Imagine Homes New 

Construction Occupied 

Test House 

Dave Stecher, Ari Rapport, and 

Katherine Allison

IBACOS, Inc.

July 2013
NOTICE

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iii

Imagine Homes New Construction Occupied 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, Ari Rapport, and Katherine Allison

IBACOS, Inc.

2214 Liberty Avenue 

Pittsburgh, PA 15222 

NREL Technical Monitor: Michael Gestwick

Task Order KNDJ-0-40341-02 

July 2013 
iv

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v 

Contents

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

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

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

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

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

2 Research Questions............................................................................................................................. 3

3 Experimental Methods ......................................................................................................................... 4

4 Results................................................................................................................................................... 9

4.1 HVAC System Evaluation ...................................................................................................9

4.2 Hot Water System Evaluation ............................................................................................13

4.3 Whole-House Energy Use and Generation ........................................................................17

4.4 Comparison of Modeled to Monitored Source Energy Use...............................................20

5 Discussion........................................................................................................................................... 25

6 Conclusions ........................................................................................................................................ 28

References................................................................................................................................................. 29

Appendix A: Imagine Homes Floor Plans............................................................................................... 30

Appendix B: Photographs of Sensor Locations from the Imagine Homes House from December 

2010...................................................................................................................................................... 32
vi 

List of Figures

Figure 1. Front elevation of the test house .............................................................................................. 1

Figure 2. Outdoor temperature and RH sensor mounted far enough from the home so it will 

not be influenced by it ......................................................................................................................... 5

Figure 3. Gas meter used to determine natural gas usage by the furnace........................................... 5

Figure 4. Temperature and RH sensor at the first-floor thermostat location ....................................... 6

Figure 5. First-floor plan with sensor locations noted............................................................................ 7

Figure 6. Second-floor plan with sensor locations noted....................................................................... 8

Figure 7. Temperature, RH, and HVAC system runtime values during the representative hot 

period of August 27, 2011, through August 31, 2011...................................................................... 10

Figure 8. Temperatures and HVAC system energy use during the representative hot period 

of August 27, 2011, through August 31, 2011.................................................................................. 11

Figure 9. Temperature, RH, and HVAC system runtime values during the representative cold 

period of February 1, 2011, through February 5, 2011 ................................................................... 12

Figure 10. Temperature, RH, and HVAC system runtime values during the representative 

nonpeak period of October 17, 2011, through October 21, 2011................................................... 13

Figure 11. Monthly DHW and electricity usage from October 1, 2010, to September 30, 2011......... 14

Figure 12. Monthly incident solar radiation and DHW electricity usage from October 1, 2010, 

to September 30, 2011........................................................................................................................ 15

Figure 13. Monthly outdoor temperature and incident solar radiation experienced by the 

solar water heater from October 1, 2010, to September 30, 2011.................................................. 16

Figure 14. Monthly electricity consumption and production and natural gas consumed from 

October 1, 2010, to September 30, 2011........................................................................................... 17

Figure 15. Monthly electricity production by PV and surplus electricity sent to the electricity 

grid from October 1, 2010, to September 30, 2011.......................................................................... 18

Figure 16. Monthly source energy consumption or production according to measured area 

from October 1, 2010, to September 30, 2011.................................................................................. 19

Figure 17. End-use source energy consumption (MBtu/yr) and percentage of total during 

the 1-yr analysis period from October 1, 2010, to September 30, 2011 ....................................... 20

Figure 18. Monitored electricity generation compared to modeled electricity generation on a 

monthly basis from October 1, 2010, to September 30, 2011 ........................................................ 23

Figure 19. Electricity produced versus electricity sent to the grid, and percent of produced 

electricity used in the house from October 1, 2010, to September 30, 2011................................ 24

Figure 20. First-floor plan (numbers on drawing refer to photos in Appendix B).............................. 30

Figure 21. Second-floor plan (numbers on drawing refer to photos in Appendix B); #3 and 

#4 are on the roof and in the attic, respectively.............................................................................. 31

Figure 22. Location #1; first-floor temperature and RH measurement at the thermostat 

location................................................................................................................................................ 32

Figure 23. Location #2; second-floor temperature and RH measurement at the thermostat 

location................................................................................................................................................ 32

Figure 24. Location #3; incident solar radiation sensor on the roof in plane with solar DHW 

and PV panels ..................................................................................................................................... 33

Figure 25. Location #4; furnace measurements .................................................................................... 33

Figure 26. Location #5; outdoor temperature and humidity measurement ........................................ 34

Figure 27. Location #6; flow and temperature measurements at the water heater location............. 34

Figure 28. Location #7; electrical measurements at the outdoor electrical panel ............................. 35

Figure 29. Location #8; wires at the data logger location near the indoor electrical panel .............. 35

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

List of Tables 

Table 1. Technical Specifications for the San Antonio, Texas, Test House and Standard House..... 2

Table 2. Monitored Source Energy Use Compared to Modeled Source Energy Use......................... 21

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

Definitions

ACH Air changes per hour

AFUE Annual fuel utilization efficiency

AHU Air handling unit

AMY Actual Meteorological Year

BA Building America

BEopt Building Energy Optimization (software)

CFIS Central fan integrated supply

CFM Cubic feet per minute

DHW Domestic hot water

EF Energy factor

HSP House Simulation Protocols

HVAC Heating, ventilation, and air conditioning

LAMEL Lighting, appliances, and miscellaneous electric load

MEL Miscellaneous electric load

PEX Cross-linked polyethylene

PV Photovoltaic

RH Relative humidity

SEER Seasonal energy efficiency ratio

SHGC Solar heat gain coefficient

TMY Typical Meteorological Year

XPS Extruded polystyrene
ix

Executive Summary

This report discusses IBACOS’ long-term monitoring plan completed to evaluate the 

performance of an energy-efficient home constructed in San Antonio, Texas, by builder-partner 

Imagine Homes in 2010. The home achieves a source energy savings of 32% without the 

installed photovoltaic (PV) system and 44% savings with the PV system contribution relative to 

the Building America (BA) House Simulation Protocols (HSP) (Hendron and Engebrecht 2010). 

