Prepared by the National Renewable Energy Laboratory

For the U.S. Department of Energy Building Technologies Program

Field Monitoring Protocol: 

Mini-Split Heat Pumps

D. Christensen, X. Fang, J. Tomerlin, and 

J. Winkler

National Renewable Energy Laboratory

E. Hancock

Mountain Energy Partnership

March 2011
ii

NOTICE

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iii

Acknowledgment

The authors thank Ben Larson of Ecotope, Inc. for his comments and input, as well as for laying 

some groundwork on the methods developed for (and omitted from) this protocol. 

Foreword

The purpose of the Building America Mini-Split Heat Pump (MSHP) Field Monitoring Protocol 

is to provide a consistent methodology for performance measurement of MSHPs when installed 

in a building. This document provides instruction on instrumentation, short-term testing, and 

long-term monitoring of these systems. It further provides an abbreviated field test report of the 

authors’ validation of the described methodologies. 

This protocol is intended for application by the U.S. Department of Energy’s Building America 

program, and optionally by outside entities, when studying MSHPs in buildings. The report 

describes the method for a single-head MSHP, but the sensors and methods can logically be 

duplicated for measuring a multi-head MSHP. It can be used in new construction and retrofit 

applications, and although the document targets residential installations, it could also be applied 

to performance measurement of small commercial building MSHP. This report focuses primarily 

on ductless MSHPs; ducted installations will be addressed in a future revision. 
iv

Contents

Acknowledgment................................................................................................................................. iii

Foreword .............................................................................................................................................. iii

Figures ................................................................................................................................................... v

Tables .................................................................................................................................................... v

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

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

Relevance.............................................................................................................................................. 3

Monitoring a Mini-Split Heat Pump..................................................................................................... 4

Site Selection .......................................................................................................................4

Monitoring Parameters ........................................................................................................................ 6

Quality Assurance Protocol .................................................................................................7

Short-Term Testing .............................................................................................................................. 9

Steady-State System Curve Measurement ...........................................................................9

Short-Term Efficiency Test ...............................................................................................10

Long-Term Monitoring ....................................................................................................................... 11

Data Collection and Documentation................................................................................................. 12

Data Collection ..................................................................................................................12

Coefficient of Performance Calculations and Uncertainty ................................................12

Other Considerations......................................................................................................................... 13

References .......................................................................................................................................... 14

Appendix A. Field Test for Protocol Validation............................................................................... 15

Objective ............................................................................................................................15

Short-Term Air Flow Test .................................................................................................18

Fan Revolutions per Minute Measurement ..................................................................18

Supply Flow Measurement With Powered Flow Hood ...............................................19

About Fan Power, Unit Power, and Control Signals .........................................................22

Short-Term Test Results ..............................................................................................22

Long-Term Monitoring ......................................................................................................23

Monitoring Package .....................................................................................................23

Results ..........................................................................................................................23

Error Analysis ....................................................................................................................26

Appendix B. Rigid Plenum Method (for reference) ......................................................................... 29
v 

Figures

Figure 1. Measured air flow versus tachometer frequency .................................................................... 9

Figure 2. Living room unit........................................................................................................................ 15

Figure 3. Bedroom unit............................................................................................................................. 16

Figure 4. Outdoor unit .............................................................................................................................. 16

Figure 5. Indoor unit nameplate information.......................................................................................... 17

Figure 6. Outdoor unit and nameplate information ............................................................................... 17

Figure 7. Proximity sensor mounted....................................................................................................... 18

Figure 8. Fan balancing weight ............................................................................................................... 19

Figure 9. Thermistors ............................................................................................................................... 19

Figure 10. Trash bag................................................................................................................................. 20

Figure 11.Trash bag plenum with wire support..................................................................................... 20

Figure 12. Inflatable plenum..................................................................................................................... 21

Figure 13. Flow test setup........................................................................................................................ 21

Figure 14. Air flow measured and submittal data.................................................................................. 22

Figure 15. Living room unit air flow measured ...................................................................................... 24

Figure 16. MSHP outdoor condensing unit total power........................................................................ 24

Figure 17. MSHP outdoor condensing unit total power on August 20, 2010...................................... 25

Figure 18. Living room unit air flow and temperatures trend on August 20, 2010............................. 25

Figure 19. MSHP EER with living room unit operating only................................................................. 26

Figure 20. Indoor living room unit air flow and outdoor unit power.................................................... 26

Figure 21. Uncertainty propagation analysis ......................................................................................... 27

Figure 22. Uncertainty propagation table............................................................................................... 28

Figure 23. Uncertainty propagation results............................................................................................ 28

Figure 24. MSHP for indoor heat air flow test rigid plenum method ................................................... 29

Figure 25. Researcher examining MSHP for indoor heat air flow test rigid plenum method............ 29

Figure 26. Researchers testing MSHP .................................................................................................... 30

Figure 27. Opening for unit room temperature thermistor sensing..................................................... 30

Tables 

Table 1. Long-Term Monitoring Instrumentation Package ..................................................................... 7

Table 2. Air Flow Measured and Submittal Data.................................................................................... 22
vi 

Definitions

AC alternating current

AHRI Air-Conditioning, Heating and Refrigeration Institute 

ANSI American National Standards Institute

CDD cooling degree day 

CFM cubic feet per minute 

COP coefficient of performance

DC direct current

DOE U.S. Department of Energy 

EER energy efficiency ratio

HDD heating degree day

HERS Home Energy Rating System 

HSPF Heating Seasonal Performance Factor

HVAC heating, ventilating, and air conditioning

MSHP mini-split heat pump 

NREL National Renewable Energy Laboratory 

RH relative humidity

RPM revolutions per minute

SEER seasonal energy efficiency ratio
1 

Introduction

The mini-split heat pump (MSHP) is an emerging technology in the United States. Worldwide, 

however, MSHPs have an established and stable market in residential applications. Throughout 

Asia, this is the dominant technology for space conditioning. MSHPs are also common in 

Europe, largely because the ductless designs provide easily retrofitted point-source climate 

control. MSHP manufacturers are thus beginning to aggressively compete with more established 

unitary systems in the United States. 

