Ductless Mini-Split Heat Pump 

Comfort Evaluation 

K. Roth, N. Sehgal, and C. Akers

Fraunhofer Center for Sustainable Energy Systems

March 2013
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iii

Ductless Mini-Split Heat Pump Comfort Evaluation

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: 

K. Roth, N. Sehgal, and C. Akers

Fraunhofer Center for Sustainable Energy Systems 

25 First Street, Suite 102

Cambridge, MA 02141

NREL Technical Monitor: Chuck Booten

Prepared under Subcontract No. KNDJ-0-40345-00

March 2013
iv

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v

Contents

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

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

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

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

1 Introduction and Field Deployments .................................................................................................. 1

1.1 Introduction...................................................................................................................................... 1

1.2 Research Questions.......................................................................................................................... 2

1.3 Experimental Design and Field Deployments ................................................................................. 2

1.3.1 Field Deployment: Austin, Texas....................................................................................... 2

2 Thermal Comfort Analysis................................................................................................................... 4

2.1 Procedures........................................................................................................................................ 4

2.1.1 Hourly Comfort Analysis.................................................................................................... 4

2.2 Daily Mean Temperature Analysis.................................................................................................. 5

2.2.1 Assumptions........................................................................................................................ 5

2.3 Thermal Comfort Based on ASHRAE Standard 55 (2004)............................................................. 6

2.3.1 Thermal Comfort Based on Either Cooling or Heating Operation ..................................... 6

2.3.2 Thermal Comfort Performance in Cooling and Heating Modes......................................... 8

3 System Costs...................................................................................................................................... 13

4 Conclusions and Recommendations ............................................................................................... 14

References................................................................................................................................................. 15

Appendix A: Thermal Comfort Performance Graphs for ASHRAE Standard 55, for Household1.... 16
vi

List of Figures

Figure 1.Austin duplex DMSHP deployment site..................................................................................... 2

Figure 2.Floor plan of Austin duplex DMSHP deployment site (second unit is mirror image, along 

garage-kitchen-dining room wall)....................................................................................................... 3

Figure 3.Thermal comfort performance of CAC for either cooling or heating operation for Living 1; 

hourly analysis...................................................................................................................................... 7

Figure 4.Thermal comfort performance of DMSHP for either cooling or heating operation for 

Living 1; hourly analysis...................................................................................................................... 7

Figure 5.Thermal comfort performance of CAC for Bed1a in cooling mode for clo 0.5 or clo 1.0; 

hourly analysis...................................................................................................................................... 9

Figure 6.Thermal comfort performance of DMSHP for Bed1a in cooling mode for clo 0.5 or clo 1.0; 

hourly analysis...................................................................................................................................... 9

Figure 7.Thermal comfort performance of DMSHP for Bed1a for 11:00 p.m. to 7:00 a.m.; hourly 

analysis................................................................................................................................................ 10

Figure 8. Thermal comfort performance of DMSHP for Bath 1 for either cooling or heating; hourly 

analysis................................................................................................................................................ 10

Figure 9. Temperature distribution with DMSHP in different rooms, household 1............................ 12

Figure 10. Temperature distribution with DMSHP in different rooms, household 2.......................... 12

Figure 11. Thermal comfort performance of CAC for Bed1a for either cooling or heating; hourly . 16

Figure 12.Thermal comfort performance of DMSHP for Bed1a for either cooling or heating; 

hourly................................................................................................................................................... 16

Figure 13. Thermal comfort performance of CAC for Bed 2a for either cooling or heating; 

hourly................................................................................................................................................... 17

Figure 14.Thermal comfort performance of DMSHP for Bed 2a for either cooling or heating; 

hourly................................................................................................................................................... 17

Figure 15.Thermal comfort performance of CAC for Bath 1 for either cooling or heating; hourly .. 17

Figure 16.Thermal comfort performance of DMSHP for Bath 1 for either cooling or heating; 

hourly................................................................................................................................................... 18

Figure 17.Thermal comfort performance of CAC for Living 1 for cooling; hourly............................. 18

Figure 18.Thermal comfort performance of DMSHP for Living 1 for cooling; hourly........................ 19

Figure 19.Thermal comfort performance of CAC for Bed2a for cooling; hourly................................ 19

Figure 20.Thermal comfort performance of DMSHP for Bed2a for cooling; hourly........................... 19

Figure 21.Thermal comfort performance of CAC for bath 1 for cooling; hourly................................ 20

Figure 22.Thermal comfort performance of DMSHP for bath 1 for cooling; hourly........................... 20

Figure 23.Thermal comfort performance of CAC for Living 1 for heating; hourly............................. 20

Figure 24. Thermal comfort performance of DMSHP for Living 1 for heating; hourly....................... 21

Figure 25. Thermal comfort performance of CAC for Living 1 for heating; hourly............................ 21

