Measure Guideline:

Air Conditioner Diagnostics, 

Maintenance, and 

Replacement

D. Springer and B. Dakin PE

Alliance for Residential Building Innovation (ARBI)

March 2013
NOTICE

This report was prepared as an account of work sponsored by an agency of the 

United States government. Neither the United States government nor any agency 

thereof, nor any of their employees, subcontractors, or affiliated partners makes any 

warranty, express or implied, or assumes any legal liability or responsibility for the 

accuracy, completeness, or usefulness of any information, apparatus, product, or 

process disclosed, or represents that its use would not infringe privately owned rights. 

Reference herein to any specific commercial product, process, or service by trade 

name, trademark, manufacturer, or otherwise does not necessarily constitute or imply 

its endorsement, recommendation, or favoring by the United States government or 

any agency thereof. The views and opinions of authors expressed herein do not 

necessarily state or reflect those of the United States government or any agency 

thereof.

Available electronically at http://www.osti.gov/bridge

Available for a processing fee to U.S. Department of Energy

and its contractors, in paper, from:

U.S. Department of Energy

Office of Scientific and Technical Information

P.O. Box 62

Oak Ridge, TN 37831-0062

phone: 865.576.8401

fax: 865.576.5728

email: mailto:reports@adonis.osti.gov

Available for sale to the public, in paper, from:

U.S. Department of Commerce

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

phone: 800.553.6847

fax: 703.605.6900

email: orders@ntis.fedworld.gov

online ordering: http://www.ntis.gov/ordering.htm

Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste
iii

Measure Guideline: Air Conditioner Diagnostics, Maintenance,

and Replacement

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:

David Springer and Bill Dakin, PE (Davis Energy Group)

Alliance for Residential Building Innovation (ARBI)

123 C Street

Davis, California 95616

NREL Technical Monitor: Cheryn Metzger

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

March 2013
iv

[This page left blank]
v 

Contents

Contents....................................................................................................................................................... v

List of Figures ........................................................................................................................................... vii

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

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

Executive Summary.................................................................................................................................... x

Acknowledgements .................................................................................................................................... x

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

1 Section 1: Planning and Decision Making Criteria............................................................................ 3

1.1 Defining the Problem...........................................................................................................3

1.2 Maintenance Approaches.....................................................................................................4

1.3 Energy Savings ....................................................................................................................5

1.4 The Consequences of No Action .........................................................................................5

1.5 Planning for Improvements and Risk Mitigation.................................................................6

2 Section 2: Technical Description........................................................................................................ 7

2.1 General Performance Interactions........................................................................................7

2.2 Airflow Impacts...................................................................................................................7

Desired Airflow ...................................................................................................................7

What Affects Airflow?.........................................................................................................7

Findings from the Field........................................................................................................9

How Airflow Affects System Operation and Energy Use .................................................11

Airflow Measurement Methods.........................................................................................12

2.3 Refrigerant System Defects, Detection, and Correction....................................................13

What Can Go Wrong .........................................................................................................13

The Importance of Correct Refrigerant Charge .................................................................13

The Effect of Non-Condensable Gases..............................................................................14

Fault Detection and Diagnosis...........................................................................................15

Methods for Setting Refrigerant Charge............................................................................16

Diagnostic Methods for Assessing Refrigerant Charge.....................................................16

Evaporator Coil Fouling and Damage to Distributor Tubes..............................................17

2.4 Repair or Replace? .............................................................................................................17

Condenser Replacement.....................................................................................................17

The Value of High Efficiency Systems .............................................................................19

Replacement of Other Components...................................................................................19

3 Section 3: Implementation................................................................................................................. 21

3.1 The Preliminary Diagnostic (PD) Method.........................................................................21

Objectives and Scope.........................................................................................................21

Skills and Tools Required..................................................................................................22

Measuring Airflow.............................................................................................................22

Before Starting Tests..........................................................................................................23

Preliminary Diagnostic Steps.............................................................................................23

3.2 The Comprehensive Diagnostic (CD) Method ..................................................................27

Objectives and Scope.........................................................................................................27

Skills and Tools Required..................................................................................................27

Refrigerant Charge Test Overview....................................................................................28

Before Starting Tests..........................................................................................................28

Test Procedures..................................................................................................................28

Measurements (Enter all values into Table A-2)...............................................................29
vi

Cold and Hot Weather Testing ..........................................................................................29

3.3 Calculations........................................................................................................................30

Temperature Split...............................................................................................................30

Condensing Temperature Over Ambient (COA)...............................................................30

Superheat............................................................................................................................30

Subcooling .........................................................................................................................31

3.4 Diagnosis............................................................................................................................31

Evaluating the Measurements............................................................................................31

Using Diagnostic Flow Charts...........................................................................................32

Removing and Replacing Refrigerant................................................................................34

Equipment Replacement ....................................................................................................35

References................................................................................................................................................. 38

Appendix A: Test Forms and Checklists................................................................................................ 41

Appendix B: Supporting Data and Calculations.................................................................................... 43

B-1: Energy Savings for Replacement with High Efficiency Air Conditioners......................43

B-2: Replacement Blower Motor Energy Savings Estimates..................................................45

B-3: Verification of Temperature Split Method for Checking Refrigerant Charge.................46

Appendix C: Detailed Methods and Procedures.................................................................................... 48

C-1: Air Conditioner Replacement Decision Tree...................................................................48

C-2: Detailed Airflow Measurement Methods ........................................................................49

General Instructions...........................................................................................................49

Powered Flow Hood (Fan Flowmeter) ..............................................................................49

Flow Grid...........................................................................................................................50

Flow Capture Hood............................................................................................................50

C-3: Detailed Condenser Coil Cleaning Instructions...............................................................51

C-4: Refrigerant Procedures to Prevent Non-Condensables and Restrictions.........................52

C-5: Vacuum Pump Maintenance............................................................................................52

C-6: Evacuation and Recovery Procedures..............................................................................53

C-7: Charging Procedures........................................................................................................56

C-8: Proper Refrigerant Procedures to Avoid Non-condensables and Restrictions................56

C-9: ASHRAE Recommendations for Proper Evacuation to Remove Non-condensable Air 

and Water Vapor................................................................................................................57

C-10: Method to Detect Blocked Distributor Tubes................................................................57

Appendix D: Test Equipment Specifications and Calibration.............................................................. 58

D-1: Recommended Accuracy of Test Equipment ..................................................................58

D-2: Calibration Method for Digital Thermometer Sensors....................................................58

D-3: Refrigerant Gauge Calibration Procedure .......................................................................58

D-4: Other Devices..................................................................................................................59
vii

List of Figures

Figure 1. Primary system fault modes...................................................................................................... 3

Figure 2. Examples of constricted ducts (Photo credit: R. Chitwood, reprinted with permission).... 8

Figure 3. Pressure loss as a function of velocity for commonly available filters. ............................... 9

Figure 4. Sources of pressure drop measured in 62 California homes (Source: Wilcox 2011, 

reprinted with permission). ............................................................................................................... 10

Figure 5. Summary of airflow measurements from 62 California homes (Source: Wilcox, reprinted 

with permission). ................................................................................................................................ 10

Figure 6. Impact of reduced airflow on cooling capacity (Source: Parker, reprinted with 

permission). ........................................................................................................................................ 11

Figure 7. Impact of reduced airflow on cooling system performance (Source: Parker, reprinted 

with permission). ................................................................................................................................ 12

Figure 8. A powered flow hood in use. ................................................................................................... 12