Monitoring of the energy use, energy generation, and temperature conditions related to this high 

performance residential project took place between July 2010 and October 2011. This report 

summarizes the research findings related to heating, ventilation, and air conditioning (HVAC) 

system performance, estimated and actual energy use of key subsystems, electricity generation 

by the PV system, and performance of the solar thermal domestic hot water (DHW) system.

Changes in the insulation strategy that eliminated the use of spray foam in wall cavities raised 

concerns from the builder about occupant comfort complaints due to increased air leakage and 

necessitated additional air sealing measures to alleviate some of these concerns. Despite the 

builder’s concerns about occupant comfort, measured air leakage was below the specified target.

Furthermore, the HVAC system effectively maintained acceptable temperatures and humidity 

levels inside the house during peak cooling and heating and nonpeak operating periods. As a 

result, occupants are likely to be comfortable in the home year round (ASHRAE 2008). System 

runtime closely tracked outdoor temperature fluctuations during peak and nonpeak periods, 

indicating acceptable sizing of the system to design loads. 

There were discrepancies between modeled and actual energy use values. After adjusting the 

model for actual weather conditions and indoor set points, actual space cooling energy use was 

the closest of all end uses to the corresponding modeled value. The energy consumption 

predicted for lighting, appliances, and miscellaneous electrical loads (LAMELs) was 

significantly higher than the measured amounts, and the energy consumption predicted for 

heating was significantly higher than the measured amounts. 

The model predicted lower energy consumption for the solar thermal DHW system than was 

actually measured; however, despite performing less efficiently than predicted by the model, the 

solar thermal system was the main source of hot water for most of the year, and it performed 

well in that function. During the summer months, electricity use by the backup system was rare. 

Energy modeling portrayed the performance of the PV system much more favorably than 

determined by measurements; based on modeling, the PV system was predicted to generate more 

energy and to offset more energy consumption than it actually did. One issue discovered was 

that, due to the usage characteristics of the house, the PV system was directly used by the house 

only 46.8% of the time. This time of production versus time of demand issue was not 

incorporated into the models used in the design of this house (e.g., Building Energy Optimization 

[BEoptTM] software) but would indicate that a greater capacity PV system is not warranted unless 

electrical utilities offer generous feed-in-tariff rates.
1 

1 Introduction and Background

Imagine Homes constructed a test house in San Antonio, Texas, under BA activities with the 

intention of achieving a source energy savings of 50% relative to the BA HSP (Hendron and 

Engebrecht 2010). The test house, shown in Figure 1, faces predominantly east, has a slab-ongrade foundation, and has 3,856 ft2

 of living area on two stories with five bedrooms. In 

collaboration with IBACOS, the builder selected the specifications for the test house on the basis 

of improved energy performance and cost effectiveness. In the case of the solar thermal water 

heater, the system selection was based on the product currently being installed by the plumber 

that included electric resistance as backup to the solar heat exchanger system. The HVAC 

contractor sized and selected the HVAC system based on the results of Air Conditioning 

Contractors of America Manual J load calculations (Rutkowski 2006). The as-built house 

utilized 2 × 6, 24-in. on center wall construction with stacked framing, three-stud corners, and 

ladder blocking at exterior walls, exterior foam sheathing, a cathedralized attic assembly with 

spray foam insulation underneath the roof deck, a high efficiency mechanical system, a solar 

thermal hot water system, and a 2-kW PV system. The house was predicted to achieve a 32% 

level of source energy savings without the PV system and 44% savings with the PV system 

contribution, relative to the BA HSP. Appendix A shows the house floor plans. 

Figure 1. Front elevation of the test house

This home achieved certification under the Build San Antonio Green and ENERGY STAR®

programs. It also was awarded the 2011 Energy Value Housing Award Silver Award for a 

production home and the National Association of Home Builders National Green Building 

Award for single-family production home Green Project of the Year. 

Table 1 shows the general building specifications for the test house and for the builder’s standard 

product. 
2 

Table 1. Technical Specifications for the San Antonio, Texas, Test House and Standard House

Component Test House Specification Standard House Specification

Concrete Slab Slab on grade, uninsulated Slab on grade, uninsulated

Exterior Walls

2 × 6 framing, 24 in. on center, with 

sprayed-in cellulose (R-20) and 1⁄2-in. 

XPS insulating sheathing (R-3), for 

R-23 nominal thermal performance

2 × 4 framing, 16 in. on center, with 

low-density spray foam (R-13), for 

R-19 nominal thermal performance 

Roof Open-cell spray foam on roof deck,

R-19 nominal thermal performance

Open-cell spray foam on roof deck, 

R-19 nominal thermal performance

Exterior Doors U-value = 0.2 U-value = 0.2

Windows U-value = 0.35, SHGC = 0.22 U-value = 0.36, SHGC = 0.29

Building 

Airtightness 1.3 ACH @ 50 Pascals (Pa) tested 2.58 ACH @ 50 Pa, average

Mechanical 

Ventilation

CFIS, AirCycler, 50% runtime, 

40 CFM, filter and damper 

CFIS, AirCycler, 50% runtime, 

40 CFM, filter and damper 

Heating 93.7% AFUE condensing furnace 92% AFUE furnace

Cooling 17 SEER condensing unit 15 SEER unit

Ductwork In conditioned space, R-6 insulation; 

no duct leakage to outside

In conditioned space, R-6 insulation; 

no duct leakage to outside

Water Heater

115-gal electric backup tank, 

0.84 EF, with 55-ft2 collector solar 

thermal system

0.82 EF tankless, natural gas 

DHW Distribution PEX “homerun” manifold system, 

no pipe insulation

PEX “homerun” manifold system, 

no pipe insulation

Appliances Gas range, all ENERGY STAR Gas range, all ENERGY STAR

Fluorescent 

Lighting 100% fluorescent lighting 80% fluorescent lighting 

PV System 2.0-kW PV array, grid-tied PV not used

% Better than BA 

HSP (Hendron and 

Engebrecht 2010)

