Sonoma House: 

Monitoring of the First U.S. 

Passive House Retrofit 

A. German, B. Weitzel, C. Backman, 

M. Hoeschele, and B. Dakin

Alliance for Residential Building Innovation (ARBI)

December 2012
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iii

Sonoma House: Monitoring of the First U.S. Passive House Retrofit

Prepared for:

The National Renewable Energy Laboratory 

On behalf of the U.S. Department of Energy’s Building America Program

Office of Energy Efficiency and Renewable Energy

15013 Denver West Parkway

Golden, CO 80401

NREL Contract No. DE-AC36-08GO28308

Prepared by: 

A. German, B. Weitzel, C. Backman, M. Hoeschele, B. Dakin

Alliance for Residential Building Innovation (ARBI)

Davis Energy Group, Team Lead

123 C Street

Davis, California 95616

NREL Technical Monitor: Cheryn Metzger

Prepared under Subcontract No. KNDJ-0-40340-00

December 2012
iv

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v

Contents

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

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

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

Foreward ................................................................................................................................................... viii

Acknowledgements ................................................................................................................................. viii

1 Introduction........................................................................................................................................... 1

1.1 Background and Motivation ................................................................................................1

1.2 Research Questions..............................................................................................................1

2 House Description................................................................................................................................ 3

2.1 Energy Efficiency Measure (EEM) Details.........................................................................4

Thermal Envelope................................................................................................................6

Mechanical and Electrical Systems .....................................................................................8

Lighting and Appliances....................................................................................................11

Photovoltaic System...........................................................................................................11

2.2 Preliminary Savings Estimations.......................................................................................11

3 Methodology ....................................................................................................................................... 12

3.1 General Technical Approach .............................................................................................12

3.2 Measurements....................................................................................................................12

Monitoring Data Points......................................................................................................12

Short Term Tests................................................................................................................12

3.3 Equipment..........................................................................................................................15

Data Logger Specifications................................................................................................15

Sensor Types and Specifications .......................................................................................15

3.4 Computation of Monitoring Variables...............................................................................16

4 Results................................................................................................................................................. 19

4.1 System Commissioning .....................................................................................................19

4.2 Short Term Test Results.....................................................................................................19

4.3 Monitoring Results and Discussion ...................................................................................20

Solar Water Heating...........................................................................................................20

Load Reduction and HVAC Strategies..............................................................................27

4.4 Annual Energy Use ............................................................................................................29

4.5 Project Costs and Builder Feedback ..................................................................................33

5 Recommendations and Conclusions ............................................................................................... 37

References................................................................................................................................................. 39

Appendix A: Building Plans..................................................................................................................... 40
vi

List of Figures

Figure 1. The Sonoma House is the first certified Passive House retrofit in the United States......... 3

Figure 2. Remodel floor plan ..................................................................................................................... 4

Figure 3. Floor layers above slab for floor type #1.................................................................................. 6

Figure 4. Wall construction........................................................................................................................ 7

Figure 5. Exterior wall showing foil-faced EPS and furring for siding.................................................. 7

Figure 6: Netted blown insulation at roof and kneewall ......................................................................... 8

Figure 7. Mechanical schematic ................................................................................................................ 9

Figure 8. Mechanical closet containing solar thermal storage system and tankless water heater . 10

Figure 9. Sensor location schematic ...................................................................................................... 14

Figure 10. Monthly solar fraction and hot water use for the domestic hot water system................. 21

Figure 11. Total 15 minute domestic hot water volume draw by size ................................................. 22

Figure 12. Tankless water heater 15-minute efficiency versus hot water draw ................................. 23

Figure 13. Summer water heater system operation .............................................................................. 24

Figure 14. Winter water heater system operation.................................................................................. 25

Figure 15. Contribution of solar thermal and mini-split to space heating and comparison to BEopt 

heating loads....................................................................................................................................... 26

Figure 16. Winter heating operation of solar hydronic heating coil and mini-split heat pump ........ 27

Figure 17. Degree day comparison ......................................................................................................... 31

Figure 18. Breakdown of monitored energy use ................................................................................... 32

Figure 19. Floor plan with wall assembly descriptions......................................................................... 40

Figure 20. Roof plan with roof assembly descriptions ......................................................................... 41

Figure 21. Wall and roof section details ................................................................................................. 42

Unless otherwise noted, all figures were created by the ARBI team.

List of Tables

Table 1. Building Energy Efficiency Measures ........................................................................................ 5

Table 2. Measurement Points List........................................................................................................... 13

Table 3. Sensor Specifications ................................................................................................................ 15

Table. 4: Results of Short Term Tests .................................................................................................... 19

Table 5. Results of ERV One-Time Testing ............................................................................................ 20

Table 6. Annual Contribution from Solar Thermal System by End Use .............................................. 20

Table 7. Energy Savings Comparison of Load Reduction and HVAC Measures ............................... 28

Table 8. Annual Energy Use Comparison of Monitored and Modeling Estimates – Site Electricity 

and Gas................................................................................................................................................ 30

Table 9. Passive House Energy Target Comparison............................................................................. 32

Table 10. Energy Related Measure Project Costs ................................................................................. 33

Table 11. Window Incremental Performance Comparison ................................................................... 35

Unless otherwise noted, all tables were created by the ARBI team.
vii

Definitions

AC

ACH

ARBI

Btu

CARB

CDD

CFL

CFM

COP

DEG

DHW

EEM

EER

EF

ERV

HDD

HERS

HSPF

HVAC

kWh

LBNL

LED

MEL

PH

PV

SEER

SHGC

SRCC

TMY3

Air conditioner

Air change per hour

Alliance for Residential Building Innovation

British thermal unit

Consortium for Advanced Residential Buildings

Cooling degree days

Compact fluorescent lamp

Cubic feet per minute

Coefficient of performance

Davis Energy Group

Domestic hot water

Energy efficiency measure

Energy efficiency ratio

Energy Factor

Energy Recovery Ventilator

Heating degree days

Home Energy Rating System

Heating Seasonal Performance Factor

Heating, ventilation, and air conditioning

Kilowatt-hour

Lawrence Berkeley National Laboratory

Light emitting diode

Miscellaneous electric load

Passive House

Photovoltaic system

Seasonal energy efficiency ratio

Solar heat gain coefficient

Solar Rating and Certification Corporation

Typical Meteorological Year 3 (weather data set)
viii

Foreword

The Sonoma Deep Retrofit is a single story deep retrofit project in the marine climate of 

Sonoma, California. The design was guided by Passive House (PH) principles, which promote 

the use of very high levels of wall, ceiling, and floor insulation along with tight envelope 

construction to maintain a comfortable indoor environment with little or no need for 

conventional heating or cooling. These concepts are gaining increasing attention and traction in 

California from custom home builders and may present an avenue for builders to reach Building 

America Program Management Milestone (PMM) goals. This research project presents a unique 

opportunity to both identify and evaluate deep retrofit measures that are practical and potentially 

cost effective for the retrofit market, as well as review the cost effectiveness of the PH design 

approach for achieving energy savings in deep retrofits. The Sonoma House was monitored both 

at the building and system level over one year to verify expected savings and system 

performance. The Sonoma house is the first certified PH in California, and the first PH retrofit in 

the United States. Source energy savings are estimated at 56% compared to the pre-retrofit case.

Acknowledgements

Davis Energy Group would like to acknowledge the U. S. Department of Energy Building 

America program and their funding and support of development of this technical report as well 

as research that informed it. In addition, we would like to thank the builder Rick Milburn of 

PassivWorks, formerly Solar Knights Construction, for his cooperation throughout the design, 

construction, and monitoring stages of this project. We would also like to acknowledge 

Lawrence Berkeley National Laboratory, who worked with Davis Energy Group to provide 

additional monitoring data for total and net building electricity use and PV production.
1

1 Introduction

1.1 Background and Motivation

Builder Rick Milburn of PassivWorks1 of Napa, California, formerly Solar Knights 

Construction, collaborated with a building owner to develop a plan for retrofitting an existing 

house in Sonoma to Passive House (PH) program standards2

. The PH concept uses very high 

levels of wall, ceiling, and floor insulation along with tight envelope construction to maintain a 

comfortable indoor environment with little or no need for conventional heating or cooling.

Davis Energy Group (DEG), technical lead for Building America research team Alliance for 

Residential Building Innovation (ARBI), adopted the project as a Building America deep retrofit 

opportunity. The goal was to identify and evaluate retrofit measure packages that achieve 

significant energy savings over pre-retrofit conditions and that are practical and potentially cost

effective for the retrofit market. In addition, the project afforded the chance to review the PH

design approach as a Building America strategy. The Sonoma house is the first certified PH in 

California, and the first PH retrofit in the United States.

Gaps and barriers related to high performance building envelopes and low load homes identified 

by the Building America Space Conditioning and Enclosures Standing Technical Committee 

members include:

• Lack of availability and documented performance of high efficiency, small capacity 

heating and cooling equipment for low load situations

• Research on distributed space conditioning

• Identify air tightness strategies and effectiveness of these strategies.

Passive House concepts are gaining increasing attention and traction in California from custom 

home builders. However, there has also been some controversy over the merits of the PH 

approach in reaching for net zero energy goals, especially in cold climates where the incremental 

cost of achieving minimal heating loads may not be warranted (Straube 2009). The PH theory 

tends to push the BEopt efficiency curve much farther to the right before applying PV than is 

suggested by NREL analysis. Note that Building America savings goals are based on efficiency 

only and do not credit PV or other onsite renewable energy generation.