Figure 9. Normalized EER versus charge and outside temperature (Source: P G &E, reprinted with 

permission). ........................................................................................................................................ 14

Figure 10. A digital manifold gauge. ....................................................................................................... 16

Figure 11. Examples of condenser and evaporator coil fouling. ......................................................... 17

Figure 12. The Energy Conservatory TrueFlow flow grid..................................................................... 22

Figure 13. Proper position of TXV sensing bulb.................................................................................... 23

Figure 14. Temperature and pressure test points. ................................................................................ 24

Figure 15. Pressure loss diagnostic flow chart. .................................................................................... 24

Figure 17. Refrigerant diagnostic flowchart: systems with fixed orifice expansion devices. .......... 33

Figure 18. Refrigerant diagnostic flowchart: systems with thermostatic expansion devices.......... 33

Figure C-1. Air conditioner replacement decision tree......................................................................... 48

Figure C-2. Pressure measurement locations. ...................................................................................... 49

Figure C-3. Removing of the top of the condensing unit for cleaning ................................................ 51

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

List of Tables

Table 1. Estimated Energy Savings, Replacement Costs, and Simple Paybacks for 15 SEER vs. 13 

SEER Air Conditioner......................................................................................................................... 19

Table 2. Target Temperature Split (Return Dry Bulb – Supply Dry Bulb)............................................ 26

Source: California Energy Commission (CEC 2008)............................................................................. 26

Table 3. Recommended COA and EST Values to be Used in Diagnostics. ........................................ 32

Table 4. Target Superheat. ....................................................................................................................... 36

Table 4 (continued). Target Superheat. .................................................................................................. 37

Table A-1. Preliminary Diagnostics Form............................................................................................... 41

Table A-2. Comprehensive Diagnostics Form (supplement to Table A-1).......................................... 42

Table B-1. Existing home assumptions by building vintage................................................................ 43

Table B-2. Annual Energy Use Determined by BEopt. .......................................................................... 44

Table B-3. Assumed Sizes of Replacement Air Conditioners. ............................................................. 44

Table B-4. Replacement Cost vs. SEER Rating. .................................................................................... 44

Table B-5. Results of Comparative Tests of a PSC and ECM Motor.................................................... 45

Table B-6. Annual Cost Savings Calculation Example. ........................................................................ 46

Table B-5. Laboratory Test Results Comparing Measured and Target Temperature Splits. ............ 47

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

Definitions

ACCA Air Conditioning Contractors of America

AHRI Air Conditioning, Heating, and Refrigeration Institute

ANSI American National Standards Institute

ARBI Alliance for Residential Building Innovation

ASHRAE American Society for Heating, Refrigeration, and Air Conditioning 

Engineers

BPI Building Performance Institute

CD Comprehensive Diagnostic (a procedure as defined in this document)

CEC California Energy Commission

CFC Chlorofluorocarbon

CFM Cubic feet per minute

COA Condensing Over Ambient”, or the difference between the condensing 

temperature and the temperature of air entering the condensing coil

DEG Davis Energy Group

ECM Electronically commutated motor

EER Energy Efficiency Ratio, in Btuh per watt

EPA Environmental Protection Agency

EST Refrigerant evaporation saturation temperature

Ft2 Square feet

HERS Home Energy Rating System

HFC Hydrofluorocarbon

HVAC Heating, ventilation, and air conditioning

kBtuh Thousand British Thermal Units per hour

kW Kilowatt

kWh Kilowatt-hour

MERV Minimum efficiency reporting value (for air filters)

NSOP Normal system operating pressure (for airflow measurement corrections)

PD Preliminary Diagnostic (a procedure as defined in this document)

PG&E Pacific Gas and Electric Company

PSC Permanent split capacitor (motor type)

SCE Southern California Edison

SEER Seasonal energy efficiency ratio
ix

SHR Sensible heat ratio

TD Temperature difference

TSOP Test system operating pressure (for airflow measurement corrections)

TXV Thermostatic expansion valve

W Watt

w.c Water column (referring to inches of pressure)
x

Executive Summary

Heating and cooling energy use represents about 54% of all energy consumed in existing 

residential buildings. Field studies have shown that more than half of installed air conditioning

(AC) systems have significant defects, and that proper maintenance of cooling systems can 

reduce their energy use by as much as 50%. Since the 1990s, an immense body of knowledge on 

the importance of proper maintenance of heating, ventilation, and air conditioning (HVAC) 

systems and the impact of defects has been accumulated, and research in this area is continuing. 

Though some of this information has found its way into standards and utility programs, general 

maintenance practices continue to miss details that could realize significant energy savings at 

minimal cost. This guideline targets home performance contractors who are relatively 

inexperienced with HVAC systems, HVAC technicians who can benefit from recent research on 

AC system faults and diagnostic methods, and program managers who may apply this 

information in structuring successful HVAC tune-up programs.

Home performance contractors who are not attentive to air conditioner performance are missing 

the opportunity to deliver optimal energy savings to their customers. Systems that are not in 

obvious need of replacement may still perform far below their rated conditions. Although these 

systems are complicated, many faults that substantially degrade performance can be easily 

identified and in some cases corrected by the home performance contractor during the normal 

course of completing home inspections and duct testing. 

Little information is available on methods to diagnose systems and particularly to detect and 

discriminate between multiple faults such as inadequate airflow, incorrect charge, liquid line 

restrictions, evaporator and condenser coil fouling, and the presence of non-condensable 

refrigerant contaminants. While there has been some progress with development of computer

diagnostic tools for use in utility programs, a systematic method is needed to detect and identify 

major system faults by field technicians who do not have access to these tools. 

This guideline provides information on the impact of various defects and the potential energy 

savings that can result from proper maintenance, and describes a two-step process for diagnosing 

and correcting faults. The first step involves a fundamental inspection and tests that can be 

completed by home performance contractors with BPI or similar training to identify or diagnose, 

and in some cases remediate, relatively non-technical problems such as low airflow and fouled 

condenser coils. The second step is designed to be implemented by experienced HVAC 

technicians and provides a systematic approach that can be used to identify system faults. 

Acknowledgements

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

America program and their funding and support of development of this guideline as well as 

research that informed it. The detailed diagnostic method described in this guideline was adapted 

from Appendix RA3 of the California Title 24 Residential Energy Efficiency Standards (CEC 

2008). Substantial guidance with the development of diagnostic methods was provided by Jim 

Phillips of The Energy Savers. Also, DEG would like to acknowledge Marshall Hunt of Pacific 

Gas & Electric Company for the direction he provided in developing the two-step approach.
1

Introduction

Space cooling and heating constitutes 54% of residential energy use nationwide (DOE 2010), 

and HVAC tune-ups are a potentially very cost-effective component of residential retrofits. 

Laboratory testing suggests that service and replacement can produce HVAC energy savings on 

the order of 30% to 50% (Messenger 2008). A recent sampling of more than 1,300 air 

conditioning (A/C) units from the Arkansas CoolSaver Program revealed that corrections in 

charge and airflow netted an average increase in delivered Btu/h of 38% and a peak kW demand 

reduction of 0.5 kW per tune-up (Kuonen 2011). However, as expressed in Kuonen’s Home 

Energy article, “Simply put, the service practices we performed in the past to get the job done are 

not getting the job done today. Past practice and best practice no longer belong in the same 

sentence.” Other programs have verified the need for air conditioner maintenance. Downey 

(2002) reported that of 8,873 systems tested, 65% required repairs. Mowris (2004) reported field 

measurements of 4,168 air conditioners where 72% of systems had improper refrigerant charge 

and 44% had improper airflow.