32% (44% with PV) 14% 

ACH is air changes per hour; AFUE is annual fuel utilization efficiency; CFIS is central fan integrated supply; CFM 

is cubic feet per minute; EF is energy factor; PEX is cross-linked polyethylene; SEER is seasonal energy efficiency 

ratio; SHGC is solar heat gain coefficient; XPS is extruded polystyrene. 
3 

2 Research Questions

The test house served as an excellent opportunity for IBACOS to capture energy consumption 

information and to measure the performance of systems unique to the project. Short-term testing

and long-term monitoring were conducted from July 2010 through October 2011 to answer the 

following research questions: 

• How did the total actual energy use and the actual energy use of key subsystems (e.g., 

furnace, water heater, central air conditioning compressor, air handling unit [AHU],

LAMELs) under occupied conditions compare to BEopt energy simulations when actual 

weather and operating conditions were used? Were differences due to weather, occupant 

behavior, modeling errors, or system performance issues?

• In what circumstances was the solar thermal DHW system used? How did energy 

consumption of the solar thermal DHW system vary during different times of the year? 

Were the delivered temperature and the capacity of DHW adequate? 

• How effective was the HVAC system in maintaining acceptable temperatures and 

humidity levels during peak and nonpeak operating periods? Does the equipment appear 

to be properly sized? Would occupants likely be comfortable in the home? 

Although this was not specifically a research question, the builder was interested in the 

effectiveness of an alternative air sealing strategy to mitigate occupant comfort issues due to air 

leakage. The builder’s standard practice of insulating wall cavities with low-density spray foam 

helped to consistently achieve air leakage rates below 3.0 ACH50, and the builder had high 

confidence in the performance of this air sealing system. The test house did not include spray 

foam as exterior wall insulation, but spray foam was used to seal around exterior penetrations, to 

insulate the band joist, and in other key air leakage locations (such as dormer transitions at 

exterior walls and hard-to-reach framing details). As an additional measure of air sealing, 

drywall was glued to the framing at the perimeter of the sheets (including around windows and 

doors), and 1⁄2 in. of XPS foam sheathing was installed on the exterior walls with the seams 

taped. 
4 

3 Experimental Methods

To answer the research questions, the research team collected measurements of energy use at key 

locations within the test house. Data were collected using remote sensing devices installed during 

construction and commissioned at the completion of the home. 

To observe the performance of the test house in every season, IBACOS conducted long-term 

monitoring for more than one year—July 2010 through October 2011. Due to later startup of the 

solar thermal and PV systems, full data collection began in early September 2010. The research 

team measured temperature and relative humidity (RH) levels at the thermostat location on each 

floor and outdoors using thermocouples and RH sensors. Electrical power for the whole house, 

the outdoor condensing unit, the furnace fan, and the water heater (including pump) was 

measured using pulse output watt-hour meters and current transducers. 

IBACOS used a gas meter to measure natural gas consumption for the furnace; the team did not 

measure gas usage for the whole house. Although the water heater was a solar thermal system 

with electrical backup, two other devices were using natural gas in the house: the fireplace and 

range. The gas consumption of the fireplace was extrapolated from the difference between 

measured furnace gas usage and modeled predictions of heating gas usage using actual 

meteorological year (AMY) weather files and measured indoor set point temperatures. The gas 

consumption of the range was not measured, but its contribution was determined according to the 

BA HSP (Hendron and Engebrecht 2010) and was included as 4.2 MBtu/yr. For the purpose of 

this research, the separate monitoring of the range and fireplace did not justify the additional 

expense and complexity of another meter. Measuring appliance gas usage is currently a gap 

identified in the working group of the BA Standing Technical Committee on Test Methods and 

Protocols.1

Water heater thermal energy output was measured using thermocouples and a high resolution 

flow meter (75.7 pulses/gal). This was accomplished by measuring the temperature of the hot 

water supply and the temperature and volume flow rate at the cold water return. All sensors were 

connected to a central data logger with a 10-s scan rate and data storage every minute or hour. 

IBACOS retrieved data by using a cellular modem connected to the logger. A family of five 

occupied the home during the monitoring period. Figure 2 through Figure 4 are representative 

images of the measurement devices installed in the test house. Sensor locations throughout the 

home can be seen on the test house floor plans shown in Figure 5 and Figure 6. 

 

1 https://sites.google.com/site/buildingamericastc/. 
5 

Figure 2. Outdoor temperature and RH sensor mounted far enough from the home 

so it will not be influenced by it

Figure 3. Gas meter used to determine natural gas usage by the furnace
6 

Figure 4. Temperature and RH sensor at the first-floor thermostat location
7 

Figure 5. First-floor plan with sensor locations noted
8 

Figure 6. Second-floor plan with sensor locations noted
9 

4 Results 

IBACOS collected data from the test home from mid-July 2010 through October 2011. The 

team’s analysis of data determined that the 12-month period from October 1, 2010, to 

September 30, 2011, provided consistent data from all sources. Consequently, the data were used 

to represent the annual measurement period.

4.1 HVAC System Evaluation

The HVAC system in the home consisted of a gas condensing furnace with variable-speed 

operation manufactured by Carrier, with the Infinity 96 model with a minimum output capacity 

of 73 kBtu/h used and a maximum output capacity of 112 kBtu/h used, and an AFUE rating of 

93.7. Cooling was provided by a condensing unit manufactured by Carrier, with the Performance 

model with a maximum output capacity of 36.8 kBtu/h used and a SEER rating of 17.00. Both 

systems were sized and selected by the HVAC contractor based on the results of Air 

Conditioning Contractors of America Manual J load calculations (Rutkowski 2006). 