Figure 26.Thermal comfort performance of DMSHP for Bed2 for heating; hourly............................. 21

Unless otherwise indicated, all figures were created by Fraunhofer.
vii

List of Tables

Table 1. Austin DMSHPs Deployed ........................................................................................................... 3

Table 2. Thermal Comfort Performance for CAC and DMSHP for Either Cooling or Heating—Hourly 

Comfort Analysis.................................................................................................................................. 6

Table 3. Thermal Comfort Performance CAC and DMSHP for Either Cooling or Heating—Daily 

Mean Temperature Analysis................................................................................................................ 6

Table 4. Thermal Comfort Performance for DMSHP and CAC in Cooling Mode Hourly Comfort 

Analysis ................................................................................................................................................. 8

Table 5. Thermal Comfort Performance for DMSHP and CAC in Cooling Mode Daily Mean 

Temperature Analysis .......................................................................................................................... 8

Table 6. Thermal Comfort Performance for DMSHP and CAC Heating Mode; Hourly Analysis ...... 11

Table 7. Thermal Comfort Performance for DMSHP and CAC Heating Mode; Daily Mean 

Temperature Analysis ........................................................................................................................ 11

Table 8. Summary of Economics of a DMSHP Retrofit ......................................................................... 13

Table 9. Summary of Economics of a CAC Retrofit .............................................................................. 13

Unless otherwise indicated, all tables were created by Fraunhofer.
viii

Definitions

COP Coefficient of performance

CAC Central air conditioning system

DBT Dry bulb temperature

DMSHP Ductless mini-split heat pump
ix

Executive Summary

Ductless mini-split heat pumps (DMSHPs) offer several advantages over conventional ducted 

heating, ventilation, and air conditioning systems. They save energy, eliminate the potential for 

in-duct mold growth, eliminate the space consumed for duct runs, and are easy to install in 

retrofits, as they do not need ducts.

The Fraunhofer team conducted field tests in two homes in Austin, Texas, from October 2011 to 

June 2012 to evaluate the comfort performance of DMSHPs. Specifically, we evaluated 

temperature and relative humidity (RH) measurements in four rooms in each home, both before 

and after retrofitting each home’s central heating and cooling system (CAC) with a DMSHP.

Our analysis of the field measurements yielded several comfort-related observations. This small 

sample relies strongly on residents’ preferences, so the findings should be used as a case study 

evaluation rather than as a general finding. 

1. ASHRAE Standard 55 Evaluation.

a. Living room. The percentage of time within the comfort zone, for either heating 

or cooling operation, was similar for the CAC and DMSHP, suggesting that the 

DMSHP indoor unit in the living room operates much like a CAC type system. 

Periods when the DMSHPs did not stay within the ASHRAE comfort zone were

primarily when the RH was higher.

b. Bedroom. A large portion of the measurements do not lie within the comfort zone 

for 0.5 clo, but do fall within the comfort zone for 1.0 clo. Thus, we conclude that 

residents preferred lower temperatures during the cooling season than those 

indicated by ASHRAE standard 55 (assuming 0.5 clo).

c. Bathroom. The DMSHP had an appreciably higher portion of time when 

conditions lay outside the 0.5 and 1.0 clo comfort zones, primarily because of

higher RH levels. This probably reflects the lack of DMSHP indoor units in the 

bathrooms; vent fan use was not, however, monitored.

d. In general, the RH levels were quite high for the DMSHP cases, which the 

occupants also noted. This suggests that the occupants did not run the DMSHP in 

its dehumidification mode, but instead tried to achieve lower RH by cooling the 

indoor spaces.

2. Homogeneity of indoor temperatures, cooling season. The maximum average hourly 

temperature difference among rooms on days where the outdoor temperature was 85°F or 

higher was around 3°F. 
1

1 Introduction and Field Deployments

1.1 Introduction

Ductless mini-split heat pumps (DMSHPs) offer several advantages over conventional ducted 

heating, ventilation, and air conditioning (HVAC) systems. They may save energy, eliminate the 

potential for in-duct mold growth, eliminate the space consumed for duct runs, and are easy to

install in retrofits, as they do not need ducts. Although DMSHPs are popular in Asia, they 

account for only 1% of residential HVAC systems installed in the United States (Baylon et al. 

2010).Most U.S. consumers, architects, and HVAC installers are wary of DMSHPs, because they 

have limited or no familiarity with these systems and how they perform in the field.

Also, DMSHPs have the potential to achieve significant energy savings in at least three ways:

• They eliminate duct losses from leakage and conduction that lose an average of 20% of 

thermal energy delivered to the ducts by HVAC systems (Roth et al. 2006, based on 

Jump et al. 1996). In homes where ducts are primarily located in unconditioned spaces, 

such as attics in southern locations, the energy penalty will tend to be greater, ranging 

from 25% to 40% (NREL 2004). 