These faults are widespread due to a lack of standards, training, information, and other market 

barriers regarding proper evacuation and maintenance procedures. Mowris (2011a) surmises that 

many systems are undercharged due to long line sets where no additional refrigerant is added or 

where systems have leaking refrigerant over time. However, there is the possibility that low 

charge diagnoses may be caused by a false diagnosis of undercharge due to refrigerant 

restrictions, lower ambient temperatures, inaccurate analog pressure sensors, or inaccurate 

refrigerant line temperature sensors. For example, a system with the correct charge when loaded 

at high ambient temperatures (i.e., 95°F or higher) may produce a false undercharge indication 

when checked at lower ambient temperatures, or a false undercharge indication can be caused by 

inaccurate temperature sensors producing higher suction line or lower liquid line temperatures. 

Superheat tables available from manufacturers (Carrier 1995) or the California Energy 

Commission (2008) account for changes in outdoor temperature, but these tables may not predict 

the proper required superheat for all systems without thermostatic expansion valves (TXV) at 

low ambient temperatures. False diagnosis of undercharge will cause a decrease in system 

efficiency if refrigerant is unnecessarily added. The lack of information and training on correct

procedures can result in an unintentional degradation of system performance. Incorrect charge 

procedures and inaccurate or badly maintained service equipment can turn a mediocre system 

into a poorly performing system.

This measure guideline attempts to bridge the gap between potential and realized whole house 

energy savings by developing field guidelines for HVAC diagnosis, repair, and equipment 

replacement. The intent of this document is to provide easy-to-implement, cost-effective, and 

reliable methods for home performance contractors and technicians to follow for the diagnosis 

and correction of AC system problems, particularly during the course of the energy upgrade 

process. It expands on ACCA Standard 4 (Maintenance of HVAC Systems) and Standard 5 

(HVAC Quality Installation Specification) as they pertain to residential AC and heat pump 

systems, and offers a way to capture a source of energy savings that is frequently ignored in the 

course of completing retrofits. 
2

This guideline primarily addresses diagnosis and repair of refrigerant systems and mechanical 

components but excludes information on ducting, which is addressed by another Building 

America measure guideline, Building America Measure Guideline: Sealing and Insulating Ducts 

in Existing Homes (Aldrich 2011). Minimizing duct leakage is critical to maintaining adequate 

system airflow. The diagnostic tests described in this guideline rely on the completion of duct 

testing and sealing prior to initiation of tests. An acceptable standard for duct tightness in 

existing homes is 15% of cooling system airflow at a duct pressurization of 25 Pa (CEC 2008). 

Refrigerant system problems can be difficult to diagnose. The art of HVAC maintenance will 

continue to evolve as simpler and more accurate techniques for evaluating refrigerant charge and 

other diagnostic methods are developed. For example, one program implementer is 

recommending further research to develop a diagnostic protocol for contractors and HERS 

verifiers to differentiate the presence of non-condensables (air and water vapor) from a 

refrigerant overcharge or blocked condenser or evaporator coil or the presence of refrigerant 

restrictions from a refrigerant undercharge or low airflow condition (Mowris 2011a). Though 

there are differing opinions on diagnostics and repair procedures, the methods included in this 

guideline are intended to represent the current state of the art. Updates will be required as 

diagnostic procedures are improved and new equipment and tools emerge. Using this guideline, 

major problems can be identified and in some cases remedied at low cost, resulting in lower 

heating and cooling energy bills and improved comfort and safety for the homeowner.

In order to accommodate different levels of training and experience as well as time and cost 

constraints, two approaches are described. One is designed to rapidly identify and resolve major 

problems, and the other involves more detailed testing, diagnosis, and remediation that will help 

to insure that equipment is operating at its optimal performance.

Air conditioner tune-ups and repair can involve exposure to high-voltage electricity and other 

risks; work involving such exposures should be conducted by trained service technicians familiar 

with standard safety practices.
3 

1 Section 1: Planning and Decision Making Criteria

1.1 Defining the Problem

Poor performance resulting from aging systems, partial equipment failure, poor original design 

and workmanship, improper service, and lack of maintenance is commonplace. These problems 

result in increased energy use and utility bills, reduced occupant comfort, accelerated equipment 

degradation, and in some cases risk to homeowners. These problems can be interrelated and 

difficult to correct without applying a systematic approach to diagnosis and maintenance. 

Deficiencies frequently found in existing homes are listed and categorized in Figure 1. In 

addition to these deficiencies, contractors should be cognizant of the effects that improvements 

to the building enclosure and ducting will have on equipment sizing so as to avoid performance 

degradation resulting from oversizing and excessive equipment cycling.

Figure 1. Primary system fault modes.
4

A key challenge to understanding refrigeration system performance is that there are many 

possible sources of degradation, and there is rarely an obvious indication which part (or often, 

parts) of the system is failing. Furthermore, system performance characteristics encountered in 

the field depend on ambient conditions and the fault status. In spite of the complexity of multiple 

faults and highly variable performance, HVAC technicians providing service calls are expected 

to improve system efficiency, often within a short scheduled time window. A systematic 

approach is needed to efficiently identify, diagnose and correct defects. 

The complexity of the HVAC maintenance problem was explored by a recent Southern 

California Edison (SCE) sponsored project. Responding to lower than expected results from an 

HVAC maintenance program, the project team reviewed prior studies on the impact of 

refrigerant charge and airflow on system performance and to what extent measurement 

uncertainty can lead to lower than expected savings (Hunt 2010). Some conclusions were:

• Though several studies of field performance have been completed, field performance is 

still not as well understood as laboratory performance due to occupant behavior, service 

frequency and quality, and other environmental factors that are difficult to control.

• There is a general lack of diagnostic methods that ensure that the service providers will 

address all the problems that might exist, and that applies improvements in the correct 

order.

• Inaccuracies in field measurements are common, and with human factors, they have a 

compounding effect that can result in high levels of uncertainty in energy savings 

estimates. With improper methods and tools, it is difficult for a simple refrigerant charge 

and airflow diagnostic adjustment to be accurately performed.

1.2 Maintenance Approaches

The increased focus on the energy performance of existing homes creates an opportunity to, at a 

minimum, identify whether a problem with AC systems exists so that the decision to make 

corrections can be offered to homeowners. However, most home performance contractors lack 

the information and training to recognize even serious problems that affect performance. 

Responding to this problem, this guideline provides a two-step approach to diagnosis and repair:

• A preliminary diagnostic (PD) approach designed for home performance contractors to 

quickly identify system problems that can be either immediately corrected or referred to 

qualified HVAC technicians

• A comprehensive diagnostic (CD) approach designed for skilled HVAC technicians that 

ensures that systems are operating at their maximum potential.

It may be unnecessary to involve a skilled HVAC technician if the major problems are obvious 

and simple to resolve, such as a clogged filter or fouled condenser coil. Results of diagnostics 

completed by a relatively unskilled person can be referred to an HVAC technician with the skills 

and equipment to complete a thorough diagnosis and maintenance process to ensure systems are 

operating to their maximum potential. This approach avoids duplication of effort and minimizes 

costs. Major problems can be identified and remedied, leaving more extensive tune-ups to be 

completed by skilled technicians at the time of retrofit, or later as budgets and circumstances 

allow.
5

The cost to diagnose and remediate HVAC system problems is as variable as the wide range of 

problems seen in the field. Basic testing and diagnosis using the PD approach could be 

completed in 30 to 60 minutes and could cost under $100, depending on the location and other 

factors (Means 2011). The identification of significant problems could lead to the need to replace 

the outdoor unit and indoor unit coil, which could range from under $1,500 to over $10,000, 

depending on the size and efficiency of the replacement equipment (according to the National 

Residential Efficiency Measures Database).