The Building America program is interested in evaluating what measures and strategies used in

PH homes are found to be commercially viable and support progress toward Building America 

goals. The Sonoma deep retrofit will be used as a case study to determine which of the energy 

efficiency measures (EEMs) are commercially viable in a marine climate and how effectively PH

design features can contribute to exceeding Building America existing home milestones.

1.2 Research Questions

The primary evaluation objective of this study is to determine how effectively building retrofit 

measures can contribute to exceeding Building America Program Management Milestone 

(PMM) goals, specifically in retrofit applications, through monitoring energy performance, and 

 1 http://www.solar-knights.com/ 2 http://www.passivehouse.us/passiveHouse/PassiveHouseInfo.html
2

evaluating the market viability of the EEMs. Specific objectives of this project are to compare 

whole house, heating, cooling, and water heating energy use from measured data with simulation 

output, and to identify the cost benefit of individual measures to the extent possible.

The following research questions have been explored:

1. How well does the measured energy use match energy use from simulations and did the 

project achieve its energy-savings goals?

2. How commercially viable are deep retrofits in the local market? 

3. How much does the solar water heater contribute to space and water heating loads and 

how well does the system perform?

4. How effective is the combination of the high performance envelope, energy recovery 

ventilator (ERV), and other measures at minimizing the heating and cooling load, and 

what are the energy savings?
3

2 House Description

The Sonoma Deep Retrofit is a one story, single family home located at 760 3rd Street East in 

Sonoma, California (Figure 1). Sonoma is located north from the San Francisco Bay Area, and 

has approximately 2,647 heating degree days and 717 cooling degree days based on a 65°F 

standard3. It is within the Building America marine climate zone (CZ) and IECC CZ 3. The 

1,975 ft2 home was originally built in the 1960s and consisted of two structures connected with 

an open breezeway. The breezeway was redesigned and enclosed to unite the two structures into 

a two bedroom, two bath, 2,380 ft2 residence (Figure 2). Because the residence was originally 

purchased in a foreclosure sale, pre-retrofit performance data is unavailable for comparison.

Figure 1. The Sonoma House is the first certified Passive House retrofit in the United States

In early 2010, DEG assisted the builder and owner with EEMs and equipment selection during 

the design and construction process. The project team used EnergyGauge4 to simulate the 

building’s energy performance with various building components and provide feedback to the 

builder on cost-effective energy efficiency packages. Based on the builder’s and owner’s interest 

in PH, the retrofit measures included features that were not initially deemed cost effective by 

Building America criteria. The home was completed and systems were commissioned at the end 

of October 2010. The project team from DEG and Lawrence Berkeley National Laboratory 

(LBNL) began monitoring the building’s energy performance once construction was completed 

and the building was occupied in November 2010.

A comprehensive monitoring system was installed to identify whole house and system 

performance, and data collection was initiated in December 2010. Key collected data includes:

• Contribution of the solar thermal system to space heating and water heating loads 

• Efficiency of the water heating system 

 3 DOE Climate Zone 4 – Station 48351 -

http://www.csd.ca.gov/Contractors/documents/Energy%20tab/References/DOE%20Climate%20Zones.pdf

4 The model was later updated using BEopt v1.0, and eventually v1.1, after the release of the 2010 House Simulation 

Protocols and the complementary BEopt software versions.
4

• Efficiency of the tankless water heater 

• Contribution of the ERV to space heating/cooling load reduction 

• Mini-split heat pump cooling and backup heating electrical energy use, and

• Individual electrical end uses, whole house electrical use and generation from the 

photovoltaic system (courtesy of LBNL).

The monitoring teams from DEG and LBNL are sharing data from the project, and preliminary 

monitoring results were reported in a 2010 final Building America report (CARB 2010). LBNL 

monitoring results are also described in a presentation by LBNL (LBNL 2011).

Figure 2. Remodel floor plan

2.1 Energy Efficiency Measure (EEM) Details

Table 1 summarizes the EEMs measures incorporated in the Sonoma House as well as preretrofit conditions.

N
5

Table 1. Building Energy Efficiency Measures

Measure Pre-Retrofit Post-Retrofit

Basic Building Characteristics

Building Type / Stories Single family, 1 story Single family, 1 story

Conditioned Floor Area 1,937 2,380

Number of Bedrooms 2 2

Envelope (See Appendix A for details)

Exterior Wall Construction 2 × 4 16 in. oc

New: 2 × 6 24 in. oc 

(~75% of existing walls remain)

Exterior Wall Insulation R-11 cavity insulation R- 21 cavity (new) R-15 cavity (existing) +

R-16 EPS + radiant wall/air gap

Wall to Garage R-11 cavity insulation R-15 cavity + R-22 EPS

Foundation Type & Insulation Slab - Uninsulated Slab - AeroGel and/or EPS (min R-13) 

above existing slab w/ R-16 at slab edges

Roofing Material & Color Asphalt shingles - dark Metal roof/custom Bilt Zincalume

Ceiling Insulation Vented, R-19 Unvented, none

Roof Deck Insulation None Minimum of R-42 blown in rafters & 

R-11 of rigid at deck

Radiant Barrier No Yes

House Infiltration - Blower Door Test N/A 0.40 ACH50 tested (0.60 target)

Thermal Bypass Inspection - QII N/A Yes

Glass Properties: U-Value / SHGC

All Windows Single metal pane: 1.28 / 

0.085 Optiwin triple pane: 0.105 / 0.52

Sliding Glass Door @ Kitchen N/A Optiwin triple pane: 0.093 / 0.52

HVAC Equipment

Heating Type & Efficiency Natural gas combined 

hydronic fan coil

Solar thermal w/ Mitsubishi Mr. Slim minisplit HP backup - HSPF 8.2

AC Type & Efficiency None Mitsubishi Mr. Slim mini-split 1-ton HP / 

SEER 17, EER 10.3

Heating & Cooling Distribution Ductwork Hydronic: central ducted; mini-split: 

ductless 

Duct Location & Insulation Attic, R-4 Conditioned attic

Mechanical Ventilation Kitchen & bath fans UltimateAir ERV

Water Heating Equipment

Water Heater Type & Efficiency Storage gas (≈0.58 EF)

(combined hydronic) Rinnai RC80HP1 tankless (0.82 EF)

Tank Capacity/Gallons N/A

HW Distribution Standard, uninsulated

Solar Water Heater Type & Solar Fraction 3 Heliodyne 4 × 6 collectors, 80 gal

Appliances & Lighting

ENERGY STAR Appliances None Dishwasher/refrigerator/washer

Dryer Fuel Electric Electric

Oven / Range Fuel Gas Gas

Fluorescent Lighting Package 100% incandescent 100% high efficacy w/ LEDs & fluorescent 

PV System

PV Solar System Type & Capacity None 10 Sanyo 215N PV modules 2.15 kW

 5 U-value and solar heat gain coefficient (SHGC) are based on default values from Table 116-A and Table 116-B of 

the California 2008 Building Energy Efficiency Standards (CEC, 2008).
6

Thermal Envelope

Slab: In most cases the existing concrete slab was maintained and a new slab was poured where 

necessary. Two styles of above-slab insulation were used. Floor type #1 received an application 

of 11⁄2 in. of expanded polystyrene (EPS) insulation above 0.6 in. Aerogel Spaceloft insulation 

below the plywood substrate. A 45/8 in. layer of EPS without the Aerogel was installed over floor 

type #2. A 33⁄4 in. of rigid stone wool insulation board was applied to the entire exterior 

perimeter. Figure 3 presents the materials used in floor type #1.

Figure 3. Floor layers above slab for floor type #1

Walls: Approximately 75% of the existing 2 × 4 walls were retained and insulated with R-15 

blown-in fiberglass insulation. All newly constructed walls were of 2 × 6 construction, 24 in. on 

center, and insulated with R-21 blown-in fiberglass. All exterior walls are clad with wood siding

and have a vented airspace under the siding, a minimum of 33⁄4 in. of foil-faced EPS rigid foam 

board insulation (taped and sealed with a Grace Ice & Water shield), and a Stego Wrap vapor 

barrier (Figure 4 and Figure 5). The wall to the garage was installed with 5 in. of EPS instead of 

33⁄4 in. See Appendix A for details of the wall assemblies.

Under the California Title-24 energy code, a Home Energy Rating Systems (HERS) credit was 

taken for Quality Insulation Installation (QII). This credit is similar to the ENERGY STAR®

Thermal Bypass Checklist with the intent to ensure proper installation of insulation materials and

minimize thermal bypass. Insulation quality and air barriers were inspected by a certified HERS 

rater before drywall was installed.

Plywood

11⁄2” EPS 0.6” Aerogel

Vapor barrier
7

Figure 4. Wall construction

Figure 5. Exterior wall showing foil-faced EPS and furring for siding

Conditioned Attic Space: The attic was converted to a conditioned, non-vented attic with 10-15

in. of netted blown cellulose in between the roof trusses and at knee walls (Figure 6). In addition, 

21⁄2 in.-31⁄2 in. of EPS rigid insulation was installed at the roof deck, which also extends over the 

top of the exterior walls. See Appendix A for details of the roof assemblies.

