NREL/CP-550-37576 Proceedings of ISEC2005 

2005 International Solar Energy Conference 

August 6-12, 2005, Orlando, Florida 

ISEC2005-76231 

EVALUATION OF A HIGH-PERFORMANCE SOLAR HOME IN LOVELAND, COLORADO 

Robert Hendron 

National Renewable Energy Laboratory 

Golden, CO 

Mark Eastment 

National Renewable Energy Laboratory 

Golden, CO 

Ed Hancock 

Mountain Energy Partnership 

Boulder, CO 

Greg Barker 

Mountain Energy Partnership 

Boulder, CO 

Paul Reeves 

Partnership for Resource Conservation 

Boulder, CO 

ABSTRACT 

 Building America (BA) partner McStain Neighborhoods 

built the Discovery House in Loveland, Colorado, with an 

extensive package of energy-efficient features, including a 

high-performance envelope, efficient mechanical systems, a 

solar water heater integrated with the space-heating system, a 

heat-recovery ventilator (HRV), and ENERGY STARTM 

appliances. 

 The National Renewable Energy Laboratory (NREL) and 

Building Science Consortium (BSC) conducted short-term 

field-testing and building energy simulations to evaluate the 

performance of the house. These evaluations are utilized by 

BA to improve future prototype designs and to identify critical 

research needs. 

 The Discovery House building envelope and ducts were 

very tight under normal operating conditions. The HRV 

provided fresh air at a rate of about 75 cfm (35 l/s), consistent 

with the recommendations of ASHRAE Standard 62.2. The 

solar hot water system is expected to meet the bulk of the 

domestic hot water (DHW) load (>83%), but only about 12% of 

the space-heating load. DOE-2.2 simulations predict wholehouse source energy savings of 54% compared to the BA 

Benchmark [1]. The largest contributors to energy savings 

beyond McStain’s standard practice are the solar water heater, 

HRV, improved air distribution, high-efficiency boiler, and 

compact fluorescent lighting package. 

NOMENCLATURE 

ACH Air changes per hour 

AFUE Annual fuel utilization efficiency 

AH Air Handler 

BA Building America 

CAE Combined annual efficiency 

DHW Domestic hot water 

EF Energy factor 

HRV Heat recovery ventilator 

RMC Remaining moisture content 

SEER Seasonal energy efficiency ratio 

SHGC Solar heat gain coefficient 

TRNSYS TRaNsient SYstems Simulation 

INTRODUCTION 

Building America is a partnership between the U.S. 

Department of Energy (DOE) and the building industry to 

develop production-ready building systems that lead to wholehouse energy savings of 50% by 2010 and 90% by 2020. 

McStain Neighborhoods is a partner to BSC, one of five 

Building America industry teams. Targeting 40-50% energy 

savings, McStain and BSC designed and built the Discovery 

House in Loveland, Colorado, with a large number of energyefficiency measures. In addition to a high-performance 

envelope and efficient mechanical systems, this home utilizes a 

passive solar design, a solar hot water loop integrated with the 

space-heating system, an HRV, compact fluorescent lighting, 

and energy-efficient appliances. In June 2004, NREL worked 

in partnership with McStain and BSC to provide 

comprehensive field-testing and analysis to evaluate key 

building systems and identify possible solutions to any 

performance issues. Figure 1 shows the completed house at the 

time field-testing was conducted. The Discovery House is a 

2512-ft2

 (233-m2), two-story home with a 636-ft2 (59-m2) 

conditioned basement. Other key design specifications are 

summarized in Table 1. Features that are not part of McStain’s 

standard practice in the Denver metropolitan area are presented 

in italics. For this particular project, NREL began its 

participation after the design and construction had been 

completed. A more complete description of the Discovery 

House and the design philosophy behind it can be found in an 

article published by McStain Neighborhoods [2]. 
Figure 1. McStain Discovery House (view from south) 

 

Table 1. McStain Discovery House Key Specifications 

Ceiling R-44+ dry-blown cellulose 

Walls 2x6 24-in (61-cm) on-center (oc), R-19 dampspray cellulose insulation and R-4 3/4-in (1.9-

cm) Dow XPS, 7/16-in (1.1-cm) OSB exterior, 

MemBrain interior; 

R-19 cellulose 2x6 24-in (61-cm) oc to garage 

Basement 

walls 

Fiberglass batts with vinyl facers draped on 

walls (R-11) 

Basement 

slab 

1-in (2.5-cm) XPS (R-5) and 6-in (15-cm) EPS 

void material (~R-1.2), radiant slab heating 

Windows 

and 

Skylights 

Vinyl frame, Low-E, spectrally selective 

double-glazing, U = 0.35, SHGC = 0.34, 

movable awnings, thermostatically-controlled 

motorized windows

Space 

Heating 

Lennox CompleteHeat in basement, 100 

kBtu/hr (29 kW), 0.90 CAE, solar assisted 

Space 

Cooling 

Lennox 19.2 SEER split system, two Tamarac 

1000-cfm whole-house fans, manual control 

DHW Lennox CompleteHeat, 0.90 CAE, 100 kBtu/hr 

(29 kW), 34-gallon tank, solar assisted, 

drainback system, three 4x8 collectors, 180-gal 

solar tank, recirculation loop on timer 

Ducts Uninsulated metal with mastic in basement, 

floor joist spaces, and interior walls; fully 

ducted returns (first and second floors) 