• DMSHPs with multiple indoor units inherently provide zoned space conditioning. 

Occupants can use these to achieve further energy savings by allowing temperatures to 

float during unoccupied periods. This capability is enhanced in units integrated with an 

occupancy sensor, which are primarily available outside the United States(Moore 2011).

• They typically have variable-capacity compressors (inverter-driven for continuous 

capacity modulation) and variable-speed fans (Roth et al. 2006).Relative to conventional 

systems with fixed cooling and heating capacities and single-speed blowers, this results 

in annual energy savings of about 35% (ADL 1999). Moreover, these variable capacity 

and volume capabilities greatly enhance their ability to efficiently dehumidify at low 

sensible heat ratios, so occupants are less inclined to select lower cooling set points solely 

to run the air conditioning for dehumidification (for example, periods of moderate 

outdoor air temperatures and higher wet bulb temperatures, such as Florida in 

winter).Such conditions are also more common in low-energy homes, where sensible 

cooling loads through the building enclosure are greatly decreased, but indoor moisture 

loads do not change (Dieckmann 2008).

The space heating capacity and efficiency of conventional air source heat pumps diminish as 

outdoor air temperatures decrease. They thus require supplementary electric resistance space 

heating during colder periods (typically when outdoor temperatures approach freezing) that 

compromises their primary energy savings potential and operational cost. Over the last few 

years, “cold climate” air source heat pumps (units optimized to meet space heating loads instead 

of space cooling loads) have come to market (Roth et al. 2006).Very recently, DMSHP 

manufacturers have also begun to offer cold-climate units. For example, the Mitsubishi 

HyperHeat produces its full heating capacity at 5o

F, with a coefficient of performance (COP) 

of 1.5–1.8 (Mitsubishi 2010).These units demonstrate superior performance at cold outdoor 

conditions, efficiently modulate capacity over a wide range of conditions, eliminate duct losses, 
2

and are operated in zones. They may thus achieve significant primary energy savings relative to 

conventional space heating options in colder climates. 

1.2 Research Questions

To address the issues noted in Section 1.1, we designed this research project to answer the 

following research question:

• What is the distribution of indoor temperature and relative humidity (RH) in different 

rooms during the cooling and heating seasons, and how do they compare to ASHRAE 

comfort criteria?

1.3 Experimental Design and Field Deployments

We made in situ measurements of temperature and RH in two households in Austin before and 

after the DMSHPs were installed. Our research partner, Mitsubishi Electric, also deployed onboard measurement systems to obtain field performance data for the real-world capacity and 

COP of the DMSHPs under the wide range of weather conditions experienced in these two 

households.

1.3.1 Field Deployment: Austin, Texas

We deployed the MSHPs in both units of a duplex, each with about 940 ft2 of floor space.

Figure 1 and Figure 2 show an image of the duplex and its floor plan, respectively, as well as the 

locations of the DMSHP indoor units and Onset HOBO (U12-011) temperature and RH

monitoring units. We took measurements at 15-minute intervals.

Figure 1.Austin duplex DMSHP deployment site
3

Figure 2.Floor plan of Austin duplex DMSHP deployment site 

(second unit is mirror image, along garage-kitchen-dining room wall)

Table 1 summarizes the three indoor and one outdoor units deployed in both units of the duplex.

Table 1. Austin DMSHPs Deployed

Location Unit Rated Cooling 

Capacity

Rated Heating 

Capacity SEER HSPF

Outdoor MXZ-3B30NA 28,400 28,600 17.5 10.5

Common Areas MSZ 17,200 21,600

Bedroom MSZ 6,000 6,600

Bedroom MSZ 6,000 6,600
4

2 Thermal Comfort Analysis

An important point of comparison between CACs and DMSHPs is their ability to maintain 

thermal comfort in the conditioned space. CACs recirculate conditioned air to a common 

temperature, but often have only a single point for temperature control that does not necessarily 

reflect temperatures in spaces with significantly different internal loads, e.g., insolation levels or 

internal heat gains and loads. The multiple indoor units of distributed cooling and heating 

sources, such as DMSHPs, can respond to in-home variations of thermal loads using the in-room 

controls, but usually do not have indoor units in all rooms. 

We used HOBO data collected from the two deployments at Austin to evaluate the thermal 

comfort in the conditioned space. Our evaluation includes data from the two housing units, both 

for the CACs (pre-retrofit) and for the DMSHPs (post-retrofit). In this report, the CAC system 

includes a ducted system with a CAC unit and furnace. We assessed thermal comfort based on 

ASHRAE Standard 55 (2004). 