Siding

11⁄2” air barrier between furring

11⁄4” EPS foil faced

21⁄2” EPS

Grace Ice & Water shield 

@ plywood

Furring strips

Foil faced EPS
8

Figure 6: Netted blown insulation at roof and kneewall

Windows: The home’s windows and doors are manufactured by Optiwin; a German company 

specializing in German PassiveHaus standard compliant windows and doors. The windows are 

low-E triple paned, with dual sealing surfaces, a solar-heat gain coefficient of 0.52, and a Uvalue of 0.105. The large sliding glass door at the north wall of the kitchen has a slightly lower 

U-value at 0.093. Total fenestration is 590 ft2

, nearly 25% of the total wall area. Overhangs 

around the entire building are 2 ft 6 in. deep at a minimum, allowing solar gain during the winter 

months and minimizing it during the summer.

Air Tightness: The building envelope was designed with the purpose of preventing building air 

leakage through the envelope and also facilitating moisture drainage. Passive House building 

standards require homes to meet the air tightness requirement of ≤0.6 Air Changes per Hour at 

50 Pascal pressure (ACH50). After envelope construction, the home’s envelope was leak tested 

by a certified HERS rater and found to have an ACH50 of 0.40 and CFM50 of 151, the equivalent 

of 0.24*10-4 specific leakage area.

Mechanical and Electrical Systems

The primary energy source for water heating and space heating is solar thermal energy, with a 

tankless water heater serving as backup for water heating and an air-source mini-split heat pump 

serving as the backup space heating system. The schematic of these integrated systems is 

depicted in Figure 7.
9

Figure 7. Mechanical schematic

Solar Thermal for water and space heating: Three 4 ft by 6 ft Heliodyne Solar Thermal Rooftop 

collectors supply hot water to an 80-gal storage tank. The thermal storage tank supplies 

preheated water to the domestic hot water supply and a hydronic coil located in the ductwork for 

the ERV system during heating season. The integrated hydronic heat exchanger was designed to 

provide most of the space heating demand. Domestic hot water takes precedence over space 

conditioning. The control strategy was custom designed for this project. The solar thermal water 

storage tank, tankless hot water heater, and circulations pumps are housed in an interior

mechanical closet (Figure 8).
10

Figure 8. Mechanical closet containing solar thermal storage system and tankless water heater

Backup space heating and cooling: A 1-ton ductless Mitsubishi Mr. Slim mini-split air source 

heat pump rated at SEER 17 and 8.2 HSPF provides a backup source for heating and supplies 

space cooling. The system design for a ductless, single indoor head unit was based on the tight 

envelope and low expected heating and cooling load.

Backup water heating: A Rinnai tankless water heater was installed downstream of the solar 

thermal storage tank supply to provide supplemental heating to maintain the hot water supply 

temperature to the home. A push button demand recirculation pump was also installed to reduce 

waste cold water supply to the master bathroom, the furthest location from the mechanical closet.
11

ERV: The ERV is manufactured by UltimateAir with a varying flow rate of 70 to 210 cubic feet 

per minute (CFM). The ERV fan is set to operate continuously to deliver 70 CFM of fresh air 

ventilation and to ramp up to deliver 200 CFM when any of the exhaust fans are activated. The 

ERV is connected to ductwork exhausting air from bathrooms, laundry, and kitchen, and 

supplying air to all bedrooms and living areas. The ductwork was sealed and tested by a certified 

HERS rater to be less than 6% of airflow. 

The original HVAC system design prior to ARBI involvement called for ground loop thermal 

heating to temper outside air before entering the ERV system. The horizontal ground loop would 

have consisted of two lines of 150 ft of 3⁄4 in. PEX tubing located 5 ft underground where the 

expected soil conductivities would be between 0.5 and 0.7 BTU/fto

F-hr. A 50-watt pump would 

circulate water to a heat exchanger placed at the inlet side of the ERV. The team’s analysis 

indicated that the payback on this design would be minimal (i.e., pre-heating intake air with 

ground loop thermal heating is an expensive investment with little return) in such a mild climate.

Lighting and Appliances

The 2008 California Title 24 standards for residential lighting require a certain percentage of 

high efficacy fluorescent fixtures in kitchens, but allow either fluorescent fixtures or 

incandescent fixtures with vacancy sensor (or dimmers in some rooms) in other locations. The 

builder used a combination of LEDs, hard-wired fluorescent linear fixtures and CFL ceiling cans 

to exceed Title 24 requirements. The dishwasher, refrigerator and clothes washers were all 

replaced with new ENERGY STAR and CEE Tier II rated appliances.

Photovoltaic System

A 2.15 kW rooftop PV system, consisting of ten Sanyo 215N PV modules provided onsite 

electricity and are mounted on the South-facing roof and controlled with a Grid-Tied SMA 

SB8000US inverter. The installed system was sized to provide about 2/3 of the total estimated 

house load.

2.2 Preliminary Savings Estimations

Based on an evaluation of the home using BEopt v1.1, the post retrofit case performed 56% 

better than the pre retrofit case without PV and 69% better with PV. This exceeds the 2013 PMM 

of 30% for existing homes in marine climates. Evaluation using BEopt was limited due to the 

following system features that couldn’t be modeled in BEopt:

• Vented wall cavities with reflective air spaces

• Solar thermal heating of ERV inlet air.

While heat pumps are easily modeled in BEopt, the lack of data on performance curves for minisplit units that have variable speed fans and compressors and electronic expansion valves most 

likely results in underestimated mini-split performance.
12

3 Methodology

3.1 General Technical Approach

The general approach of this research is to employ system commissioning, short term tests, long 

term monitoring, and detailed analysis of results to identify the performance attributes and cost 

effectiveness of the whole house measure package. Whole house energy usage from simulations 

is compared to monitored performance in an attempt to identify the applicability of individual 

measures to Building America retrofit standards. The house was monitored for a full year.

The water heating system was monitored to determine solar fraction of the solar thermal system 

as a percentage of domestic hot water load, the overall efficiency of the water heating system, the 

heat delivered by the solar thermal system to the ERV, and the gas consumption by the tankless 

water heater. The ERV was monitored to determine energy consumption and its contribution to 

the heating and cooling load. Mini-split heat pump energy delivery was not directly measured as 

it is extremely difficult to measure airflow at the indoor unit given the variable nature of the 

supply fan. The power consumed by the unit was monitored to quantify run times and energy 

consumption. 

Whole house electrical energy usage was monitored by DEG. During construction, LBNL’s 

desire to monitor individual electrical circuit loads left little room in key electrical panels areas 

for simultaneous monitoring efforts. A mutual agreement was reached with LBNL to provide 

systems monitoring data in return for electrical energy usage data.

Control settings for the hydronic and ventilation systems were verified, and operation of the heat 

pump, controls, fan, and other components, checked. Long-term monitoring also provided

“continuous commissioning” support in identifying failure of any components.

Monitoring data has been carefully reviewed and analyzed in an effort to respond to the research 

questions and to identify sources of energy savings, such as from reduced heating and cooling 

load, improved equipment efficiency, etc.

3.2 Measurements

The site was equipped with a Data Electronics data logger and modem for continuously 

collecting, storing, and transferring data via telephone lines. Sensors were scanned every 15 

seconds, and data summed or averaged (as appropriate) and stored in data logger memory every 

15 minutes. Data was downloaded to a server every 24 hours, and range checks were 

automatically performed to identify problems with monitoring sensors or the systems being 

monitored.

Monitoring Data Points

Table 2 lists all the measurement points that were monitored on a continuous basis. Total house 

electricity and PV production were measured by LBNL and provided to DEG for analysis. 

Location of key data points are shown in the diagram in Figure 9.

Short Term Tests

Short-term tests were conducted to verify air-tightness with blower door testing and to develop 

performance data for the ERV fan. Duct leakage was verified by a HERS rater.
13

Table 2. Measurement Points List

Point 

No. Abbrev. Description Location Sensor Type Sensor Mfg./Model

Channel # of 

Cond.