Indoor and outdoor temperature and RH data were analyzed at peak and nonpeak (shoulder) 

temperature periods to determine how the HVAC system performed in those cases. On 

August 28, 2011, the outdoor temperature reached its peak at 109°F. Figure 7 shows the 

temperature and RH conditions inside the home and the HVAC system runtimes during the 4-day 

period surrounding this peak temperature. The runtime of the air conditioner outdoor unit 

compressor and fan and the AHU are shown as a fraction of an hour, with a value of 1 indicating 

continuous operation during the hourly data period. 

Note that the HVAC system has two-stage cooling and variable-speed operation, and, as shown 

in Figure 8, its energy consumption varied substantially during a runtime period as it was 

operating at different airflows and system capacities to match the load on the home. Specifically, 

from 9:00 to 13:00 on August 27, 2011, the system was operating in low-stage cooling mode, 

whereas from 13:00 to 21:00 on the same day, the system was operating in high-stage cooling 

mode. Similar behavior occurred in the following days as the system matched its capacity with 

the lower loads on the home in the morning and the higher loads in the afternoon and evening. 
10 

Figure 7. Temperature, RH, and HVAC system runtime values during the representative 

hot period of August 27, 2011, through August 31, 2011 

0

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8/29/2011 0:00

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8/30/2011 0:00

8/30/2011 12:00

8/31/2011 0:00

Outdoor 

Temperature

First-Floor 

Temperature

Second-Floor 

Temperature

Outdoor RH

First-Floor RH

Second-Floor RH

CU Runtime

AHU Runtime

Temperature (°F) and Relative Humidity (%)

Runtime (Hour)
11 

Figure 8. Temperatures and HVAC system energy use during the representative 

hot period of August 27, 2011, through August 31, 2011

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8/27/11 12:008/27/11 15:008/27/11 18:008/27/11 21:00

8/28/11 0:008/28/11 3:008/28/11 6:008/28/11 9:00

8/28/11 12:008/28/11 15:008/28/11 18:008/28/11 21:00

8/29/11 0:008/29/11 3:008/29/11 6:008/29/11 9:00

8/29/11 12:008/29/11 15:008/29/11 18:008/29/11 21:00

8/30/11 0:008/30/11 3:008/30/11 6:008/30/11 9:00

8/30/11 12:008/30/11 15:008/30/11 18:008/30/11 21:00

8/31/11 0:00

Temperature (F)

AC Outdoor Unit 

Electricity

AHU Electricity

Outdoor 

Temperature

First-Floor 

Temperature

Electricty (kWh) 
12 

On February 2, 2011, the outdoor temperature reached a low of 18.5°F. Figure 9 shows the 

temperature and RH conditions inside the home and the HVAC system runtimes during the 4-day 

period surrounding this peak temperature. During this period, the furnace operated in first-stage 

heating mode (it is a dual-stage unit) with an AHU runtime average of 26.3% each hour. 

Figure 9. Temperature, RH, and HVAC system runtime values during the representative 

cold period of February 1, 2011, through February 5, 2011 

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2/3/2011 0:00

2/3/2011 12:00

2/4/2011 0:00

2/4/2011 12:00

2/5/2011 0:00

Outdoor 

Temperature

First-Floor 

Temperature

Second-Floor 

Temperature

Outdoor RH

First-Floor RH

Second-Floor RH

CU Runtime

AHU Runtime

Temperature (°F) and Relative Humidity (%)

Runtime (Hour)
13 

The average outdoor temperature experienced during the annual analysis period was 72.1°F. On 

October 18, 2011, this temperature was observed. Figure 10 shows the temperature and RH 

conditions in the home and HVAC system runtimes during the 4-day period surrounding this 

nonpeak temperature. During this period, the AHU was operating in cooling mode, first-stage, 

and the runtime average was 18.5% each hour. 

Figure 10. Temperature, RH, and HVAC system runtime values during the representative 

nonpeak period of October 17, 2011, through October 21, 2011

4.2 Hot Water System Evaluation 

The DHW system consisted of a 115-gal electric backup tank, 0.85 EF, with 55 ft2

 of solar 

thermal collector (two panels). Figure 11 displays the monthly DHW and electricity usage of the 

system for the 1-yr period. Figure 12 shows the average monthly solar radiation incident on the 

solar collectors along with the monthly electricity usage of the system for the 1-yr period. The 

solar collectors are tilted approximately 34° (8/12 pitch) from the horizontal. The system used a 

total of 737 kWh of electricity and 25,987 gal of water during this period (an average of 

71 gal/day). For half of the year, the DHW system used 20.7 kWh or less in electricity per 

month. In August 2011, the solar thermal system handled all hot water demands, with several 

events calling for more than 40 gal of hot water in 1 h. The amount of electricity usage of the 

system directly tracked the available solar radiation incident to the collectors. 

0

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10/17/2011 0:00

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10/19/2011 0:00

10/19/2011 12:00

10/20/2011 0:00

10/20/2011 12:00

10/21/2011 0:00

Outdoor 

Temperature

First-Floor 

Temperature

Second-Floor 

Temperature

Outdoor RH

First-Floor RH

Second-Floor RH

CU Runtime

AHU Runtime

Temperature (°F) and Relative Humidity (%)

Runtime (Hour)
14 

Figure 11. Monthly DHW and electricity usage from October 1, 2010, to September 30, 2011

0.0

20.0

40.0

60.0

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Oct-10 Nov-10 Dec-10 Jan-11 Feb-11 Mar-11 Apr-11 May-11 Jun-11 Jul-11 Aug-11 Sep-11

Hot Water 

Consumption

Water heater 

electricity usage

Gallons DHW usage

kWh Electric Usage
15 

Figure 12. Monthly incident solar radiation and DHW electricity usage from 

October 1, 2010, to September 30, 2011

0

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Oct-10 Nov-10 Dec-10 Jan-11 Feb-11 Mar-11 Apr-11 May-11 Jun-11 Jul-11 Aug-11 Sep-11

Watts per m2, Average Monthly

kWh Electric Usage

Water heater 

electricty usage

Incident Solar 

Radiation
16 

Figure 13 displays, on a monthly average basis, the outdoor temperature and incident solar 

radiation experienced by the solar water heating system. The solar radiation measured is not 

global horizontal, but rather is the solar radiation incident on the PV and solar thermal systems 

that are on the southwestern roof orientation, which has a slope of 8/12 that corresponds to a 

33.75° angle with respect to the horizontal. Incident solar radiation varies seasonally, with an 

anomaly seen in May 2011 that is most likely due to the increased rainfall and cloud cover that 

occurred in that month. 

