One of my colleagues, a thoughtful energy auditor and building scientist, is fond of saying, “If you don’t test, you don’t know.” One of the things

we don’t currently know much about is how ductless heat pumps perform in the real world. On paper, these systems promise very high heating and cooling efficiency. But, as we might expect, actual mileage may vary. A 2015 study sponsored by DOE’s Building America program measured twofold differences in efficiency in cold-climate heat pumps installed in New England. (See “learn more” at the end of the article for a link to the study.)

Performance Testing Ductless Systems by Jon Harrod

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Although the rated performance of the heat pumps studied was similar, their real-world performance varied widely with siz-ing, installation, and operator behavior. The results of the Building America study have informed best-practice recommendations.

As a contractor, I know that it’s one thing to read a report and attempt to apply its lessons; it’s another to test one’s own instal-lations in the field. The methods used by the Building America team were quite elaborate. To measure airflow, they built a large foam-board box around the outlet of each unit, then used a Duct Blaster to maintain neutral pressure between the outlet and the room. Temperature and electrical consumption were record-ed with data loggers over a several month period. While this in-depth approach is critical for evaluating long-term perfor-mance, contractors need simpler, quicker methods that provide snapshots of how a system is performing.

In this article, I share a set of techniques for measuring air-flow, delivered heating and cooling capacity, and efficiency of ductless heat pumps. I was first exposed to these techniques in a video by Bryan Orr on the HVAC School site (see learn more). These techniques do not require specialized tools. Because they do not involve connecting pressure gauges, they do not re-quire EPA certification nor risk the release of refrigerants. These techniques may prove useful in several ways:

START-UP AND COMMIS-SIONING. Testing done at the time of start-up can be used to verify proper operation and pro-vide the designer and installer with immediate feedback on their work. It also provides a benchmark against which future tests can be compared.

PREVENTIVE MAINTENANCE. Testing done at routine maintenance visits provides snapshots that can be compared with previous measurements. A drop in performance may indi-cate a problem—for example, a refrigerant leak—that warrants further investigation.

TROUBLESHOOTING. Comfort complaints with ductless systems can be notoriously difficult to troubleshoot. The same sophisticated control algorithms and variable-speed opera-tion that help these systems achieve high efficiency can some-times make them seem like they have “minds of their own.” Performance measurements allow a technician to determine that the system is delivering its rated heating and cooling Btu/hr. If so, the technician can rule out several equipment-related issues and focus troubleshooting on questions of sizing, con-trols, unit placement, and operator behavior (for example, the use of deep temperature setbacks, which are not recommended with ductless systems).

ENERGY AUDITING. Ductless heat pumps are becom-ing increasingly common as both primary and supplemental heating/cooling systems. Just as we use combustion analyzers to determine the steady-state efficiency of a fossil-fuel heating system, the techniques described here can allow energy auditors to calculate efficiencies of ductless heat pumps they encounter.

Understanding Heat Pump Capacity and Efficiency MetricsHeat pumps move thermal energy using the vapor compres-sion cycle. A refrigerant (R-410A in most modern residential systems) cycles through a closed loop, which is divided into high- and low-pressure sides. The two sides are separated by the compressor (a pump-like, motor-driven device that increases the temperature and pressure of the gaseous refrigerant) and a metering device (essentially a narrow orifice which causes a temperature and pressure drop). On the high-pressure side of the system, which includes the condensing coil, the refrigerant releases heat as it condenses from gas to liquid. On the low-pres-

sure side, in the evaporator coil, the refrigerant absorbs heat as it boils from liquid to gas.

In a conventional air-condi-tioning system, heat moves in only one direction; the condensing coil, which releases heat to the sur-rounding air, is always outside the house; the evaporator coil, which absorbs heat, is always inside. Heat pumps contain an additional com-ponent, the reversing valve, which changes the direction of refriger-

ant flow. In heating mode, the high-pressure, high-temperature gas leaving the compressor is routed to the indoor coil, where it condenses and releases heat. The outdoor coil becomes the evap-orator, in which low pressure liquid boils and absorbs heat from the surrounding air. Fans are present in both the indoor and outdoor units to move air across the coils.

The capacity calculations described here are based on measurements taken at the indoor unit. In heating mode, the amount of heat delivered to the conditioned space depends on the airflow (measured in cubic feet per minute, CFM) and the change in dry-bulb air temperature across the indoor coil (ΔT, measured in °F) according to the following formula:

Btu/hr = CFM x ΔT x 1.08The 1.08 is a constant representing the product of the specific

heat and density of air at sea level (an additional altitude-correc-tion factor is needed for higher-elevation sites).

The delivered capacity calculation is slightly more complicat-ed in cooling mode. Cooling reduces the temperature of the air

As a contractor, I know that it’s one thing to read a report and attempt to apply its lessons; it’s another to test one’s own installations in the field.

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HVAC

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Top: A hot-wire anemometer (top) or vane anemometer (bottom) can be used to measure airflow. Bottom: Checking wet- and dry-bulb bulb temperatures of air entering and leaving a floor-mounted ductless unit with thermohydrometers.