• MSHPs have large turndown ratios and can address wider variation in thermal loads than 

equipment manufactured by major U.S. heating, ventilation, and air-conditioning 

(HVAC) companies. Through innovative control methods, a variable-speed compressor, 

and variable-speed fans, the refrigerant system operation can be quickly adjusted through 

feedback control methods to provide cooling over a broad range rather than stepping 

between stages. In this way, part-load conditions are accommodated more efficiently with 

lower losses.

• An MSHP is often installed in a ductless configuration, and can serve one or more rooms 

individually. Originally these systems were designed with a single small outdoor 

fan/coil/compressor unit and a wall-mount “head” (fan/coil). Current products often have 

a remote-control interface, which may also contain the system thermostat. 

• Multiple indoor coils can be installed for a single outdoor coil. This system configuration 

is termed a multi-split heat pump, and the methods described in this document are 

expected to apply to that configuration as well. 

Manufacturers claim substantially higher coefficients of performance (COPs) for these systems 

than for the ducted unitary splits common in America, often claiming seasonal energy efficiency 

ratio (SEER) 22/heating seasonal performance factor (HSPF) 9 or higher. Some manufacturers 

do not provide engineering data to support these claims, so achieving rated performance in real 

homes is uncertain. Further, the novel features of MSHPs must be studied to understand wholehouse impacts. For example: 

• Variable-speed components enable “load-matching” rather than cycling on and off. 

• Point-source space conditioning may leave some rooms or areas uncomfortable.

• Latent performance (humidity control) is less understood. 

• COP variability as outdoor temperatures change is less certain. Some equipment claims 

to provide high performance at very low ambient temperatures in heating mode. 

• Operating modes and control methods are not uniform between manufacturers. Different 

control and operation methods allow optimization to climate and to occupant loads. They 

also enable the occupant to use this equipment differently than other heating and cooling 

equipment. 

MSHP merits significant evaluation because: 

• It is all-electric and can provide heating at a COP well above 1.0, making it an attractive 

replacement system for electric resistance heating where natural gas is not available. 

• Its high SEER rating promises substantial energy savings compared with more common 

unitary-split equipment. 
2 

• It can supplement air-conditioning or heating, and can provide both. 

• It is minimally invasive, which simplifies installation in homes without ductwork. 

• It is small and quiet. 

• It can meet the minimal heating and cooling needs of many extremely high-performance 

Building America new homes. 

In a recently initiated pilot project in the Northwest, customer approval ratings are high and 

energy impacts appear compelling (Larson and Allan 2010). Further study in the laboratory and 

field will permit a true “lessons-learned” and “best practices” analysis. Topics of interest include:

• System design

• Distribution of cooling and heating throughout the home 

• Real-world installed efficiency (for comparison to published and laboratory data), 

including multiple head systems 

• Occupant usage characteristics

• Marketability, including installation issues and retrofit potential

• Climate and regional applicability

• System costs versus energy and carbon savings potential 
3 

Relevance 

More than 20% of the electricity supply in the United States is consumed by residential buildings 

(DOE 2009); roughly 40% of that goes to space conditioning. The U.S. Department of Energy 

(DOE) has increasingly raised minimum appliance efficiency standards, but inefficient legacy 

equipment abounds throughout the U.S. housing stock. Ideally, aging equipment is replaced by 

space conditioning systems that provide high efficiency at appropriate cooling, heating, and 

dehumidification loads. 

Building America has recently been directed to expand retrofit opportunity analysis, and MSHP 

is a key retrofit technology in several applications. Emerging technologies entering the U.S. 

HVAC market must be evaluated early in their adoption period. MSHP technology promises to 

be a key component of the energy efficiency strategy of reducing HVAC loads by 30% 

compared with best-available technology (Anderson and Roberts 2008). Laboratory testing to 

enable simulation of these systems is underway. Installing equipment in homes and monitoring 

its performance with occupants will address the “real-world” complications. 

An excellent Market Progress Evaluation Report was recently published (NEEA 2010). 
4 

Monitoring a Mini-Split Heat Pump

A key field test objective is to measure the energy tradeoffs between a ductless MSHP and any 

other heating or cooling devices that are used in the home, such as backup resistance heaters.

This evaluation may be performed as part of a whole-house monitoring effort, or may be a 

standalone study of the MSHP installation. In either case, basic characteristics of the home must 

be understood to enable simulation of the MSHP. For retrofit cases, a Home Energy Rating 

System rating (HERS Rating) is acceptable, but a less extensive home evaluation may be 

adequate. Of particular interest is the amount of heating energy use offset by the ductless heat 

pump, so historical utility bills are typically required.

When arriving at a new field test site, information about the home and equipment must be 

gathered. Second, monitoring equipment and a data logger must be installed. Then the field test 

technician must perform several short-term tests to correlate certain measurements to 

performance parameters, and to verify the system was installed and is functioning correctly. 

Finally, the site will be monitored in the “long-term” to capture seasonal performance of the 

MSHP. The measurement equipment will be retrieved following the long-term monitoring 

period. 