1.3 Energy Savings

Just as the range of problems encountered is extremely wide, benefits accruing from maintenance 

are highly variable. Efforts to verify energy savings from California HVAC maintenance 

programs have been confounded by problems with measurement (Hunt, 2010). However, the 

finding of 38% energy savings for airflow and refrigerant maintenance reported for the Arkansas 

CoolSaver Program is encouraging (Kuonen 2011). Researchers have been attempting to find 

ways to use diagnostics to predict energy savings, but since multiple defects can produce similar 

measurable effects, specific defects cannot be identified without initiating corrective actions 

(Braun 2011).

The potential for energy savings through maintenance has been identified in laboratory testing 

that measures performance as a function of airflow, charge condition, and other parameters. 

Recent laboratory testing compared the effects of hot attic environment, line-set length, 

refrigerant charge, TXV devices, airflow, duct leakage, evaporator coil blockage, condenser coil 

blockage, refrigerant restrictions, measurement equipment, and non-condensables on air 

conditioner performance (Mowris 2012). Baseline comparisons were made to a 3-ton unit tested 

using ANSI/AHRI Standard 210/240 test methods. Tests showed a reduction of the “application 

SEER”1 by 67% for a poorly installed unit with ducts installed in a hot attic with 30% duct 

leakage, 300 cfm per ton airflow, 10% refrigerant undercharge, and 50% condenser coil 

blockage. Moderate to severe non-condensables (0.3 to 1% by weight) were found to reduce the 

application EER and SEER by 12% to 38% and refrigerant restrictions by 30% to 59%.

1.4 The Consequences of No Action 

If, in the course of performing home energy retrofits HVAC maintenance is ignored, a significant 

energy savings opportunity will be lost. In addition to unrealized energy savings, the 

consequences can include:

• Reduced comfort due to inadequate airflow, poor airflow distribution, or refrigerant 

system and other defects that result in diminished capacity

• Poor humidity control due to faulty equipment and/or equipment short-cycling resulting 

from restrictions or post retrofit over-sizing

• Occupant health hazards of mold growth due to plugged condensate drains

• Low equipment reliability due to faulty components such as contactors, capacitors, 

condenser fan motors, and compressors 

 1 An “application SEER” is not a formal SEER rating but is developed using the same ANSI/ASHRAE Standard 201/240-2008 test methods used 

for developing official ratings. Hereafter, informal performance metrics are denoted by an asterisk.
6

• Electric shock hazards because of failure to identify incorrect wiring and equipment 

grounding

• Shortened equipment life resulting from coil freezing from low refrigerant charge or 

restrictions, defective TXV, improper refrigerant charge, and non-condensable air and 

water vapor or other contaminants in the refrigerant lines.

1.5 Planning for Improvements and Risk Mitigation

Except where systems have failed catastrophically, the condition of systems cannot be 

determined until basic diagnostic procedures are implemented. This causes difficulty in planning 

for improvements and uncertainty in anticipating energy savings. For the home performance 

contractor, failure to conduct simple diagnostics may impose a higher risk than taking the time to 

investigate and at least, solve the major deficiencies. In the course of inspecting systems for 

performance-related problems, risks such as faulty wiring, plugged condensate drains, and 

imminent equipment failure can be brought to light.

There is also a market-related risk to taking no action. If homeowner expectations of energy 

savings and comfort are not met, news of dissatisfaction may infect the regional market for home 

energy upgrades. Interviewing the homeowner to determine the maintenance history of the 

system and to identify any comfort issues prior to proceeding with diagnostics can provide clues 

to possible system or design deficiencies. The two-stage approach described in the 

Implementation section of this document is designed to identify obvious problems with a small 

investment of time, and provides for efficient diagnosis and remediation by skilled HVAC 

technicians when significant problems are found. This approach will increase the likelihood that 

attention will be given to minimizing risks in HVAC systems.
7

2 Section 2: Technical Description

2.1 General Performance Interactions

In addition to their role in maintaining comfortable indoor temperatures, heating and cooling 

systems are responsible for moisture removal (latent cooling) and maintaining indoor air quality 

through the removal of particulates and distribution of ventilation air. If ducts are leaky and/or if 

there are inadequate provisions for return air, they can induce infiltration, thereby adding to the 

heating and cooling load. As such, defects can inhibit the ability of HVAC systems to maintain 

comfort, indoor relative humidity, and air quality. Poor humidity control can result in mold 

formation and even structural damage. The consequences of defects related to airflow and 

refrigerant charge are discussed in the following sections.

2.2 Airflow Impacts

Desired Airflow

Air conditioners are typically designed to have 350 to 400 cfm of air flowing across the indoor 

coil for each ton of cooling capacity. For humid climates, 350 cfm per ton is recommended to 

increased moisture removal. In hot-dry climates where latent loads are minimal, 500 cfm per ton 

is recommended (CEC 2011). As pointed out by Parker (1997), correct airflow achieves the 

desired balance between sensible heat transfer and moisture removal. ACCA Manual S (1995) 

prescribes a method for determining airflow based on the sensible heat ratio (SHR). For example, 

at SHRs below 0.80, an airflow is selected to achieve a supply air temperature that is 21°F below 

the return air temperature, whereas at SHRs above 0.85, this temperature split is reduced to 17° 

by increasing the airflow. The Manual S equation for calculating airflow is:

 CFM = Sensible Load / (1.1 x TD) Equation 1

Where: Sensible Load (Btuh) = load calculated using ACCA Manual J

TD (°F) = temperature difference across the cooling coil

What Affects Airflow?

In the field, system airflow is affected by the furnace or air handler equipment used, motor speed 

settings, installed duct system, air filter, and the condition of the air distribution system. 

Restrictions in the air distribution system have a direct impact on airflow and include:

• Duct undersizing and excessive length, turns, fittings, kinks, and obstructed grilles

• Blower motor type

• Filter undersizing, design and type of filter, and/or filter blockage (dirt or debris)

• Cooling coil restrictions due to sizing and/or dirt or debris accumulation

• Zoned systems with incorrectly sized ducts.

Duct Constrictions. It is not uncommon for flex ducting to be constricted by hangers or crimped 

by tight bends. Section 3 (Implementation) provides a method for quickly verifying issues with 

supply duct sizing or restrictions that result in substantial pressure losses. The method can also 

detect excessive pressure loss at cooling coils, filters, and other points in the system.
8

Motor Types and Characteristics

_____________________________________________________

Permanent split capacitor (PSC) motors are the 

most common type used in furnace and air 

handlers and operate at a relatively fixed RPM. As 

the static pressure increases they deliver less air 

and the power falls off slightly. Their efficiency is 

in the 55-70% range.

Electronically commutated motors (ECMs), also 

called brushless permanent magnet (BPM) motors, 

are used in higher efficiency systems. The variable 

speed variety has the ability to vary RPM and 

torque to maintain a constant airflow as static 

pressures increase. Others maintain a constant 

RPM and respond to static pressure in a similar 

fashion as PSC motors. The efficiency of ECM’s is 

in the 70-80% range.

Calculating Filter Velocity _____________________________________________________

If filter velocity exceeds 300 fpm the filter is almost 

certain to reduce airflow to unacceptable levels 

when used with high efficiency filters. To check 

this, the equation below can be used to calculate 

the filter velocity for a given system: 

Filter Velocity = Tons x 57,600/(LxW) 

where:

Tons = air conditioner rated tons

L = filter length in inches

W = filter width in inches

Figure 2. Examples of constricted ducts (Photo credit: R. Chitwood, reprinted with permission).