Ventilation Lennox HRV2-150 HRV, 123 CFM (58 l/s), 60-

66% sensible effectiveness, fan-cycling control 

for intermittent mixing, temperature-controlled 

window operation 

Other Compact fluorescent lighting (CFL) package 

and ENERGY STAR appliances (weight sensing 

horizontal axis clothes washer, soil sensing 

dishwasher, dryer with temperature and 

moisture feedback) 

SHORT-TERM FIELD TEST RESULTS 

 Building envelope and duct leakage tests were conducted 

by BSC personnel using a blower door and duct blaster. A 

summary of the measurements, which are not adjusted for high 

altitude, is shown in Table 2. 

Table 2. Air Leakage Characteristics Measured by BSC 

Blower 

Door CFM50 (l/s) 1052 (497) 

Estimated ACHnat 0.163 

Duct 

Blaster CFM25total (l/s) 365 (172) 

CFM25outside (l/s) 21 (10) 

CFM25total, supply (l/s) 289 (137) 

CFM25outside, supply (l/s) 20 (9) 

 These measurements verified compliance with the BSC 

air-leakage specifications of less than 2239 CFM50 (1057 l/s 

@50 Pa), or 2.5 in2

 (16 cm2) EqLA per 100 ft2 (9.3 m2) of 

surface area, and less than 5% duct leakage to the outside, 

corresponding to 60 cfm (28 l/s) based on 1200 cfm (566 l/s) 

total airflow. The design target of 10% total duct leakage, or 

120 cfm (57 l/s), was not met during initial testing, but 

additional air-sealing steps taken by the builder reduced duct 

leakage from 289 cfm (136 l/s) to about 135 cfm (64 l/s) on the 

supply side, which is very close but still would not quite meet 

the target level of 120 cfm (57 l/s) total. 

 The NREL multi-zone tracer-gas monitoring system was 

installed in the Discovery House from June 13 to 17, with 

sample points on each floor, including the basement. Air 

exchange rates were measured for several different operating 

conditions, with and without the air-handler fan and HRV 

operating. The measured hourly average air exchange rates are 

displayed in Figure 2. The outside temperature during the test 

period was fairly mild and peaked as high as 95°F (35°C) on 

some days while remaining below 60°F (16°C) on other days 

(usually when overcast). Active space conditioning was 

unnecessary and was not employed during the test period. 

 From 1800 h Sunday, June 13, through 0700 h Monday, 

June 14, the house was in normal operating mode with the 

HRV on at low speed and the air-handler operating 

continuously. The measured air exchange rate during this 

period was between 0.15 and 0.20 air changes per hour (ACH), 

depending on wind speed and temperature difference. The 

HRV was turned off at 0800 h on Monday, turned on again at 

1400 h and off again at 1800 h. This is commonly referred to 

as a "bump" test. The difference between the on and off 

periods was about 0.12-0.13 ACH (69-75 cfm, or 33-35 l/s) and 

represents the net air exchange attributable to operation of the 

HRV. Another bump test was performed on June 16 with 

similar results. From 1800 h Monday until 0800 h Wednesday, 

the house operated without ventilation. The measured airexchange rate without ventilation was between 0.02 and 0.05 

ACH. Additional ventilation was clearly an important and 

necessary feature of this house. The design ventilation rate for 

the HRV was 75 cfm (35 l/s) at low speed and 175 cfm (83 l/s) 

at high speed. Based on a conditioned floor area of 3148 ft2

(292 m2

), including a basement and three bedrooms, the 

ventilation rate recommended by ASHRAE Standard 62.2 is 75 

cfm (35 l/s). The design target appears to have been met within 

the accuracy of the tracer gas measurements. 
 The air handler (AH) was turned off from 0800 h on 

Tuesday, June 15, until 0800 h on Wednesday, June 16. The 

effect on ACH was negligible, perhaps 0.01-0.02 ACH (6-12 

cfm, or 3-6 l/s). This is consistent with our expectations for 

ducts located in conditioned space and is also consistent with 

the duct blaster results measured by BSC (21 cfm, or 10 l/s @ 

25 Pa). 

0.00

0.05

0.10

0.15

0.20

0.25

18 0 6 Mon 18 0 6 Tue 18 0 6 Wed

Hour

ACH

AH on, HRV on AH on, HRV on

AH on, HRV off

AH off, HRV off

AH on, HRV on

Figure 2. Tracer gas measurements at the Discovery House 

 The interior temperatures of the house during the test 

period are shown in Figure 3. Because the heating and cooling 

functions were not active during the test period, these profiles 

show that the air was well mixed during the tracer gas test. In 

addition, the energy efficiency measures appeared to be 

effective in keeping the interior temperature stable during mild 

weather conditions without the need for space conditioning. 