1 TAO Temp, air, outdoor Target is under East 

Eave RTD, 4-20ma Gen Eastern 1+ 4

2 RHO RH, air, outdoor Target is under East 

Eave RH, 4-20mA Gen Eastern 1- "

3 TAI1 Temp, air, indoor, living 

area

Next to T-Stat 1 RTD, 4-20ma ACI 2+ 4

4 RHI1 RH, indoor, living area " " RH, 4-20mA ACI 2- "

5 TAI2 Temp, air, indoor, 

sleeping area

In living and dining 

room RTD, 4-20ma ACI 3+ 4

6 RHI2 RH, indoor, sleeping area " " RH, 4-20mA ACI 3- "

7 TAERVL Temp, air, ERV leaving ERV supply duct 

before HC RTD, 4-20ma Vaisala 4+ 4

8 RHERVL RH, air, ERV leaving " " RH, 4-20mA Vaisala 4- "

9 TACL Temp, air, coil leaving Duct leaving HC TT Omega, Thermex 10+ 2

10 TWSL Temp, water, solar 

storage leaving Mechanical Room

Immersion 

Thermocouple

Omega, 

Thermex, or 

Watlow Gordon

5+ 2

11 TWCS Temp, water, cold water 

supply Mechanical Room 5- 2

12 TWHL Temp, water, water heater 

leaving Mechanical Room 6+ 2

13 TWHE Temp, water, water heater 

entering Mechanical Room 6- 2

14 TTNK Tank midpoint 

temperature Mechanical Room 7+ 2

15 TWCE Temp, water, coil entering Supply line to heating 

coil 7- 2

16 TWCL Temp, water, coil leaving Return line from 

heating coil 8+ 2

17 ECP Power, coil pump Mechanical Room Power Meter Wattnode D1

18 INSOL

Solar radiation (incident 

to solar water heater 

surface)

On roof adjacent to 

solar Pyranometer Licor LI-200 12+ 2

19 EERV Power, ERV Attic in ERV cabinet Power Meter Wattnode D2 2

20 ECOND / 

EFAN Power Mini-split At condenser or power panel Power Meter Rochester RIS 1000 D3 2

21 FWC Flow, cold water supply 

to solar storage tank Mechanical Room Flow meter Onicon F-1300 D8 3

22 FWD Flow, domestic hot water Mechanical Room Flow meter Onicon F-1300 D9 3

23 FWH Flow, water, heating coil 

loop Mechanical Room Flow meter Onicon F-1300 D6 2

24 SCIRC Status, domestic hot water 

recirculation pump Mechanical Room Current status meter Hawkeye D4 2

25 GAS Flow, gas, water heater Mechanical Room Gas Meter 

with Pulser Equimeter D5 2
14

Figure 9. Sensor location schematic TAEE FWH TWCL TWCETACL FWD TWHL HEATING COIL

TWCS FWC

TAI2/RH2

WATER

DHWR

TWHE

TWSL

HEATER

TV

SOLAR

STORAGE

TANK

TO SUPPLYGRILLES

RHEL

DHW

CW

EXHAUST

INTAKE

TAI1/RH1

ERV

TAO/RHO

TAEL

FROM BATHEXHAUST
15

3.3 Equipment

Data Logger Specifications

A Model DT-800 Data Electronics data logger was used to collect and store monitoring data. 

Analog inputs are single-ended type (all referenced to ground). Digital inputs are used for power 

monitors and status signals; high speed counter inputs were used with water flow meters. The 

data loggers are provided with an RS232 communications interface and battery backup. They 

also include integral cold junction circuitry for direct measurement of Type T thermocouples.

Manufacturer: dataTaker, Inc.

Model: DT-800

Analog Inputs: Up to 36 single-ended and 24 double-ended

Digital Inputs: 16 total, 8 bidirectional, 1 kHz

Analog Accuracy: 0.02% of reading plus 0.02% of full scale.

Memory: 2 MB flash, 4 MB SRAM, 24 system variable registers

Sensor Types and Specifications

Standard specifications for the sensor types used are listed in Table 3. Sensor selection was based 

on functionality, accuracy, cost, reliability, and durability. Specific model numbers are listed as 

examples; similar models by other manufacturers may be used. Signal ranges for temperature 

sensors correspond approximately to listed spans.

Table 3. Sensor Specifications

Type Application Mfg/Model Signal Span Accuracy

RTD Outdoor temp and 

RH GE MRHT3 4-20 mA 32 - 132°F ±1.5%

0 – 100% +2%RH

RTD Indoor / Duct 

temperature / RH Vaisala HM*60 4-20 mA 32 - 132°F ±1.5%

0 – 100% +2%RH

Type T 

Thermocouple

Immersion Water 

temperatures

Gordon Watlow 

Type T special limits

~11mV @ 

500oF

Range = 0.4% -328 to 662oF

Surface / Air 

temperatures Omega -99 to 500 oF

24VAC Relay

Fresh air Damper 

Status, zone damper 

status 

Hawkeye dry 

contact n/a n/a

Small power 

monitor

Fan and condenser 

power

WattNode pulse CTA/40 ±0.5%

WNA-1-P-240-P

Large power 

monitor

Total house power, 

PV production

Watt Node pulse CTA/40 ±0.5%

WNB-3D-240-P

Flow meter Water flow Onicon F-1300 pulse varies by meter ±0.5%

Pyranometer Insolation LiCor Analog varies by sensor ±5%

Pressure 

Transducer Air Pressure Auto Tran 4-20mA 0-4inWC ±1%FS

Diaphragm 

Gas meter Tankless gas use IMAC/Rockwell Pulse 250 SCFM ±1ft3
16

3.4 Computation of Monitoring Variables

Whole house electricity use: Electricity supplied to the house and electricity produced by the PV 

system is used to identify total house electric use. PV system efficiency is calculated from

monitored energy data and insolation values.

Water heating system performance: Heat transferred from the solar water heater to the tankless 

water heater, and from the tankless water heater to the house is measured to determine the actual 

solar fraction and the overall efficiency of the water heating system. Losses from the 

recirculation system are treated as part of the DHW load. Gas flow into the water heater is 

measured with a dedicated gas meter.

For total domestic hot water load (Btu) delivered to the house, the following equation is used:

Equation 1: Qdelivered = FWD * (TWHL – TWCS) * 8.33 (Btu)

Where: FWD = domestic hot water flow (gallons)

TWHL = supply water temperature from water heater (°F)

TWCS = cold water supply temperature (°F)

The value of 8.33 in Equation 1 represents both the specific heat of water, 1 Btu/°F-lb, and the 

density of water, 8.33 lbs/gal, at a certain temperature. Over the range of expected temperatures 

the less than 0.5% variation is considered to be within acceptable measurement error.

Equation 2 is used to calculate total energy (Btu) provided by the tankless water heater.

Equation 2: Qtankless = FWD * (TWHL – TWSL) * 8.33 (Btu)

Where: TWSL = supply water temperature from solar storage tank (°F)

Energy consumption of the gas water heater and recirculation pumps is converted to Btu using 

Equation 3 and Equation 4. Recirculation pump energy use (EPMPR) is calculated based on 

pump status, SCIRC. 

Equation 3: Edwh = GAS * 1,013 (Btu)

Equation 4: Erecirc = EPMPR * 3,412 (Btu)

Where: GAS = gas use of water heater (ft3

)

EPMPR = electric use of recirculation pumps (kWh)

The effective thermal efficiency of the tankless water heater is calculated according to Equation 

5. Electrical energy use of the tankless water heater was not measured and was ignored for this 

analysis. Field monitoring has shown that the annual impact of electrical consumption on 

tankless annual efficiency is on the order of 1-3% (Hoeschele et al, 2011).

Equation 5: Tankless effective thermal efficiency = Qtankless / (GAS)
17

The solar Energy Factor (EF) and associated solar fraction are calculated according to 

methodology developed by the Solar Rating and Certification Corporation (SRCC). These values 

are calculated and reported on a seasonal basis according to Equation 6 and Equation 7. The 

solar collector pump power was not monitored but is estimated based on solar insolation values, 

tank temperatures, and rated pump power.

Equation 6: SEF = Qdelivered / (Edhw + Esolar pump)

Equation 7: SF =1 – EF / SEF

Where: SEF = Solar Energy Factor 

Esolar pump = electric use of solar collector circulation pump (in Btu)

SF = Solar Fraction

EF = average daily efficiency of the gas water heater

The contribution of solar thermal to space heating was calculated through flow and temperature 

sensors on the heating coil loop. Total delivered heating (Btu) is calculated using Equation 8.

Equation 8: Qcoil = FWH * (TWCE – TWCL) * 8.33 (Btu)

Where: FWH = hot water flow through the hydronic coil (gallons)

TWCE = supply water temperature to the coil (°F)

TWCL = return water temperature from the coil (°F)

ERV system performance: ERV supply temperature and relative humidity measurements are 

used to determine its efficiency and impact on heating and cooling loads. Energy use was 

continuously monitored.

During the cooling season, Equation 9 is used to calculate the sensible load reduction due to the 

ERV. ERV airflow rates were measured once for the two possible airflow settings, and power 

measurements are used to identify changes in airflow and calculate the CFM using in the 

following equations.

Equation 9: Q ̇ ERV_cool_sensible = CFMERVs * (TAO – TAERVL) * 1.08 (Btu/h)

where: CFMERVs = calibrated air flow of supply (cubic feet per minute)

TAERVL = supply air temperature (°F)

TAO = outdoor air temperature (°F)

Total cooling (sensible plus latent) is calculated based on calculated enthalpies of the supply and 

outdoor air streams.

Equation 10 through Equation 14 represent a non-iterative approximation of enthalpy based on 

supply and outdoor air temperature and relative humidity.

Equation 10: X = (18.678 - Tc / 234.5) * Tc / (Tc + 257.14)

Equation 11: Y = 1 + X + 0.5*X2 + 0.16393*X3 + 0.041667*X4 + 0.0123457*X5
18

Equation 12: Pw = RH * 6.112 * Y / 100

Equation 13: W = 0.6219 * Pw / (1013.26 - Pw)

Equation 14: h = 0.24 * Tf + W * (1060.9 + 0.443 * Tf)

where: Tc = supply or outdoor air temperature in °C

Tf = supply or outdoor air temperature in °F

RH = percent relative humidity of the supply or outdoor air

Pw = water vapor partial pressure (hPa)

W = humidity ratio

h = enthalpy of supply or outdoor air (Btu/lbm)

The density of air in Equation 15 is calculated using the supply air temperature. Total cooling 

load reduction is calculated according to Equation 16.

