath 2 Bedroom 2 (NE) Bedroom 3 (E) Master Bedroom Master Bath

Pass % 39.5 39.9 46.5 66.7 92.6 91.7 92.5 95.8

Fail 

%

Mode

Cyc. 59.4 59.4 49.5 32 6.4 7.5 7.2 4.2

30 1 0.7 3.9 1.3 0.9 0.8 0.2 0

60 0.1 0 0.1 0 0.1 0 0 0

120 0 0 0 0 0 0 0 0

240 0 0 0 0 0 0 0 0

Table 13. Failure Mode Heating No Ducted Distribution—Non-ASHRAE Conditions—37 Days Kitchen Family/ Dining Hallway Bath 2 Bedroom 2 (NE) Bedroom 3 (E) Master Bedroom Master Bath

Pass % 33.3 33.4 40 66.9 100 100 100 100

Fail 

%

Mode

Cyc. 66.7 66.6 56.5 32.5 0 0 0 0

30 0 0 3.5 0.6 0 0 0 0

60 0 0 0 0 0 0 0 0

120 0 0 0 0 0 0 0 0

240 0 0 0 0 0 0 0 0

In general, the data collected during the conditions meeting ASHRAE Standard 55-2010 

Section 7.4 (Measuring Conditions) included time periods with greater duration and frequency of 

HVAC system operation, with correspondingly greater failure rates due to cyclic temperature 

movement than the data collected during time periods that did not meet the measurement 

standard. In all cases, rooms with no active conditioning tended to most frequently meet the 

cycles, drifts, and ramps standard (Table 14). When the peak load of the entire house was 

supplied into the living space, it caused a high rate of failure not only in that space but also in 

immediately adjacent spaces connected by open doorways, such as the hallway and the hallway 

bathroom. The performance of the low airflow system is between the performance of the typical 

airflow system and the no ducted distribution system, consistent with its operation methodology

of delivering less conditioned air to the bedrooms than the typical airflow system. Failures due to 

drifts or ramps 30 min or greater were rare in all cases. During conditions not meeting ASHRAE 

Standard 55-2010 Section 7.4 (Measuring Conditions), all systems had a higher passing rate

(Table 15).
16

Table 14. Passing Rate for All Systems and Modes—ASHRAE Conditions

Mode System

Kitchen

Family/ 

Dining

Hallway

Bath 2

Bedroom 2 

(NE)

Bedroom 3 

(E)

Master 

Bedroom

Master Bath

ASHRAE Conditions

Cooling Typical 

Airflow 89.5 0 98.7 100 36.7 61 1.1 100

Cooling Low 

Airflow 10 5.3 49.7 39 83.4 92.1 55.1 96.8

Cooling No Ducted 

Distribution 0.5 0 30.6 25.1 100 100 100 100

Heating Typical 

Airflow 0 0.1 13.5 93.8 45.9 19 86.1 99.6

Heating Low 

Airflow 12.2 12.2 16 33.4 88.5 91.5 92.4 94.5

Heating No Ducted 

Distribution 1 1 2.6 35.6 100 100 100 100

Table 15. Passing Rate for All Systems and Modes—Non-ASHRAE Conditions

Mode System

Kitchen

Family/ 

Dining

Hallway

Bath 2

Bedroom 2 

(NE)

Bedroom 3 

(E)

Master 

Bedroom

Master Bath

Non-ASHRAE Conditions

Cooling Typical 

Airflow 91.7 16.3 98.6 100 59.6 77.6 29.1 100

Cooling Low 

Airflow 35.8 32.8 73.8 55.7 88.5 92.3 78.6 91.2

Cooling No Ducted 

Distribution 33.2 31.7 51 48.6 100 99.9 100 100

Heating Typical 

Airflow 15.6 16.8 38.6 99.7 66.6 54.8 98.3 99.6

Heating Low 

Airflow 39.5 39.9 46.5 66.7 92.6 91.7 92.5 95.8

Heating No Ducted 

Distribution 33.3 33.4 40 66.9 100 100 100 100

3.2 ACCA Manual RS Analysis

Results from the ACCA Manual RS (Rutkowski 1997) analysis using the methodology described 

in Section 2.1 are shown in Table 16 through Table 21. Again, there is one table for each system 

(typical airflow, low airflow, and no ducted distribution). For consistency of data, the analysis 

was performed only on days that met the ASHRAE Standard 55-2010 Section 7.4 (Measuring 

Conditions) test conditions standard (ASHRAE 2010a).
17

In cooling mode, the typical airflow system maintained all rooms except the living room within 

± 3°F of the temperature at the thermostat. Room-to-room temperature differences are all within 

ACCA Manual RS (Rutkowski 1997) standards (Table 16). The low airflow system showed 

greater extremes in temperature and had a higher percentage of time outside the ± 3°F band for 

the living room. The northeast bedroom also spent 64% of the time outside the acceptable range. 