20

22

24

26

28

30

32

1800 0 600 Mon 1800 0 600 Tue 1800 0 600 Wed

Hour

Temperature (°C)

Dining

Kitchen

Living

MBR

SWBR

SBR

NEBR

Basement

Figure 3. Interior temperatures during the first week of shortterm testing 

 Three of the energy-efficient appliances (dishwasher, 

clothes washer, clothes dryer) used in the Discovery House 

were evaluated as part of this test. Because occupant behavior 

can have such a large effect on energy and hot water use for 

these appliances, NREL was interested in characterizing their 

performance beyond the basic information provided on the 

EnergyGuide labels. Energy savings calculations for the 

appliances in both the Discovery House and the BA Benchmark 

are based on the 2005 version of an appliance analysis 

spreadsheet developed by NREL [3]. This spreadsheet 

performs energy-savings calculations using the energyconsumption data collected by the manufacturer in accordance 

with the DOE standard test procedures [4,5,6]. Usually these 

data can be found on the EnergyGuide label, the manufacturer’s 

web site, the ENERGY STAR web site 

(www.energystar.gov/index.cfm?c=appliances.pr_appliances), 

or in the appliance database published by the California Energy 

Commission (www.energy.ca.gov/appliances/appliance). The 

NREL spreadsheet accepts published test results as inputs and 

calculates energy and hot-water consumption for the Prototype 

and Benchmark based on the operating conditions and analysis 

guidelines documented in the BA Benchmark Definition [1]. 

 Energy savings predictions for these three energy-efficient 

appliances are provided in Table 3. The results suggest that 

substantial end-use energy savings can be expected for all three 

appliances. Electricity, natural gas, and hot water usage are all 

significantly less than the Benchmark values. 

Table 3. Energy Savings Calculations for Discovery House 

Appliances (Pre-test) 

Dishwasher Clothes 

Washer 

Clothes 

Dryer 

Benchmark Electricity 

(kWh/yr) 240 122 89 

Discovery House Electricity 

(kWh/yr) 102 41 73 

% Electricity Savings 58% 66% 18% 

Benchmark Gas (kWh/yr) N/A N/A 1817 

Discovery House Gas 

(kWh/yr) N/A N/A 850 

% Gas Savings N/A N/A 53% 

Benchmark DHW (liters/day) 22.0 66.2 N/A 

Discovery House DHW 

(liters/day) 8.7 19.3 N/A 

% DHW Savings 60% 71% N/A 

 The clothes washer in the Discovery House was an 

ENERGY STAR-rated Whirlpool Duet Model GHW9200LT1. 

This model is a 3.18-ft3

 (0.09-m3) horizontal-axis machine with 

a thermostatic control valve to adjust the ratio of hot and cold 

water entering the tub, an internal heater to boost the hot-water 

temperature for the “sanitary” cycle, and a weight sensor to 

adjust water level based on the size of the load. The washer 

was run with 3 lb (1.4 kg) and 7 lb (3.2 kg) test loads using 

each of the five available wash/rinse cycles (cold/cold, 

warm/cold, warm/warm, hot/cold, extra hot/cold). This series 

of tests was intended to duplicate as nearly as possible the DOE 

standard appliance test procedures, used as the basis for 

calculating the information published on the EnergyGuide 

label. The test loads consisted of clean white 100% cotton tshirts; laundry detergent was not used. 

 The measured hot water and machine energy use during 

the tests are shown in Figures 4 and 5. All test cycles 

consumed significantly less hot water than the BA Benchmark, 

and all of the cycles except the sanitary wash cycle (extra 

hot/cold) used less machine energy. (In fact, the sanitary cycle 

not only used a large amount of energy to heat the water to 

about 150oF (66oC), but the cycle duration exceeded 2 hours.) 

As expected, the automatic water-level control feature 

significantly reduced the amount of hot water consumption 

when the smaller 3 lb (1.4 kg) test load was used. Also evident 

is the significant difference in hot water use for both cold 

washing and cold rinsing. However, it is noteworthy that this 

machine actually did use a small amount of hot water during 
the cold wash cycle to maintain a temperature of about 70oF 

(21oC) for the purpose of detergent activation. 

0

10

20

30

40

50

60

70

Benchmark Cold/Cold Warm/Cold* Warm/Warm* Hot/Cold Extra Hot/Cold

Cycle (Wash Temp/Rinse Temp)

Hot Water (liters)

3 lb (1.4 kg) test load

7 lb (3.2 kg) test load

Figure 4. Clothes washer hot water use under various operating 

conditions. Extra rinse used for Warm/Cold and 3 lb (1.4 kg) 

Warm/Warm cycles. 

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Benchmark Cold/Cold Warm/Cold* Warm/Warm* Hot/Cold Extra Hot/Cold

Cycle (Wash Temp/Rinse Temp)

Machine Energy (kWh)

3 lb (1.4 kg) test load

7 lb (3.2 kg) test load

Figure 5. Clothes washer machine energy use under various 

operating conditions. Extra rinse used for Warm/Cold and 3 lb 

(1.4 kg) Warm/Warm cycles. 