Equation 15: Dair = (518.67 / (459.67 + TAERVL)) * 0.075028 (lb/ft3)

Equation 16: Q ̇ ERV_cool_total = CFM * (houtdoor – hsupply) x Dair (Btu/h)

During heating mode, Equation 17 is used to calculate the heating load reduction due to the ERV 

system.

Equation 17: Q ̇ ERV_heat = CFMERV * (TAERVL – TAO) * 1.08 (Btu/h)

Equation 18 shows the total EER calculation for the ERV system.

Equation 18: EERERV = Q ̇ ERV / EERV

where: EERV = power of ERV unit (kW)

The equation to calculate sensible effectiveness of the ERV is shown in Equation 19 according to 

AHRI Standard 1060 (AHRI, 2005). 

Equation 19: EFFERV_sens = [m ̇ ERVs * (TAO – TAERVL)] / [m ̇ ERVmin * (TAO – TAI1)]

where: TAI1 = indoor or entering ERV exhaust air temperature in °F

m ̇ ERVs = mass flow rate of the supply air (lb/hr)

m ̇ ERVmin = minimum mass flow rate of the exhaust and supply air (lb/hr)

Since the ERV supply and exhaust airflow rates are balanced, Equation 19 can be simplified as 

shown in Equation 20.

Equation 20: EFFERV_sens = (TAO – TAERVL) / (TAO – TAI1)
19

4 Results

4.1 System Commissioning 

The focus of commissioning was to verify correct operation of the mechanical systems, verify 

correct operation of the monitoring equipment including sensors and communications, and 

record one-time measurements of pertinent data points.

The indoor unit for the mini-split heat pump was originally installed in the hallway bathroom, 

both for its central location between the living space and bedrooms and to be out of view from 

the main hallway. The design assumption was that the tight envelope conditions and continuous 

supply of air from the ERV would assist in evenly distributing heat and reducing thermal 

stratification within the home. In the month following system commissioning, it was observed 

that the location of the unit resulted in overheating of the bathroom and inadequate distribution 

of conditioned air throughout the rest of the home. The head unit was moved in January, 2011 

into the main hallway. This appeared to resolve the issue of overheating the indoor space as there 

were no further complaints from the homeowner. Since indoor temperature was not monitored 

where the head unit was relocated, there was no way to properly evaluate the distribution of 

conditioned air from the mini-split heat pump.

4.2 Short Term Test Results

Following are results of short term tests conducted either by DEG or other consultants. Test 

results showed alignment with design expectations. 

Table 4: Results of Short Term Tests

Short Term Test Design Target Results

Blower Door Infiltration 0.60 ACH50 0.40 ACH50

Measured building envelope leakage was below the PH target of 0.60 ACH50. Leakage testing of 

the ERV ducts was completed by another HERS rater but the tested value was less than 6% of 

rated airflow (< 12 CFM).

ERV airflow measurements were made using an Alnor Model 6200 Low Flow Balometer. The 

ERV was set to deliver the maximum airflow of 200 CFM for testing purposes. Using the 

adjustable diffusers, the room-by-room airflows were reduced to a maximum of 40 CFM for 

measurement readings. In Bedroom 1 and the Master Closet, supplies were adjusted to lower 

flow rates (about 20 CFM) in accordance with the guidelines provided by the builder. 

Results of these measurements are provided in Table 5. With various sources of measurement 

error such as indoor-outdoor pressure differences and pressure drop imposed by the balometer, 

the differences between the sum of the exhaust and supply airflows and the exhaust and 

supply/intake readings is not unusual. The larger discrepancy between the outdoor intake 

measurement, the sum of the supply measurements, and the outdoor exhaust measurement 

suggests the house may have been pressurized by drafts through open doors when the outdoor 

intake measurement was taken. A leak in the intake duct would also explain this difference, but 

based on observations, duct sealing was extremely thorough.
20

Overall, the data indicates that total airflow on both the exhaust and supply sides is within an 

acceptable range of 10% of the 200 CFM maximum. 

Table 5. Results of ERV One-Time Testing

Room CFM % 

Difference

Exhaust: East Bath 30

Laundry 36

Kitchen 40

Powder 40

M. Bath 38

Total 184 8%

Supply: Bedroom 1 28

Den 31

Living 35

Craft/Family 32

M. Bedroom 34

M. Closet 21

Total 181 10%

Outdoor Exhaust: 190 5%

Outdoor Intake: 173 14%

4.3 Monitoring Results and Discussion

Solar Water Heating

The three solar collectors and 80-gal storage tank serve both domestic hot water and a heating 

coil for space heating located within the ERV ductwork. The majority of this analysis focuses on 

the contribution to domestic hot water. Complete water heating monitoring data was available for 

the majority of 2012, from the second week in January through the end of December. Since the 

solar circulation pump was not directly monitored, estimates of total operating hours were made 

based on the first order collector performance equation, monitored insolation, and technical 

information published by the SRCC for the installed collectors. The rated power of the pump was 

used to calculate total pump energy. 

Table 6 shows the relative contribution of the solar system to both space heating and domestic 

hot water. Over the course of the year, the system supplied almost identical quantities of energy 

to each end use. 

Table 6. Annual Contribution from Solar Thermal System by End Use

Space 

Heating 

Domestic 

Hot Water

Energy Delivered (kBtu) 3,540 3,485

% of Total 50.4% 49.6%
21

Analysis shows that domestic hot water solar fraction was 76% on an annual basis with monthly 

solar fractions reaching 90% in the summer (see Figure 10). Even during the winter months,

solar fractions were over 60% and monthly average tank temperature was greater than 90o

F.

Average daily hot water use was relatively low, under 30 gal/day except in December. This is 

significantly less than the DOE EF water heater test assumption of 64.3 gal/day per household. 

However, the Sonoma House had only a single occupant, where the 64.3 gal/day assumes three 

people. Figure 11 shows that a majority of the draws (55%) are less than one gallon; however, 

over 50% of the total annual draw volume was from draws greater than 10 gallons.

Figure 10. Monthly solar fraction and hot water use for the domestic hot water system
22

Figure 11. Total 15 minute domestic hot water volume draw by size

While gas water heating use was low because of the large contribution from the solar thermal 

system, the observed tankless water heater efficiencies were very low. Average annual efficiency 

was 48% and average monthly summer efficiencies were as low as 26%. This is partially a result 

of tankless cycling due to high entering water temperatures coming from the solar pre-heat tank. 

Tankless water heaters fire the burner whenever flow is sensed through the heat exchanger. Once 

flow has been initiated, water temperature is measured to determine the firing rate. When the 

tankless entering temperature is close to or above the water heater setpoint, the tankless water 

heater will fire on momentarily before determining that water heater setpoint is met and shutting 

off. In this situation, most of the firing energy results in heating the heat exchanger, not the 

flowing water.

Tankless efficiencies are also affected by hot water draw volumes. Figure 12 shows 15-minute 

tankless efficiencies as a function of total 15-minute hot water draw volume. Typically, tankless 

water heaters operate at lower efficiencies when draws are small, due to the fact that heat 

exchanger thermal cycling reduces efficiency. However, this relationship is not readily apparent 

from the data. Due to the resolution of the gas meter, which only logs at increments of one cubic 

foot of gas, and the resolution of the data, at 15-minute averages, it is difficult to fully distinguish 

full load water heater operation from part load. As a proxy, full load 15-minute data was 

identified by isolating any 15-minute period during which the tankless consumed greater than 3
23

cubic feet of gas and the temperature difference across the tankless heat exchanger was greater 

than 20o

F. The full load data points are also shown in the graph below. The “higher gas use” data 

set does reveal an increase in operating efficiencies with the majority of points above 70%.

These values are in the range for what is expected from a tankless with an EF of 0.82.

Figure 12. Tankless water heater 15-minute efficiency versus hot water draw

Figure 13 and Figure 14 show water heating system operation for a typical summer and winter 

two-day period, respectively. During the summer, even with large hot water draws, the supply 

water from the storage tank is hot enough to supply the load. However, the tankless still fires on 

initiation of any single water draw, resulting in unnecessary gas usage and low efficiencies. The 

water from the storage tank is tempered with cold water through the mixing valve. There are 

some draws that don’t coincide with measured gas use, due to the resolution of the gas meter, 

which records on increments of 1 cubic foot. In the winter, lower tank temperatures and larger 

hot water draws result in higher operational efficiencies, but still fairly low for draws below 10 

gallons. Note that due to temperature decay in the pipes, temperature measurements in between 

water draws will not be relevant.
24

Figure 13. Summer water heater system operation

Despite the low efficiencies, it should be acknowledged that overall annual gas usage of the 

tankless water heater is relatively low. Annual consumption based on 2011 data is 30 therms. 

This is less than the 40 therms that storage gas water heaters use on average annually in storage 

losses alone6. While system issues have resulted in lower than ideal operational efficiencies, the 

overall impact is minimal.

 6 Based on results of field research conducted by Davis Energy Group as reported in a preliminary project report 

titled “California Field Performance of Advanced Residential Gas Water Heating Technologies”. The final project 

report will be published in the end of 2012.
25

Figure 14. Winter water heater system operation

Total heating and cooling loads were not available from monitoring. Due to the difficulties in 

measuring airflow, detailed system monitoring of the mini-split was not conducted and therefore 

operational system efficiencies and capacities are unknown. Based on published data by 

Mitsubishi, heating efficiency at rated conditions (47o

F outdoor air temperature) is 3.4 COP. This 

value was applied as an average operating efficiency to monitored heat pump power to estimate 

delivered heating energy. This is shown in Figure 15 along with energy supplied by the solar 

system to the hydronic heating coil (calculated from water side monitoring).