Blower Reaction to Increased Pressure. Many 

existing systems operate under static pressure 

conditions than are higher than anticipated by 

manufacturers, resulting in decreased airflow 

(typical for blowers with PSC motors) or high fan 

watt draw (typical of ECM-driven blowers). 

Reduced airflow degrades air conditioner 

performance, which increases compressor energy 

use and decreases cooling capacity, and decreases 

the SHR of the equipment. Nearly all furnaces and 

air handlers installed before 1990 use constant 

RPM PSC motors that deliver less air as the 

pressure differential across them increases. 

However, many high efficiency furnaces and heat 

pump air handlers use variable speed ECM motors 

that vary RPM and torque to maintain a relatively 

constant airflow under varying static pressure 

conditions, but do it at the expense of increased 

fan power.

Filter Pressure Loss. Most existing HVAC 

systems have filter grilles or furnace filter slots 

that were sized for fiberglass filters. The tendency 

is to replace them with the newer “high minimum 

efficiency reporting value (MERV)” filters that 

are much more effective at removing small 

particles such as pollen. While filters with MERV 

ratings of 6 and higher improve indoor air quality, 

they also impose a much greater restriction to 

airflow, and result in either a decrease in airflow 

(with PSC blower motors) or increased power 

consumption (with variable speed blower motors), 
9

or both. Figure 3 shows the results of tests performed to determine the pressure loss for five 

commonly available filters (Springer 2009). The MERV 2 filter is a conventional 1 in. fiberglass 

type and the others are 1 in. pleated filters. Note that pressure loss does not always correspond to 

MERV rating, as two of the MERV 8 filters have quite different pressure drop characteristics. 

Unfortunately, manufacturers do not provide the pressure loss value filter on packaging, but as a 

general rule, velocity should be limited to 300 fpm or less to avoid excessive pressure loss.

Figure 3. Pressure loss as a function of velocity for commonly available filters.

Zone Dampers. As the thermal integrity of building enclosures improves, it is becoming more 

common for contractors to install zone dampers instead of multiple systems, particularly for twostory homes. When zone dampers close, the air velocity in ducts serving the remaining zones 

increases, which increases overall system pressure drop and reduces the volume of air delivered. 

Many zoned systems utilize bypass dampers to maintain airflow across the cooling coil. When 

air bypasses the return-supply grilles, it results in a lower coil entering air temperature, which 

has a similar effect on system performance as reduced airflow. Zoned distribution systems 

deserve particular attention and may require modification before air conditioner charge can be 

properly set. Improvements could include eliminating the bypass damper and installing 

additional ducting to supply excess air to central living spaces. 

Duct Leakage. It is important that ducts be tested for leaks and sealed before any other 

diagnostics are initiated since leakage in the return ducting will affect airflow measured at the 

return grille. Depending on the method used, duct leakage can result in false airflow 

measurements and also leads to incorrect temperature split measurements. 

Findings from the Field

Data from California field studies suggest that system restrictions are prevalent and have a 

significant impact on airflow. Figures 4 and 5 display results from tests of 62 California homes 

built around 2007 (Wilcox 2011). Pressure measurements showed the median static pressure to 

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

150 200 250 300 350 400 450 500 550

Velocity, FPM

Inches Static Pressure

MERV 2

MERV 8

MERV 8

MERV 11

MERV 6
10

be greater than the typical manufacturer’s maximum recommended value of 0.8 in. w.c. (Figure 

4). Filters were responsible for the majority of return side pressure losses, averaging 0.28 in. w.c. 

ACCA Manual D suggests that a standard clean furnace filter will have a pressure drop of about 

0.10 in. w.c. Referring to Figure 3, achieving 0.10 in. with the best MERV 8 filter would require 

a velocity of about 220 fpm. For a 2-ton system at 800 cfm, the filter size would need to be about 

20 in. x 25 in. to maintain this velocity. 

Figure 4. Sources of pressure drop measured in 62 California homes (Source: Wilcox 2011, 

reprinted with permission).

Figure 5. Summary of airflow measurements from 62 California homes (Source: Wilcox, reprinted 

with permission).

Figure 5 confirms the connection between high pressure drop and depressed airflow rates. About 

two-thirds of the homes studied by Wilcox had airflow rates lower than the minimum 

recommended value of 350 cfm per ton. Three of the houses had airflows near or below 200 cfm 

per ton, which can lead to coil icing (Parker 1997).

Minimum 

recommended

airflow
11

How Airflow Affects System Operation and Energy Use

Very low airflow (below 200 cfm per ton) can lead to system failure due to icing of the indoor 

coil and can shorten equipment life. When airflow falls below 350 cfm, it begins to affect the 

efficiency of the system as shown in Figures 6 and 7 (Parker 1997). The figures were developed 

by testing a typical 3-ton cooling system under ARI standard conditions. Figure 6 shows how 

sensible, latent, and total capacity varied, and Figure 7 shows the impact on power, capacity and 

effective (as opposed to rated) EER. As airflow falls, latent capacity increases and peaks at about 

250 cfm/ton, but then declines. At the same 250 cfm/ton airflow, sensible capacity falls to 75%,

total capacity to 85%, and EER is reduced to about 92% of the value at 400 cfm/ton.

Though the impact on EER is not enormous, the shift to lower SHR is undesirable for dry 

climates and can cause comfort problems. Most importantly, airflows below 350 cfm/ton render 

standard tests for determining refrigerant charge invalid and can lead to improper charging. 

Airflows below 200 cfm/ton can lead to coil icing and greatly shorten compressor life. 

Figure 6. Impact of reduced airflow on cooling capacity (Source: Parker, reprinted with 

permission).
12

Figure 7. Impact of reduced airflow on cooling system performance (Source: Parker, reprinted 

with permission).

Airflow Measurement Methods

Methods for measuring airflow include the use of direct measurement devices such as flow 

hoods (balometer or powered types) and flow grids, and velocity measurement devices. Tests of 

a variety of balometers under laboratory conditions have shown they have potential errors of 

20% to 30%, making them unsuitable for measuring total flow of residential systems (Wray 

2002). Powered flow hoods (also known as fan flowmeters), were found to have an order of 

magnitude less error. They use a variable speed (ductblaster) fan to neutralize the pressure at the 

grille or match the pressure in the supply plenum. With a small amount of additional effort, 

airflow can be measured using a duct blaster at the beginning or end of a duct pressurization test 

by removing tape from the supply registers and running the system fan ( +/- 3% error).

The Energy Conservatory TrueFlow flow grid, which inserts 

into the filter slot, has a measurement error of about 7%, which 

is acceptable for this application. Vane or hotwire anemometers 

can be used to measure velocity at grilles or for traverses of 

ducts, and can convert velocity to airflow rate. Measurement 

accuracy of anemometers is highly subject to the skill and 

diligence of the user and other factors.

HVAC diagnostic procedures required by the California Energy 

Commission use temperature split (or difference between 

supply and return temperature) to check airflow across the 

evaporator and as a means of determining whether the 

refrigerant charge is valid (CEC 2008). Measurement 

with a calibrated airflow measurement device is 

preferred.

Figure 8. A powered flow hood in use.
13

2.3 Refrigerant System Defects, Detection, and Correction

What Can Go Wrong 

There are several faults that can occur with air conditioner refrigerant systems, and they can exist 

individually or in any combination creating a multitude of symptoms. Typical faults can include:

• Under or overcharge

• Refrigerant leaks

• Restrictions in the liquid line or expansion device

• Defective TXV or other expansion device

• Non-condensable gases in the system

• Condenser and evaporator fouling.