We clearly need more data on ductless airflow.“

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including flow plates, static pressure measurements, and heat rise tests. But none of these methods translates well to the ir-

regularly shaped openings, adjustable louvers, and turbulent airflow present at the outlets of most ductless systems. The Building America researchers tried several approaches, ultimate-ly settling on the flow-matching approach de-scribed above.

Given the measurement challenges, is it bet-ter to rely on manufacturer’s published data? After all, ductless systems, by their very na-ture, don’t have restrictive site-built ductwork. As long as the filters, blower fan, and coil are

clean, shouldn’t airflow closely match the manufacturer’s specs? The Building America study yielded some surprising results. In the systems they measured, airflow was typically 20-50% be-

(removing sensible heat); it also reduces its moisture content (re-moving latent heat). The total change in the energy content of the air (the change in its enthalpy) is the sum of the sensible and latent heat removal. In addition to measuring the dry-bulb temperature of the air entering and leaving the unit, the technician uses a thermohygrometer to measure its wet-bulb temperature or relative humidity. These measurements are converted to enthalpy (H) using a lookup chart or online calculator, which can also provide corrections for elevation; I use the “Psychrometer °F” app for this purpose.

Delivered cooling capacity is calculated us-ing the following formula:

Btu/hr = CFM x ΔH x 4.5where CFM is airflow in cooling mode, ΔH is the difference

in enthalpy between air entering and exiting the indoor unit, and 4.5 is a multiplier based on the density of air.

The delivered capacity calculations described above can be automated using mobile apps and wireless probes. Some apps can also capture a time- and location-stamped record of the test.

Delivered capacity of heat pumps will vary with indoor and outdoor temperature and humidity; by comparing field measure-ments with a manufacturer’s performance charts, it’s possible to show that a system is performing as designed under a specific set of conditions. By measuring power consumed (Watts) with a mul-timeter or energy monitor, it’s possible to take the analysis a step further to measure real-world heating Coefficient of Performance (COP) and cooling Energy Efficiency Ratio (EER).

COP, used for heating, is a unitless ratio of heat energy deliv-ered to electrical energy consumed:

COP = Btu/hr / (Watts x 3.412 Btu/Watt)Electric baseboard has a COP of 1—for each kWh (3412 Btu)

delivered, one kWh is consumed. A COP above one indicates that more heat energy is being delivered to the conditioned space than is being consumed by the indoor and outdoor equipment. EER, used for cooling, is measured in Btu/hr of cooling per Watt of power:

EER = Btu/hr / WattsBoth COP and EER are instantaneous measures, represent-

ing performance at a moment in time. They differ from seasonal ratings (Heating Season Performance Factor, HSPF, and cool-ing Seasonal Energy Efficiency Ratio, SEER, both measured in Btu/Watt-hr like EER), in the same way that fossil fuel Annual Fuel Utilization Efficiency (AFUE) differs from Steady State Efficiency (SSE).

Ductless Air Flow—Measured or Rated?Airflow is a critical input to delivered capacity equations. In ducted systems, we have several methods for estimating airflow,

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Figure 1. Extended performance charts, provided by the manufacturer, show capacity ratings at specific indoor and outdoor conditions. In the example calculation in Table 1, the measured indoor intake wet bulb temperature is 62.8 °F, so I use data for 63 °F IWB. The outdoor dry bulb temperature is 79°F, so I average the total capacity ratings (TC, listed here in 1,000’s of Btu/hour) for 75 °F and 85°F. (Source: www.mylinkdrive.com/viewPdf?srcUrl=http://enter.mehvac.com.s3.amazonaws.com/DAMRoot/Original/10005\M-Series_Engineering_Manual.pdf)

low manufacturer’s specs. The position of adjustable louvers, not listed as a factor in airflow charts, also made a measurable difference. In the wall-mounted units they studied, flows were highest when louvers pointed horizontally and lowest when they pointed down, as is commonly recommended in heating mode. If you don’t test, you don’t know.

In the HVAC School video, Orr demonstrates a different method for estimating ductless airflow. With the ductless vanes removed and the unit running at maximum speed, he moves a hand-held anemometer in a regular pattern (traverse) across the opening. The anemometer transmits data wirelessly to an app, which averages air-speeds and converts them to CFM based on the size of the opening. In the video, and in my own trials with both vane- and hot-wire anemometers, this method yields good agreement with manu-facturer’s specs.

We clearly need more data on ductless airflow. If you can, use an anemometer to measure air-flow. If you need to rely on published data, double check that all system components are clean; set the ductless vanes to the least restrictive position (horizontal for high-wall mounted units), and make sure you are looking at the correct value in the manufacturer’s chart. Many units have four or more speed settings, and rated airflows will differ depending on whether the indoor coil is dry, as in heating mode, or wet, as it will become in cooling mode.