Room-to-room temperature differences were more extreme and more frequent in occurrence, 

with all rooms spending some time outside the ± 6°F band (Table 17). The no ducted distribution

system showed the greatest extremes in temperature, and the percentage of time outside the 

± 3°F band increased for all rooms compared to the low airflow system. The northeast bedroom 

again had the greatest duration of time outside the acceptable range at 87%, along with the most 

extreme temperature: 8.7°F above the thermostat set point. Room-to-room temperature 

differences were also more extreme and more frequent in occurrence, with all rooms spending at 

least 30% of the time outside the ± 6°F band (Table 18). The apparent effect of outdoor 

temperature is unclear, with no obvious patterns and extreme indoor conditions occurring for 

outdoor temperatures between 70°F and 108°F.

Table 16. Typical Airflow Volume System During Cooling Mode Kitchen Family/ Dining Hallway Bedroom 2 (NE) Bedroom 3 (E)

Master 

Bedroom

Room-to-Thermostat 

Set

Point Temperature

Difference

Largest positive 

difference (°F) 0.9 0.5 2.8 2.8 0.8 0.8

Largest negative 

difference (°F) –2.7 –4.5 –0.8 –2.1 –2.6 –3.1

Percentage of time 

outside ± 3°F band 0 31 0 0 0 0

Room-to-Room 

Temperature 

Difference

Average (°F) –0.3 –1.5 1.5 1.2 –0.3 –0.6

Maximum (°F) –2.6 –4.8 4.8 4.3 –3.0 –3.6

Percentage of time 

outside ± 6°F band 0 0 0 0 0 0
18

Table 17. Low Airflow System During Cooling Mode Kitchen Family/ Dining Hallway Bedroom 2 (NE)

Bedroom 3 

(E)

Master 

Bedroom

Room-to-Thermostat 

Set

Point Temperature

Difference

Largest positive 

difference (°F) 1 0.7 3.4 8.3 5.9 4.1

Largest negative 

difference (°F) –3.6 –6.1 –1.2 –1.3 –2.4 –2.6

Percentage of time 

outside ± 3°F band 5 45 0 64 13 4

Room-to-Room 

Temperature 

Difference

Average (°F) –2.2 –3.6 0.4 3.5 1.6 0.3

Maximum (°F) –9.9 –13.8 –8.7 13.8 11.5 9.4

Percentage of time 

outside ± 6°F band 25.4 40.2 6.6 40.9 28.3 15.2

Table 18. No Ducted Distribution System During Cooling Mode Kitchen Family/ Dining Hallway Bedroom 2 (NE) Bedroom 3 

(E)

Master 

Bedroom

Room-to-Thermostat 

Set

Point Temperature

Difference

Largest positive 

difference (°F) 1.7 0.4 3.4 8.7 6.9 4.8

Largest negative 

difference (°F) –4.3 –6.9 –3.9 –4.3 –3.3 –3.8

Percentage of time 

outside ± 3°F band 18 49 2 87 58 24

Room-to-Room 

Temperature 

Difference

Average (°F) –3.2 –4.8 –1.1 4.8 3.2 1

Maximum (°F) –12.3 –14.6 –9.8 14.6 12.7 10.1

Percentage of time 

outside ± 6°F band 57.8 67.7 36.7 68.1 55.9 35.2
19

In heating mode, the typical airflow system had infrequent but significant temperature excursions 

outside the ACCA required ± 2°F of the thermostat set point temperature. An exception to this is 

the master bedroom, which was outside this temperature range 80% of the time, with a maximum 

temperature of 6°F below the thermostat set point. Room-to-room temperature differences were 

significant but infrequent, occurring less than 15% of the time (Table 19). The master bedroom 

was similarly out of bounds for the low airflow system. The low airflow system also showed 

greater extremes in temperature and had a higher percentage of time outside the ± 2°F band for 

all rooms. In particular, the northeast bedroom and the east bedroom spent at least 70% of the 

time outside the ± 2°F band. Room-to-room temperature differences were more extreme and 

more frequent in occurrence, with all rooms spending at least 30% of the time outside the ± 4°F 

band (Table 20). The no ducted distribution system showed the greatest extremes in temperature,

and the percentage of time outside the ±3°F band increased for all rooms compared to the low 

airflow system. The master bedroom, northeast bedroom, and east bedroom were outside the 