 The clothes dryer in the Discovery House was a Whirlpool 

Duet Model GGW9200L. Dryers do not have EnergyGuide 

labels and cannot qualify for ENERGY STAR. However, this 

dryer does have moisture and temperature sensors designed to 

reduce drying time and save energy by optimizing the amount 

of heat added and by automatically turning off the dryer when 

the clothes are dry. In addition, the ENERGY STAR clothes 

washer was expected to yield indirect energy savings for the 

dryer by reducing the remaining moisture content (RMC) in the 

clothes at the end of the spin cycle. RMC is defined as the 

weight of the water remaining in the damp clothes after the 

wash cycle divided by the dry weight. 

 Figures 6 and 7 present dryer electricity and natural gas use 

corresponding to the five clothes washer test cycles discussed 

above. Unfortunately, electricity data for one of the 3 lb (1.4 

kg) cycles were accidentally overwritten following the test, and 

could not be reported. Energy use was substantially less than 

the Benchmark value for nearly all of the test cycles. However, 

the drying times did not seem to be faster than usual, averaging 

30 minutes for the 3 lb (1.4 kg) loads and 40 minutes for the 7 

lb (3.2 kg) loads. In fact, several 12.9 lb (5.9 kg) loads (full 

loads according to the DOE test procedures) were run, and the 

drying times averaged about 90-120 minutes. One of the 7-lb 

(1.4 kg) loads used much more machine energy than the others 

and tended to cycle on and off much more frequently. There 

was no clear explanation suggested by the data, so the 

unusually high dryer energy was likely caused by an anomaly 

in the dryer load, such as bunching or some other random 

effect. 

0

0.05

0.1

0.15

0.2

0.25

BA Benchmark Cold/High/High Cold/High/High Warm/High/High Cold/High/High Cold/Extra

High/High

Conditions (Rinse Temp/Spin Speed/Dryer Temp)

Electricity (kWh)

3 lb (1.4 kg) test load

7 lb (3.2 kg) test load

Figure 6. Clothes dryer electricity use under various operating 

conditions 

0

1

2

3

4

5

BA Benchmark Cold/High/High Cold/High/High Warm/High/High Cold/High/High Cold/Extra

High/High

Conditions (Rinse Temp/Spin Speed/Dryer Temp)

Natural Gas (kWh)

3 lb (1.4 kg) test load

7 lb (3.2 kg) test load

Figure 7. Clothes-dryer natural gas use under various operating 

conditions 

 The dishwasher installed in the test house was an 

ENERGY STAR-rated Whirlpool GU1200XTLT3. Features of 

this model include five different wash levels, a soil sensor that 

adjusts wash time based on the dirtiness of the dishes, and a 

heated drying option. The dishwasher was operated using a test 

load consisting of eight place settings of typical ceramic dishes 

and stainless steel silverware. Most cycles were run with clean 

dishes. For cycles with dirty dishes, a controlled amount of 

spaghetti sauce was brushed on the dishes, which were then 

cooked in the microwave for about 20 seconds to simulate the 

effects of a typical meal. 

 The electricity and hot water use of the dishwasher under a 

variety of operating conditions are summarized in Figure 8. 

The dishwasher used less energy than the BA Benchmark for 

both the machine and hot water under each of the conditions 

tested. However, the results indicate that the electric heaters 

used in the power dry option nearly double the amount of 

machine electrical energy compared to the air-dry option. The 

data also suggest that the soil sensor has a very large effect on 

both the hot water use and the machine energy. Ultimately, 

user-controlled operating choices will determine whether this 

ENERGY STAR dishwasher saves energy compared to a 

typical dishwasher. 
0

5

10

15

20

25

30

35

40

BA Benchmark Clean Dishes, Air Dry Clean Dishes, Air Dry

(Repeat)

Clean Dishes, Power

Dry

Dirty Dishes, Air Dry

Hot Water (liters) or Electricity (source kWhx10)

Hot Water Volume

Electrical Energy

Figure 8. Dishwasher electricity and hot water consumption 

under various test conditions 

SOLAR WATER HEATER 

 TRNSYS simulations were performed to evaluate the 

contribution of the solar hot water system toward meeting the 

DHW and space-heating loads. A schematic of the hot water 

system in the Discovery House is shown in Figure 9. The 

thermostat on the solar storage tank controls two solenoid 

valves wired in parallel, one normally open and one normally 

closed. When the storage tank temperature is above the 

thermostat setting (currently 110oF, or 43oC) and if either space 

heat through the air handler heating coil or space heat through 

the hydronic floor in the basement is called for, the respective 

circulation loop will be directed through the solar storage tank 

to collect heat. If the storage tank is not hot enough then the 

loop will be directed through the boiler to collect heat. 