2.1 Procedures

We evaluated comfort based on hourly and daily mean outdoor temperatures, using indoor air 

DBT (dry bulb temperature) and RH that were measured every 15 minutes. We applied two 

procedures to analyze the real-time data (indoor temperature and RH) for thermal comfort 

performance. 

2.1.1 Hourly Comfort Analysis

1. The data for indoor DBT and indoor RH for the living room, master bedroom, and 

bathroom were acquired through real-time monitoring at the site.

2. Assumption 1 (described in Section 2.2.1) was used to determine those readings when the 

system (either CAC or DMSHP) was considered “in operation.” A system is considered 

in operation when the building is expected to have either heating or cooling loads.

3. Assumption 2 was used to select those readings when the system was considered to be in 

heating or cooling mode.

4. For ASHRAE Standard 55:

a. Two areas were plotted in a temperature versus RH graph. These areas correspond 

to two levels of clothing: 1.0 clo and 0.5 clo. These areas together define the 

comfort zone for the ASHRAE Standard 55. (A “clo” is a unit used to quantify the 

thermal insulation provided by garments and clothing ensembles, where 1 clo = 

0.155 m2 °C/W.) Put in more intuitive terms (ASHRAE 2009):

i. Clo 1.01 corresponds to trousers, long-sleeved shirt, long-sleeved sweater, 

T-shirt.

ii. Clo 0.57 corresponds to trousers, short-sleeved shirt.

iii. Clo 0.36 corresponds to walking shorts, short-sleeved shirt.

b. We used the indoor temperature and RH measures to determine if the given 

condition lay within the comfort zone.
5

c. The indoor conditions (temperature and RH at a given time) were plotted on this 

graph for a visual estimate of how many points lay within the comfort zone.

d. We performed a similar analysis by plotting the points for cooling mode only (comfort 

zone corresponding to 0.5 clo) and heating mode only (comfort zone corresponding to 1.0 

clo).We applied the above procedure for all the conditioned spaces.

2.2 Daily Mean Temperature Analysis

We applied this method while evaluating thermal comfort performance based on ASHRAE 

Standard 55 only. All the steps under hourly comfort analysis, for ASHRAE Standard 55, are 

followed in this procedure except for one difference. For this procedure, the analysis used the 

outdoor daily mean DBT instead of hourly outdoor temperatures (as used in Hourly Comfort 

Analysis). Assumptions 3 and 4 (see Section 2.2.1) hold true for this case. 

The rationale for this approach is that the mean outdoor temperature over a 24-hour period takes 

into account how building thermal mass may impact indoor comfort conditions.

2.2.1 Assumptions

We did not measure occupancy patterns nor occupants’ preferences, so we made the following 

assumptions for hourly comfort analysis and daily mean temperature analysis to predict the 

outdoor conditions when the systems would most likely operate in either heating or cooling 

mode.

For the hourly comfort analysis, the CAC and DMSHP most likely provided heating or 

cooling when the hourly outside DBT was either ≤60°F or ≥77 °F. Between these two 

temperatures, we assumed that both systems did not operate to meet the set points in the 

conditioned space.

The system was in heating mode when the outdoor DBT was ≤60°F and in cooling mode 

when the outdoor DBT was ≥77°F.

For the daily mean temperature analysis, the systems (CAC and DMSHP may have operated

in either heating or cooling mode when the outside mean DBT was either ≤60°F or ≥70°F. 

Between these two temperatures, both systems did not operate to meet the set points in the 

conditioned space.

The system was in heating mode when the outdoor DBT was ≤60°F and in cooling mode 

when the outdoor DBT was ≥70°F.

Residents in the conditioned space were dressed per typical ASHRAE summer clothing 

assumption of 0.5 clo when the systems were in cooling mode and the winter clothing 

assumption of 1.0 clo when the systems were in heating mode. 

The data for CACs are primarily for the month of October 2011.1

The data acquisition period for DMSHPs was November 2011 to June 2012.

 1 The average high and low temperatures for Austin in October are 82o

F and 61oF, respectively. Thus, significant 

space cooling typically occurs during this period (Weather Underground 2012).
6

2.3 Thermal Comfort Based on ASHRAE Standard 55 (2004)

The following sections describe the results from the thermal comfort analysis performed based 

on ASHRAE Standard 55. 

Before installing the DMSHPs in Austin, we acquired a month of baseline indoor comfort data 

for the prior air-based CAC using HOBOs to record indoor temperature and RH. Similar data 

were collected when DMSHPs were installed. The outdoor weather data used are from AustinBergstron International Airport (Weather Underground 2012). The results based on the 

procedures described in Section 2.1.1 follow.

2.3.1 Thermal Comfort Based on Either Cooling or Heating Operation

We first evaluated the data assuming either typical summer or winter clothing. Table 2 and Table 

3 summarize the relative performance of the two systems for the hourly comfort analysis and 

daily mean temperature analysis, respectively, for both test households. Zones in test household 

1 are identified by the number “1” and those of test household 2 with the number “2.”