Site Selection

Installations of multiple manufacturers’ systems are sought in each climate region. Application 

to retrofits and new homes will be needed. New homes permit better prediction of heating and 

cooling loads (more controlled experimental conditions), but present fewer challenges for 

installation. Retrofits present many complications, but provide a much greater opportunity for 

energy and carbon savings. Selection of test sites should be based on the following 

considerations:

• Geographical location 

• Home heating and cooling type (for retrofits) 

• System configuration (single or multiple heads, distribution methods) 

• Homeowner approval of monitoring for at least one year 

• Homeowner approval for release of historical utility bills (for retrofits)

• Building energy audit or evaluation. For retrofits, the audit must be performed before the 

retrofit and aspects related to the retrofit must be repeated after work is complete.

• Reasonable expectation that MSHP can provide acceptable heating and cooling for 

occupants and climate, and can be measured.

The following information should be obtained during the initial phone screening, literature 

review, or exploratory site visit:

• Building use (number of occupants and typical occupancy schedule) 

• Location of indoor heat pump heads (which rooms, square feet and use, 

ducted/ductless) 

• Measured refrigerant line length from outdoor unit to each indoor head 

• Heating and cooling distribution method, if any, and relevant details 

• Equipment specifications (MSHP model numbers and backup heating equipment) 
5 

• Documentation of contractor’s proposed scope of work and design specifications

• HVAC contractor’s information, including the point(s) of termination between the 

separate scopes of work covered by each contractor. Determine installing contractor 

experience (number of previous jobs) and certifications.

• System sizing calculations (Manual J or other), name of party responsible for sizing, and 

cooling and design heating month run fraction (either heating degree day [HDD] or bin 

analysis)

• Climate design conditions (outdoor dry-bulb design temperature, HDD, cooling degree 

day [CDD])

• System controls, including: thermostat setpoints, deadband, available operating modes, 

zoning, and ventilation control settings 

• Home construction details (or audit information), including blower door and duct 

blaster results if applicable, for later use in identifying the cause of any discrepancies 

between predicted and actual energy use 

• Installation cost and date. Include any known installation issues and duration. 

• Recent operating costs. Monthly utility bills and costs of alternative fuels such as wood 

burning, oil, and propane tanks 

• System commissioning and quality installation verification by contractor (startup 

documentation, in-field testing after startup, and nature of any post-installation service 

calls)

• Installer-supplied and manufacturer-recommended maintenance requirements

• Temperature limits or cutout (heating mode only) 

• Complete list and details of other retrofits occurring along with MSHP installation, and 

during the monitoring period. 
6 

Monitoring Parameters

Only standard heat pumps are widely available in MSHP format. As other implementations 

become available, this report will be revised to address monitoring details particular to that type 

of system as those products gain market share. 

To estimate system performance, two measurements are needed: power consumed and load 

delivered. To estimate MSHP as a retrofit measure, a good estimate of the cooling/heating offset 

by the MSHP must also be obtained. Power consumption is easily measured with a standard Watt 

node of appropriate size and accuracy. Delivered load is much more difficult to estimate, and 

must be calculated from several measurements: temperatures, relative humidity (RH), and air 

flows. This report focuses on methods to cost-effectively estimate these parameters, particularly 

indoor head air flow.

Data points required for all field monitoring sites: 

• Whole-house power/energy 

• MSHP system power/energy

• Backup heat power/energy 

• Outdoor air temperature and humidity

• Indoor air temperature and humidity taken in each living zone supplied by system 

• Location of outdoor unit. 

Data points required for all sites where MSHP efficiency or performance is sought: 

• Return air temperature, measured near each indoor head inlet, but shielded from indoor 

coil and other radiant heat sources

• Supply air temperature for each indoor head 

• RH measurement at return and supply of each indoor head 

• Air flow rate for each indoor head (obtained via system curve using tachometer 

correlation; see Short-Term Testing)

• Vapor line temperature, for device status 

• System runtime during the measurement interval (%) – Power measurement can be used 

to calculate this quantity 

• Any unit controls, sensors, or outputs that can be accessed and recorded.

In addition to installing the data logger and setting up the sensors, the site manager may need to 

perform a basic audit to collect occupant and thermostat information. The audit must include 

blower door (and duct blaster, for ducted installations) if this was not previously performed by a 

third party.

Optional measurements providing important information: 

• Condensate flow for each indoor head – useful for rough validation of RH measurements

• Operation of dehumidification subcool/reheat mode, if system has this capability, at each 

indoor head 

• Operation of defrost cycle.
7 

Recommended monitoring points and make/model for common sensors are provided in Table 1. 

The table lists specific monitoring equipment used by Building America researchers in the past. 

Equivalent sensors that meet expected operating ranges and accuracies are acceptable.

The indoor unit fan tachometer is used to measure the unit’s fan speed. It must be installed in a 

convenient location to sense the fan without dramatically affecting flow. The recommended 

tachometer requires ferrous material for its magnetic measurements, so 0.005-in. shim material 

must be attached to the fan. Two pieces should be used to maintain fan balance, thus the reading 

will be twice the fan revolutions per minute (RPM). For fans made of ferrous material, an optical 

tachometer may be necessary. Some fan motors have a tachometer output, which can sometimes 

be read directly by the data system.

Quality Assurance Protocol

At or before monitoring installation, test personnel will assemble test equipment to verify proper 

operation and measuring tolerances. Equivalent quality assurance measures, such as an ice bath 

calibration test for thermocouple probes, are acceptable. Test personnel should maintain a 

sufficient supply of spare parts and sensors for the installation site visit. It is also advised that the 

field technician verify that the equipment installation follows manufacturer guidelines, 

particularly refrigerant line length.

