Final Expert Meeting Report: 

Simplified Space Conditioning 

Strategies for Energy Efficient

Houses

Dave Stecher

IBACOS, Inc.

July 2011
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i 

Final Expert Meeting Report: Simplified Space Conditioning 

Strategies for Energy Efficient Houses

Prepared for: 

Building America

Building Technologies Program

Office of Energy Efficiency and Renewable Energy 

U.S. Department of Energy 

Prepared by: 

IBACOS, Inc.

2214 Liberty Avenue 

Pittsburgh, Pennsylvania 15222

NREL Technical Monitor: Michael Gestwick

Task Order KNDJ-0-40341-02 

Deliverable 1.3.4 

July 2011 
ii

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iii

Contents

List of Figures ............................................................................................................................................ iv

List of Tables ............................................................................................................................................... v

Definitions ................................................................................................................................................... vi

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

Background on Simplified Space Conditioning Systems and Heat Flow Characteristics within 

Houses and Justification of Need....................................................................................................... 1

Meeting Logistics........................................................................................................................................ 2

Topic ..........................................................................................................................................2

Location and Time .....................................................................................................................2

Attendees....................................................................................................................................2

Meeting Objectives and Agenda................................................................................................................ 4

IBACOS’ Research Questions ...................................................................................................4

Meeting Specific Objectives ......................................................................................................5

Agenda .......................................................................................................................................5

Presentations .............................................................................................................................................. 7

Experiences in Implementing Mini-Split Heat pumps – Carter Scott .......................................7

Massachusetts Two Point Heating: Monitoring Results – Kohta Ueno ....................................7

Single Point Cooling and Heating: Temperature and Energy Monitoring Results from Six 

houses – Duncan Prahl .........................................................................................................8

Simpler HVAC: Design Methods and Monitoring Results – Robb Aldrich ...........................10

Preliminary Temperature Data from GHI Townhomes – Lieko Earle ....................................10

Retrofit Strategies Incorporating Mini-Split Heat Pumps– Dave Robinson ............................11

IBACOS Simplified Space Conditioning Research Plan – Dave Stecher ...............................12

Research Findings from Electric Resistance Heat Displacement Program – David Baylon ...12

Discussion ................................................................................................................................................. 13

Answers to Key Questions .......................................................................................................13

Critical Issues .....................................................................................................................14

Technology and Research Gaps .........................................................................................16

Future Research .................................................................................................................16

Conclusions and Next Steps.................................................................................................................... 17

References................................................................................................................................................. 18
iv

List of Figures

Figure 1. Photo taken during Expert Meeting .......................................................................................... 4

Unless otherwise noted, all figures were created by IBACOS.
v 

List of Tables 

Table 1. Expert Meeting Contributors/Listen-only Participants (*)........................................................ 2

Table 2. Expert Meeting Agenda................................................................................................................ 5

 

Unless otherwise noted, all tables were created by IBACOS
vi 

Definitions

ACCA Air Conditioning Contractors of America

ACH air changes per hour

AHU air handling unit

ASHRAE American Society of Heating, Refrigeration and AirConditioning Engineers 

Bth British thermal unit

CFD computational fluid dynamics

cfm cubic feet per minute

CFM50/SF cubic feet per minute air leakage per square foot of enclosure 

area at 50Pa depressurization

COP coefficient of performance

DHU ductless air handler unit

DWH domestic water heating

EF energy factor

EER energy efficiency ratio

ERV energy recovery ventilator

GSHP ground source heat pump

HP heat pump

HPWH heat pump water heater

HRV heat recovery ventilator

HSPF heating seasonal performance factor

kWh kilowatt-hours

Low-E low emissivity

MBH thousand Btu/h
vii 

MMBtu million British thermal units

Pa pascals

PV photovoltaic system

RF recovery factor

RH relative humidity

R-x resistance value

SEER seasonal energy efficiency ratio

SF square foot or square footage

SHGC solar heat gain coefficient

SIPS structural insulated panel system

U-x overall heat transfer coefficient

VT visible transmittance

ZEH zero energy home
viii

Executive Summary

A Building America Expert Meeting was held on March 11, 2011, at the Seaport Hotel in 

Boston, Massachusetts on the topic of simplified space conditioning systems in low load homes. 

This meeting provided a forum for presentations and discussions on the interrelationship between 

advanced thermal enclosures, space conditioning systems, and comfort; and an outside peer 

review of IBACOS’ research plan for the topic. This report presents background on the topic, 

meeting objectives, logistics, invitees, and presentations. 

Seven presentations were given describing current implementation efforts in addition to field 

research documenting the thermal comfort and energy characteristics of installed simplified 

space conditioning systems in both new and retrofit applications. IBACOS’ 2011 research plans 

in the area were also presented and evaluated. The meeting was attended by 15 people with 

experience in the areas of design, construction, instrumentation, and analysis of houses with 

simplified space conditioning systems. The first major outcome of the meeting was a realization 

that although space conditioning with reduced size and complexity have been successfully 

implemented in many cases, more research is needed to characterize the means of interaction 

between defined airspaces in the house to enable simple systems to be designed to meet the 

demands of a mass market audience. The second major outcome of the meeting was a thorough 

critique of, and many improvements to, IBACOS’ research plans to define these characteristics. 

 
1 

Background on Simplified Space Conditioning Systems and Heat 

Flow Characteristics within Houses and Justification of Need

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

to the Building America House Simulation Protocol (HSP) (Hendron 2010) will be “low load.” 