Performance here refers to the portion of time that the system maintained indoor conditions 

within the bounds specified by the comfort zone in either heating or cooling mode. 

Table 2. Thermal Comfort Performance for CAC and DMSHP for 

Either Cooling or Heating—Hourly Comfort Analysis

CAC Thermal Comfort Performance DMSHP Thermal Comfort Performance

Living 1 Bed1a Bed1b Bath1 Living 1 Bed1a Bed 1b Bath1

87% 88% 88% 94% 87% 74% 93% 80%

Living 2 Bed2a Bed2b Bath2 Living 2 Bed2a Bed2b Bath2

100% 97% 100% 99% 94% 89% 92% 85%

Table 3. Thermal Comfort Performance CAC and DMSHP for 

Either Cooling or Heating—Daily Mean Temperature Analysis

CAC Thermal Comfort Performance DMSHP Thermal Comfort Performance

Living 1 Bed1a Bed1b Bath1 Living 1 Bed1a Bed1b Bath1

85% 87% 88% 93% 86% 73% 91% 79%

Living 2 Bed2a Bed2b Bath2 Living 2 Bed2a Bed2b Bath2

93% 94% 95% 87% 91% 87% 93% 83%

The plots for the living room (Living 1) that were prepared per the procedures described in 

Section 2.1.1 are shown in Figure 3 and Figure 4. We created similar plots for other conditioned 

spaces for both test households (see Appendix A for the plots for other conditioned spaces in 

household 1).

2.3.1.1 Observations and Findings 

Based on the portion of time that indoor conditions lay within the ASHRAE comfort zone, the

following points emerged.
7

Based on Table 2, the percentage of time when the conditions are within the comfort zone,

for either heating or cooling operation, in the living room is almost the same for the CAC and 

DMSHP. This suggests that the DMSHP indoor unit in the living room operates much like a 

CAC, particularly because the CAC master thermostat is often in the living room. Periods 

when the CAC and DMSHP do not stay within the ASHRAE comfort zone are primarily 

when the indoor DBT (CAC) or indoor RH (DMSHP) are higher. It is not clear the extent to 

which this reflects the households’ setting up the temperature (e.g., when out of the house).

The RH levels are quite high for the DMSHP case, something that the inhabitants also noted. 

This suggests that they did not run the DMSHP in its dehumidification mode, but instead 

tried to achieve lower RH by cooling the indoor spaces. 

Figure 3.Thermal comfort performance of CAC for either 

cooling or heating operation for Living 1; hourly analysis

Figure 4.Thermal comfort performance of DMSHP for either 

cooling or heating operation for Living 1; hourly analysis

The results from daily mean temperature analysis also closely mirror the trends noted for the 

hourly data analysis. Thus, the effect of thermal mass may even out temperature over a 

period of 24 hours. 
8

2.3.2 Thermal Comfort Performance in Cooling and Heating Modes

We also evaluated the performance of the two systems separately for heating and cooling modes. 

2.3.2.1 Thermal Comfort Performance in Cooling Mode 

Table 4 and Table 5 compare the thermal comfort performance in cooling mode of the two 

systems, assuming the typical ASHRAE clothing levels 0.5 clo. Later in this section, we also 

analyze the case where the systems are in cooling mode, but the thermal comfort is based on both 

1.0 clo and 0.5 clo. 

Table 4. Thermal Comfort Performance for DMSHP and CAC in 

Cooling Mode Hourly Comfort Analysis

CAC Thermal Comfort Performance 

(%)

0.5 clo/0.5 clo or 1.0 clo

DMSHP Thermal Comfort Performance 

(%)

0.5 clo/0.5 clo or 1.0 clo

Living 1 Bed1a Bed1b Bath1 Living 1 Bed1a Bed1b Bath1

80/82 44/89 77/88 80/97 27/75 9/68 62/70 14/65

Living 2 Bed2a Bed2b Bath2 Living 2 Bed2a Bed2b Bath2

73/95 68/92 81/90 68/97 67/77 11/74 27/86 62/88

Table 5. Thermal Comfort Performance for DMSHP and CAC in 

Cooling Mode Daily Mean Temperature Analysis

CAC Thermal Comfort Performance 

(%)

0.5 clo/0.5 clo or 1.0 clo

DMSHP Thermal Comfort Performance 

(%)

0.5 clo/0.5 clo or 1.0 clo

Living 1 Bed1a Bed1b Bath1 Living 1 Bed1a Bed1a Batha

79/82 44/88 74/88 78/97 29/71 7/63 64/91 11/63

Living 2 Bed2a Bed2b Bath2 Living 2 Bed2a Bed2b Bathb

68/71 25/66 33/79 41/66 33/80 16/68 31/80 28/70

The plots for conditioned spaces for household 1, prepared per the procedures described in 

Section 2.1.1, can be found in Appendix A.