Table 1. Long-Term Monitoring Instrumentation Package

Monitored Parameter Sensor 

Description 

Sensor 

Manufacturer

Sensor 

Part Number

Sensor 

Accuracy

Whole house electrical

Watt node power 

meter

Continental Control 

Systems 

WNB-3D-240-P 100HZ 

output ± 0.5%

Split core current 

transformer

Continental Control 

Systems CTS-0750-100 ± 1%

Outdoor Unit

MSHP system power

Watt node power 

meter

Continental Control 

Systems 

WNB-3D-240-P 100HZ 

output ± 0.5%

Split core current 

transformer

Continental Control 

Systems CTS-0750-030 ± 1%

Outdoor temperature and 

RH

Temperature and 

humidity sensor Vaisala HMP60A12A0A3B0

±0.6 °C,

± 3% RH

Outdoor unit radiation 

shield

Radiation shield 

from solar radiation Campbell Scientific 41303-5A NA

Data logger Main logger Campbell Scientific CR1000 ± 0.06%

For Each Indoor Unit

Indoor fan RPM Tachometer Extech 461950 Panel Tachometer 

w/Proximity Sensor ± 2%

Supply air temperature

and RH

Temperature and 

humidity sensor Vaisala HMP60A12A0A3B0

± 0.6°C, 

± 3% RH

Supply air sectional 

temperature Thermistor Omega 44000 ± 0.1°C

Return air temperature

and RH

Temperature and 

humidity sensor Vaisala HMP60A12A0A3B0

± 0.6°C, 

± 3% RH
8 

Monitored Parameter Sensor 

Description 

Sensor 

Manufacturer

Sensor 

Part Number

Sensor 

Accuracy

Return air sectional 

temperature Thermistor Omega 44000 ± 0.1°C

Space air temperature

and RH

Temperature and 

humidity sensor Vaisala HMP60A12A0A3B0

± 0.6°C,

± 3% RH

Condensate flow rate Tipping bucket Campbell Scientific TB4-25

Data logger Wireless data 

logger Campbell Scientific CR206X

± (0.25% of 

reading +1.2 

mV offset)

Comments:

A. Supply and return thermistors need to be properly located per velocity profile measurements. 

Refer to Short-Term Test in Appendix A. 

B. Only used for each additional indoor unit. Wireless logger only has up to 5 channels for analog 

signals, 2 loggers are required for each additional indoor unit.

C. Temperature accuracy is over a temperature range of –40°–60°C. RH accuracy is over 

temperature range of 0° –40°C, and 0%–90% RH.
9 

Figure 1. Measured air flow versus tachometer frequency

Short-Term Testing

After monitoring instrumentation is installed, short-term testing is used to exercise the new 

system and establish baseline performance characteristics under set, known, and well-controlled 

conditions. The researcher may seek to compare the results with previous laboratory data, 

manufacturer’s data, or a performance map to establish that the equipment was properly

installed. A system curve for the indoor head must be established for use in later COP 

monitoring. Additional sensors and equipment are required for these tests. 

Steady-State System Curve Measurement

The supply air flow from the indoor unit is the most challenging measurement to make and 

monitor in-situ. Accurate measurement of supply air flow with corresponding fan speed is the 

key to success of a MSHP system performance measurement. A system curve must be created.

Follow the procedure shown in the Duct Blaster Operation Manual (The Energy Conservatory 

2009) to connect a powered flow hood to the indoor head’s supply vent. Fashion a plenum from 

a simple trash bag or heavy plastic sheet, using internal support (springy metal rings from an 

automotive windshield shade work well) to hold the trash bag open and to aid in visible 

monitoring of plenum pressure. Operate the powered flow hood with zero static pressure (gauge) 

at the MSHP supply outlet. Actuate the MSHP from the lowest to highest fan speeds during 

heating or cooling mode (preferably both), or adjust the thermostat set point to carefully induce 

the heat pump to operate in several modes. Record air flow and fan speed at several flow rates 

across the fan’s operating range (one near low end, one at maximum flow, and up to three in 

between). If the system has 

discrete fan speeds rather 

than being fully variable, 

measure each unique speed. 

Plot the data and fit with a 

line; use Tachometer 

frequency on x-axis and

Measured Flow on y-axis. 

(See blue points and linear 

fit in Figure 1.) Take

several additional air flow 

measurements and compare 

them to the line to ensure 

repeatability. If they fit 

well, remove the flow hood. 

Repeat the heating methods 

and calls for cooling as 

closely as possible, and 

ensure that at each point the 

RPM measurement matches 

the previous dataset. 

Finally, perform error analysis to determine the uncertainty associated with the system curve. 

y = 15.025x - 79.006

0

50

100

150

200

250

300

350

400

15 17 19 21 23 25

Measured Air Flow vs. Frequency

Measured Air Flow (CFM)

Submittal Cooling (Dry CFM)

airflow (cfm)

Hz
10 

A few air flow test details require special attention by the field test technician:

• Ensure the pressure probe inserted into the supply plenum is clean and unclogged. If it is 

a pitot-type bent tube probe, ensure it is oriented properly. 

• The steel wire rings support the inflatable plenum, preventing the bag from collapsing. 

You must still examine the area around the pressure probe and ensure the bag is not 

locally collapsed and generating more local turbulence. 

• Slowly adjust the pressure in the supply plenum to 0.00-in. static pressure (gauge). Overor underpressurizing the inflatable plenum will result in discrepant flow readings. Use the 

plastic bag as a visual cue to confirm the pressure reading.