Based on this estimate of total building heating load, solar thermal contributed 20% of the total 

load on an annual basis. Actual contribution may be greater or even lower due to a number of 

factors. The average operating efficiency of the heat pump is an estimate only and could be off 

substantially. If performance curves for the system were available, a better estimate could be 

obtained by applying the rated efficiency to the 15-minute data based on outdoor temperature. 

Additionally, due to the location of the mini-split (in a narrow hallway) and the lack of a 

distribution system, it may not have efficiently satisfied the house heating demands. During 

periods when solar was unable to satisfy a call for heating, the heat pump may have operated 

additional hours resulting in temperature stratifications within the home.
26

Figure 15. Contribution of solar thermal and mini-split to space heating and comparison to BEopt 

heating loads

Heating system operation during a March winter week is shown in Figure 16 showing mini-split 

heat pump and hydronic heating coil operation and relevant temperatures. The mini-split picks 

up most of the load during the nighttime when solar storage tank temperatures drop below an 

adequate supply temperature and the heat pump ramps down during the day to supplement the 

heating load not provided by the solar system. Because the mini-split is a variable capacity 

system with inverter compressor technology, it is able to throttle the output capacity and meet the 

required load more efficiently than constant speed systems with higher power draws and greater 

cycle degradation. 
27

Figure 16. Winter heating operation of solar hydronic heating coil and mini-split heat pump 

Load Reduction and HVAC Strategies

The Sonoma House incorporates a number of EEMs designed to significantly reduce the heating 

and cooling load on the building, consequently reducing energy use and allowing for equipment 

downsizing. These measures include high performance wall and roof assemblies, triple pane high 

performance windows, slab insulation, air sealing, and an ERV. BEopt was used to investigate 

the benefits of these various measures by estimating source energy use percent reductions. The 

base case used for comparison purposes was the pre-retrofit house specifications (Table 1) with 

the geometry, orientation and window area of the post-retrofit house. Since the pre-retrofit house 

did not have a cooling system, a standard efficiency air conditioner (SEER 13) was added for the 

purpose of this analysis. A 78 AFUE gas furnace was used in the model to represent the standard 

efficiency combined hydronic fan coil used for heating in the pre-retrofit case. The R-4.2 

ductwork was located in the attic with an estimated 30% leakage. Table 7 summarizes the energy 

savings of the load reduction measures.

Upgrading the windows from single pane metal frame (1.28 U-value/0.8 SHGC) to the triple 

pane windows (0.105 U-value/0.52 SHGC) (Parametric Run #6) had the largest total source 

energy savings of all the envelope measures based on the BEopt analysis. Second to this is 

converting the vented attic to an unvented attic with R-52 insulation. 
28

The option of upgrading the HVAC system in place of the envelope measures was also evaluated 

for comparison. Since mini-split heat pumps cannot be modeled in BEopt, a high efficiency 

single-speed heat pump was selected from the standard BEopt library for evaluation with a 12.7 

EER and 8.8 HSPF. Replacing the HVAC system with a high efficiency heat pump provides 

greater savings than any individual load reduction strategy (Parametric Run #8), resulting in 29% 

source energy reduction. A portion of these savings can be attributed to the ductless distribution. 

Parametric run #10 combines all the envelope measures together (runs 2-7) resulting in 59% total 

source energy savings, which is twice the savings as upgrading HVAC alone. If the HVAC 

system has reached the end of its useful life and is to be replaced anyway, there are capital cost 

savings associated with load reduction measures through HVAC system downsizing, helping 

offset incremental costs for the envelope measures. The useful measure life of all of the envelope 

measures is significantly longer than the useful life of the HVAC equipment, which has an

estimated useful life of 20 years.

BEopt modeling does not predict any energy benefit for the ERV compared to the base case 

which assumed exhaust ventilation only. Monitoring data confirms high fan energy use (12% of 

total source energy) and minimal load reduction benefit based on the mild climate. High 

electrical usage is due partly to the filter clogging and partly to the high fresh air ventilation rate. 

Heat exchanger effectiveness is reduced because of the small temperature differential across the 

heat exchanger due to the mild climate.

Table 7. Energy Savings Comparison of Load Reduction and HVAC Measures

Parametric 

Run # Description

Total 

Source 

Energy 

(kBtu/yr)

% 

Total 

Energy 

Savings

Heating 

Source 

Energy 

(kBtu/yr)

% 

Heating 

Energy 

Savings

Cooling 

Source 

Energy 

(kBtu/yr)

% 

Cooling 

Energy 

Savings

1 Pre Retrofit Base Case 

(BC) 284 - 166 - 16.9 -

2 BC + New Walls (R31+Radiant Wall) 262 8% 147 11% 14.3 15%

3 BC + White Metal Roof 

(Cool Roof) 281 1% 169 -2% 11.8 30%

4 BC + Unvented Attic (R53+Radiant Barrier) 223 21% 113 32% 9.3 45%

5 BC + Slab insulation 255 10% 134 19% 19.8 -17%

6 BC + Triple Pane 

Low-e windows 208 27% 99 40% 8.0 52%

7 BC + Reduced 

infiltration (0.40 ACH50) 256 10% 139 16% 16.5 2%

8 BC + Mini-Split HP 202 29% 91 45% 10.2 40%

9 BC + ERV 293 -3% 171 -3% 17.1 -1%

10 BC + Envelope Package 

(Runs 2-7) 115 59% 13 92% 2.1 87%
29

4.4 Annual Energy Use

Total building energy use, net electricity use, and energy by end use for heating, cooling, 

ventilation, and DHW were calculated annually from monitored data for comparison with BEopt 

estimates. Utility bills from PG&E were provided by LBNL to inform total building gas usage. 

Building electrical metering was managed by LBNL. Due to system difficulties, LBNL data was 

unavailable from December, 2010 through the second week in February, 2011. Consequently, 

gross and net building energy use data was only available for 39 weeks in 2011. This data was 

extrapolated to estimate annual totals. While more granular monitoring of individual electrical 

panels was conducted by LBNL, due to the difficulties of isolating end uses, specifically 

lighting, further disaggregation of appliance, lighting, and miscellaneous electric load (MEL)

electricity use was not possible. The BEopt model was calibrated to reflect actual blower door 

infiltration measurements, average seasonal interior temperature during the summer and winter, 

which were 76oF and 73oF, respectively, and actual airflow and fan efficiency of the ERV. The 

annual energy figures are presented in Table 8. 

BEopt is limited in its ability to evaluate a number of features in the Sonoma House including 

vented wall cavities with reflective airspaces, solar thermal space heating, and mini-split heat 

pumps. However, total gross annual electricity use aligns reasonably well between monitored 

and expected, within 14%. Annual PV production was underestimated by BEopt by 17%, 

resulting in a difference in net energy use of 40%. While pre-retrofit energy data is unavailable 

because the residence was purchased as a foreclosure, given the overestimate by BEopt of total 

energy use, it is reasonable to assume that at the building level this project has achieved its 

energy savings goals. Given the deep energy savings (56% source) that were estimated, at a 

minimum, the PMM goal for existing homes have been met.

The most significant discrepancies between monitored and estimated energy exist in space 

cooling and mechanical ventilation. As previously described, the ERV was set to deliver 70 CFM

of outdoor air continuously, approximately 30% greater than the minimum required by ASHRAE 

62.2 of 54 CFM. The increase in electricity use is primarily a result of increased fan power draw 

due to a clogged inlet vent. During the 2011 summer, review of monitoring data revealed that the 

ERV power was substantially higher than the expected baseline and had been increasing over 

time. A site visit confirmed that the ERV outdoor intake grille was severely clogged with debris, 

primarily plant matter. Once the grille was cleaned, the power measurement immediately 

dropped back down to its previous value. After another few months, the power began to creep up 

as the grille became clogged once again. This occurrence resulted in a substantial increase in 

system annual electricity use as well as efficiency decrease due to performance degradation. 

Unfortunately, the outdoor grille was not designed to be removed and cleaned; however, regular 

maintenance will need to be performed to ensure continued proper ERV operation. Systems that 

are designed with outdoor filters that are easy to be replaced or cleaned would help simplify this 

problem. Also, care should be taken to properly locate the outdoor air intake away from any 

obvious source of debris; however, it wasn’t clear that this was the problem at the Sonoma 

House.