The difficulty with diagnosing refrigerant problems is that there can be multiple faults that are 

interactive. For example, a problem with a liquid line restriction or non-condensable

contaminants in the system can lead to a false diagnosis of refrigerant charge. To arrive at a 

correct diagnosis requires that the system be inspected for faults in a particular order using a 

systematic approach that considers the environmental conditions during testing and other factors. 

Systematic diagnostic procedures are detailed in the Implementation section.

The challenge in performing diagnostic tests is to identify the particular source of problems that 

affect performance so that they can be corrected. Combinations of faults can yield false clues 

about the condition of refrigerant charge and can be very difficult to sort out. As reported by 

Purdue researchers, “evaluating fault detection and diagnosis is hard” and “false alarms happen” 

(Braun 2011). The methods described in this guideline are an attempt to significantly improve 

field diagnostic procedures and catch major problems, but there is no pretense they can produce 

ideal outcomes 100% of the time, even when followed precisely.

The Importance of Correct Refrigerant Charge

The amount of refrigerant in an air conditioner affects both the efficiency and capacity of the 

system. In addition to energy and capacity impacts, incorrect charge can also damage equipment. 

If the system is overcharged, there is a chance that liquid refrigerant will not completely 

evaporate and could slug the compressor. If there is too little charge in the system, the low 

suction line pressure and corresponding saturation temperature of the refrigerant can lead to ice 

formation on the evaporator, which restricts heat transfer, increases airflow resistance, and 

reduces airflow. This will further reduce air conditioner performance and can shorten compressor 

life (Siegel 2001).

Tests of more than 4,000 residential cooling systems in California indicated that about 34% were 

undercharged, 28% were overcharged, and only 38% had the correct charge (Proctor 1998). 

Additional field data for residential cooling systems gathered by Blasnik et al. (1996) and 

Proctor (1997) indicated that an undercharge of 15% is common. Temple (2006) identified how 

many units of a sample of 289 were within 4°F of the nominal charge for subcooling and 8°F for 

superheat. Of these, 28% had an acceptable charge, 31% were high, and 41% were low. Mowris 

(2004) reported field measurements of 4,168 air conditioners where 72% of systems had 

improper refrigerant charge and 44% had improper airflow. These studies point to the conclusion 
14

that a large percentage of systems are operating below their rated efficiencies2. It should be noted 

that the accuracy of large sets of field measurements may be brought into question by technicians 

using analog pressure gauges and pipe temperature measurement instruments that might cause 

false undercharge diagnostics.

The impact of under and overcharge on EER is shown in Figure 9, which was developed from 

data obtained by Pacific Gas & Electric Company at their San Ramon test facility using systems 

with orifice and TXV expansion devices. These results verify that TXVs can mitigate the effects 

of undercharge provided they are properly installed. If the TXV sensing bulb does not have good 

thermal contact with the suction line, refrigerant charge can be misdiagnosed. 

Figure 9. Normalized EER versus charge and outside temperature (Source: P G &E, reprinted with 

permission).

The Effect of Non-Condensable Gases

The presence of non-condensable gases in refrigerant systems is not given the attention it 

deserves. The existence of non-condensable gases (NCGs), such as air, nitrogen, or argon in 

refrigerant systems results in performance reductions. These gases, which have low boiling 

points, remain in gaseous phase under operating conditions, and will remain trapped in the 

condenser tubing. Detailed tests of air conditioners in California homes revealed that two of 

seven had refrigerant contaminated with NCGs. The efficiency of one of these systems was 

increased by 19% and the other by 35% when the existing refrigerant was removed and replaced 

with pure refrigerant (Proctor 2011). Contaminants can be left in systems as a result of 

inadequate evacuation or can enter the system during charge tests if proper procedures are not 

 2 It should be noted that the accuracy of large sets of field measurements may be brought into question by technicians using analog pressure 

gauges and pipe temperature measurement instruments that might cause false under-charge diagnostics.

Deviation from Full Charge (10 lb)

Normalized Energy Efficiency Ratio
15

Definitions

________________________________________________

Superheat is the difference between the suction 

line refrigerant temperature and the vapor

saturation temperature at the measured pressure.

Subcooling is the difference between the liquid line 

refrigerant temperature and the liquid saturation 

temperature at the measured pressure.

followed. A CEC report (CEC 2011) describes a scenario where the installer pressurized the 

inside coil and line set with 20 psig of nitrogen but failed to fully remove it prior to releasing the 

refrigerant into the system, resulting in a 42% reduction in sensible EER. Failure to frequently 

change the oil on a vacuum pump can prevent the pump from achieving the desired 240 micron3

vacuum needed to properly evacuate refrigerant lines, resulting in nitrogen left in the line during 

charging.

Fault Detection and Diagnosis

Detection of faults such as liquid line restrictions, faulty TXVs, and the presence of noncondensables require several diagnostic measurements, including:

• Temperature split is the difference between return air and supply air dry bulb 

temperatures measured upstream of the blower and downstream of the cooling coil. 

Temperature split is evaluated using a table (see Table 2). This table was developed by 

Carrier Corporation and modified for use in California’s Title 24 standards (CEC 2008). 

Proper temperature split (within +/-3°F) should be further validated with refrigerant 

charge diagnostic tests since proper temperature split can be found in systems that are 

overcharged or that contain non-condensables (Mowris 2011a). The temperature split 

method can be used to diagnose low capacity, which can be caused by refrigerant 

undercharge or liquid line restrictions. 

• Evaporator coil airflow. Accurate measurement of airflow is needed to eliminate the 

uncertainty that test results may be a result of airflow that is below manufacturers’ 

specifications.

• Superheat and subcooling. For proper 

diagnosis, both superheat and 

subcooling should be measured. Other 

defects besides under/overcharge can 

yield false indications of charge 

condition, leading to a diagnosis that 

would result in incorrect actions to 

remediate the problem. 

• Condensing temperature over 

ambient (COA), or the difference between the condensing saturation temperature and the 

temperature of the air entering the condensing coil is useful in differentiating between 

overcharge and the presence of non-condensables.

• Evaporator saturation temperature (EST) can provide clues about defects other than 

improper refrigerant charge, such as the presence of non-condensables and liquid line 

restrictions.

The use of these measurements in diagnosing problems is described in the Implementation 

section of this document.

The importance of using accurate, well calibrated test equipment cannot be overemphasized as a 

means of avoiding false diagnostics and inappropriate application of refrigerant charge. Findings 

 3 “Micron” refers to microns of Mercury. One micron is equivalent to 0.0046 psia (14.7 psia is standard atmospheric pressure).
16

from a study of the uncertainty of HVAC measurements concluded that with the methods and 

tools commonly used today, a simple refrigerant charge adjustment cannot be accurately 

implemented, measured, and verified. It is impossible to eliminate all sources of measurement 

uncertainty, but they should be mitigated where possible (for example, by using higher accuracy, 

calibrated instruments). The effects of measurement uncertainty need to be better understood by 

contractors and technicians (Hunt 2010).

Methods for Setting Refrigerant Charge

When new air conditioners are installed, the correct charge can be approximated by weighing in 

a prescribed amount of refrigerant that takes into account the length and size of the refrigerant 

lines between the condensing unit and the cooling coil. This method can also be used with 

existing systems if they are fully evacuated (e.g. to 240 microns) beforehand, and is useful when 

outdoor temperatures are too low to apply superheat or subcooling methods. Manufacturers 

generally recommend verification of the charge using the methods described below.