• Use the correct flow ring for the supply air flow. For steady-state fan flow measurement, 

allow the fan speed enough time to stabilize. To ensure repeatability, make at least two 

test series with fan speed from low to high and from high to low. Use the mean air flow 

measurement at each fan setting if the numbers are close to each other.

According to Ecotope (2009), a typical inverter-driven ductless MSHP indoor unit does not have 

a brushless DC motor or electronically commutated motor. Thus clogged filters will reduce air 

flow and system performance. Inspect the cleanliness of the air filter before conducting the air 

flow test. On older MSHP installations, you should conduct air flow tests and compare the dirty 

filter to the cleaned filter. On new MSHP installations, several air flow tests during the 

monitoring period will be valuable. The air filter must be cleaned regularly; remind the 

homeowner about this maintenance task.

Short-Term Efficiency Test

Set the system to provide maximum cooling (or heating). Use your measurements and 

calculations from ANSI/AHRI 210/240-2008 to estimate the loads being removed from the 

space. Compare load, power consumption, and supply conditions to manufacturer submittals or 

engineering data sheets for the specific articles being measured. The measurements described in 

this protocol, when applied correctly, are expected to result in load calculations that are accurate 

within ±20%, compared to manufacturer data. If any measured performance value is more than 

20% off the expected value, double-check sensor performance.
11 

Long-Term Monitoring

Long-term monitoring should take place to ensure multi-seasonal operating performance of the 

MSHP is captured across the common operating conditions specific to that climate. A typical 

field test for a heat pump would occur over at least 6 months which overlap both heating and 

cooling seasons. In addition, the performance limits during extremely hot summer days, or very 

cold winter nights, should be inspected where possible. These explorations should be driven by 

research questions specific to the field test and technology. 
12 

Data Collection and Documentation

The following data and documentation are required. 

Data Collection

A data logger shall be used to collect data from the installed sensors. The logger will be 

configured to take measurements at short intervals (such as every 10 or 15 seconds) and to store 

data as appropriate. For most measured parameters, recording a mean or sum at 1-minute and 

hourly periods is appropriate. At minimum, every parameter’s average (or total) shall be 

recorded at 15-minute intervals. The logger must be configured for remote communication via a 

land-line modem, wireless modem, Internet, or equivalent method. Data will be transferred at an 

interval such that modem faults and sensor issues may be noticed and corrected before critical 

data are lost.

Coefficient of Performance Calculations and Uncertainty

Performance metrics such as HSPF, EER, and SEER shall be calculated per the most recent 

relevant standards. That standard is currently ANSI/AHRI 210/240-2008. Efficiency calculations 

must consider the runtime fraction for the given period. Error analysis providing uncertainty 

estimates is strongly encouraged when data will be presented, particularly in public reports. 
13 

Other Considerations

Optional: Discussion or results from measurements on air mixing in the space during high load 

situations. How well is heating/cooling distributed around the served space(s), as determined by 

temperature, relative humidity, or occupant comments. Which zones in the house are warm and 

cold, and when (time of year, weather) does the discomfort seem to occur? How is the thermal 

distribution from room to room? Have any thermal comfort issues been reported? Other times of 

interest are low-load periods such as shoulder seasons. Are other heating sources, such as woodburning stoves or gas fireplaces, used in the home? 

Occupants should be reminded of maintenance issues pertaining to their MSHP, including when 

to clean air filters. The Field Technician should demonstrate the filter cleaning process. 

Refrigerant lines for MSHP systems can often travel long distances to remote indoor heads. If 

these lines are within the thermal enclosure of the home, they may be providing some unintended 

load to the space. This occurs because most MSHPs have expansion devices in the outdoor unit, 

so cold, two-phase refrigerant flows through one line to each indoor head (in cooling mode, hot 

in heating mode). If those lines are poorly- or un-insulated, they may provide a non-negligible

cooling or heating load which, if neglected, will lead to calculated efficiencies that do not fairly 

represent the system’s performance.
14 

References 

Anderson, R.; Roberts, D. 2008. Maximizing Residential Energy Savings: Net Zero Energy 

House (ZEH) Technology Pathways. Golden, CO: National Renewable Energy Laboratory. 

NREL/TP-550-44547. Available at www.nrel.gov/docs/fy09osti/44547.pdf. 

“ANSI/AHRI 210/240-2008: 2008 Standard for Performance Rating of Unitary Air-Conditioning 

& Air-Source Heat Pump Equipment.” Air Conditioning, Heating and Refrigeration Institute. 

2008. Available at www.ahrinet.org/. 

DOE. 2009. Energy Data Book. Available at http://buildingsdatabook.eren.doe.gov/. 

Ecotope. 2009. “Mini-Split Ductless Heat Pump Bench Test Results.” Available at 

www.bpa.gov/energy /n/emerging_technology/BPA-Report_Ductless-Heat-PumpJune2009_FINAL.pdf

Larson, B.; Allan, A. 2010. Private communications. 

NEEA. 2009. Report: Northwest Ductless Heat Pump Pilot Project, Market Progress Evaluation 

Report #1. Portland, OR: Northwest Energy Efficiency Alliance. Report #E10-215. Available at 

http://neea.org/research/reportdetail.aspx?ID=773. 

The Energy Conservatory. 2009. Minneapolis Duct Blaster Operation Manual (Series B 

Systems). Available at www.energyconservatory.com/download/dbmanual.pdf. 
15 

Appendix A. Field Test for Protocol Validation 

Objective

This test report documents a pilot test on validating the above MSHP field test monitoring 

protocol. Alternate test methods were proposed and tried, and final recommendations 

documented. The main focus was to determine: 

• How to measure the installed indoor unit fan flow under different speeds with the system 

under different operating modes. 