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

space heating and cooling load of less than 10 BTU/hr-ft2. IBACOS hypothesizes that heating 

and cooling energy in low load houses will be distributed sufficiently throughout the house via 

convective currents through open doors or transfer grilles, buoyancy, and conduction through 

interior partition walls. This hypothesis is based on research performed by Fiest (2005) where 

computational fluid dynamics (CFD) models of dwellings meeting the Passivhaus energy 

standard indicate that, at 1.8°F delta T, it is possible to have conductive transfer of 0.3 to 0.6 

BTU/hr-ft2 of wall area and convective transfer of 300 to 600 BTU/hr per open interior door. 

Field test data obtained by IBACOS (2008, 2010a, 2010b) from a Passivhaus in Climate Zone 5 

also supports this hypothesis. This data indicates that sufficient thermal comfort for the occupant 

(according to ASHRAE 55-2010, ACCA, and other metrics based on air temperature, relative 

humidity, air speed, and mean radiant temperature) can be obtained in all parts of the house from 

cooling air supplied to a single point. There has also been work in the area of displacing 

resistance electric space conditioning with heat pump technology in existing homes in New 

England (Swift 2010) and the Pacific Northwest (Baylon 2010). While neither of these studies 

specifically focused on documenting occupant comfort, they did show 35% to 44% electric 

savings for heating.

There are alternatives to conventional central ducted space conditioning systems system, such as 

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

buoyant force or ventilation driven distribution. A recent Building America (BA) Meeting 

identifying gaps and barriers established “distributed space conditioning strategy” as a key 

research need in the space conditioning area.

One proven, general strategy for significant energy savings is to bring all of the ducts of a forced 

air system inside the conditioned space. During a retrofit of a conventional system, it is generally 

impractical to bring the ducts inside the conditioned space unless the attic roof deck is insulated, 

which can be costly. Advances in space conditioning equipment, such as multi-splits (heat 

pumps which combine a single outdoor unit with multiple indoor ducted or ductless air handler 

units), may provide distributed space conditioning solutions that would allow for abandoning 

duct systems in unconditioned spaces. 

More research is needed to evaluate the level of energy efficiency and the conditions where 

simplified space conditioning systems will work in new and retrofitted houses. Guidance is 

needed on the design and installation of these systems to support a wider adoption throughout the 

new construction and retrofit market. 

The purpose of this expert meeting was to recap the current state of knowledge in this area and to 

provide a peer review of IBACOS’s research plan for new and existing unoccupied test houses 

with minimized space conditioning systems. 
2 

Meeting Logistics

IBACOS held this session on Friday, March 11, 2011 at the Seaport Hotel in Boston, 

Massachusetts. Since the meeting was held immediately following the Northeast Sustainable 

Energy Association’s (NESEA) Building Energy 11 conference, many of the expert meeting 

participants were already in the area for the conference. Furthermore, webinar technology was 

used to enable one speaker from the West Coast to present without the need for two days of 

travel, round trip, to physically attend. 

Topic

Simplified Space Conditioning Strategies for Energy Efficient Houses 

Location and Time

Seaport Hotel 

200 Seaport Blvd, Boston, Massachusetts 02210-2031 

March 11, 2011 

Attendees

The meeting included 15 contributions from 15 industry representatives plus two IBACOS team 

members. Additionally, 18 individuals participated in some or all of the meeting via the webinar 

in listen-only mode. All meeting attendees are listed in Table 1. 

Table 1. Expert Meeting Contributors/Listen-only Participants (*) 

Name Organization

Linda Wigington* Affordable Comfort Inc.

Patrick O’Malley* Building Knowledge

Kohta UenoΥ Building Science Corp.

Honorata Wytrykowska Building Science Corp.

Daniel Bergey Building Science Corp.

Dan Dempsey Carrier Corporation

Hugh Henderson* CDH Energy/ARIES

Tom Hartman Coldham & Hartman Architects

Andrew Webster Coldham & Hartman Architects

Joe Swift Connecticut Light and Power 
3 

Jane Bugbee Connecticut Light and Power 

David Springer* Davis Energy Group

David BaylonΥ Ecotope

El Hassan Ridouane Fireside Hearth & Home

Patrick Gillis Florida Solar Energy Center

Peter Thibeault Fraunhofer CSE

Larry Brand Gas Technology Institute

Dave RobinsonΥ GreenEarthEquities

Thom Phillips* Habitat for Humanity of Michigan

Dave Stecher IBACOS, Inc.

Duncan PrahlΥ IBACOS, Inc.

Anthony Grisolia* IBACOS, Inc.

Glenn Cottrell* IBACOS, Inc.

Amber Wood* NAHB Research Center

Michael Gestwick National Renewable Energy Laboratory (NREL)

Jon Winkler NREL

Lieko EarleΥ NREL

Dane Christensen* NREL

Eric Wilson* NREL

Hassan Ridouane* NREL

Xia Fang* NREL

Marc Rosenbaum South Mountain Company

Robb AldrichΥ Steven Winter Associates Inc.

Srikanth Puttagunta* Steven Winter Associates Inc.
4 

Jordan Dentz* The Levy Partnership/ARIES

R. Carter ScottΥ Transformations, Inc.

Υ Speaker

Figure 1. Photo taken during Expert Meeting

Meeting Objectives and Agenda

This meeting provided a forum for presentations and discussions on the interrelationship between 

advanced thermal enclosures, space conditioning systems, and comfort. The meeting consisted of 

presentations and discussion related to energy and comfort measurement and metrics for low 

load space conditioning strategies; and evaluation of IBACOS’ research plan on these systems. 