2.3.2.2 Observations and Findings : 

Table 4 and Table 5 show a difference in performance of the two systems in some areas. For 

instance, the comfort performance for Bed1a based on 0.5 clo only is 44% of CAC to 9% for 

DMSHP. Similarly, the ratios for Bed2b differ by as much as 81% to 27%. 

Figure 5 and Figure 6 show plots of measured temperature and RH, for cooling mode only 

(0.5 clo and 1.0 clo ), and for Bed1a for the two systems. A large portion of the 

measurements do not lie within the comfort zone for 0.5 clo, but do fall within the comfort 

zone for 1.0 clo. Thus, we conclude that the residents prefer lower set point temperatures. 

These values are also listed in Table 3 and Table 4. Indeed, we noted during site visits that 

the occupants of both units wore clothing consistent with 1.0 clo. 
9

Figure 5.Thermal comfort performance of CAC for Bed1a in 

cooling mode for clo 0.5 or clo 1.0; hourly analysis

Figure 6.Thermal comfort performance of DMSHP for Bed1a in 

cooling mode for clo 0.5 or clo 1.0; hourly analysis

We investigated the significant difference in performance for the master bedrooms, Bed1a,

and Bed2a, further hypothesizing that this could reflect distinct occupancy patterns for this 

space and the difference between central and distributed space conditioning systems. 

Specifically, CACs typically have only one thermostat that is usually located in the living 

room. Consequently, they cool other spaces even if those spaces are not occupied. In 

contrast, a DMSHP indoor unit might run only when that room is occupied, because 

DMSHPs have an individual control for each indoor unit. To explore this further, we 

analyzed bedroom comfort during times when we expected that the occupancy would be 

high(11:00 p.m. to 7:00 a.m.). We assumed that the DMSHP is always in operation (either in 

cooling or heating mode) for this time window. Figure 7 shows the performance of the 

DMSHP in Bed1a. In this case, the thermal comfort performance was 86% compared to 73% 

in the previous case. However, similar plots for the other bedrooms did not necessarily show 

different performance.
10

The RH levels are quite high for the DMSHP case, which the occupants also noted. This 

suggests that they did not run the DMSHP in its dehumidification mode, but instead tried to 

achieve lower RH by cooling the indoor spaces. 

Figure 7.Thermal comfort performance of DMSHP for Bed1a for 

11:00 p.m. to 7:00 a.m.; hourly analysis

1. The DMSHP also had an appreciably higher portion of time when conditions lay outside 

the ASHRAE comfort zones, primarily because of higher RH levels (see Figure 7). This 

probably reflects the lack of DMSHP indoor units in the bathrooms; vent fan use was not, 

however, monitored.

Figure 8. Thermal comfort performance of DMSHP for Bath 1 for either 

cooling or heating; hourly analysis

2. The results presented in Table 3 and Table 4 show that the daily mean and hourly 

temperature analyses yield similar conclusions, signifying that even when thermal mass 

of the building is taken into consideration the thermal comfort performance of DMSHP 

may be approximately the same as that calculated in hourly comfort analysis.
11

2.3.2.3 Thermal Comfort Performance for Heating Mode Only

We also considered the thermal comfort performance of the two systems in heating mode only. 

Table 6 and Table 7 summarize the results of this analysis. Although the performance during 

heating season appears to be broadly comparable for both systems, limited data were collected 

for the heating season with the CAC. Moreover, most of those data were taken during October, 

when outdoor temperatures were mild. Hence, it is difficult to draw meaningful conclusions 

about the relative performance of the two systems.

Table 6. Thermal Comfort Performance for DMSHP and CAC Heating Mode; Hourly Analysis

CAC Thermal Comfort Performance 

1.0 clo(%)

DMSHP Thermal Comfort Performance 

1.0 clo(%)

Living 1 Bed1a Bed1b Bath1 Living 1 Bed1a Bed1b Bath1

82 % 89% 83% 97% 83% 75% 93% 87%

Living 2 Bed2a Bed2b Bath2 Living 2 Bed2a Bed2b Bath2

97% 78% 84% 91% 94% 78% 90% 84%

Table 7. Thermal Comfort Performance for DMSHP and CAC Heating Mode; 

Daily Mean Temperature Analysis

CAC Thermal Comfort Performance 1.0 

clo [%]

DMSHP Thermal Comfort Performance 

1.0 clo [%]

Living 1 Bed1a Bed1b Bath1 Living 1 Bed1a Bed1b Bath1

88% 77% 78% 73% 83% 74% 89% 79%

Living 2 Bed2a Bed2b Bath2 Living 2 Bed2a Bed2b Bath2

92% 78% 73% 83% 85% 82% 90% 83%

Additional plots for household 1 that we prepared per the procedures described in Section 2.1.1 

can be found in Appendix A.