Figure 13. Monthly outdoor temperature and incident solar radiation experienced 

by the solar water heater from October 1, 2010, to September 30, 2011

The greatest amount of hot water was used during the late evening of May 23, 2011, when 

79.8 gal were used in 1 h and 94.2 gal were used over a 2-h period. The DHW system was able 

to supply water at a temperature between 111°F and 113°F for this event. The reason for this 

peak event is unclear, although presumably the family occupying the house may have hosted 

guests during this time period when additional showers and laundry use may have occurred. In 

addition to this peak event, there are approximately 85 hot water draws of 35 gal or more in 1-h 

time periods that occurred during the monitoring period, with an average supply water 

temperature of 117°F. Most of the remaining water draw events were less than 35 gal (3 gal 

average). Because the system was able to provide sufficient water temperatures during the peak 

event periods, this indicates the ability of the system to provide sufficient hot water. 

70

61

53 50

53

65

74 77

84 86 88

82

108

86

79

68

72

96

100

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107

111

105

0

20

40

60

80

100

120

140

Oct-10 Nov-10 Dec-10 Jan-11 Feb-11 Mar-11 Apr-11 May-11 Jun-11 Jul-11 Aug-11 Sep-11

Outdoor 

Temperature

Incident Solar 

Radiation

Solar Incident Radation (Wh/cm2) and Temperature (oF)
17 

During a 3-h span on the morning of December 26, 2010, the system used the most electrical 

energy, 10.1 kWh, with 71.1 gal of hot water used and the hot water temperature reaching 120°F. 

4.3 Whole-House Energy Use and Generation 

Figure 14 shows, on a monthly basis, the electricity consumption and production and the natural 

gas consumption of the home. During the study period, 10,615 kWh of electricity were used, 

1,633 kWh were produced (15% of total consumption) by the PV system, and 11.3 thousand 

cubic feet or 113 therms of natural gas were used. Monthly electricity consumption and 

production peaked in August 2011. The lowest electricity consumption occurred in November 

2010, and the lowest electricity production occurred in January 2011; January 2011 is also the 

month with the lowest recorded incident solar radiation, which supports the measurements of 

lowest electricity production by the PV system. 

Figure 14. Monthly electricity consumption and production and natural gas consumed from 

October 1, 2010, to September 30, 2011

Figure 15 shows the electricity produced and sent to the electricity grid on a monthly basis. From 

October 2010 through March 2011, more than half of the electricity produced in those months 

was sent to the grid. Of the 1,633 kWh of electricity produced annually by the PV system, 

749 kWh (46.8%) were sent to the electricity grid.

In November 2010, 91.2% of all electricity produced was sent to the grid, which contrasts with 

August 2011, when this value was only 4.9%. An examination of hourly data determined that, on 

0.0

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Oct-10 Nov-10 Dec-10 Jan-11 Feb-11 Mar-11 Apr-11 May-11 Jun-11 Jul-11 Aug-11 Sep-11

Gas (MCF)

Electricity Consumed

Electricity Produced

Natural Gas Consumed

Electricity (kWh)
18 

a typical day in November 2010, electricity was produced during daylight hours but was seldom 

used because the heating system is gas fired, peak heating loads occurred at night, and there were 

few other electrical loads.

Figure 15. Monthly electricity production by PV and surplus electricity sent to the 

electricity grid from October 1, 2010, to September 30, 2011

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Oct-10 Nov-10 Dec-10 Jan-11 Feb-11 Mar-11 Apr-11 May-11 Jun-11 Jul-11 Aug-11 Sep-11

Electricity Produced

Electricity Sent to Grid

Electricity (kWh)
19 

Figure 16 shows the source energy consumed or produced monthly for the 1-yr period. The 

energy used by the furnace (red bar) clearly outlines the heating season, and the energy used by 

the air conditioner outdoor unit/compressor (light blue) outlines the cooling season, which is 

predominant. Energy use from LAMELs is significant; lighting energy is slightly higher in the 

winter months (probably due to a greater number of hours of darkness), and appliance/plug loads 

show great variability (3:1) between cold and warm months. 

Figure 16. Monthly source energy consumption or production according to 

measured area from October 1, 2010, to September 30, 2011

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Oct-10 Nov-10 Dec-10 Jan-11 Feb-11 Mar-11 Apr-11 May-11 Jun-11 Jul-11 Aug-11 Sep-11

Appliance and Plug 

Electricity

Lighting Electricity

Water Heater 

Electricity

Air handler unit 

electricity

AC outdoor unit 

electricity

Furnace gas

PV Generation

Source Energy Consumption or Production (MMbtu)
20 

Figure 17 shows the distribution of source energy according to end use for the 1-yr study period. 

The air conditioning system consumes the most source energy, followed by LAMELs. 

Figure 17. End-use source energy consumption (MBtu/yr) and percentage of total during the 

1-yr analysis period from October 1, 2010, to September 30, 2011

4.4 Comparison of Modeled to Monitored Source Energy Use

The actual measured energy consumption of five major subcategories (heating, cooling, hot 

water, LAMELs, and PV panel generation) was compared to projected energy consumption 

using BEopt version 1.3. Table 2 shows the results. As stated in Section 3 (Experimental 

Methods), the gas consumption of the range and fireplace were not directly measured, but their 

estimated influence is included.