Ventilation strategies, while important, were not a primary focus of this session. IBACOS feels 

that this could be a future session as this research area progresses. 

IBACOS’ Research Questions

IBACOS has defined the following research questions relative to this area of study: 

• Provided that the DOE’s long-term research is successful in other thermal enclosure areas 

(new and existing houses), what are the alternative strategies to conventional central 

ducted space conditioning systems that will provide thermal comfort for the occupant 

according to ASHRAE 55-2010, ACCA, and others (air temperature, relative humidity, 

air speed, and mean radiant temperature)? 

• What are the terminal conditions and parameters needed for simplified space 

conditioning systems (e.g. face velocity, Btu/cfm, duration of run cycle) to provide 

thermal comfort in new and existing homes in different U.S. climate regions? 
5 

• What is the ventilation distribution efficacy of simplified space conditioning strategies? 

What kinds of ventilation distribution strategies are needed?

• What technology gaps exist where new product development is needed to enable 

simplified space conditioning strategies to succeed in providing good thermal comfort, 

humidity control, and ventilation air distribution? What are the characteristics of the new 

products? 

• How do low load new houses and significantly retrofitted houses respond to externally 

induced peak load situations (e.g. when the outside temperature and RH are at or above 

design conditions) or internally induced peak loads (e.g., large gatherings of people in 

one space, significant home electronics waste heat)? What is the impact on simplified 

space conditioning strategies and operation? 

Meeting Specific Objectives

The major objectives of this meeting were to:

• Review the results of existing field data and participant experiences with respect to 

energy savings and occupant comfort in houses with minimized space conditioning 

systems. 

• Receive feedback on IBACOS’ simplified space conditioning research objectives and test 

plan. 

• Identify relevant and useful outputs for stakeholders using the results of this research. 

• Identify the current gaps between the existing market-based space conditioning products 

and future needs. 

Agenda

The meeting closely followed the agenda found in Table 2. 

Table 2. Expert Meeting Agenda

8:00 am – 8:15 am Introductions and Project Overview; IBACOS

8:15 am – 8:45 am Carter Scott – Strategies attempted and lessons learned from 

installing simplified space conditioning systems in houses with good 

thermal enclosures

8:45 am – 9:15 am Kohta Ueno – Monitoring results from a house built by Carter Scott, 

heated and cooled from two points (one per floor), including 

measurement of individual room temperatures, door state (open or 

closed), and system runtime

9:15 am – 9:45 am Duncan Prahl – One year of monitoring results from a house in 

Central Illinois with cooling provided from one point. Winter results 

from four houses in Massachusetts using a mini-split in the main 

living space, with electric resistance in other rooms. 

9:45 am – 10:00 am Break 
6 

10:00 am– 10:30 am Robb Aldrich – Design tool used to provide a reasonability check 

when considering single point space conditioning in a house. 

Application of tool in a duplex in Massachusetts. Monitoring Results 

and lessons learned.

10:30 am – 10:45 am Lieko Earle – Initial monitoring results from NAHBRC/NREL study 

of twelve 1930s era townhomes in Maryland with electric baseboard 

heat.

10:45 am – 11:15 am Dave Robinson – Strategy for and lessons learned from using 

concealed unit mini-splits with all ducts inside the envelope in 

retrofit homes in Fresno, California. Also a model whereby Energy 

Wise investors can do deep energy retrofits even Near Zero with little 

or no program assistance. 

11:15 am – 11:45 pm Presentation of IBACOS’ Simplified Space Conditioning Research 

Plan

11:45 pm- 12:45 pm Lunch

12:45 pm – 1:15 pm David Baylon – Pertinent results from multi-year, multi home study 

in the Pacific Northwest where, in each house, a mini-split heat pump 

was installed to supplant electric resistance heat.

1:15 pm – 2:15 pm Roundtable Discussion: Discussion of Research Strategy, Industry 

and Technology Gaps, and Opportunities; IBACOS

2:15 pm – 2:30 pm Break

2:30 pm – 3:15 pm Roundtable Discussion: Discussion of Research Strategy, Industry 

and Technology Gaps, and Opportunities (Continued); IBACOS 

3:15 pm – 3:30 pm Summary and Next Steps

3:30 pm Meeting Conclusion 
7 

Presentations

Seven presentations were given describing current research efforts related to simplified space 

conditioning systems in both new and retrofit applications. Additionally, IBACOS’ 2011 

research plans were presented. The presentations are summarized below.

Experiences in Implementing Mini-Split Heat pumps – Carter Scott

Transformations, Inc. is a builder in New England specializing in highly efficient houses. 

During his presentation, Carter Scott discussed several houses and the various attempts to install 

small mini-split heat pumps. Seven houses were discussed with installed system capacity from 

12 MBH for the Solar Ranch to 36 MBH for the Greek Revival and the Colonial, listed below: 

• Needham (one 9 MBH upstairs, one 12 MBH downstairs), 

• Farmhouse (two 9MBH upstairs, one 12MBH downstairs), 

• Greek Revival (2 x 9MBH upstairs, 2 x 9 MBH downstairs), 

• Colonial (originally 18 MBH down, 15MBH up, upgraded to 36 MBH with third zone), 

• Solar Ranch (one 12 MBH DHU) 

• Farmhouse II (2 x 12 MBH units) 

Wall-mounted, ductless air handler units (DHU) were used in most houses with at least one DHU 

per above grade floor. The upstairs DHU typically was mounted in the hallway and the main 

floor DHU mounted in the living room. In houses with multiple DHUs per floor, DHUs were 

placed in rooms with the greatest expected occupancy, while others were passively conditioned. 