• Which test parameters—fan power, control signal, and fan speed—can be adopted as 

repeatable and valid monitoring parameters to correlate to the system air flow.

• The definition of the basic set of instrumentation package to monitor the installed system 

performance.

Test Equipment Description 

The pilot test was conducted in downtown Denver, Colorado, from late July to mid-August 2010. 

The test home is a two-bedroom condominium. The MSHP system has one outdoor condensing 

unit connected with two indoor wall-mounted units. The test home is located on the third floor of 

a renovated historic building, an ideal application for ductless mini-split systems. Indoor Unit A 

is mounted in the master bedroom and Unit B is mounted in the living room. The outdoor 

condensing unit is on the deck (see Figure 2 through Figure 4). The MSHP is the sole cooling 

source and a supplemental heating source. The main heating source is hydronic baseboard 

heaters fed by building hydronic loops with two condensing boilers in a mechanical room. Each 

condo has its own thermostat control for heating. 

Figure 2. Living room unit

NREL/PIX 18324, Credit: Xia Fang 
16 

Figure 3. Bedroom unit

Figure 4. Outdoor unit

Figure 5 and Figure 6 provide nameplate information of the outdoor and indoor units. The 

outdoor unit model is Mitsubishi MXZ2A20NA. The indoor unit A and B model is Mitsubishi 

MSZA09NA. 

NREL/PIX 18325, Credit: Xia Fang 

NREL/PIX 18326, Credit: Xia Fang 
17 

Figure 5. Indoor unit nameplate information

Figure 6. Outdoor unit and nameplate information

NREL/PIX 18328 and 18329, Credit: Xia Fang

NREL/PIX 18326, Credit: Xia Fang 
18 

Electrical power is connected to the outdoor unit, and branches off to Unit A and Unit B. One 

remote controller controls both indoor units. The remote controller has the following modes: 

• On / Off with Timer

• Fan: Auto – Low – Med – High

• Vane: Auto – 1 – 2 – 3 – 4 – Swing

• Mode: Auto – Cool – Dry – Heat

• Econo Cool (SWING vane mode, with 4°F high temperature set) 

• Powerful Cool (POWERFUL fan speed, HORIZONTAL (1) vane position, no 

temperature setting, automatically cancelled in 15 minutes) 

Short-Term Air Flow Test 

Fan Revolutions per Minute Measurement

In preparation for the air flow measurement, fan RPM recording was first installed. The 

proximity sensor (see Figure 7 and Table 2) is mounted next to the flow fan (Figure 7, 8 and 9). 

The proximity sensor detects the steel shim as the fan blade rotates. By capturing the sensor 

detection rate, the fan RPM or rotation frequency is calculated. In the case of a non-magnetic 

fan, a section of thin steel shim can be bonded to the fan for detection by the proximity sensor. 

Note that application of additional shim pieces on opposing fan blades may be necessary to keep 

the fan balanced. 

Figure 7. Proximity sensor mounted

NREL/PIX 18330, Credit: Xia Fang 
19 

Figure 8. Fan balancing weight

Figure 9. Thermistors

Supply Flow Measurement With Powered Flow Hood

Ecotope (2009), section 13.3 describes the test procedure followed in our test. This flow 

calibration method uses a DG-700 digital gauge, according to the manual guideline, to introduce 

± 3% error. Flow ring 1 was used for the low-speed measurement; ring 2 for the medium and 

high speeds. The MSHP indoor unit has long, narrow slot supply opening. It requires a custombuilt plenum to cover the opening, capture the air flow, and allow the air flow development. Our 

study tried both a field-built inflatable plenum and a rigid plenum method. Our final 

recommendation is the inflatable plenum method because of its ease of construction and equally 

reliable measurement repeatability. (Refer to Appendix B for the rigid plenum method.) 

Figure 10 through Figure 13 show the construction of the inflatable plenum. It is built from a 

heavy-duty, construction-grade trash bag supported by two steel wire rings from a windshield 

sunshade that eliminated some sensitivity on the pressure probe location when the bag collapsed. NREL/PIX 18331, Credit: Xia Fang NREL/PIX 18332, Credit: Xia Fang 
20 

Figure 10. Trash bag

Figure 11.Trash bag plenum with wire support 

Windshield sunshade

wire inside

NREL/PIX 18333, Credit: Xia Fang 

NREL/PIX 18334, Credit: Xia Fang 
21 

Figure 12. Inflatable plenum 

Figure 13. Flow test setup

NREL/PIX 18336, Credit: Xia Fang

NREL/PIX 18335, Credit: Xia Fang 
22 

About Fan Power, Unit Power, and Control Signals

We did not measure indoor unit power or control signal methods. The indoor unit power (S1 / S2 

– 208 / 230 V, 1Ph, S3 – 12-24 V DC) is fed to an electronic PC board. The PC board has DC 

power to indoor fan motor, vane motor, and other sensors such as operator indication lights and 

coil and room temperature thermistors. Indoor unit power or control signal measurements will 

not be good indicators. 

Short-Term Test Results 

Table 2 and Figure 14 show the comparison of field test data and manufacturer’s equipment 

submittal data. The cooling coil observed was mostly dry during the short term test period, so dry 

coil cooling air flow was used for comparison with corresponding fan frequency. The air flow 

tests using inflatable plenum method were very repeatable with the rigid plenum method. 