As indicated in Table 2, there were significant differences in predicted versus actual energy 

consumption for several end use categories, particularly for space heating and for 

appliances/miscellaneous electrical loads (MELs). Temperature readings at the first- and secondfloor thermostats are consistent with the 75°F cooling set point used in BEopt, and settings for 

miscellaneous loads (electric, gas, hot water) in the model are based on the HSP for the size and 

presumed occupancy of the house. Actual occupancy of the house is unknown; however, it 

would make sense if actual occupancy were less than the BA HSP predictions because fewer 

occupants would likely result in reduced appliance/MELs energy consumption than predicted by 

the model. At this time, there is no catalog of actual plug-in loads for this test house. 

Air Conditioner, 59, 44%

Air Handler Unit Fan, 7, 5% Hot Water, 8, 6%

Lighting, 12, 9%

Appliances and MELS, 37, 27%

Furnace, 13, 9%
21 

Table 2. Monitored Source Energy Use Compared to Modeled Source Energy Use

Description

Modeled Annual Source Energy Monitored Annual 

Source Energy

BA

HSP* 

Regional 

Standard 

Practice

Builder’s 

Standard 

Practice

Prototype 

Home 

(TMY)

Prototype 

Home 

(AMY)

Prototype 

Home 

(AMY and 

Set Points)

Prototype Home 

Monitored Source 

Energy 

(Sept. 2010–Aug. 2011) 

End Use (MBtu/yr) (MBtu/yr) (MBtu/yr) (MBtu/yr) (MBtu/yr) (MBtu/yr) (MBtu/yr)

Space Heating 43 28 29 22 18 29 14 

Space Cooling 61 76 52 37 49 55 63 

DHW 41 19 12 8 5 5 9 

Lighting 35 35 28 26 26 26 13 

Appliances and 

MELs 84 82 76 76 76 76 37 

Outdoor Air 

Ventilation 3 0 2 2 2 2 6 

Subtotal 267 240 196 170 176 194 141

PV Panel 

Generation 0 0 0 –31 –34 –34 –18 

Total 267 

(250 Adj) 240 196 139 142 160 123 

*Hendron and Engebrecht (2010). TMY is Typical Meteorological Year. 
22 

An AMY weather file for the local weather station at the San Antonio International Airport was 

purchased from Weather Analytics2 and was used in place of the local TMY3 weather file in the 

BEopt energy model. Also, the indoor set point temperatures in the model were adjusted to 

match the measured indoor set point temperatures of 75°F for cooling and 75°F for heating. 

Table 2 includes the results of these predictions. As indicated, there is a large gap in predicted 

versus measured heating energy consumption. Because gas consumption of the fireplace was not 

measured, IBACOS assumed that much of this gap in heating energy use is due to fireplace use. 

This assumption has not been confirmed with the occupants. 

As indicated in Table 2, the predicted cooling energy consumption using the AMY file and 

measured set point temperatures is closely aligned with actual usage, indicating fairly accurate 

prediction by the model. 

Measurements indicated that the PV system produced 1,633 kWh of electricity annually. 

Modeling predicted that the PV system would generate 2,955 kWh of electricity annually. There 

is no clear explanation for this large discrepancy between the modeled prediction and actual 

performance of the PV system, except perhaps for inherent errors in the modeling software 

calculations or operational issues due to installation errors or equipment malfunction. However, 

follow-up conversations with the builder, homeowners, and solar installer did not indicate any 

system malfunction during the monitoring period, and additional modeling using the National 

Renewable Energy Laboratory’s PVWatts (version 1) online performance calculator for gridconnected PV systems (NREL 2012) verifies the predicted PV system production. Figure 18

shows a comparison of the monthly values for modeled and measured electricity generation 

during the study period. Each month the predicted electricity generation was greater than the 

actual amount. Figure 19 shows the electricity produced versus the electricity sent to the grid 

during the study period, and the percent of electricity produced that was used by the house. The 

highest percentage of electricity produced was used by the house in August 2011 at 95%, and the 

lowest percentage of electricity was used by the house in November 2010 at 9%. 

 

2

Weather Analytics, Inc., Winchester, MA, http://www.weatheranalytics.com. 
23 

Figure 18. Monitored electricity generation compared to modeled electricity generation 

on a monthly basis from October 1, 2010, to September 30, 2011

0

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300

350

Oct-10 Nov-10 Dec-10 Jan-11 Feb-11 Mar-11 Apr-11 May-11 Jun-11 Jul-11 Aug-11 Sep-11

Measured Electrical 

Generation

Modeled Electricial 

Generation

Electricity (kWh)
24 

Figure 19. Electricity produced versus electricity sent to the grid, and percent of produced 

electricity used in the house from October 1, 2010, to September 30, 2011

0%

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Oct-10 Nov-10 Dec-10 Jan-11 Feb-11 Mar-11 Apr-11 May-11 Jun-11 Jul-11 Aug-11 Sep-11

Percent of Generated 

Electricity Used in 

House

Measured Electrical 

Generation

Modeled Electricial 

Generation

Electricity (kWh)
25 

5 Discussion 

Prior to moving forward with this test house project, the builder had expressed concern about the 

HVAC system maintaining comfortable temperatures and humidity levels because of the changes 

that were made to the thermal enclosure. With a standard practice of insulating the floors, walls, 

and roof deck with low-density spray foam, the builder had high confidence in achieving a tight, 

well-insulated enclosure and in meeting the comfort needs of the occupants. The only additional 

air sealing performed by the builder as a standard practice was the use of expanding foam or 

caulk to seal around penetrations. Because this test house eliminated the use of spray foam in the 

walls and floors in favor of the lower cost cellulose insulation, the builder was unsure if the 

completed home would be as comfortable for the occupants as the builder’s standard home and 

whether the HVAC system would sufficiently maintain acceptable temperature and humidity 

levels. To allay some of these concerns, the builder performed additional air sealing practices 

such as sealing the bottom plates of the exterior walls to the subfloor, gluing the perimeter of the 

drywall to the wall framing, taping the seams of the 1⁄2 in. of XPS sheathing, and using spray 

foam insulation in several key locations (e.g., at all band joists and at several dormer wall 

locations with difficult access for hand-applied air sealing). To the satisfaction of the builder, 

tested air leakage measured 1.3 ACH50, which is about half of the average leakage of 2.58 

ACH50 for the builder’s standard product. Also, the HVAC system of the test house performed 

efficiently during the hottest and coldest periods of the analysis, with first- and second-floor 

temperatures consistently maintained between 75°F and 76°F and RH maintained at levels no 

greater than 45% during peak heating and 30% during peak cooling. Similar indoor temperature 

and RH values were observed during the nonpeak period. Occupants should have been 

comfortable during all of these periods because the indoor temperatures varied very little and the 

RH levels were within healthful levels (ASHRAE 2008). 