Two houses used ducted-concealed air handler units (AHUs). Although the duct runs were 

generally short, the installation cost of the ducted AHUs was higher than the DHUs. Since the 

DHUs were installed prior to drywall with no additional work after, the entire space conditioning 

system was installed during a single visit to the house by the mechanical contractor.

Despite a design peak load of 17 MBH, the 33 MBH system in the Colonial house needed to be 

increased in capacity after the first winter, due to the heat pump derating during very cold 

weather. The change in total capacity was small, 33 MBH to 36 MBH, and a third zone was 

added in the basement to remedy one unintended consequence of removing the heating system 

and ductwork, and their associated “accidental” heat gains from the basement: cold floors on the 

first floor.

Any of these strategies might be considered risky by a traditional HVAC contractor, but in all 

cases (apart from the initially undersized Colonial house) no occupant comfort complaints were 

noted. 

Massachusetts Two Point Heating: Monitoring Results – Kohta Ueno

Monitoring results were presented by Building Science Corporation for the winter of 2011 from 

a house built by Transformations, Inc. The house was heated and cooled by two DHUs (one per 

floor). Measurements included individual room temperatures, door state (open or closed), and 

system runtime. 
8 

Thermal enclosure specifications of this 1,835 square feet, two-story house include R-45 walls, 

R-60 attic, R-28 Slab, U 0.2 (R-5) / 0.26 SHGC windows. One issue that was discussed was the 

apparent poor performance of one of the mini-split systems based on what appeared to be long 

runtime with low system output. However, as many of the participants noted, these mini-splits 

are not simply on or off, they vary system capacity and operational energy based on the what the 

system perceives the need of the house to be. System operation was measured using a state 

logger, which could only tell when the system was not completely off. So although the system 

appeared to be on, it may have just been operating at very minimal capacity. Other graphs Kohta 

presented indicating no correlation between exterior temperature and runtime supported this 

hypothesis. Additional concerns included potential snow blockage of airflow through the outdoor 

unit which had been experienced by many people using this system in the northeast due to the 

abnormally large amount of snow received by the region in the winter of 2011. It was noted that 

many traditional heat pump units account for this issue by using an upflow design instead of a 

side flow design. The exact physics of how this works were not discussed. 

The occupants used deep setbacks on the second floor, and a noticeable spike occurred in the 

hallway (where the mini-split was located) upon recovery. Data indicated that the master 

bedroom lagged behind other rooms. However, this was determined to be a sensor that fell off its 

original mounting location. 

The most notable result of this research is the impact of interior partition door state on room 

conditions, which until this point, had not been well documented. Door state results showed a 

much better correlation between rooms and the hallway when there was a high frequency of the 

door being open. Two bedrooms had door open frequency of 92%. While the third bedroom, 

which was used as storage, had a door open frequency of 44%. Generally the rooms were within 

7°F of the temperature in the hallway. However the third bedroom did experience a greater 

number of occurrences with a temperature differences larger than 7°F. The magnitude of these 

temperature differences was also greater than the other two upstairs bedrooms.

The results of this study showed that single point heating is effective at maintaining rooms close 

to set point, however leaving doors closed and deep setbacks can result in large temperature 

differences. 

 

Single Point Cooling and Heating: Temperature and Energy Monitoring Results 

from Six houses – Duncan Prahl

Key summer and winter data from one year of monitoring results from a house in Central Illinois 

with cooling provided from one point was presented and discussed. 

Specifications of the central Illinois 1,200 square feet (net), two-story house include R-60 walls, 

R-80 attic, R-20 or 50 underslab (depending on location), U 0.16 (R-6.25) / 0.61 SHGC windows 

(south orientation only, others low SHGC). The heating and cooling system consists of one 

inverter-driven mini-split heat pump with DHU located in the stairwell and electric resistance 

baseboard heaters in all rooms which are controlled by a central thermostat with individual unit 

restriction control.

Electric consumption monitoring results showed that heating energy was on par with energy 

required for plug loads, and cooling energy was slightly less than the ERV fan energy. 
9 

Temperature data showed that during cooling, all rooms were generally within 2°F of the 

temperature at the thermostat. Data also showed that during several of the hottest days of the 

season, the system was able to cool the house below the set point of the electric resistance heat, 

causing a brief period of simultaneous heating and cooling. The mini-split was able to maintain 

temperature despite full operation of the electric resistance heat. 

Although it is impressive that the cooling system maintained temperature, it brings to light an 

important issue of the need for controls integration for mini-split heat pumps when used in 

conjunction with other heating systems. During the winter time, lack of controls integration was 

a factor in the lack of operation of the heat pump. Due to the relatively confined space the 

stairwell where the DHU is located, it would heat up the stairwell quickly and turn itself off, as 

the temperature sensor is located in the DHU. The location of the DHU also prevented sufficient 

heating energy from reaching the downstairs living space, leaving the thermostat for the 

baseboard heat dissatisfied, and resulting in the operation of the electric resistance heat. Due to 

this issue, the occupant chose to not operate the heat pump during the winter. Room 

temperatures remained uniform within 2°F of the temperature at the thermostat location.