* Return through cold water lines

C

H M

Collectors

*

M

C

M

H

S1

S2

Mains

Mains

DHW

Solar Storage Boiler

F4

F5

F1

F3

F2

F1

Temperature Sensor

Flow Meter

Check Valve

Flow Direction

Solenoid Valve

Mixing Valve

Ball Valve

Heating Coil

Hydronic Floor

Tstat

hydronic 

dhw 

Figure 9. Schematic design of domestic hot water and spaceheating system 

 There are limitations to using the solar storage tank as a 

source for DHW, space heat through the air handler, and space 

heat through the hydronic floor. The heating coil in the air 

handler is typically designed to deliver the expected heating 

energy using a circulating water temperature of 120oF (49oC) or 

higher. If the temperature is lower, then the heat exchange rate 

at the air handler coil may be too low to meet the load. The 

hydronic loop, on the other hand, can deliver heat at a lower 

temperature, perhaps 90oF (32oC). With the system as it is 

now, the thermostat on the storage tank must be set to about 

110oF (43oC) to be able to meet the load at the air handler. 

Because there is also a load on the storage tank to heat DHW, 

the tank will rarely reach 110oF (43oC) and, therefore, only a 

small fraction of the space-heating load is likely to be met by 

solar energy. 

 Figures 10 and 11 show the predicted contribution of the 

solar storage tank toward each end-use, based on the DHW 

volume and operating profile specified for Building America 

analysis [7]. Figure 10 shows the results for the system as 

installed, and Figure 11 shows the effect of reducing the 

minimum supply temperature for the hydronic floor slab to 

90oF (32oC), and disconnecting the heating coil loop from the 

solar tank. In either case, the solar hot water system is 

expected to meet a very large percentage of the DHW load 

(83% as-built, 77% if modified), which for the purpose of this 

analysis includes both the energy to heat the mains water to the 

set point of 120oF (49oC) and the standby losses associated with 

the boiler tank. Based on our analysis, it appears that a greater 

fraction of the combined DHW and space-heating loads can be 

met by making the system modifications. We would expect the 

fraction of the basement space-heating load met by the solar 

system to increase from about 25% to nearly 56%. This would 

represent an increase from 12% to 16% of the total spaceheating load, and an increase from 28% to 30% of the 

combined DHW and space-heating loads. 

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

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

Month

Million Joules

Solar to Hydronic Loop

Solar to Heating Coil

Solar to DHW

Figure 10. TRNSYS simulation results for the solar hot water 

system as currently implemented 

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

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

Month

Million Joules

Solar to Hydronic Loop

Solar to Heating Coil

Solar to DHW

Figure 11. TRNSYS results if heating coil water does not run 

through the solar storage tank and the minimum supply 

temperature for the solar storage tank is set to 90oF (32oC) 

 It is important to note that for Loveland, Colorado, an 

idiosyncrasy in the BA analysis guidelines results in active 

space heating throughout the year with a set point of 71oF 
(22oC), which is the optimal temperature for the average person 

when the house is in heating mode. This is an unlikely 

operating condition during the summer months even in cold 

climates. For the Discovery House, combining this year-round 

heating set-point with night ventilation, cool night-time 

temperatures at high altitude, relatively small internal heat 

gains, and reduced DHW demand, the analysis predicts what 

may be an unrealistically high solar contribution toward 

summertime space heating. 

 During short-term testing of the solar hot water system, 

two performance issues became apparent. The system did not 

always begin circulating water to the collector when weather 

conditions indicated it should, and the system was short cycling 

once it got started. The short cycling problem was traced to an 

oversized pump, which was circulating too much flow (about 6 

gpm, or 0.38 l/s) to the collector, resulting in a temperature rise 

that was too small. This issue was partly corrected by adjusting 

a valve to increase the pressure head, thereby reducing the flow 

rate to a more reasonable level (about 2 gpm, or 0.13 l/s). As a 

longer-term solution, NREL has recommended that two smaller 

pumps be used in place of the current pump. These pumps 

should be installed in series to provide sufficient pressure head 

during start-up, after which time one pump could be turned off 

to reduce the flow. 

 The system start-up issue appeared to be a result of 

temperature readings that were not representative of the actual 

supply and return temperatures to the solar collector. The 

return temperature sensor on the collector was originally 

located outside of the collector on the return pipe. This sensor 

was moved to the back of the absorber plate inside the collector 

to better indicate the temperature of the empty collector when 

the system is off for a period of time. In addition, the sensor at 

the bottom of the stratified solar tank, which was supposed to 

measure the collector supply temperature, was actually 

providing readings closer to the average tank temperature. The 

temperature sensors providing these readings have been 

relocated since the time of the test, and it appears that the 

system is now operating as intended. 

ANNUAL ENERGY SIMULATIONS 

 A computer model of the Discovery House was created 

using the DOE-2.2 hourly simulation program. Inputs to the 

model were derived from the design specifications, short-term 

test results, TRNSYS simulations, and appliance spreadsheet 

calculations presented earlier in this paper. Simulations were 

performed in accordance with the BA Performance Analysis 

Procedures [7]. Graphical representations of the model 

generated using eQuest are shown in Figures 12 and 13. 