The appliance and miscellaneous gas consumption is attributed solely to gas range use. Due to 

occupant use patterns, actual gas cooking use is much lower than estimated by BEopt.
30

Table 8. Annual Energy Use Comparison of Monitored and Modeling Estimates – Site Electricity 

and Gas

End Use

Annual Site Energy

Monitored Data BEopt Modeling Estimates

kWh Therms kWh Therms

Space Heating 1,310 - 1,619 -

Space Cooling 369 - 207 -

Domestic Hot Water 78 28 97 19

Appliances, Lighting + MELs 4,626 4 5,738 24

OA Ventilation (ERV) 922 - 638 -

Total Usage 7,305 32 8,299 43

 Site Generation (3,419) - (2,844) -

 Net Energy Use 3,886 32 5,455 43

Estimated space cooling energy from BEopt is 44% less than actual, while estimated heating 

energy use is 24% more than actual. A calibrated BEopt run was not completed using actual 

weather data, but heating degree days (HDDs) and cooling degree days (CDDs) from the 

monitored period were compared to the weather file7

. Figure 17 compares heating and cooling 

degree days for the TMY3 weather file used in the BEopt model (Santa Rosa - AWOS) and 

monitored weather data at the Sonoma House. Monthly and annual heating degree days align 

fairly well. However, annual cooling degree days from the monitoring period are twice that of 

those based on the TMY3 data. This is especially true in July, which saw the highest cooling 

energy use of the season. The differences in cooling energy use could easily be explained by the 

differences between last summer’s cooling weather and the weather file. Differences in heating 

energy use are not as significant and monitored heating energy use is expected to be lower than 

modeled values because the solar heating contribution through the fan coil in the ERV was not 

able to be modeled in BEopt.

 7 Daily HDDs and CDDs were calculated using daily mean temperatures estimated as the average between the daily 

maximum and minimum temperatures. 
31

Figure 17. Degree day comparison

Monitored energy use by end use was converted to source energy using national average figures 

for energy consumption and is shown in Figure 18. Over 60% of total source energy is attributed 

to appliances, lighting, and miscellaneous energy use. In this mild marine climate, space heating 

and space cooling only represent 17% and 5% of total energy, respectively. The miscellaneous 

category also includes loads such as irrigation and a pump for an outdoor water fountain. Water 

heating energy use is significantly reduced due to the solar water heating system.
32

Figure 18. Breakdown of monitored energy use

Modeling was also conducted by other consultants for PH program participation. The PH

program has maximum heating energy and maximum total source energy use requirements of 

4.75 kBtu/ft

2

/yr and 38.1 kBtu/ft

2

/yr, respectively. Table 9 compares actual energy use to both 

the PH requirements and the design targets determined by the PH consultant. Annual heating 

energy use was taken from the full year of monitoring heating energy use. Total annual source 

energy use was extrapolated from the 39 weeks of data provided by LBNL. Actual energy use 

was significantly lower than the design target, especially for heating energy, and met the PH

performance standards.

Table 9. Passive House Energy Target Comparison

Annual Energy (kBtu/ft2/yr)

PH 

Standard

Design 

Target Actual

Space Heating (Site) 4.75 4.1 1.88

Total Energy (Source) 38.1 27.6 20.0
33

4.5 Project Costs and Builder Feedback

The builder provided total project costs and a breakdown for each of the major EEMs. Table 10

summarizes these costs, which are actual costs to the builder over the pre-retrofit base case and 

include both material and labor. All costs are gross costs and do not include any available 

incentives. Since this project incorporated an addition of almost 450 ft2 of conditioned floor area, 

these project costs do not completely isolate the cost of the energy upgrade to the existing home 

with a portion of some of the costs attributed to the addition, such as the framing of new walls.

Also, the costs for air sealing measures are difficult to isolate and potentially may be greater than 

the $1,600 listed below. Some of this incremental cost may be wrapped up in the wall and roof 

assembly costs.

The cost effectiveness of individual measures is difficult to quantify due to the interactive effects 

of combining EEMs in a building. Defining cost effectiveness for retrofit applications presents 

additional complexities due to all the non-energy benefits inherent to many upgrades in existing 

homes. Adding insulation, replacing windows and air sealing can turn a previously 

uncomfortable home into a comfortable, quiet, and enjoyable space.

Table 10. Energy Related Measure Project Costs

EEM Description Measure Cost

Exterior walls: framing of new walls, R-15 or R-21 

cavity insulation + R-16 EPS + radiant wall/air gap $39,450

Unvented attic & roof insulation (R-53+Radiant 

Barrier) $30,000

Slab insulation $16,500

Triple pane low-e windows $91,000

Reduced infiltration (0.40 ACH50) $1,600

Mini-split heat pump $12,000

Hydronic coil in ERV & integration of solar thermal

for space heating $2,100

ERV $8,400

Tankless water heater $6,500

Solar water heater $13,650

ENERGY STAR appliances $19,455

LED & fluorescent lighting $62,000

PV solar system $18,600

Total $321,255

The “builder standard” for this project is a house built to qualify as LEED-Homes Silver. The 

builder specializes in “green” construction and retrofits. The area in which he builds (Sonoma 

County) includes a population that is environmentally conscientious and interested in reducing 

building energy use. The town of Sonoma requires that all construction perform at least 15% 

better than the 2008 California Title-24 energy code. The builder bid the project as both a PH

retrofit and a standard “green” retrofit. According to the builder, a conventional “green” retrofit 

for this project would include the following measures:
34

• Dual pane low-e windows

• Existing walls: 2 × 4 16 in. oc with R-15 cavity insulation. New walls: 2 × 6 24 in. oc 

with R-21 cavity insulation

• Vented attic with a high level of blown attic insulation

• No slab insulation

• Air sealing to ~ 5 ACH50

• Solar thermal DHW

• High efficiency ducted heat pump, no solar space heating

• Rooftop solar PV

• Similar lighting and appliances measures.

The incremental cost for the Sonoma Deep Retrofit above a conventional “green” retrofit is an 

estimated $96,000, or a premium of 11% cost. Assuming a 5.5% interest rate on a 30-year 

mortgage, the monthly incremental cost would be approximately $545. Predicted savings based 

on BEopt for the Sonoma Deep Retrofit over the conventional “green” retrofit are estimated at 

40% source energy8

, resulting in $619 monthly utility bill savings. While these savings are 

substantial, they do not justify this cost increase. Because of the relatively mild (marine) climate, 

heating and cooling energy costs are much lower than seen in more extreme climates. The PH

concept originated in Germany, which is a cold climate with high heating loads and more 

expensive energy costs. Provided this is a test house and the builder’s first PH project, it is 

expected that this price increase can be reduced based on builder feedback discussed below.

A majority of the incremental cost was spent on envelope measures, which provided a highly 

insulated and tightly constructed building. Wall, roof and foundation insulation and air sealing 

practices were both costly and labor intensive and involved education and training on the part of 

the builder and subcontractors. However, this strategy has the potential in similar marine 

climates to eliminate cooling altogether and in smaller homes, where internal gains dominate,

perhaps even eliminate heating.

The triple pane windows, imported from Germany, were one of the most expensive elements of 

the project, and can’t be justified by energy savings alone. Table 11 shows a cost and payback 

comparison for the triple pane windows that were installed and a standard dual glazed low-e 

window. The high SHGC of 0.52 is the same for both windows, so the energy savings reflect the 

difference in U-value only. Both incremental window costs are high, partly due to the portion of 

total cost attributed to non-energy characteristics. These non-energy benefits need to be taken 

into account when selecting windows; however, there is not a straightforward way to do this with 

standard cost effectiveness analyses. It also should be noted that total glazing area in the house is 

high at around 25% of total wall area, also increasing total costs.

 8 Estimated savings for the conventional green retrofit over the pre-retrofit case are 26% source energy based on 

BEopt modeling. 9 Based on a national average electricity rate of $0.1126. There are no gas savings.
35

Table 11. Window Incremental Performance Comparison

Base Case Incremental 

Measure

Source 

Energy 

Savings 

(kBtu/yr)

Incremental 

Cost ($)

Annual 

Utility 

Savings 

($/yr)

Simple 

Payback 

(yr)

Pre Retrofit Base 

Case (BC)

BC + Dual standard 

Low-e windows 42 $59,500 $1,320 45

Pre Retrofit Base 

Case (BC)

BC + Triple pane 

Low-e windows 76 $91,000 $1,631 55

While mini-split heat pumps are readily available, their market share remains low and 

incremental costs are high. Additionally, their benefits have yet to be completely quantified and 

performance over a range of operating conditions adequately understood. However, eliminating 

duct losses allows for system downsizing and, in homes where a single indoor unit sufficiently 

satisfies the loads, the savings achieved from eliminating a centrally ducted system may justify 

the costs. Also, previous lab and field studies by Ecotope have shown good agreement between 

lab, field and manufacturers’ rated COPs (NEEA 2011). 

Even with the high incremental costs, the builder is still committed to building to PH standards. 

This test house has allowed for identification of more cost-effective means of achieving similar 

deep energy savings. Based on his experience of installing the advanced building retrofit 

measures, the builder decided on the following changes for future projects. 

• Simplified wall assemblies: Exterior foam insulation above a certain thickness requires 

unique means of properly securing the sheathing, such as furring strips. These methods 

were labor intensive and unfamiliar to the installers. The builder conveyed that he would 

use a staggered double stud assembly for future deep retrofits and new homes to reduce 

thermal bridging. This approach incorporates construction practices that are familiar to 

builders, reducing both labor and material costs. This practice may not be feasible for 

many existing homes where converting single stud walls to double stud walls would be 

complicated, expensive, and reduce indoor floor area. In these cases, reducing exterior 

foam insulation to 1 in. of EPS can reduce costs significantly and improve the cost 

effectiveness of exterior wall retrofits in mild climates.