± 2°F band at least 95% of the time, with temperature extremes of approximately 9°F below the 

thermostat set point. Room-to-room temperature differences also were more extreme and more 

frequent in occurrence, with all rooms spending at least 60% of the time outside the ± 4°F band

(Table 21). The apparent effect of outdoor temperature is unclear, with no obvious patterns and 

extreme indoor conditions occurring for outdoor temperatures between 32°F and 62°F.

Table 19. Typical Airflow Volume System During Heating Mode Kitchen Family/ Dining Hallway Bedroom 2 (NE) Bedroom 3 

(E)

Master 

Bedroom

Room-to-Thermostat 

Set

Point Temperature

Difference

Largest positive 

difference (°F) 2.8 2 3.3 2 1.9 0.3

Largest negative 

difference (°F) –3.4 –4.1 –2.6 –4.4 –5.2 –6.0

Percentage of time 

outside ±2°F band 41 54 5 7 13 80

Room-to-Room 

Temperature 

Difference

Average (°F) –0.1 –0.8 1.5 0.8 0.4 –1.9

Maximum (°F) 7.8 –3.2 5.5 4.9 5.3 –5.7

Percentage of time 

outside ±4°F band 1.7 0.1 13.1 5.2 4.1 13.8
20

Table 20. Low Airflow System During Heating Mode Kitchen Family/ Dining Hallway Bedroom 2 (NE) Bedroom 3 

(E)

Master 

Bedroom

Room-to-Thermostat 

Set Point Temperature

Difference

Largest positive 

difference (°F) 6.2 4.3 3.8 2.3 4.3 4.4

Largest negative 

difference (°F) –4.0 –6.4 –3.5 –7.8 –7.3 –8.4

Percentage of time 

outside ± 2°F band 49 56 15 71 81 85

Room-to-Room 

Temperature 

Difference

Average (°F) 1.7 1.1 2.3 –1.7 –1.4 –1.9

Maximum (°F) 12.9 11.8 11 –11.6 –11.7 –12.8

Percentage of time 

outside ± 4°F band 38.8 31.8 50.5 44.8 43.5 53.3

Table 21. No Ducted Distribution System During Heating Mode Kitchen Family/ Dining Hallway Bedroom 2 (NE) Bedroom 3 

(E)

Master 

Bedroom

Room to Thermostat 

Set Point Temperature

Difference

Largest positive 

difference (°F) 5.8 4.7 3.4 –0.7 –1.6 –2.2

Largest negative 

difference (°F) –4.1 –5.1 –4.1 –9.8 –9.3 –9.1

Percentage of time 

outside ± 2°F band 61 66 24 96 99 100

Room-to-Room 

Temperature 

Difference

Average (°F) 3.6 2.8 3.3 –3.0 –3.2 –3.6

Maximum (°F) 15 13.5 12.2 –14.5 –14.3 –14.1

Percentage of time 

outside ± 4°F band 71.4 63.2 77 75.4 75.7 80
21

4 Discussion

In general, the typical airflow volume ducted distribution system to the bedrooms and a single 

point of delivery into the main living space was the most effective at meeting the ACCA 

Manual RS (Rutkowski 1997) and ASHRAE Standard 55-2010 (ASHRAE 2010a) guidelines. It 

is notable that the system had difficulty in meeting the guidelines in the room that contained the 

thermostat. Ostensibly, this should be the easiest room in which to meet the requirements

because it contains the control; however, this indicates the inherent challenge in supplying

sufficient airflow and temperature difference to ensure the room load is met while minimizing 

the perceived temperature movement by the occupant. This room did not meet the guidelines 

because the shielded aspirated thermocouple for the living room temperature measurement was 

able to detect the partially mixed airflow moving through the room. Using a register that allowed 

more precise airflow placement could have helped to mitigate this problem.