 The Discovery House prototype was compared to three 

base case houses: the BA Benchmark (representing typical 

practice in the 1990s), Regional Standard Practice, and Builder 

Standard Practice. The key features of these three base cases 

and the prototype are summarized in Table 4. Builder Standard 

Practice and Regional Standard Practice were estimated based 

on inputs from McStain and our experience with other builders 

in central Colorado. Although the stated efficiency of the 

prototype air conditioner was SEER 19, we modeled it as an 

effective SEER 16.5 based on discussions with BSC, an 

examination of the HVAC components, and published data 

from Lennox. 

Figure 12. DOE-2 model geometry as viewed from the 

southwest, generated using eQuest 

Figure 13. DOE-2 model geometry as viewed from the 

northeast with shading surfaces hidden, generated using eQuest 

Table 4. Summary of model inputs for the Discovery House 

and three BA base cases 

BA Benchmark: 

Frame walls, 2x4, R-14 cavity insulation, wood siding 

R-33 ceiling insulation 

Double-pane clear windows 

Uninsulated vented crawlspace 

R-10 basement walls 

Infiltration rate = 0.65 ACH (annual average) 

10 SEER 4-ton (14 kW) air-conditioner 

78% AFUE forced-air furnace 

40-gal (151-liter) Gas DHW, standard 0.54 EF 

90% incandescent lighting 

Standard appliances 

Regional Standard Practice: 

Same as Benchmark except: 

R-27 ceiling insulation 

Uninsulated basement walls 

Infiltration rate = 0.35 ACH (annual average) 

Builder Standard Practice: 

Same as Regional Standard Practice except: 
Frame walls, 2x6, 16 in (41 cm) oc, R-19 cavity insulation 

R-38 ceiling insulation 

Low-E, double-pane windows 

Conditioned crawlspace and basement 

R-5 insulated basement walls 

Infiltration rate <0.35 ACH (annual average) 

12-SEER air conditioner 

92.1% AFUE furnace 

Power-vented gas DHW, 0.58 EF 

ENERGY STAR refrigerator and dishwasher 

Prototype (Discovery House): 

Same as Builder Standard Practice except: 

Frame walls, 24 in (61 cm) oc, sprayed cellulose insulation 

R-4 insulating sheathing 

R-11 insulated basement walls 

R-44 ceiling insulation 

Low-E, spectrally selective, vinyl frame windows 

Infiltration rate = 0.1 ACH 

75 cfm (35 l/s) HRV 

Temperature-controlled natural ventilation 

Movable awnings 

16.5 SEER 3-ton (11 kW) air-conditioner 

Active solar DHW system with 0.86 EF recovery backup 

90% AFUE boiler with solar assist 

Radiant slab and forced-air distribution 

90% compact fluorescent lighting 

All ENERGY STAR appliances

 Source energy calculations sorted by end-use for the 

Discovery House prototype and base case houses are presented 

in Table 5. Source energy, or primary energy, is defined as the 

energy delivered to the house (site energy) plus the energy 

required for generation, transmission and distribution. The 

national average site-to-source energy multiplier for electricity 

is about 3.16, and for natural gas is about 1.02 [7]. Table 5 

indicates that energy for space cooling and DHW are nearly 

eliminated. Space heating and lighting are also significantly 

reduced, and there is a noticeable reduction in appliance 

energy. Total source energy savings for the Discovery House 

compared to the Benchmark is predicted to be 54%, 

significantly exceeding the design target of 40-50%. 

 Estimated energy and cost savings for packages of 

efficiency measures are shown in Table 6. Descriptions of the 

measures included in each package are listed in Table 7. The 

effect of each measure on end-use energy consumption is 

shown in Figure 14. A significant fraction of the total savings 

is attributable to the quality of construction that McStain has 

already implemented, as indicated by the bar labeled “Builder 

Std.” Beyond McStain’s standard features, the most significant 

energy savings are associated with the high-efficiency boiler, 

efficient air distribution, solar hot water system, HRV, and 

compact fluorescent lighting package. 

 As often happens in a showcase home with many pieces of 

equipment that are donated or intended to be more educational 

than cost-effective, the Discovery house has a few redundant 

energy efficiency measures. Low solar heat gain windows, 

exterior shading, high-SEER air conditioner, whole-house fan, 

and thermostatically controlled windows are all measures that 

reduce summertime cooling energy for a house that already has 

limited cooling loads because of a tight building envelope, heat 

recovery ventilation, efficient lighting and appliances, and a 

climate with relatively few cooling degree days. NREL 

performed some additional simulations in an attempt to identify 

a more cost-effective package, but the effect of each measure 

on annual energy use was heavily dependent on the sequence in 

which the measures were ordered. Other important 

considerations, including comfort, health and safety, durability, 

and physical appearance must also be factored into the trade-off 

analysis, along with energy and cost. 