2.4 Temperature Distributions Among Rooms During Cooling Season 

As discussed earlier, the ability to individually control DMSHP indoor units raises the possibility 

that “uncomfortable” conditions may occur in spaces when the indoor units are switched off. 

This contrasts with a CAC, which provides cooling or heating to all conditioned spaces 

whenever the system runs in either cooling or heating mode. Therefore, it is worthwhile to note

how the mean temperature distribution varies throughout the household. We performed the 

following analysis to determine the temperature variations in the test households during the 

cooling season.

1. Selected the days when the maximum outdoor air DBT ≥85°F; this limited the analysis to 

days expected to have significant cooling loads. 

2. Calculated mean hourly temperatures for each day selected in step 1.

3. Calculated the average value of each mean hourly temperature, calculated in step 2, of all

days where the maximum outdoor air DBT ≥85°F. 
12

4. Plotted these averages to observe the variations over 24 hours (see Figure 9 and 

Figure 10).

Figure 9. Temperature distribution with DMSHP in different rooms, household 1

Figure 10. Temperature distribution with DMSHP in different rooms, household 2

2.4.1.1 Observations and Findings 

The maximum average temperature difference among rooms is around 3°F.

65

68

71

74

77

80

12:00 PM 3:00 AM 6:00 AM 9:00 AM 12:00 PM 3:00 PM 6:00 PM 9:00 PM

Average Indoor Temperature [oF]

Time of Day

Bath1

Small Bed1

Master Bed1

Living1

65

68

71

74

77

80

12:00 PM 3:00 AM 6:00 AM 9:00 AM 12:00 PM 3:00 PM 6:00 PM 9:00 PM

Average Indoor Temperature [oF]

Time of Day

Living2

Master Bed2

Small Bed2

Bath2
13

3 System Costs

One objective of this study is to determine the impact of retrofitting a DMSHP unit to an existing 

home with CAC. Bergey and Ueno (2011) found that the DMSHP is a less costly option (by 

approximately $3,000) than installing a CAC + condensing furnace in a very low-energy home, 

so we gathered data for the retrofit case in Austin.

We used the actual cost of the DMSHP retrofits for the deployment in Austin, based on capital 

costs from Mitsubishi Electric and installation costs from Foundation Communities. Table 8 and 

Table 9 summarize the economics of a retrofit.

Table 8. Summary of Economics of a DMSHP Retrofit

System Component Cost Specifications

One Outdoor Unit Plus 

Three Indoor Units $2,800 1 MXZ-3B30 outdoor unit, 1 MSZ-GE18 indoor 

unit, and 2 MSZ-GE06 indoor units

Installation Cost for One 

Household $4,163 Actual cost of installation at the Texas-Austin site

Total per Household $6,963

Table 9 summarizes the cost for a complete retrofit of a CAC, using the same capacities as the 

DMSHP (28,400 Btu cooling and 28,600 Btu heating). We assume that the new CAC has 

seasonal energy efficiency ratio (SEER) of 15, and that the new furnace has an annual fuel 

utilization efficiency of 92.5%. The CAC retrofit cost comes from BEopt; the furnace costs come

from BEopt and RSMeans (2011). Unfortunately, a full BEopt analysis could not be completed, 

because BEopt cannot yet model DMSHPs (Winkler 2012).

Table 9. Summary of Economics of a CAC Retrofit

System Component Cost Specifications

Cooling System $5,032 Retrofit: SEER=15; old: SEER=13

Furnace $1,2762 Retrofit: AFUE=92.5%;old: AFUE=72%

Total $6,308

Assuming that the existing CAC and furnace both require replacement, the data show a $655 

incremental cost for the DMSHP. 

 2

Based on: $995 for furnace cost (BEopt), $165 for installation labor (RSMeans), and 10% overhead and profit 

(RSMeans).
14

4 Conclusions and Recommendations

The Fraunhofer team conducted field tests in two homes in Austin, Texas, from October 2011 to 

June 2012 to evaluate the comfort performance of DMSHPs. Specifically, we measured 

temperature and RH in four rooms in each home, both before and after retrofitting each home’s 

CAC with a DMSHP.

Our analysis of the field measurements yielded several comfort-related observations. This small 

sample relies strongly on residents’ preferences, so the findings should be used as a case study 

evaluation rather than as a general finding. 