Although the typical airflow system performed almost perfectly in cooling mode, no system was 

perfectly effective in heating mode. The discrepancy between cooling mode and heating mode 

performance can be partially attributed to the difference in temperature criteria in ACCA

Manual RS (Rutkowski 1997). When the same temperature boundaries are used for each mode, 

the performance is similar. Why two different temperature boundaries are used for the heating 

mode and cooling mode is unclear, and its applicability in energy efficient, low-load houses 

could be a topic of future research.

Additionally, there is no specified amount of time that a room needs to be out of bounds. In this 

study, which used data averaged every minute, even temperature extremes occurring for a very 

short period of time would trigger a failure. This prompts the question of the minimum 

acceptable duration of time that a temperature extreme needs to occur before failing the ACCA 

Manual RS guidelines (Rutkowski 1997). The application of ASHRAE Standard 55-2010 

(ASHRAE 2010a) to a time period greater than that outlined in Section 7.4 (Measuring 

Conditions) of that standard shows there is a slight risk to researchers in the field of getting a 

result during a short-term test that is not necessarily representative of the performance of the 

house as a whole. The study also showed that, in this particular house, there is a lower risk of 

occupant discomfort during midseason part-load conditions than during conditions closer to 

peak. This conclusion should be taken with caution because the house in this study had no 

internal gains and little time-dependent solar gain due to well-shaded windows. Both of these 

variables are likely to have a significant impact in most houses.

One unexpected result of the method used for operating the low airflow volume ducted 

distribution system to the bedrooms and a single point of distribution delivery into the main 

living space was that the system delivered more energy into the bedrooms than originally 

anticipated. During the system cycles when airflow was provided to the bedrooms, the system 

ran for approximately 40% longer than during the cycles when no airflow was provided to the 

bedroom. This increase in delivered energy should be considered when looking at the results for 

the low airflow system. Although this system yielded sufficient performance in many cases, the 

methods used to create a low average flow rate—operating the bedroom registers at the standard 

airflow for a reduced period of time—introduced variations to the data significant enough such 

that the results from this study should not be used as a generalization of the performance of low 

airflow systems. The method of operation likely exaggerated the swings in temperature with 

respect to the ASHRAE 55-2010 analysis (ASHRAE 2010a) and created the opportunity for 
22

more significant excursions outside the acceptable ACCA Manual RS values (Rutkowski 1997). 

This made the performance of the system appear worse than that of what would likely be 

installed in the field—a low airflow system that delivered a small amount of airflow every 

system cycle. 

The transfer grilles used on the no ducted distribution system appeared to be ineffective at 

enabling heat transfer with the bedrooms via natural convection. To help verify this conclusion,

the doors to the bedrooms were left open for a 2-week period, with the no ducted distribution

system operational. The results showed that the bedrooms were almost the same temperature as 

the hallway and were closer in temperature to the main living space than with the doors closed. 

However, there remained a significant temperature difference between the bedrooms and the 

main living space because the cross-sectional area of the hallway acted as a bottleneck to the 

amount of airflow that could occur via natural convection. Factors that could have influenced the 

effectiveness of this strategy include the cross-sectional area, shape, and length of the hallway, 

the placement of the single point of conditioned air supply to the house, and its horizontal throw

parameters. The transfer grilles themselves enabled less airflow than previously anticipated due 

to several possible factors, including the sound-deadening material used inside them and the 

angle and shape of the fins on the grilles. To determine the energy contribution of the transfer 

grilles to the bedrooms, a CFD analysis was performed and is documented in this report in 

Section 5.

The lack of a clear and obvious association of failures with specific outdoor conditions indicates 

that a time series analysis should be considered in future research. For example, if the most 

extreme temperature deviations occurred after a very cold night followed by a mild day, there 

may be a delay in the load reaching a room.
23 

5 Calibration of a Computational Fluid Dynamics Model for 

Transfer Grille Airflow

To determine the expected flow rate through the over-door and bottom-of-door transfer grilles

used in the air distribution strategy with no ductwork to the bedrooms (distribution system 3) at a 

given temperature difference between the spaces on either side of the transfer grilles, the 

IBACOS research team created a CFD model. The team then used the field test results to 

calibrate this model. 

5.1 Computational Fluid Dynamics Model Description

The IBACOS team created a finite element CFD model using ANSYS CFX (ANSYS). The 

purpose of the model was to determine the expected flow rates through the over-door and 

bottom-of-door transfer grilles at a given temperature difference between the hallway and 

bedroom. The CFD model consists of a single fluid material (air) encompassing the volume of 

the hallway, hallway bathroom (henceforth referred to as “bathroom”), and northeast bedroom 

(henceforth referred to as “bedroom”) of the Fresno retrofit unoccupied test house (see Figure 7). 