Table 5. Predicted Annual End-use Source Energy Consumption and Energy Savings for the McStain Discovery House 

Annual Source Energy Savings 

Annual Source Energy Percent of End-Use Percent of Total 

BA Bench Region Builder Proto BA Reg Bldr BA Reg Bldr 

End-Use kWh/yr kWh/yr kWh/yr kWh/yr Bench Base Base Bench Base Base 

Space Heating 53,932 38,303 33,554 20,881 61% 45% 38% 33% 22% 18% 

Space Cooling 9,520 3,583 2,617 1,036 89% 71% 60% 8% 3% 2% 

DHW 8,514 8,514 7,652 1,049 88% 88% 86% 7% 9% 9% 

Lighting 8,517 8,517 8,517 3,407 60% 60% 60% 5% 6% 7% 

Appliances + Plug 19,136 19,952 18,425 18,137 5% 9% 2% 1% 2% 0% 

Ventilation 651 651 651 2,048 -214% -214% -214% -1% -2% -2% 

Total Usage 100,272 79,519 71,416 46,559 54% 41% 35% 54% 41% 35% 

Site Generation 0 0 0 0 0% 0% 0% 

Net Energy Use 100,272 79,519 71,416 46,559 54% 44% 35% 54% 41% 35% 
Table 6. Predicted Annual Energy and Cost Savings for Major Energy Efficiency Measures 

National Average Builder Standard (Local Costs) 

Annual Site 

Energy 

Annual Source 

Energy 

Annual Energy 

Cost 

Annual Energy 

Cost 

Savings for 

Measure 

Savings for 

Package 

Increment kWh kWh Savings $/yr Savings $/yr Savings value ($/yr)

savings 

($/yr) 

BA Benchmark 65,658 100,357 $2,828 $2,516 

Regional Standard Practice 51,599 79,556 21% $2,244 21% $1,987 

Builder Standard Practice 45,943 71,459 29% $2,016 29% $1,778 

Improved Wall and Ceiling 

Insulation 43,583 68,387 32% $1,931 32% $1,695 5% $83 $83 

Basement Wall and 

Crawlspace Ceiling Insulation. 41,357 65,987 34% $1,865 34% $1,624 9% $71 $154 

Automatic Exterior Shading 41,477 66,026 34% $1,866 34% $1,627 9% $(3) $151 

Automatic Natural Ventilation 41,855 66,196 34% $1,870 34% $1,635 8% $(9) $143 

HRV 38,080 62,462 38% $1,769 37% $1,519 15% $116 $259 

Improved DHW 35,471 59,395 41% $1,684 40% $1,432 19% $87 $346 

Improved HVAC 31,000 53,679 47% $1,525 46% $1,277 28% $156 $501 

Improved Cooling 30,918 53,273 47% $1,513 47% $1,270 29% $7 $508 

Solar DHW and Space Heat 27,610 50,886 49% $1,451 49% $1,181 34% $89 $597 

Lighting and Appliance 26,889 46,633 54% $1,325 53% $1,109 38% $72 $669 

"Source Energy Savings %" and "National Average Energy Cost Savings %" are compared to the Building America Benchmark, 

whereas the "Local Energy Cost Savings %" and the "Package savings $/yr" are compared to Builder Standard Practice. 

National Average Electric Cost: 0.087 $/kWh 

National Average Gas Cost: 0.032 $/kWh 

Local Average Electric Cost: 0.059 $/kWh 

Local Average Gas Cost: 0.033 $/kWh 

Table 7. Descriptions of Energy Efficiency Measures included 

in Each Step of the Analysis 

Measure Description 

BA Benchmark Establishes the baseline energy use for 

the analysis. 

Regional Standard 

Practice 

Regional Standard Practice models a 

building typical of the region 

Builder Standard 

Practice 

Builder Standard Practice models a 

building typical of this builder 

Improved Wall and 

Ceiling Insulation 

Walls improved to R-27 (2x6 with R-4 

sheathing), Ceiling to R-44 

Basement Wall and 

Crawlspace Ceiling 

Insulation 

Basement wall insulation increased to R11 full length, Crawlspace Ceiling 

insulation to R-15 

Exterior Shading Movable Shading added 

Automatic Natural 

Ventilation 

Automatically opening ventilation added 

(windows) 

HRV 70% Effective, 0.15 ACH HRV system 

Improved DHW DHW improved to a boiler with 

EF=0.86 

Improved HVAC Duct system 95% efficiency, 

electronically commutated fan motor, 

downsized 3-ton (10.5-kW) A/C 

Improved Cooling Improved cooling system (SEER 16.5) 

Solar DHW & Space 

Heat 

Active solar hot water system supplies 

heat to DHW and space heat 

Lighting & Appliance CFLs, Energy-Star appliances, 

0

20,000

40,000

60,000

80,000

100,000

120,000

BA Benchmark

Regional Std

Builder Std

Imp. Walls & Ceil

Bsmt/Crwl Ins.

Ext. Shad

Auto Vent

HRV

Impr. DHW

Impr. HVAC

Impr. Cool

Solar

Lighting & Appl

Source kWh/yr

DHW

Space Cooling

Space Heating

Appliance & Plug

Lighting

Figure 14. Influence of efficiency measures on end-use energy 

consumption 
LONG-TERM MONITORING 

 NREL plans to record detailed measurements at short time 

intervals over an extended period of time to provide insights 

and interpretations that are not available from short-term 

testing, simulations, or monthly utility bills. The results of this 

long-term monitoring program (including direct measurements, 

utility bills, and homeowner interviews) will be documented in 

a follow-up paper. 