Winter results from four houses in Massachusetts using a mini-split in the main living space, 

with electric resistance radiant panel heaters in other rooms were then presented and discussed. 

Specifications of the Massachusetts two-story houses include R-31walls, R-49 attic, R-20 

foundation and under slab, U 0.19 (R-5.25) windows. One inverter driven mini-split is used with 

DHU located in the main living space, and electric radiant panels are installed in bedrooms, 

which were controlled by individual thermostats in each room.

Monthly energy consumption of the heat pump and electric resistance heat were each submetered. Temperature was measured in each of the three bedrooms and compared with the 

measured temperature in the main living area by categorizing the hourly sampled temperature 

differences into 4°F temperature bins for each month. The results showed that the two houses 

with greater operation of electric resistance heat had more consistent temperatures throughout the 

house, without substantially more total electricity consumption. In these two houses, between 

50% and 60% of the time all rooms were within +/- 2°F of the temperature in the conditioned 

zone, with remaining samples evenly distributed into bins immediately adjacent to the +/- 2°F 

bin. For the two houses that had greater heat pump operation, temperatures of all rooms were 

within +/- 2°F only 25% to 40% of the time. Outlying bins were consistently grouped in the 

range of temperature below -2°F of the thermostat. What was unknown from this study was the 

impact of door state (open/closed). 

ASHRAE standards indicate that none of these houses would be comfortable to the typical 

occupant. However, in all of these houses, occupants had individual thermostat control and 

chose to operate the house in the manner indicated by the results. 

Finally, test data from a two-week test from February 16th to 28th, 2011 at IBACOS’ cold 

climate new construction unoccupied test house was presented. During outdoor temperatures 

averaging approximately 40°F, when all second floor bedroom doors were closed and 

conditioned air was provided only to the hallway, the bedrooms were on average 7°F cooler. 

Within 3 hours of opening the doors, temperatures stabilized to about +/- 1°F. 
10 

Simpler HVAC: Design Methods and Monitoring Results – Robb Aldrich

Robb presented results from work performed on four houses in Massachusetts including the 

design tool used to determine the amount of fan forced airflow necessary in non-directly 

conditioned rooms along with monitoring results and lessons learned from the built houses. 

Specifications of these four, two-story, two-bedroom houses in western Massachusetts built by 

RDI include R-40+ walls, R-50 attic, with triple pane windows. A two-stage (10.2 MBH low 

stage, 16 MBH high stage) gas fired unit heater is located in the main living space. To facilitate 

circulation from the main living space to the upstairs bedrooms, an exhaust fan was installed that 

pulled air from the ceiling of the living room and supplied it to the upstairs bedrooms. 

Robb demonstrated the Excel-based spreadsheet tool that was used during the design process to 

estimate the contribution steady state heat transfer to the rooms through interior partition 

structures and to determine the amount of fan airflow necessary to maintain uniform relative age 

of air in each room and to help equalize temperatures in these houses. This type of calculation is 

appropriate when designing buildings with any habitable rooms that will not be actively 

conditioned. 

After the houses were completed, the modeling results were validated when tracer gas tests 

conducted by NREL indicated that the operation of the fan provided sufficient circulation to the 

rooms with doors closed, equivalent to having the doors open. This study also showed that when 

the doors were closed with the fan off, room air circulation was greatly reduced. 

Temperature data collected through the winter of 2011 indicated that for three houses, all spaces 

in the home were within 4°F at least 80% of the winter. The fourth house had cooler bedrooms, 

most likely due to an oversized heater being specified and the use of an aggressive thermostat 

setback strategy by the occupants. 

Also presented were monitoring results from a two-story house using a mini-split heat pump with 

one wall mounted AHU on the main floor and one in one of the two bedrooms in the finished 

walkout basement. The occupants disliked the aesthetics of the wall-mounted AHUs, reported 

that there were thermal and acoustic comfort issues with the ERV unit and supply air locations. 

The occupants also disliked not having warm floors as in a radiant floor system, but did have 

desirably low energy bills.

Preliminary Temperature Data from GHI Townhomes – Lieko Earle

Lieko presented initial monitoring results from a study by NAHBRC/NREL of twelve 1930s era 

townhomes in Maryland with electric baseboard heat. 

Greenbelt Homes is a cooperative housing community consisting of 1,571 homes constructed by 

the federal government between 1936 and 1941. It consists of two-story townhomes with three 

different construction types: concrete masonry unit (CMU), wood framed with brick façade or 

wood framed with vinyl siding. Initially constructed with essentially no insulation, in the 1980s, 

insulation was installed where possible (e.g. attics, framed wall cavities, and crawlspaces).

Heating is provided by baseboard heaters with individual thermostats located in each room. 

Cooling is via window or through wall air conditioners where installed. 
11 

Greenbelt Homes has planned several upgrades for 2015 including: windows, doors, and 

baseboard heaters. Twenty-eight units are planned to be upgraded via a phased implementation 

with monitoring occurring before upgrade and between each phase. Occupant concerns range 

from economy-oriented desire to control the temperature in each space, to comfort-oriented 

desire for in-floor heating and whole house cooling. 

Data from three units was presented for the period between November 28th and December 22nd. 