The boundary conditions are represented in the figure as follows: green lines are the exterior 

walls, gray lines are the interior walls, red lines are the windows, and the curved line is a curved 

air boundary that allows air to enter and leave the model from the remainder of the house and is 

where temperature data from the living room space are input as values. 

Figure 7. Plan view of the CFD model, showing the upper and 

lower transfer grilles and fan locations 

Bathroom

Bedroom

Hallway

Bedroom Window

Bathroom

Window

Aspirated 

Thermocouple at 

43” from floor

Closet

Closet doorway 

(open)

Bathroom 

Doorway (Open)

Transfer Grilles

Curved Air 

Boundary 
24

The model is of only the air; the interior and exterior walls and floor are not included in the 

model but instead are represented as boundary conditions. The interior partition walls and floor 

assembly were assumed to be adiabatic in the model. The exterior wall, ceiling, and window 

boundaries were treated with convection boundary conditions, based on specified R-values

(Table 22) and outside air temperatures from monitored data. A curved-face, free-flowing air 

boundary layer was established at the end of the hallway where it meets the main living space. 

This type of boundary layer enables convective heat transfer across it while ensuring that the 

amount of air in the model remains constant. This boundary condition used inputs of temperature 

data measured in the living room. The mesh was primarily tetrahedral, but hexahedrals, prisms, 

and pyramids also were used when appropriate. It contains 61,151 nodes and 201,663 elements. 

The residuals value of the mesh was less than 2 × 10-5. Mesh size was the smallest at the transfer 

grille locations (Figure 8) and served as a limit for a computational time step, which was 2 s.

Table 22. Exterior Boundary R-Values and Temperatures

Assembly R-Value

Exterior Wall 13

Ceiling 60

Window 3

Interior Wall Adiabatic

Floor Adiabatic

Figure 8. Close-up of the CFD model mesh, showing the upper transfer grille (pink) 

connecting the bedroom (light blue) and hallway (tan)
25

5.2 Methods

To refine the model based on measured data, average temperatures were taken for the three air 

spaces: the hallway, the bathroom, and the bedroom. Point temperature measurements also were 

taken at the locations coinciding with the measurements used in the actual unoccupied test house. 

These locations included the center of the bedroom at 43 in. from the floor, the center of the 

bathroom at 43 in. from the floor, and the hallway at 43 in. from the floor. In the actual test 

house, each of these measurements was shielded from radiation and was mechanically aspirated. 

The model also had a temperature data point at the geometric center of each transfer grille to 

coincide with the unshielded non-aspirated measurements used in the actual house. Data

collected on December 2, 2011, for the 12-h period between 12:00 a.m. and 12:00 p.m. were 

used for all iterations of the model.

Three iterations of the model were performed to assess the influence of two factors: the aspirated 

thermocouple fan and the transfer grille pressure drop versus flow coefficient. The temperatures

at the center of the room, the upper transfer grille, and the lower transfer grille were used as the 

primary factors for comparison between measured results and modeled performance for each 

iteration (also referred to as a case). Case 1 was the base case model. In Case 2, to assess the 

impact of the thermocouple aspiration fan used in the built test house, the research team 

incorporated into the CFD model an assembly consisting of the radiation shielding tubes and a 

7-CFM fan pointing toward the ceiling, and then the team recalculated the results with all other

input data, the same as those used in the first run. In Case 3, to assess the impact of the 

assumed—based on a curve fit to a data point for each transfer grille obtained from the 

manufacturer—pressure versus flow coefficients, the team evaluated the transfer grilles in a 

separate CFD model to determine a more accurate pressure drop to flow rate coefficient. The 

research team then input this value into the main model, which included the fan assembly used in 

Case 2. In all cases, the interior walls and the floor were assumed to be adiabatic, while the 

exterior walls, windows, and the ceiling were determined to have the R-values shown in 

Table 22.