CONCLUSIONS 

 Based on the test and analytical results discussed in the 

preceding sections, we were able to draw several conclusions: 

• The building envelope of the Discovery House was very 

tight. Tracer-gas testing indicated about 0.02 to 0.05 ACH 

during mild summer weather. Blower-door tests conducted 

by BSC suggested an annual average infiltration of 0.16 

ACH. 

• Duct leakage to the outside was well within the design goal 

of 60 cfm (28 l/s) at 25 Pa, as measured by BSC using a 

duct blaster. Tracer-gas test results were consistent with 

duct-blaster measurements, indicating that duct leakage 

was less than 12 cfm (6 l/s) to the outside while the air 

handler was operating. NREL did not evaluate total duct 

leakage, but duct-blaster testing by BSC showed it to be 

slightly higher than the design goal. 

• Based on tracer-gas measurements, the HRV provided 

fresh air at a rate of about 75 cfm (35 l/s) when operating 

at low speed, consistent with the recommendations of 

ASHRAE Standard 62.2. 

• The dishwasher, clothes washer, and clothes dryer each 

consumed significantly less energy than the BA 

Benchmark under normal operating conditions. However, 

certain operating modes, including the “sanitary” clothes 

washer cycle and the dishwasher power-dry option, used 

much more machine energy and/or hot water than other 

operating modes. The soil sensor in the dishwasher also 

dramatically increased energy use, while the weight sensor 

in the clothes washer significantly reduced energy use. 

• The solar hot water system is expected to meet a large 

fraction of the DHW load (~83%), but only about 12% of 

the space-heating load. We recommended that the air 

handler heating coil loop be re-plumbed so that it is not 

allowed to circulate through the solar storage tank. The 

storage tank temperature setting could then be lowered to 

90oF (32oC), and the basement hydronic loop could make 

greater use of the solar-heated water. 

• DOE-2.2 simulations predict whole-house source energy 

savings of 54% compared to the BA Benchmark. The 

largest contributors to the energy savings (other than the 

efficiency improvements that are already standard practice 

for McStain) are the solar water heater, high-efficiency 

boiler, air-distribution improvements, HRV, and compact 

fluorescent lighting package. 

• The measures designed to reduce cooling energy 

(including a high SEER air conditioner, exterior shading, 

low solar heat gain glass, heat recovery ventilation, tight 

envelope, automatic window control, and night ventilation) 

appear to be a bit redundant given the relatively mild 

summertime weather in Loveland. However, because the 

benefits of each measure are dependent on the order in 

which the measures are analyzed, it is difficult to say 

which are most cost-effective. It is also important to note 

that occupant comfort, durability, and other considerations 

must factor into the decision-making process when 

evaluating these features. 

ACKNOWLEDGMENTS 

We express our appreciation to Justin Wilson, Jeff 

Medanich, and the rest of the team at McStain Neighborhoods 

for providing NREL with extensive field support, including 

generous access to the Discovery House during and after the 

test period. We would also like to acknowledge the hard work 

of the BSC team in assisting with the design of the Discovery 

House and offering their insights throughout the test and 

analysis phase of the project. Finally, we want to thank Ed 

Pollock and George James of DOE for the leadership and 

resources necessary to perform this work. 

REFERENCES 

[1] Hendron, R. 2005. Building America Research Benchmark 

Definition, Updated December 29, 2004. NREL Report No. 

37529, Golden, CO: National Renewable Energy Laboratory. 

[2] McStain Neighborhoods. 2004. The McStain Discovery 

House Breaks Ground! Louisville, CO. 

[3] Eastment, M., Hendron, R. In Preparation. Method for 

Evaluating Energy Use of Dishwashers, Clotheswashers and 

Clothes Dryers Based on DOE Test Procedures. Draft NREL 

Technical Report. 

[4] U.S. Department of Energy (DOE). 2001. Code of Federal 

Regulations Title 10, Part 430, Subpart B, Appendix J – 

Uniform Test Method for Measuring the Energy Consumption 

of Automatic and Semi-automatic Clothes Washers, DOE, 

Washington, DC. 

[5] U.S. Department of Energy (DOE). 1981. Code of Federal 

Regulations Title 10, Part 430, Subpart B, Appendix D – 

Uniform Test Method for Measuring Energy Consumption of 

Clothes Dryers, DOE, Washington, DC. 

[6] U.S. Department of Energy (DOE). 2003. Code of Federal 

Regulations Title 10, Part 430, Subpart B, Appendix C – 

Uniform Test Method for Measuring Energy Consumption of 

Dishwashers, DOE, Washington, DC. 

[7] Hendron, R., Anderson, R., Judkoff, R., Christensen, C., 

Eastment, M., Norton, P., Reeves, P., Hancock, E. 2004. 

Building America Performance Analysis Procedures: Revision 

1. NREL Report No. TP-550-35567, Golden, CO: National 

Renewable Energy Laboratory.