The delivered capacity test procedure is as follows:1. Visual inspection. Perform a quick visual inspection of the

indoor and outdoor components of the system. Verify that outdoor coil, indoor coil, filters, and fan blower wheel are clean. This is also an opportun ty to look for visible defects (bent fins on indoor and outdoor coils, missing line set in-sulation, kinked line sets, and so on).

2. Measure the outdoor dry-bulb temperature.3. Set the fan speed to maximum and the target temperature

(setpoint) several degrees above (heating) or below (cool-ing) the room temperature. Allow the system to run for 10 to 15 minutes to reach steady state.

4. If measuring airflow, remove the ductless vanes and per-form a traverse measurement of the supply air opening. Use an app to convert to CFM or calculate CFM as (av-erage air velocity in feet per minute) x (opening size in inches / 144).

5. Heating: Measure return air temperature (dry bulb tem-perature) near the inlet of the unit. For a typical high-wall unit, this will be the air temperature near the ceil-ing. If possible, keep the probe out of direct line-of-sight of the indoor coil to avoid radiant effects. Measure sup-

ply air dry-bulb temperature at the outlet of the indoor unit. Because of temperature variations in the supply air, it may be necessary to average multiple measurements. Calculate ΔT as the difference between the supply and return air temperatures.

Location, elevation Enfield, NY, 1,420 ft

Test date 7/11/2020

Outdoor temperature (dry bulb) 79°F

Manufacturer Mitsubishi

Model MFZ-KJ12NA (indoor)MUFZ-KJ12NAHZ (outdoor)

Installation date 5/5/2020

Style Floor-mounted

Mode Cooling

Rated airflow 354 CFM

Measured airflow 368 CFM

Return air (dry bulb/wet bulb/enthalpy) DB=70.5°F/ WB=62.8°F/ H=28.9 Btu/lb

Supply temp (dry bulb/wet bulb/enthalpy)

DB=52.0°F/ WB=50.9°F/ H= 21.2 Btu/lb

Delivered capacity 12,751 Btu/hr

Rated Capacity 12,600 Btu/hr

Measured Volts, Amps, Power consumption (W)

242V/4.48A/1,084W

Measured COP/EER EER=11.8 Btu/W-h

Table 1. Sample Delivered Capacity and EER Calculations for a Ductless Unit.

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>> learn more

To share your results measuring ductless systems, or for a blank copy of the delivered capacity worksheet shown, you can email me at jharrod@snugplanet.com.“Field Performance of Inverter-Driven Heat Pumps in Cold Climates”https://apps1.eere.energy.gov/buildings/publications/pdfs/build-ing_america/inverter-driven-heat-pumps-cold.pdfHow to Tell if a Ductless AC is Working

(https://www.hvacrschool.com/how-to-tell-if-a-ductless-ac-is-working/)

Cooling: Follow the procedure above but use a thermo-hygrometer to measure dry bulb temperature and either wet-bulb temperature or relative humidity of the return and supply air. Use a chart, app, or online calculator to calculate enthalpy (H) of the return and supply air, then calculate ΔH as the difference between them.

6. Calculate delivered capacity using the formulas above. Compare your results to manufacturer’s data for given in-door and outdoor conditions.

For a sample calculation, see Table 1 and Figure 1.

Left: Measuring voltage and current draw at the outdoor unit.

Measuring Electrical Power and Calculating EfficiencyIn most ductless systems, the fan and electronics in the indoor unit are powered through the outdoor unit, so that the total power consumption of the system can be measured at a single location. To calculate power, we need to know voltage (V, in volts) and current (A, in amperes). Voltage is measured with a multimeter at the line-voltage terminals (L1 and L2); cur-rent can be measured with a clamp-style ammeter around one leg of the line-voltage power supply. Power is approximated as Watts = V x A. (A more precise measurement of real power can be obtained using a meter that corrects for phase and wave-form variations typical of inverter-driven systems; in practice, ductless systems running a maximum speed appear to have power factors close to one, so that V x A provides a good esti-mate of power consumed.)

Power measurements, together with delivered capacity, can be used to calculate heating efficiency (COP) and cooling effi-ciency (EER) using the formulas described above.

Future DirectionsWhat we don’t test, we don’t know. But what we measure, we can improve. As ductless systems continue to grow in popularity, measuring their performance can give us the feedback we need to improve our design, installation, and service. Increasingly, mobile apps like MeasureQuick and Testo Smart Probes are making it easier to perform and document these measurements. I expect that we’ll soon see this functionality integrated into the ductless equipment itself, as it is with many ground-source heat pumps. Performance tests may also show up as requirements for utility and state incentive programs. In the meantime, I’d love to know what you’re finding when you measure ductless systems.

JON HARROD is president of Snug Planet, a building performance and HVAC company based in Ithaca, New York.

This article was supported with funding from DOE’s Weatherization Assistance Program.

Comfort complaints with ductless systems can be notoriously difficult to troubleshoot. The same sophisticated control algorithms and variable-speed operation that help these systems achieve high efficiency can sometimes make them seem like they have minds of their own.

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