Table 2. Air Flow Measured and Submittal Data

Mode Low Medium High Powerful 

Fan Frequency (Hz) 16.2 19.2 21.6 23.8

Measured Air flow (CFM) 164 208 250 276

Submittal Air flow (Dry CFM) 152 229 307 338

Difference (%) –8% 9% 19% 18%

Figure 14. Air flow measured and submittal data

y = 15.025x - 79.006

0

50

100

150

200

250

300

350

400

15 17 19 21 23 25

Measured Air Flow vs. Frequency

Measured Air Flow (CFM)

Submittal Cooling (Dry CFM)

airflow (cfm)

Hz
23 

Linear fit for air flow correlation was as follows:

Qairflow = 15.025 * Frequencyfan – 79.006 (1) 

where

Qairflow is Fan air flow, in CFM 

Frequencyfan is measured proximity sensor frequency, in Hz

As-installed or field operating conditions are normally not comparable to manufacturer’s data. In 

this prepackaged integrated unit, a dirty filter and mounting the unit close to ceiling may have 

affected the air flow delivery. The data listed here are for reference only. 

Testing the system air flow and developing air flow correlation with fan rotation speed enable 

long-term MSHP performance monitoring. 

Long-Term Monitoring

Monitoring Package

Table 1 details the long-term monitoring package used for this field test. Data were recorded on 

one-minute intervals. 

During the test period in the local dry climate, the indoor unit delivered sensible cooling only. 

RH had very slight variations across the coil face. Table 2 specifies four thermistors and one 

temperature and RH sensor mounted on the supply and return air grilles separately (see Figure 

9). For other climate studies, more temperature and RH sensors may be installed.

During the monitoring period, a variance of 1.6°C was observed across the supply-side 

thermistor measurements, along with a variance of 3.0°C across the return-side thermistor 

measurements. 

During the entire monitoring period, the indoor units operated at “Fan Auto” and “Cooling” 

modes, except that the bedroom unit was turned off on August 18 for part of the day. 

Results

The purpose of this pilot study is to find field test methods to test and monitor the performance 

of MSHP system. The bedroom unit was not instrumented and monitored. This affected the 

results analysis on obtaining full MSHP system performance with indoor unit(s) on/off and 

outdoor unit matching compressor modulating. This analysis is for illustration only. 

Figure 15 is the trend of indoor unit fan air flow with recorded fan rotation frequency and linear 

correlation developed during the monitoring period. Data recording frequency is 1 minute. 
24 

Figure 15. Living room unit air flow measured

Figure 16 shows the MSHP outdoor condensing unit total power draw as a function of outdoor 

dry bulb temperature and indoor unit (living room Unit A only) return air wet bulb temperature. 

Both indoor units were running during the monitoring period, except on August 18 between 6:30 

a.m. and 2:30 p.m., when the bedroom unit was turned off. 

With a relatively constant indoor unit return air wet bulb temperature, the outdoor unit power 

followed the rise and fall trend of outdoor dry bulb temperature. On August 20, 21, and 22, the 

unit power draws oscillated up and down, indicating unstable system operation. 

Figure 16. MSHP outdoor condensing unit total power 

Figure 17 and Figure 18 are zoom-in views on the system power draw and air flow rate on 

August 20. From midnight (0:00) to early morning (7:00) on August 20, the outdoor temperature 

stayed at 17°–18°C (~ 62°–64°F) and the indoor temperature stayed at 21°–22°C (70°–72°F). 

Both the total system power draw and living room unit (the tested unit) air flow oscillated up and 

100

120

140

160

180

200

220

240

260

8/18/2010 0:008/18/2010 4:488/18/2010 9:36

8/18/2010 14:248/18/2010 19:12

8/19/2010 0:008/19/2010 4:488/19/2010 9:36

8/19/2010 14:248/19/2010 19:12

8/20/2010 0:008/20/2010 4:488/20/2010 9:36

8/20/2010 14:248/20/2010 19:12

8/21/2010 0:008/21/2010 4:488/21/2010 9:36

8/21/2010 14:248/21/2010 19:12

8/22/2010 0:008/22/2010 4:488/22/2010 9:36

8/22/2010 14:248/22/2010 19:12

8/23/2010 0:008/23/2010 4:488/23/2010 9:36

Living Room Indoor Unit CFM

Calculated CFM

0

5

10

15

20

25

30

35

40

0

0.5

1

1.5

2

2.5

3

8/18/2010 0:008/18/2010 2:248/18/2010 4:488/18/2010 7:128/18/2010 9:36

8/18/2010 12:008/18/2010 14:248/18/2010 16:488/18/2010 19:128/18/2010 21:36

8/19/2010 0:008/19/2010 2:248/19/2010 4:488/19/2010 7:128/19/2010 9:36

8/19/2010 12:008/19/2010 14:248/19/2010 16:488/19/2010 19:128/19/2010 21:36

8/20/2010 0:008/20/2010 2:248/20/2010 4:488/20/2010 7:128/20/2010 9:36

8/20/2010 12:008/20/2010 14:248/20/2010 16:488/20/2010 19:128/20/2010 21:36

8/21/2010 0:008/21/2010 2:248/21/2010 4:488/21/2010 7:128/21/2010 9:36

8/21/2010 12:008/21/2010 14:248/21/2010 16:488/21/2010 19:128/21/2010 21:36

8/22/2010 0:008/22/2010 2:248/22/2010 4:488/22/2010 7:128/22/2010 9:36

8/22/2010 12:008/22/2010 14:248/22/2010 16:488/22/2010 19:128/22/2010 21:36

8/23/2010 0:008/23/2010 2:248/23/2010 4:488/23/2010 7:128/23/2010 9:36

8/23/2010 12:00

EP_OutdoorUnit_kW_Avg T_Hum_Outdoor_C_Avg Return air Avg WB (°C)

MSHP Power Draw Trend

kW

DEG C

Power jump indicating 

bedroom unit turned on. 
25 

down, indicating the system was operating into the unstable region. The temperature and 

pressure driving potential between the condenser and evaporator sides probably dropped below 

the compressor modulating range. 