The standard method for measuring refrigerant charge involves the use of pressure gauge sets 

and clamp-on temperature sensors to measure subcooling and/or superheat. Fixed orifice systems 

require measurements of outdoor dry bulb and return air wet bulb temperatures, and charge is 

typically set based on superheat. However, subcooling should also be measured to insure that the 

expansion device is receiving a solid stream of liquid. 

Proper refrigerant charge for systems with TXVs is typically determined by measuring 

subcooling, but measuring superheat will verify that the TXV is operating correctly. Measured 

values are compared against target values provided by manufacturers.

Refrigerant charge should be checked while temperature conditions are favorable, that is when 

the outdoor temperature is between 70°F and 100°F and the indoor temperature is between 70°F 

and 80°F (Carrier 2010). Provided the indoor dry bulb temperature can be maintained above 

70°F, refrigerant charge adjustments for systems with TXVs may be made at outdoor 

temperatures as low as 65°F (Mowris 2010, CEC 2008). Systems without TXVs should not be 

tested at outdoor temperatures below 70°F.

Diagnostic Methods for Assessing Refrigerant Charge

The most accurate way to verify proper refrigerant charge is to measure 

subcooling and superheat using manifold gauges. If proper airflow has been 

verified and is greater than 300 cfm per ton, the temperature split across the 

cooling coil can provide an indication that that there is a charge problem. This 

diagnostic method can identify undercharge conditions in the absence of an 

EPA-certified technician, but should be followed up with refrigerant charge 

tests and other diagnostics. 

For systems that are very low on charge, refrigerant leaks can be detected 

by pressurizing them with nitrogen. Systems should be able to maintain a 

pressure of 150 psig for 10 minutes. Also, test equipment is available for 

detecting refrigerant leaks.

Figure 10. A digital 

manifold gauge. 
17 

The Evolution of Refrigerants

___________________________________________________ 

Chlorofluorocarbon (CFC) refrigerants including 

R-12 were phased out of production in 1996

because of their high ozone depletion potential 

(ODP). R-22, an HCFC, began being phased out 

in 2010 and is being replaced by R-410a, an HFC.

By virtue of not containing chlorine, R-410a has 

zero ODP. R134a, another HFC, was also used in 

some systems in the 1990’s, but is being phased 

out due to its high global warming potential.

Evaporator Coil Fouling and Damage to Distributor Tubes

Fouling of the evaporator reduces airflow, will eventually cause the coil to freeze over, and may 

damage the compressor. If fouling affects one or more coil circuits, the circuit receiving less 

airflow will boil off less refrigerant. If a TXV is installed, it will throttle back, starving the rest of 

the coil. Superheat will be in the correct range, but capacity and efficiency will be reduced. Coil 

fouling can be checked by removing the coil inspection cover. The first indication may be low 

airflow and high pressure drop. If access to the evaporator is difficult, measuring the pressure 

drop across the coil will indicate significant fouling, but minor fouling can affect coil 

performance without substantially increasing pressure drop. For an evaporator that is properly 

sized and clean, the pressure differential should not be more than 0.25 in. w.c. (about 62 Pa). 

Though rare, kinked or blocked distributor tubes will reduce cooling capacity. A method for 

detecting this problem is provided in Attachment C. 

Figure 11. Examples of condenser and evaporator coil fouling.

2.4 Repair or Replace? 

Condenser Replacement

In some cases replacement of the air conditioner condensing unit and indoor coil is necessary to 

restore proper function or to head off an impending equipment failure. In such cases, improving 

efficiency becomes secondary to providing comfort. Other indications that the condensing unit 

should be replaced include:

• The system is more than 15 years old (especially if it contains R-12 refrigerant)

• The condensing unit is oversized due to incorrect 

initial sizing or reduced loads due to 

improvements to the building enclosure 

• Compressor amps are higher than rated and/or 

the compressor is noisy or failing

• The condenser coil is damaged (fins compressed 

or corroded). 

Replacement affords an opportunity to reduce cooling 

costs by installing a more efficient system. Federal 

standards for air conditioners were set at SEER 10 in 
18

1987 and raised to SEER 13 in 2006. To meet ENERGY STAR® specifications, split system air 

conditioners must have a SEER rating of 14.5 or higher and an EER of 12 or higher. A decision 

tree for guiding the replacement decision is provided in Attachment C.

It is typical for the evaporator coil and expansion valve to be replaced at the same time that the 

condenser is replaced. Most coils designed for R-22 are not rated for the higher pressures used 

with R410A refrigerant. At the time of replacement, both the size of the system and the size of 

the indoor coil should be re-evaluated. If improvements were made to the home since the 

previous system was installed, proper load and equipment sizing should be performed using 

ACCA Manual J, D, and S sizing methodologies. Equipment and operating costs can be saved by 

installing a correctly sized system.

Installing an indoor coil that is one size larger than the condensing unit (for example installing a 

3-ton coil with a 2-ton condenser) will improve airflow by reducing coil pressure drop4

. The 

larger coil surface area will also improve system efficiency but will increase the SHR and may 

not be appropriate for humid climates. At time of replacement, the sizing and tightness of ducts 

and particularly the size of the filter grille should be evaluated and upgraded as needed.

Many of the same defects that are seen in existing systems can appear in newly installed systems 

if proper procedures for insuring adequate airflow and providing the correct refrigerant charge 

that is free of non-condensables are not taken. Air, nitrogen and other gases that are left in the 

system and mixed with the refrigerant can have severe impacts on performance. Noncondensables accumulate in the condenser and blanket heat transfer surfaces, which impacts 

condenser coil performance and efficiency. ASHRAE (2010) recommends evacuation to 240 

microns of mercury (Hg) to remove non-condensable air or nitrogen. Removal of moisture is a 

much more difficult task. According to ASHRAE (2010) “Excess moisture in refrigeration 

systems may lead to freeze-up of the capillary tube or expansion valve. Contaminants can cause 

valve breakage, motor burnout, and bearing and seal failure. Except for freeze-up, these effects 

are not normally detected by a standard factory test. Therefore, it is important to use a 

dehydration technique that yields a safe moisture level without adding foreign elements or 

solvents. In conjunction with dehydration, an accurate method of moisture measurement must be 

established. Many factors, such as the size of the unit, its application, and the type of refrigerant, 

determine the acceptable moisture content.” 

Proper evacuation is important for R22 and much more important for R410A since the synthetic 

polyolester (POE) oil is highly hygroscopic and moisture in R410A systems will cause 

significant problems (Maier 2009). Installation of a liquid line filter drier is required for all 

systems and a new filter-drier must be properly installed whenever a system is opened. When 

POE oils are exposed to moisture and heat, they may react, forming acid that is harmful to the 

system. If a R410A POE oil system is open for the same amount of time service technicians are 

used to having R22 mineral oil systems open, there is a much greater chance of moisture 

contamination of the oil. It is imperative that technicians keep containers of POE oils sealed, 

except when the oils are actually being dispensed. POE oils should also be stored properly in 

their original container because many plastics used to package oils are permeable to moisture. It 

is also important to keep compressors and systems closed, except when work is actually being 

 4 Codes in some locations require that evaporators and condensing units be matched and that their ratings be included in the AHRI database.
19

Electric Utility Savings 15 SEER

 Hot-Dry $82

 Hot-Humid $95

 Mixed-Humid $59

 Marine $11

 Cold $29

Incremental Cost

 Hot-Dry $360

 Hot-Humid $360

 Mixed-Humid $300

 Marine $180

 Cold $240

Simple Payback, years

 Hot-Dry 4.4

 Hot-Humid 3.8

 Mixed-Humid 5.1

 Marine 16.5

 Cold 8.2

Table 1. Estimated Energy Savings, 

Replacement Costs, and Simple 

Paybacks for 15 SEER vs. 13 SEER Air 

Conditioner.

done on the equipment, and to filter out undesirable contaminants. This can be achieved with 

proper installation and service techniques as well as the use of correct filters and driers.