3. ASHRAE Standard 55 Evaluation.

a. Living room. The percentage of time within the comfort zone, for either heating 

or cooling operation, was similar for the CAC and DMSHP, suggesting that the 

DMSHP indoor unit in the living room operated much like a CAC. Periods when 

the DMSHPs did not stay within the ASHRAE comfort zone were primarily when 

the RH was higher. 

b. Bedroom. A large portion of the measurements did not lie within the comfort 

zone for 0.5 clo, but did fall within the comfort zone for 1.0 clo. Thus, we 

conclude that residents preferred lower temperatures during the cooling season 

than those indicated by ASHRAE standard 55 (assuming 0.5 clo). 

c. Bathroom. The DMSHP had an appreciably higher portion of time when 

conditions lay outside the 0.5 and 1.0 clo comfort zones, primarily because of

higher RH levels. This probably reflects the lack of DMSHP indoor units in the 

bathrooms; vent fan use was not, however, monitored.

d. In general, the RH levels were quite high for the DMSHP cases, which the 

occupants also noted. This suggests that the occupants did not run the DMSHP in 

its dehumidification mode, but instead tried to achieve lower RH by cooling the 

indoor spaces.

4. Homogeneity of indoor temperatures, cooling season. The maximum average hourly 

temperature difference among rooms on days when the outdoor temperature was 85°F or 

higher was around 3°F. 

A key recommendation from our study is to obtain detailed field performance data, i.e., unit 

power draw and heating and cooling capacity as functions of indoor and outdoor temperatures 

for the DMSHP units. These data are crucial for further validating and enhance EnergyPlus

models for DMSHPs (see, for example, FSCE 2012).Similarly, it is crucial to obtain the same 

data for DMSHPs optimized for cold-climate performance to develop adequate models for these 

products as they become more prominent in the residential HVAC market. We continue to work 

with our field test partners to generate these data, that we hope to obtain and publish as part of a 

future study.
15

References

ADL. 1999. “Opportunities for Energy Savings in the Residential and Commercial Sectorswith 

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ANSI/ASHRAE Standard 90.2. 2007. “Energy Efficient Design for Low Rise Residential 

Buildings.” ASHRAE: Atlanta, GA.

ASHRAE. 2009. ASHRAE Handbook: 2009 Fundamentals. ASHRAE: Atlanta, GA. 

Baylon, D. 2010. “Performance of Ductless Heat Pumps in the Pacific Northwest”. Proc. 2010 

ACEEE Summer Study on Energy Efficiency in Buildings, Pacific Grove, CA: August.

Bergey, D. and K. Ueno. 2011. “New England Net Zero Production Houses”. Proc. ASHARE 

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Dieckmann, J. 2008. “Improving Humidity Control with Energy Recovery Ventilation” ASHRAE 

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NREL. 2004. “Better Duct Systems for Home Heating and Cooling”. Produced for the U.S. 

Department of Energy (DOE) by the National Renewable Energy Laboratory 

(NREL).November. Available at: http://www.nrel.gov/docs/fy05osti/30506.pdf.

Roth, K., D. Westphalen, and J. Brodrick. 2006. “Ductless Split Systems”. ASHRAE Journal.

July, pp. 115,117.

Weather Underground. 2012. Data from Austin, TX Airport Weather Station (KATT). Accessed 

on from: www.wunderground.com/history/airport/KATT/2011/10/31/DailyHistory.html.

Winkler, J. 2012. Personal Communication, National Renewable Energy Laboratory (NREL).
16

Appendix A: Thermal Comfort Performance Graphs for ASHRAE 

Standard 55, for Household1

Figure 11. Thermal comfort performance of CAC for Bed1a for either cooling or heating; hourly

Figure 12.Thermal comfort performance of DMSHP for Bed1a for 

either cooling or heating; hourly
17

Figure 13. Thermal comfort performance of CAC for Bed 2a for either cooling or heating; hourly

Figure 14.Thermal comfort performance of DMSHP for Bed 2a for 

either cooling or heating; hourly

Figure 15.Thermal comfort performance of CAC for Bath 1 for either cooling or heating; hourly
18

Figure 16.Thermal comfort performance of DMSHP for Bath 1 for 

either cooling or heating; hourly

Figure 17.Thermal comfort performance of CAC for Living 1 for cooling; hourly
19

Figure 18.Thermal comfort performance of DMSHP for Living 1 for cooling; hourly

Figure 19.Thermal comfort performance of CAC for Bed2a for cooling; hourly

Figure 20.Thermal comfort performance of DMSHP for Bed2a for cooling; hourly
20

Figure 21.Thermal comfort performance of CAC for bath 1 for cooling; hourly

Figure 22.Thermal comfort performance of DMSHP for bath 1 for cooling; hourly

Figure 23.Thermal comfort performance of CAC for Living 1 for heating; hourly
21

Figure 24. Thermal comfort performance of DMSHP for Living 1 for heating; hourly

Figure 25. Thermal comfort performance of CAC for Living 1 for heating; hourly

Figure 26.Thermal comfort performance of DMSHP for Bed2 for heating; hourly
DOE/GO-102013-3814 ▪March 2013

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