• Standardized air sealing: The builder used a variety of air sealing tape products in the 

Sonoma House. While this allowed him to test the effectiveness and usability of a 

number of brands, it was costly and complicated. From this experience, he has identified 

a single brand that is both cost effective and performs well. In addition, substituting the 

use of tape around wire and plumbing penetrations with EPDM gaskets would further 

reduce cost. While building gaskets are more expensive than standard caulk and foam, 

many in the PH industry endorse their benefits because of superior sealing capabilities, 

longer lifetime, and ease of installation and verification of proper application. The Grace 

Ice & Water Shield® used on the Sonoma House as a waterproofing layer on the exterior 

walls added unnecessary cost, and the builder felt that standard house wraps would 

suffice for this application if they are properly taped, and are about one third of the cost. 

• Solar thermal for DHW only: The complexity of integrating the solar thermal system 

with space heating (using the ERV fan) was not cost effective in the Sonoma House. 
36

Higher heating loads would be required to improve cost effectiveness. Increased solar 

storage capacity would increase the solar contribution but would likely not improve the 

cost effectiveness in the mild climate.

This project also identified measures and techniques that the builder considered successful and 

will continue to incorporate in future projects.

• Windows: As previously discussed, windows were the single most costly component of 

the project. However, in the builder’s opinion, the German-manufactured triple pane 

windows are unparalleled in performance, quality, and durability to any other products 

available in the United States. He has worked with products from many national 

manufacturers, including Serious Materials, and has not found a window with 

comparable glazing properties, both thermal performance and visible light transmittance, 

and longevity. Other benefits include superb air sealing. He will continue to import 

German windows in all future jobs that are able to support the costs. 

• Unvented attic: The builder felt that converting the attic to a non-vented space for 

mechanical equipment provided substantial energy benefits that were justified by the 

cost. For retrofit applications, the builder’s takeaway was that installing blown cellulose 

using netting in between the roof trusses and at knee walls was fairly simple. When the 

roof is being replaced anyway, adding rigid foam insulation is an easy modification. He 

would reduce the thickness of rigid to 1 in. for simplicity and cost savings. However, 

truss configuration and space constraints in some existing attics may complicate this 

process and it may be difficult to achieve an R-value equivalent to R-38 or R-49 at the 

roof, which is easily attained with a typical depth of blown-in cellulose or fiberglass at 

the ceiling level. Unless mechanical equipment and ducts are located in the attic, it may 

be more cost effective to seal the attic floor and blow in insulation. 

The extreme building envelope and window features will not likely become cost effective in mild 

climates with low heating and cooling energy needs anytime soon without significant material 

and labor cost reductions or significant increases in energy costs. While the incremental costs of 

the measures implemented to meet PH standards are much higher than can be justified by current 

energy savings, there still seems to be a growing market for PH. The builder indicated he 

receives one or two inquiries per week from homeowners inquiring about PH projects and none 

for conventional construction. The current market seems to lie in high end custom homes and 

deep retrofits, where the incremental costs for the energy features may be small in comparison to 

other non-energy features included in the house. In the case of the Sonoma project, the $96,000 

price tag for the energy features was only 10% of the overall cost of the deep retrofit. Regardless 

of the questionable cost effectiveness of homes built to PH standards, the program is having an 

impact on the general market for high performance homes, and is helping to educate builders on 

better building practices. Because of overlapping practices and standards, it is probable that the 

PH program may also yield benefits to other programs aimed at improving efficiency. The 

Sonoma project has received a lot of attention both nationally and locally, and the builder is 

currently working on a new construction PH project in Sonoma and is in the bid process for 

another PH deep retrofit. In this respect, the Sonoma House project was both marketable and 

commercially viable in selected markets.
37

5 Recommendations and Conclusions

Measured energy use of the Sonoma House matches reasonably well with expectations from 

BEopt modeling and confirms that the project has attained its energy savings goals. Savings over 

the pre-retrofit case, estimated from BEopt, are 56% of total source energy. Envelope measures 

including high R-value wall, floor, and ceiling assemblies and tight air sealing were effective at 

reducing building loads. Builder air sealing techniques were very successful and decreased

infiltration to 0.40 ACH50. The incremental costs to reduce building loads to this level were not 

insignificant. Retrofitting the house to the levels of insulation used is not currently cost effective 

in the mild climate where heating and cooling loads are relatively low. Through the process,

methods were identified to achieve similar results more cost effectively, although additional test 

houses will also be valuable to contribute further to this knowledge set. Parametric modeling 

with BEopt showed that the installed package of envelope measures provides twice the source 

energy savings than simply upgrading the heating and cooling system to a high efficiency unit.

The solar water heating system performed very well and was able to support the majority of the 

domestic water heating load on an annual basis. The average annual solar fraction was 78% with 

solar fractions during the summer months exceeding 90%. Tankless efficiencies were lower than 

expected, due in part to unnecessary firing of the tankless water heater during the summer when 

the solar tank was capable of supplying the entire load for a hot water draw, and also due to low 

daily hot water use. Tankless performance, when paired with solar hot water systems, can be 

improved if proper controls are in place. At its simplest, the tankless water heater can be 

unplugged during the summer months since monitoring shows that tank temperatures remained 

above 120o

F most of the summer (June-September) and never dropped below 104oF. A better 

solution would be to provide a bypass valve around the tankless water heater when storage tank 

temperatures are adequate for hot water loads. Additionally, mixing valve location can affect 

system performance. Some manufacturers recommend locating the mixing valve before the 

tankless to reduce the incoming temperature to a range so that the tankless may operate without 

cycling or thermally stressing the heat exchanger. However, this can cause unnecessary firing of 

the water heater, especially during the summer when entering temperatures may be sufficient to 

supply the water heating load. Controls developed by the manufacturers to better monitor 

entering conditions and only fire the water heater when necessary may also provide a solution.

The solar water heating system was not nearly as effective in offsetting space heating loads, only 

providing an estimated 20% of the annual heating load. Given the added complexity and cost, it 

does not make sense in mild climates. Additionally, Sonoma is relatively dry for a marine 

climate with fewer overcast days that other more humid marine locations, which may experience 

less solar radiation on an annual basis for conversion to useful thermal energy.

The installed windows do not qualify for the federal energy efficiency tax credit as an ENERGY 

STAR window because of the high SHGC. The ENERGY STAR window performance 

requirements are based on broad geographic regions that do not adequately reflect the subclimates within, and particularly marine climates. Sonoma has a very minimal cooling load with 

annual (HDD six times greater than CDD. Sonoma is in the same ENERGY STAR climate 

region as Fresno, California, which has over three times the CDD as Sonoma. We recommend 

that ENERGY STAR consider relaxing the requirement for low SHGCs in these climates, as 

modeling shows they increase annual energy use.
38

This test house has provided valuable insight into the performance and cost effectiveness of PH

strategies as a means of achieving deep energy retrofit savings in a marine climate. Builders, 

such as PassivWorks, who are willing to experiment with non-industry standard construction 

methods, as well as homeowners who are willing to fund such efforts, are invaluable to the 

process of identifying, defining, and refining cost effective practices for deep energy savings. 

The Sonoma House has made a valuable contribution in advancing research aimed towards zero 

net energy homes. Information gained from the construction process and detailed building 

monitoring has identified best practices, “lessons learned,” and other practical recommendations 

that will contribute to future deep retrofit and new construction projects. 
39

References

AHRI (2005). ANSI/AHRI Standard 1060. “2005 Standard for Performance Rating of Air-to-Air 

Exchangers for Energy Recovery Ventilation.” Air-Conditioning, Heating, and Refrigeration 

Institute. 

CARB (2010). 2010 Annual Progress Report Budget Period 3 (BP3). Attachment NNN: Sonoma 

Deep Energy Retrofit Summary Report. Prepared for the U.S. Department of Energy by the 

Consortium for Advanced Residential Buildings. December, 2010.

CEC (2008). “2008 Building Energy Efficiency Standards for Residential and Nonresidential 

Buildings.” California Energy Commission.CEC-400-2008-001-CMF. December, 2008. 

Hoeschele, M.; Weitzel, E.; McNeil, J.; Kosar, D. (2011). “California Field Performance of 

Advanced Residential Gas Water Heater Technologies.” Prepared by Davis Energy Group 

(DEG). December, 2011. 

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

CO: National Renewable Energy Laboratory. NREL/TP-550-49246. 

LBNL (2011). “Measured Performance of a Passive House Retrofit in Sonoma, CA. Lawrence 

Berkeley National Laboratory Monitoring Study of Deep Energy Retrofits in 

California. http://www.passivehouse.us/phc2011/2011%20Presentations%20PDF/Measured%20

Performance%20of%20a%20Passive%20House%20Retrofit%20in%20Sonoma,%20CA.pdf. 

Accessed 12/9/11.

NEEA (2011). “Ductless Heat Pump Impact & Process Evaluation: Lab-Testing Report”. 

Prepared for the Northwest Energy Efficiency Alliance by Ecotope, Inc. Report No. E11-225. 

July 14, 2011.

Straube, J. (2009). “The Passive House (Passivhaus) Standard - A Comparison to Other Cold 

Climate Low-Energy Houses.” Building Science Insights – 025. October 2009.
40

Appendix A: Building Plans

Figure 19. Floor plan with wall assembly descriptions 
41

Figure 20. Roof plan with roof assembly descriptions 
42

Figure 21. Wall and roof section details
DOE/GO-102012-3632 ▪ December 2012

Printed with a renewable-source ink on paper containing at 

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