During the peak cooling period, the HVAC system operated at least once during most hours of 

the day; in some cases, it ran for several hours without interruption. There were periods 

overnight when the system ran less frequently as the outdoor temperatures dropped. Considering 

the peak temperature reached 109°F, which is substantially higher than the 0.4% cooling dry 

bulb temperature of 98.5°F (ASHRAE 2009) often used for mechanical system design, the 

system did very well to maintain temperatures at both locations within close range of the set

point. If the system did not have adequate cooling capacity, set point temperatures would not 

have been reached, and there would not have been any system downtimes. During the coldest 

period, the system never ran for a full hour during any hourly measurement period. Temperature 

levels were consistently maintained at both set point levels, indicating the HVAC system had 

ample load capacity. 

A total of 61% of electricity use by the DHW system occurred during the months when heating 

occurred. During the other nine months, the solar water heating system was more active, with 

that system predominantly being used six months of the year. During August 2011, the solar 

thermal system handled all hot water demands. Examination of data indicated that the DHW 

system had enough capacity to meet all hot water demands. 

Electricity consumption in the home was greatest during the cooling season, from May to 

September, when the air conditioning system was operating frequently. Overall, the occupants 
26 

did not use much electricity, considering the size and climatic location of their home, with 

2.75 kWh used per square foot of gross finished floor area (30 kWh/m2). 

The PV system consistently produced energy, with production varying from a high in August 

2011 (a level of 157.4 kWh) to a low in January 2011 (a level of 102.1 kWh). On a monthly 

basis, the PV system did not have the capacity to cover all of the electricity needs of the home. In 

November 2010, on a monthly basis, the PV system supplied 20% of the electricity that was 

used. However, an examination of the amount of electricity sent to the grid reveals that, for

November 2010, only 9% of the electricity produced that month actually was used in the home. 

Utilization of less than half of the PV-generated electricity occurred in the six coldest months of 

the year (when air conditioning was not operating). Every month some generated electricity was 

sent to the grid. Annual electricity generation predicted by modeling was 67% greater than the 

amount of electricity actually produced. Some of this discrepancy could be due to the modeling 

assumptions built into the default input for a 2.0-kW PV system in BEopt, which may be 

different from the actual performance characteristics of the PV system (e.g., panels, inverter). 

Although PV azimuth and tilt were selected in the model to most closely match the actual 

installation of the system on the test house, there might have been some discrepancy in the actual 

insolation achieved on site (e.g., a neighboring house that was constructed midway through the 

monitoring period may have cast a shadow on the installed PV). Follow-up conversations with 

the builder, homeowners, and solar installer did not indicate any system malfunction during the 

monitoring period, and additional modeling using the National Renewable Energy Laboratory’s 

PVWatts (version 1) online performance calculator for grid-connected PV systems (NREL 2012) 

verified the production of the PV system as predicted by BEopt. Future investigation into this 

issue could involve measuring PV production directly from the installed inverter instead of using 

the wattnodes and comparing these measurements with the predictions; this method would need 

to be performed on site and with the permission of the current homeowners. 

Total space conditioning source energy usage measurements were 9 MBtu/yr (11%) lower than 

the space conditioning source energy use predicted by modeling (adjusted for AMY weather 

conditions and actual set point temperatures), with space cooling measurements 8 MBtu/yr 

(15%) higher than modeled and space heating measurements 15 MBtu/yr (52%) lower than 

modeled. As indicated earlier, the fireplace could account for much of the large gap in heating 

energy consumption, although the results of other occupied test house research would suggest 

that variations in occupancy and errors in modeling could account for some of this gap (Stecher 

and Allison 2011, Stecher and Brozyna 2013). 

Measured DHW source energy use was 4 MBtu/yr (80%) higher than the modeled value. The 

model predicted that 5 MBtu/yr of electricity would be used for hot water production, which is 

based on 55 ft2 of solar collectors and an average hot water demand of 80 gal/day (in accordance 

with BA HSP simulation procedures) (Hendron and Engebrecht 2010). This was compared to the 

measured value of 9 MBtu of energy use, which does not include pump energy for the system 

and is based on 55 ft2 of solar collectors and an average hot water demand of 71 gal/day. Again, 

this discrepancy may be due, in part, to differences in the modeled and actual efficiency of the 

solar thermal system (e.g., panels, tank) or to differences in actual insolation versus the modeled 

insolation to the panels. No damage or installation errors were identified by the builder or 

contractor during the installation and operation of this system. 
27 

The month with the highest level of source energy usage was August 2011, which was the month 

with the most space cooling energy usage. Appliances and MELs measured source energy use 

was highest during the months when heating occurred. 

Measured source energy for LAMELs was 52% of the modeled value and was the primary 

reason that the actual source energy usage (without electricity generation contribution) of the 

home was close to 40 MBtu/yr less than modeled. The model predicted that the home would use 

194 MBtu/yr of energy (160 MBtu/yr with PV), which compares to the measured value of 

141 MBtu/yr (123 MBtu/yr with PV). Actual source energy contribution from the PV system 

was 55% of the modeled value. 