5.3 Results

Three iterations of the model were performed. Their resulting bedroom air temperatures are 

shown in Figure 9. The bedroom air temperature for the second iteration (Case 2)—which 

included the aspiration fan—more closely follows measured data than the first iteration. The 

third iteration (Case 3) differs most from the measured data.
26

Figure 9. Plot comparing temperatures of the three runs

The room air temperature results also show a difference in the two different methods of 

determining air temperature in the model. The first is represented in Figure 9 by labels in the 

legend that do not contain the number 43. These values are determined in the model using the 

volume averaged air temperature of the room. The labels that do contain the number 43 are 

values that used a single measurement point in the model meant to emulate the actual sensor 

location used in the test house. Case 1 shows a larger difference between the two methods than 

Case 2 or Case 3 shows. Case 2 shows that the single point of measurement is generally slightly 

warmer than the volume weighted average, whereas Case 1 and Case 3 are cooler. The RMS 

error for the two measurement types shows that the volume weighted average is generally closer 

in temperature to the measured data, as shown in Table 23. This table also shows that the RMS 

error for the volume average room air temperature improved 39% from 1.95°F for Case 1 to 

1.18°F for Case 2. A relatively high RMS error remained for the lower transfer grille at 5.58°F 

(Table 23). This value increased to 7.13°F when the updated flow to pressure coefficient was 

input into the model in Case 3. 

54

55

56

57

58

59

60

61

62

63

64

65

12:00 AM 1:00 AM 2:00 AM 3:00 AM 4:00 AM 5:00 AM 6:00 AM 7:00 AM 8:00 AM 9:00 AM 10:00 AM 11:00 AM 12:00 PM

Temperature (F)

Time

Case 1: Tbed

Case 1: Tbed43

Case 2: Tbed

Case 2: Tbed43

Case 3: Tbed

Case 3: Tbed43

Measured: Tbed43
27

Table 23. RMS Error Values for Modeled Temperatures at Each Location

Location RMS Error (°F)

Case 1 Case 2 Case 3

Bedroom (Volume Average) 1.946 1.182 4.328

Bedroom (at 43 In. from Floor) 2.511 1.260 4.505

Hallway (Volume Average) 1.513 0.786 1.457

Hallway (at 43 In. from Floor) 0.933 1.237 0.916

Bathroom (Volume Average) 0.394 0.982 0.335

Bathroom (at 43 In. from Floor) 0.511 1.987 0.542

Lower Transfer Grille 5.415 5.583 7.130

Upper Transfer Grille 0.554 1.222 0.553

The change in the flow to pressure difference coefficient reduced the modeled airflow into the 

room between Case 2 and Case 3 by 39%. Average airflow in Case 2 was approximately 

30 CFM at an average temperature difference between the hallway and bedroom of 6.2°F.

Average airflow in Case 3 was approximately 22 CFM at an average temperature difference 

between the hallway and bedroom of 9.6°F. In each of the two cases, the rate of airflow followed 

the same trend and remained consistent over the 12-h period, varying by approximately 10%.

Based on these flow rates and temperature differences, the transfer grilles in Case 2 provided a 

continuous 270 Btu/h of heat to the room; in Case 3, they provided 228 Btu/h.

5.4 Discussion

The results from the model are telling, in that the amount of energy transported is low, regardless 

if the flow rate is 30 CFM or 22 CFM. The flow rates also occur at room-to-hallway temperature 

differences that are outside the bounds of ACCA Manual RS (Rutkowski 1997) heating 

requirements. These temperature differences also are 3.4 to 5.3 times greater than the 1.8°F that 

Feist et al. (2005) used to observe between 300 Btu/h and 600 Btu/h of convective heat transfer 

through an open door. The amount of heat transfer occurring through the transfer grilles via a

1.8°F temperature difference—a temperature difference that also would meet the requirements of 

ACCA Manual RS—would be correspondingly lower. Thus, this strategy is not viable in 

buildings where ACCA Manual RS guidelines must be met. Given the low energy transfer rates 

in the model and the test house, it is not surprising that the aspiration fan had a significant impact 

on the output of the CFD model. Figure 10 visualizes the whole-room air mixing induced by the 

7-CFM fan (i.e., the size of fan typically used in a laptop computer).
28

Figure 10. Vertical section view of the bedroom through the fan centerline, 

showing fan-influenced air movement (scale is air temperature in Kelvin)