Figure 17. MSHP outdoor condensing unit total power on August 20, 2010

Figure 18. Living room unit air flow and temperatures trend on August 20, 2010

Figure 20 is a plot of cooling system EER on August 18, when only the instrumented living room 

unit operated. The system EER plotted is much lower than the rated efficiency of SEER 17. 

When the outside air dry bulb temperature increases, system EER drops and system unit power 

consumption increases (Figure 20). 

0

5

10

15

20

25

30

35

40

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

8/20/2010 0:008/20/2010 0:288/20/2010 0:578/20/2010 1:268/20/2010 1:558/20/2010 2:248/20/2010 2:528/20/2010 3:218/20/2010 3:508/20/2010 4:198/20/2010 4:488/20/2010 5:168/20/2010 5:458/20/2010 6:148/20/2010 6:438/20/2010 7:128/20/2010 7:408/20/2010 8:098/20/2010 8:388/20/2010 9:078/20/2010 9:36

8/20/2010 10:048/20/2010 10:338/20/2010 11:02

8/20/2010 11:318/20/2010 12:00

EP_OutdoorUnit_kW_Avg T_Hum_Outdoor_C_Avg Return air Avg WB (°C) Avg Supply Temp (°C) Avg Return Temp (°C)

deg C

kW

10

15

20

25

30

35

100

120

140

160

180

200

220

240

260

8/20/2010 0:008/20/2010 0:288/20/2010 0:578/20/2010 1:268/20/2010 1:558/20/2010 2:248/20/2010 2:528/20/2010 3:218/20/2010 3:508/20/2010 4:198/20/2010 4:488/20/2010 5:168/20/2010 5:458/20/2010 6:148/20/2010 6:438/20/2010 7:128/20/2010 7:408/20/2010 8:098/20/2010 8:388/20/2010 9:078/20/2010 9:36

8/20/2010 10:048/20/2010 10:338/20/2010 11:028/20/2010 11:318/20/2010 12:00

Calculated CFM T_Hum_Outdoor_C_Avg Avg Supply Temp (°C)

deg C

CFM
26 

Figure 19. MSHP EER with living room unit operating only

Figure 20. Indoor living room unit air flow and outdoor unit power

Error Analysis

Figure 21 through Figure 23 show an uncertainty propagation analysis example with the listed 

instrumentation accuracy range for a general test condition, assuming 

• ± 15% experimental error introduced by the air flow measurement method 

• Additional ± 3% error on supply air RH introduced by using only one temperature and 

RH sensor on the supply grille, (not a concern for this dry climate study). 

The final system cooling capacity and associated system cooling EER have error of ± 16.6%. 

5

10

15

20

25

30

35

40

0

2

4

6

8

10

12

14

8/18/2010 6:288/18/2010 6:578/18/2010 7:26

8/18/2010 7:55

8/18/2010 8:248/18/2010 8:528/18/2010 9:218/18/2010 9:50

8/18/2010 10:198/18/2010 10:488/18/2010 11:16

8/18/2010 11:45

8/18/2010 12:148/18/2010 12:438/18/2010 13:128/18/2010 13:408/18/2010 14:098/18/2010 14:38

Cooling EER T_Hum_Outdoor_C_Avg Avg Supply Temp (°C) Supply air Avg WB (°C) Return air Avg WB (°C) Avg Return Temp (°C)

deg C

EER

0

0.5

1

1.5

2

2.5

3

0

50

100

150

200

250

300

8/18/2010 6:288/18/2010 6:578/18/2010 7:268/18/2010 7:558/18/2010 8:248/18/2010 8:528/18/2010 9:218/18/2010 9:50

8/18/2010 10:198/18/2010 10:488/18/2010 11:168/18/2010 11:458/18/2010 12:148/18/2010 12:438/18/2010 13:128/18/2010 13:408/18/2010 14:098/18/2010 14:38

Calculated CFM EP_OutdoorUnit_kW_Avg

CFM

kW
27 

Figure 21. Uncertainty propagation analysisa 

a

(F-Chart Software. 2011. EES: Engineering Equation Solver. http://www.fchart.com/ees/) 
28 

Figure 22. Uncertainty propagation table

Figure 23. Uncertainty propagation results 
29 

Appendix B. Rigid Plenum Method (for reference)

Figure 24. MSHP for indoor heat air flow test rigid plenum method

Figure 25. Researcher examining MSHP for indoor heat air flow test rigid plenum method NREL/PIX 18320, Credit: Xia Fang NREL/PIX 18321 Credit: Xia Fang 
30 

Figure 26. Researchers testing MSHP

Figure 27. Opening for unit room temperature thermistor sensing NREL/PIX 18322, Credit: Xia Fang NREL/PIX 18323, 

Credit: Xia Fang 
F1147-E(10/2008) 

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4. TITLE AND SUBTITLE

Field Monitoring Protocol: Mini-Split Heat Pumps

5a. CONTRACT NUMBER

DE-AC36-08GO28308

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14. ABSTRACT (Maximum 200 Words)

The report provides a detailed method for accurately measuring and monitoring performance of a residential MiniSplit Heat Pump. It will be used in high-performance retrofit applications, and as part of DOE's Building America 

residential research program.

15. SUBJECT TERMS

Mini Split Heat Pump; MSHP; Air Conditioning; HVAC; Field Test; Monitoring; Protocol; Building America; 

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