Overall, the builder viewed this project as a success. The builder’s initial concern for meeting the 

comfort needs of the occupants was subsequently alleviated. In general, the annual energy 

consumption of this house was lower than predicted, and the builder was able to achieve a low 

enough incremental cost for these upgrades to his standard specifications to justify the 

investment in the higher performance package of measures. Moving forward, the builder has 

continued to use 1⁄2 in. of XPS insulation as an exterior sheathing for a standard product on his 

homes in place of the noninsulating sheathing. The use of higher performing windows on this 

project also has become standard for all of this builder’s homes. The builder continues to install 

solar thermal water heating systems and PV as an option and has utilized advanced framing 

techniques on other projects as well. 

This builder strives to lead the local market with energy-efficient green construction practices 

and is now looking closely at the space conditioning and ventilation systems currently being 

installed to identify additional opportunities for efficiency and cost-effective energy savings. 
28 

6 Conclusions

The HVAC system effectively maintained acceptable temperatures and humidity levels during 

peak cooling and heating and nonpeak operating periods. Occupants would likely be comfortable 

in the home year round (ASHRAE 2008). The equipment did not run continuously for the entire 

peak operating 4-day period, indicating that it can contribute more capacity if needed. In the San 

Antonio climate, ensuring that the HVAC system is properly sized for cooling is the main 

objective of equipment-sizing calculations. The ability of the system to maintain consistent 

temperatures and humidity levels and to have extra capacity to offer during outdoor conditions 

that greatly exceed the summer design situation indicates that the system is not undersized and 

may even be oversized. However, the system has a high enough level of energy efficiency and 

performance to discourage any lowering of equipment load capacity. Future opportunities exist 

to improve the efficiency and performance of the space conditioning system, and the builder is 

actively pursuing research into the next generation of HVAC for his company. 

Airtightness levels for this test house well exceeded the standard levels achieved by the builder, 

helping to reduce additional loads on the HVAC system and to maintain sufficient comfort levels 

in the house. Strategic air sealing of the band joists and dormer walls using spray foam, gluing 

drywall to the interior framing, and taping the seams in the exterior sheathing were all helpful in 

achieving the lower levels of airtightness.

The solar thermal DHW system was the main source of hot water for most of the year, and it 

performed well in that function. During the summer months, electricity use by the backup system 

was rare. The whole system provided adequate delivery temperature and capacity year round as 

it met all loads. Going forward, the builder will offer a solar thermal system as a standard option 

and is considering opportunities to improve performance and to reduce costs for these systems.

Of the 1,633 kWh of electricity produced annually by the PV system, 46.8% was sent to the 

electricity grid. Utilization of less than half of the PV-generated electricity occurred in the six 

coldest months of the year. This time of production versus time of demand issue was not 

incorporated into the models used in the design of this house (e.g., BEopt) but would indicate 

that a greater capacity PV system is not warranted unless electrical utilities offer generous feedin-tariff rates. The builder currently offers PV as a standard option. 

There were discrepancies between modeled and actual energy use values. After adjusting the 

predicted consumption using actual weather and indoor set point temperatures, actual space 

cooling energy use was the closest of all end uses to the corresponding modeled value. Heating 

energy use did not include the installed gas fireplace, which was likely a significant contributor 

to heating energy usage. The energy consumption predicted for LAMELs was significantly 

higher than the measured amounts. The energy consumed in these end uses depends greatly on 

occupant preferences, which are difficult to predict.
29 

References 

ASHRAE. (2008). HVAC Systems and Equipment. Atlanta, GA: American Society of Heating, 

Refrigerating and Air-Conditioning Engineers, Inc.

ASHRAE. (2009). Fundamentals. Atlanta, GA: American Society of Heating, Refrigerating and 

Air-Conditioning Engineers, Inc. 

BEopt. BEoptTM (Building Energy Optimization) software, version 1.3. Golden, CO: National 

Renewable Energy Laboratory. http://beopt.nrel.gov/. 

Hendron, R.; Engebrecht, C. (2010). Building America House Simulation Protocols. Golden, CO: 

National Renewable Energy Laboratory. NREL/TP-550-49426. 

Rutkowski, H. (2006). Manual J—Residential Load Calculation, 8th edition. Arlington, VA: Air 

Conditioning Contractors of America. 

Stecher, D.; Allison, K. (2011). Long-Term Results: New Construction Occupied Test House, 

Urbana, Illinois. Golden, CO: National Renewable Energy Laboratory. Accessed January 4, 

2013: www.nrel.gov/docs/fy13osti/53707.pdf. 

Stecher, D.; Brozyna, K. (2013). Long-Term Results from Evaluation of Advanced New 

Construction Packages in Test Homes: Lake Elsinore, California. Golden, CO: National 

Renewable Energy Laboratory (to be published in summer 2013).

NREL. (2012). PVWatts: A Performance Calculator for Grid-Connected PV Systems (version 1). 

Golden, CO: National Renewable Energy Laboratory. Accessed March 26, 2012: 

http://rredc.nrel.gov/solar/calculators/pvwatts/version1/. 
30 

Appendix A: Imagine Homes Floor Plans

Figure 20. First-floor plan (numbers on drawing refer to photos in Appendix B)

6

5

8

1 7
31 

Figure 21. Second-floor plan (numbers on drawing refer to photos in Appendix B); #3 and #4 are 

on the roof and in the attic, respectively

2

3

4
32 

Appendix B: Photographs of Sensor Locations from the Imagine

Homes House from December 2010 

Figure 22. Location #1; first-floor 

temperature and RH measurement at the 

thermostat location

Figure 23. Location #2; second-floor 

temperature and RH measurement at the 

thermostat location
33 

Figure 24. Location #3; incident solar radiation sensor on the roof in plane 

with solar DHW and PV panels 

Figure 25. Location #4; furnace measurements
34 

Figure 26. Location #5; outdoor temperature and humidity measurement

Figure 27. Location #6; flow and temperature measurements at the water heater location
35 

Figure 28. Location #7; electrical measurements at the outdoor electrical panel

Figure 29. Location #8; wires at the data logger location near the indoor electrical panel
DOE/GO-102013-3911 ▪ July 2013 

Printed with a renewable-source ink on paper containing at 

least 50% wastepaper, including 10% post-consumer waste.