Although the Case 2 modeled room air temperature tracks the closest to measured data, the 

observed RMS values of 1.18°F and the fluctuations observed in Figure 9 indicate an issue 

remains with some aspect of the model. One possible explanation for the fluctuations may be due 

to the lack of thermal mass in the model that is in the test house, such as drywall and the concrete 

floor. Furthermore, that the ostensibly “more accurate” CFD determined pressure versus flow 

ratio used in Case 3 resulted in a bedroom temperature that was significantly colder than 

measured could indicate that the CFD results are inaccurate. On the other hand, it could indicate 

that other factors, such as conduction through the interior partition walls, are significant. The 

impact of conduction through interior partition walls and thermal mass of the structure should be 

assessed before using this model elsewhere. 
29

6 Conclusions

The first two research questions answered by the report are as follows:

• To what extent do alternative space conditioning distribution strategies meet ACCA and 

ASHRAE guidelines for room-to-room temperature uniformity and stability, 

respectively? Under what weather conditions do indoor temperatures rise above or fall 

below the guidelines?

• How substantially do the alternative strategies differ from typical ducted forced-air

systems in their ability to meet ACCA and ASHRAE guidelines? 

As shown in Section 3.1, failures according to ASHRAE Standard 55-2010 Section 5.2.5 

(Temperature Variations with Time) (ASHRAE 2010a) for all three systems tended to be cyclic 

in nature. Failures due to drifts or ramps greater than 15 min were rare in all cases. All systems 

had a greater than 85% failure rate in the living room. This was due, in part, to the sensor 

location and the amount of energy delivered to the space. The no ducted distribution system was 

most successful in the bedrooms; because no conditioned air was supplied directly to them, they 

showed no cyclic failure. The typical airflow system was most successful in the kitchen, hallway, 

and hallway bathroom. The performance of the low airflow system ranked between the 

performance of the typical airflow system and the no ducted distribution system for these rooms 

but had the best performance of the three systems in the living room.

As shown in Section 3.2, the typical airflow system was the best at meeting ACCA Manual RS 

guidelines (Rutkowski 1997) for room temperature variation from the thermostat set point, with 

only the living room showing failures in cooling mode and with all rooms showing the fewest 

and least extreme failures in heating mode. The no ducted distribution system performed the 

worst, with a high percentage of failures in the non-actively conditioned bedrooms. In the 

bedrooms, this system failed approximately 50% of the time in cooling mode and almost 100% 

of the time in heating mode. By contrast, the typical airflow system never failed in cooling mode 

and failed approximately 10% of the time in heating mode.

The typical airflow system was the most effective at meeting the ACCA Manual RS guidelines 

(Rutkowski 1997) in the bedrooms, although it did this at the expense of meeting the ASHRAE 

Standard 55-2010 Section 5.2.5 (Temperature Variations with Time) requirements (ASHRAE 

2010a). It is notable that the system had difficulty in meeting both guidelines in the room that 

contained the thermostat—the living room. Based on the system’s failure to meet ACCA 

Manual RS guidelines (Rutkowski 1997), the transfer grilles used with the no ducted distribution

system appeared to be ineffective at enabling heat transfer with the bedrooms via natural 

convection. Despite this, the lack of any failure in the non-actively conditioned bedrooms via 

drifts, ramps, or cycles during the operation of this system indicates the success of the thermal

enclosure at mitigating the effects of external loads placed on the house. 

The third research question answered by the report is as follows:

• Where applicable, in what ways and by how much does the measured performance of the 

distribution system differ from the computer-modeled performance?

The measured performance differs significantly from the modeled performance, with a bedroom 

RMS error ranging from 1.18°F to 4.51°F, depending on the iteration of the model and the 
30

measurement type as shown in Section 5.3 in this report. To reliably use this model, the RMS 

error should be approximately 0.5°F to allow confident application of the ACCA Manual RS 

(Rutkowski 1997) and ASHRAE Standard 55-2010 (ASHRAE 2010a) criteria to the model to 

use for predicting the performance of over-door and bottom-of-door transfer grilles in 

conjunction with a single point space conditioning system.

Cutting-edge builders are installing single point space conditioning systems with no means of 

providing conditioned air to bedrooms except via open doors. This report offers insight into a 

strategy for providing conditioned air to those rooms when the bedroom doors are closed. The 

report also provides heretofore unmeasured data for the range and frequency of potential thermal 

discomfort that occupants may experience when using this strategy. The results from this study 

of a single-story, three-bedroom house in Fresno, California, should not be generalized for other 

house types and climates; instead, it is information for the research community to use as it works 

with builders toward the end goal of being able to discuss space conditioning options with their 

clients to determine the level of potential discomfort the occupants are willing to accept to have a 

cost-optimized, cutting-edge, energy efficient house.
31

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