Each unit had a distinctly different operation with average heating electrical energy of 5, 50, and 

100 kWh per day for the three units. The units had correspondingly different operational 

temperatures: 55 to 65°F, 65 to 70°F, and 70 to 75°F respectively. The range of electricity 

consumption that was broader than the range of temperatures for the three units was likely due to 

conduction through party walls between units. It appears that the 5kWh/day house was being 

heated by its neighbors, and the 100kWh/day house was heating its neighbor’s house. In many 

cases the temperature readings indicate that occupants are using the thermostat knob as a throttle 

instead of leaving it at a fixed set point. 

Retrofit Strategies Incorporating Mini-Split Heat Pumps– Dave Robinson

This presentation was on strategies and lessons learned by Green Earth Equities from installing 

mini-split heat pumps with wall-mounted DHUs and concealed ducted AHUs with all ducts 

inside the envelope in retrofit homes in Fresno, California. A model whereby energy wise 

investors can do deep energy retrofits up to near zero energy homes with little or no program 

assistance was discussed. 

Energy efficiency upgrades, combined with interior and exterior cosmetic makeovers have been 

executed successfully on 12 foreclosed houses (3 bedroom, 2 bath) in Fresno, California and sold 

at a profit. Specifications for the 1,200 square feet, one-story one house presented include R-16 

walls, R-55 attic, U 0.30 (R-3.33) windows. The HVAC system has been retrofitted by 

eliminating all ductwork from the unconditioned attic and replacing the traditional ducted forced 

air heat pump with an ACCA properly-sized inverter driven mini-split connected to a DHU 

located in the main living space and an AHU concealed in the hallway ceiling and ducted to the 

bedrooms via short straight sheet metal ducts located within the thermal enclosure. House air 

sealing performed with blower door verification and an ERV provides balanced and continuous 

ventilation by delivering fresh air directly into the main living space near the hallway return for 

the ducted AHU. Utility bill data collected through the first summer of occupancy shows 

summertime electric consumption for cooling of between 100 and 200 kWh per month. 

Based on Dave’s experience, the successful retrofit business is a balance between remodeling, 

real estate and building performance. The basic strategy includes choosing the right house, 

designing it properly, building it well, marketing it effectively and selling it for a profit. Energy 

package hard costs generally constitute 25% of the total upgrade cost while the remainder is 

spent for key components necessary for the market viability of the retrofit house including: 

granite counters, new tile and wood floors, carpeting, new roof, outdoor plantings and other 

cosmetic details. Ideal houses for this strategy are 30 to 50 year old tract houses with three or 

four bedrooms and two baths. These have the best opportunity for both cosmetic and energy 

upgrades with the least potential for more difficult repair problems.
12 

One notable benefit for contractors working for a foreclosure retrofitter is that since the houses 

are unoccupied and they have repeatable business, they end up doing more projects with less 

marketing costs and inconveniences than would occur performing retrofits with individual clients 

in occupied houses.

IBACOS Simplified Space Conditioning Research Plan – Dave Stecher

IBACOS presented research plans for three unoccupied test houses. Test Plans (KNDJ-0-40341-

02 Deliverables 2.1.1, 7.1 and 7.3) have been submitted to NREL for each of these houses. The 

executive summary appears below.

“The purpose of the three houses is to create research facilities to determine the level of 

reduction of fan forced distribution that is possible while maintaining satisfactory occupant 

comfort in a well insulated, airtight house by comparing the seasonal performance of three 

different distribution systems. This test plan includes questions addressing the specific thermal 

energy output performance aspects of the equipment, but moreover, the relationship between the 

distribution method, the house and the occupants. Primarily: 

• To what magnitude does the interior temperature and relative humidity vary in different 

rooms under various weather conditions? 

• How does the system respond to extreme load conditions? 

• How does the system respond to erratic thermostat behavior?” 

Research Findings from Electric Resistance Heat Displacement Program – David 

Baylon

David presented pertinent results from multi-year, multi-home study conducted by Ecotope in 

the Pacific Northwest where a mini-split heat pump was installed to displace the use of electric 

resistance heat throughout much of the year to provide energy savings. The total house electricity 

consumption before and after retrofit was analyzed for 84 homes, in addition to measurement of 

the electricity consumption of the mini-split heat pump installed in each house. The goal is to 

determine to what degree the mini-split heat pump reduced house heating electricity 

consumption. 

The results showed a wide range of mini-split heat pump operation. Some houses had almost noheat pump operation, with almost all heating continuing to be done by electric resistance heat. In 

other cases, no electric resistance heat was used by the occupants after the installation of the 

ductless heat pump. Although no individual room temperature measurements were taken, this 

indicates that at least for some houses, the single point mini-split heat pump was able to satisfy 

the occupants comfort requirements. 

Preliminary results indicate electricity savings from measured houses in climate zone 4 was 

between 41% and 51%. Houses in climate zone 5 and 6 ranged from 16% to 27% savings. 

However, the absolute savings were similar across all climates. The percentage savings appeared 

to be greater in milder climates primarily because the amount of heating required in these cold 

periods is much higher in zones 5 and 6 than in zone 4 and the mini split was not sized for the 

total heating needs of the house. COP reduction was secondary, in most cases less than 30%, 

occurring only as temperatures fell below 20 °F. 
13 

Discussion 

The primary purpose of the discussion portion of the meeting was to critique IBACOS’ 

unoccupied test house Test Plans. The key questions, critical issues, gaps and barriers, and 

future research discussed by the meeting participants are summarized below.

Answers to Key Questions

Several questions were discussed regarding research goals and test equipment setup in the 

houses: 

• What should the range of supply air temperatures be for short term airflow tests?

The range of temperatures to be tested during short term airflow testing was 

discussed and it was agreed that for heating the range should be from 90°F to 

140°F. Temperatures outside of this range were considered to be unreasonable for 

any production heating system to produce. 

• What room-to-room temperature differences are considered “acceptable”?

As the temperature difference between the indirectly conditioned rooms and the 

directly conditioned spaces changes, the amount of heat transfer increases, 

however if the rooms are occupied, a strict +/- 2°F temperature range is applicable 

(although thoroughly debated by members of the meeting). If the room is 

unoccupied, larger temperature variations may be acceptable.

• Where should the thermostat be located?

In all test cases the thermostat will be located in the main living space which is 

actively conditioned in all distribution test cases.

• How should heating and cooling be generated in the house? 

The option of using a single air handler unit containing a hydronic coil with 

heated and cooled water was well received by those in attendance as a good 

system to use to facilitate easy measurement of the amount of thermal energy 

entering the room. Some questions arose around whether or not the system 

should be operated as a modulating system. The challenges are that there was 

little understanding of exactly how modulating systems work (e.g. controls 

programming for appropriate response). There was consensus among the group 

that modulating systems would provide inherently better comfort than nonmodulating systems. So it was decided that operating the houses as a nonmodulating system would represent the worst case scenario for occupant comfort 

characteristics of the three distribution systems. 

• What “load” will the test respond to?

The load of the house and system size will be according to ACCA calculations. 

The true load of the house will be determined via field measurements of the 

thermal output of the air handler unit. One suggestion from the attendees was to 

modify “tonnage” to evaluate remote room comfort. The example was given that 

in a house with two 12MBH mini-split heat pumps, it would be possible to just 

operate one heat pump, cutting the total system capacity in half, and measure the 
14 

response of the house to this undersizing to determine if dramatic temperature 

fluctuations occur during peak conditions. This example strategy will not be 

implemented initially, but depending on the results of the first year of operation, 

IBACOS will consider evaluating the house in an undersized condition. 

• How many temperature measurements are necessary?

Another question that was raised was how many temperature measurements 

would be needed per room on a long-term basis. It was the opinion of some of the 

experts there that as few as one air temperature measurement per room could be 

sufficient to characterize the performance of the different distribution systems.

• What additional house characteristics do designers need to know to implement simplified 

space conditioning systems? 

A goal that developed through this meeting is to develop a target thermal 

enclosure area weighted thermal conductivity value (UA value) and load (Btu/h) 

for each climate that are thresholds, that if achieved will allow each particular 

distribution system to work. Furthermore, in a given room, the relationship 

between the heat transfer through the thermal enclosure and the heat transfer 

through the interior partitions or openings must be balanced if active distribution 

is to be avoided for that room. 

Critical Issues

Although there are many critical issues related to space conditioning outlined in the IBACOS 

unoccupied test house Test Plans, during this meeting the following issues were brought to light 

by the experts as key issues that were not overtly addressed in the plan and should be addressed 

in some way by IBACOS or other researchers:

• Comfort expectations in retrofit houses

o In existing houses, comfort levels may already be well below ASHRAE levels. 

But in retrofits, how much of an improvement is necessary to satisfy occupants 

that the house is more comfortable? Are strategies improving energy efficiency 

by reducing the total usable area of the house by segmenting and retrofitting only 

a few viable rooms? 

o How quickly can the house recover from a deep temperature setback (e.g grid 

failure, vacation)? 

• The impact of basements

o Basements add an additional level of complexity when trying to reduce the 

amount of active distribution in a house. Many issues were discussed regarding 

what the possible influences of an insulated but unfinished basement might be on 

the comfort levels in the habitable portions of the house. This includes: what 

temperature should a designer expect or require the basement to be conditioned 

to, and if there are ASHRAE or GRI requirements or existing research in the area. 

Depending on the level of active conditioning in the basement, does it present a 

parasitic load on the rooms above it or is it actively heating those spaces? It was 

determined that this was out of the project scope of the two slab on grade hot-dry 

unoccupied test houses that IBACOS is proposing, however, pending a suitable 
15 

house in the cold or mixed humid climate with basement, some of these issues 

could be explored. 

• Air tightness requirements in new and retrofit houses 

o The level of air tightness in houses can have a substantial impact on occupant 

comfort; in new construction houses it was accepted by the group that air leakage 

rates of 1 to 1.5 ACH50 are reasonable to achieve. However, in existing 

construction houses, the situation is much more variable. Those with experience 

in retrofit of existing houses indicated that air leakage rates of 3.5 to 5 ACH50 

would be readily achievable without extensive changes to the house, provided that 

combustion safety is addressed when decreasing the air infiltration rate of an 

existing house. 

• Measuring and documenting air leakage 

o Another question that arose was the way in which air leakage is documented. The 

ACH50 metric allows houses with good volume to surface area ratios to score 

better, which generally favors large houses while punishing small houses. Some 

in attendance prefer the metric of air leakage flow rate (cfm) at 50 Pascals (Pa) 

depressurization divided by the total building thermal enclosure surface area (all 

six sides, also known as CFM50/SF shell). Using this type of metric is also 

important in the design of houses capable of incorporating minimized space 

conditioning strategies, as individual room loads can be influenced by the amount 

of exterior surface area. 

o It was agreed that .1 CFM50/SF shell was an appropriate goal for new 

construction houses or deep energy retrofits. Green Earth Equities has been able 

to achieve 0.2 CFM50/SF shell on several houses, albeit these houses are slab on 

grade with stucco finish, which inherently lends itself to an airtight structure. This 

number is less than what Building Science Corporation uses (0.25 CFM50/SF

shell) as their new construction air tightness standard for houses built on a large 

scale production basis. However more research should be done on a national basis 

to determine what levels of airtightness are reasonable to achieve during moderate 

retrofits based on regional house construction typology. 

o After some discussion it became clear that just applying the per unit area air 

leakage rate would be insufficient for calculating room loads, and that the position 

of the room in the house must also be considered. There are two primary means 

by which air leakage leads to occupant comfort: cold jets of air hitting the 

occupant directly (commonly referred to as drafts), and whole house stratification 

due to the stack effect enabled by leakage points near the base and top of the 

house. Although the unit area air leakage rate may help in identifying the impact 

of the latter, it does not help with the former. Cold jets of air that may hit 

occupants will create local areas of occupant discomfort, and should be identified 

during the air leakage test and fixed. The impact of air leakage on natural 

convective air movement inside the house should be identified and documented in 

test houses.
16 

Technology and Research Gaps

The following technology and Research Gaps were identified during the meeting: 

• When a mini-split heat pumps is oversized for heating, does its ability to vary capacity 

produce adequate cooling and dehumidification to maintain comfort? 

• Measuring equipment power draw/stage for variable capacity systems and, in general, 

understanding of the operation of variable capacity systems is challenging. 

o On – Using simple electrical measurements, it is difficult to determine what the 

amount of conditioning energy that a variable capacity mini-split is outputting 

into a house because there are hundreds of thermal output possibilities as opposed 

to the more than two or three distinct output stages of a typical forced air system. 

It is not clear what factors the manufacturers use in their control algorithms to 

have the system operate at a particular capacity, and for how long before 

modifying that capacity. 

o Off – When the system is not providing any heating or cooling, does it 

periodically or continually run a circulation fan through the unit? What are the 

standby functions that require energy when the unit is “off”? 

• Snow buildup around outdoor unit impacts performance. 

o Issue for mini-splits, however, according to an attendee, up flow standard (nonmini-split) heat pumps designed for the American market are less susceptible to 

this issue. 

• There is a need to standardize reporting and measurement methodologies. 

• What are the energy and comfort implications of setbacks in houses with minimized 

space conditioning systems? 

• Control systems and strategies are needed that can integrate the use of products that were 

not originally designed to be used together such as mini-split heat pumps and electric 

baseboard heat, but also consider heat pump water heaters, and whole house ventilation 

strategies.

Future Research

Several possible interesting research areas came up during the course of discussion. While these 

will not be performed by IBACOS in 2011, they are worth noting and may be included in future 

research plans:

• What is the human response expectation to satisfy comfort? Research presented through 

this meeting has shown that occupant comfort is variable based on the personal desires of 

the occupants. Have these expectations changed since the last time ASHRAE comfort 

guidelines were developed? 

• Once basic concepts are proven, it would be useful to determine what ASHRAE/ACCA 

Guidelines are impacted, such as space conditioning loads, airflow, and sizing strategies, 

and modify those guidelines appropriately to accommodate space conditioning systems 

with minimal or no active distribution.
17 

• There is a large opportunity to reduce wintertime heating energy consumption of 

buildings with electric resistance heat. One way presented in the meeting was via the 

installation of air source mini-split heat pumps. Are there other viable systems that can 

be implemented? 

• After the physical requirements are known, determine the size of market for simplified 

space conditioning systems. 

Conclusions and Next Steps

IBACOS has incorporated the results of this meeting into its Test Plans for three unoccupied test 

houses to be constructed in 2011 and operated through 2012. 
18 

References 

Baylon, David, et al. Performance of Ductless Heat Pumps in the Pacific Northwest: Proceedings 

of the 2010 ACEEE Summer Study on Energy Efficiency in Buildings. ACEEE, Pacific Grove, 

California, August 15-20, 2010. 

Feist, W., et al., 2005. Re-inventing Air Heating: Convenient and Comfortable within the Frame 

of the Passive House Concept,. Energy and Buildings 37, 1186-1203. 

Hendron, Robert and Engebrecht, Cheryn. Building America House Simulation Protocols. 

Golden, CO: National Renewable Energy Laboratory. October 2010 

IBACOS. DE-FC26-08NT02231. Building America Program Annual Report Budget Period 1. 

Pittsburgh: IBACOS. December, 2008. 

IBACOS. DE-FC26-08NT02231. Building America Program Annual Report Budget Period 2. 

Pittsburgh: IBACOS. February 2010a. 

IBACOS. DE-FC26-08NT02231. Building America Final Technical Report. Pittsburgh: 

IBACOS. December 2010b. 

Swift, Joseph, and Rebecca Meyer, Ductless Heat Pumps for Residential Customers in 

Connecticut. Proceedings of the 2010 ACEEE Summer Study on Energy Efficiency in Buildings. 

ACEEE, Pacific Grove, California, August 15–20, 2010. 
DOE/GO-102011-3344 ▪ July 2011

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