The Value of High Efficiency Systems

If replacement is required, installing an air conditioner that has an efficiency that exceeds the 

national standard of 13 SEER can be a good investment. Table 1 lists annual energy savings and 

costs for 15 and 18 SEER air conditioners that were calculated from BEopt simulations of a 

typical 1800 ft2 house in five climate zones. Savings and costs are relative to a SEER 13 system. 

Attachment B lists all the assumptions that were used in this analysis.

These estimates suggest that replacement with a SEER 15 system may be justified, particularly if 

there is a source of low interest financing. For example, for a 5%, 15-year loan, a SEER 15 

system would save enough energy to produce a positive cash flow in all but the marine climate 

zone. In this case, analysis of a SEER 18 system yielded negative cash flows in all climate 

zones5

.

Replacement of Other Components

Furnace. If the air conditioner is being replaced, 

replacement of the furnace should be considered. 

Furnace efficiency has improved significantly over the 

past 15 years, and the older the furnace, the greater the 

risk that cracks in the heat exchanger could introduce 

carbon monoxide into the house. The cost to replace the 

furnace when the air conditioner is replaced will be 

lower than if it is done independent of the air conditioner 

because the cooling coil must typically be removed in 

both cases. Some newer coils can accept either R-22 or 

R-410A, and should be labeled as such.

Blower Motor. With many furnaces it is possible to 

replace the existing PSC motor with a more efficient 

brushless DC permanent magnet (ECM) motor. Because 

the ECM motors are usually 1-2 in. longer than PSC 

motors, the dimensions of the furnace should be checked 

before a new motor is ordered. The larger motor will not 

fit in some of the slim furnaces that are found in the 

field.

To determine the energy savings and economic viability 

of replacing a PSC motor with an ECM motor, a Regal 

Beloit (Genteq) “Evergreen” motor was obtained and 

tested in a 1970’s vintage furnace. Energy use of the two motors was used to estimate energy 

savings as function of operating hours. A detailed description of the test and results is provided 

in Attachment B. Findings indicate that, unless it is necessary to replace a defective motor, 

energy savings will probably not offset the incremental cost if amortized at 5% and 15 years. 

 5 Discrepancies between modeling results of typical homes and utility survey data could raise questions about the validity of these results. See 

Attachment B.
20

Replacement is easier to justify in buildings that have very long blower run times, and run time 

depends on the climate, the integrity of the thermal enclosure, temperature setpoints, and other 

factors.

Condensing Unit Components. Generally when a compressor fails, the HVAC contractor 

replaces the entire condensing unit. However, contractors should assess the practicality of 

replacing only the compressor. The size and condition of the condensing coil is a determining 

factor of efficiency, and replacing compressors in older systems that have large coils in good 

condition could be much more cost-effective than full replacement. If the evaporator coil is 

designed for R-22 refrigerant, then replacing the compressor would also eliminate the cost of 

trading out the R-22 evaporator coil with one rated for R-410A.

Parker (2005) researched the energy savings from improved condenser fan and exhaust 

configuration, as well as replacement of PSC motors with brushless DC motors. He concluded 

fan improvements could reduce motor power by about 49 W (26%), and combined with motor

replacement could reduce power by about 100 W. More efficient and quiet condenser fan blades 

are commercially available.
21

3 Section 3: Implementation 

Two approaches to air conditioner diagnosis and repair are provided: the Preliminary Diagnostic 

(PD) approach, for quickly identifying and correcting serious system design or installation 

problems without the need for special training or test equipment; and the Comprehensive 

Diagnostic (CD) approach that provides a procedure and methodology for qualified HVAC

technicians to accurately identify system defects using the proper diagnostic tools. Measurement 

of “temperature split” is used as a key determinant that there are significant refrigerant problems 

and that the CD method must be employed.

It is important that duct testing and sealing is done in advance, as return duct leakage can result 

in false airflow readings and incorrect temperature split readings. For proper procedures on 

testing and sealing ductwork in existing homes, refer to the Building America Measure 

Guideline: Sealing and Insulating Ducts in Existing Homes (Aldrich 2011).

Some states, such as Florida, have specific requirements regarding who is qualified to perform 

work on HVAC systems. While tasks described in the PD method require little training, it may 

be necessary to have a licensed HVAC contractor perform this work.

3.1 The Preliminary Diagnostic (PD) Method

Objectives and Scope

The PD Method is designed for use by the home performance contractor and has three primary 

objectives:

• Find major defects

• Fix what can be fixed given available skills, tools, and time

• Identify what information should be referred to an HVAC technician if further repairs are 

needed.

The scope of the PD method includes:

• Observation of the general condition of the system and obvious maintenance needs

• Verification of proper airflow and quick diagnosis of the source of airflow restrictions if 

airflow is too low

• A temperature split test to identify low system capacity, efficiency, and overall condition.

If problems are identified, the contractor can inform the homeowner of completed repairs and/or 

provide a recommendation and referral for further diagnostics and maintenance. 

It is understandable for home performance contractors to strive to minimize their time on site to 

keep costs down. Less than one hour is required to complete the PD process, provided duct and 

seal has already been completed. The PD process first evaluates overall system performance by 

measuring supply-return temperature difference (temperature split) to diagnose airflow and 

capacity problems. The process either ends with this test or continues with an airflow test to 

isolate the problem if airflow is too low.
22

IMPORTANT!

__________________________________________________________________ 

For systems with gas furnaces, a combustion safety 

check should be completed in accordance with BPI 

specifications before starting any work.

Accuracy of Test Equipment

__________________________________________________________________ 

Accuracy of test equipment can vary widely 

resulting in false diagnosis. Refer to Attachment C 

for recommended specifications.

Skills and Tools Required

The PD is designed to fit the skill level of most home performance contractors who have had BPI 

training, and to only require test equipment that is likely to be in their toolbox. The following 

equipment is required:

• Digital manometer with hoses and static pressure probes

• Duct blaster, flow grid, or accurate balometer

• Digital thermometer, preferably with two 

probes that can simultaneously measure 

supply and return temperatures (see 

Attachment D for specifications)

• Digital relative humidity device that can also 

report wet bulb temperature, or a psychrometric chart or tool (available as smart phone 

“app”)

• Portable drill and 5/16 in. high speed bit

• Aluminum tape for covering holes drilled into ducts

• Tape measure, screwdrivers, nut drivers, lights, ladders, and other commonly carried 

tools.

A hose with a spray nozzle and coil cleaning solution will also be useful if the outdoor coil 

requires cleaning.

Measuring Airflow

A duct blaster can be used as a powered flow hood to measure 

airflow, and measurements can be completed as an extension of 

duct pressurization tests by simply removing the seals from the 

registers and leaving the duct blaster attached to the return 

grille. A flow grid (see Figure 12) or an accurate balometer may 

also be used. The supply plenum pressure (NSOP) must be 

measured before starting tests; for the duct blaster, it is needed 

to match the plenum pressure; and for the flow grid and 

balometer, to adjust the measured airflow using the 

equation below:

Corrected airflow = Measured airflow x �(NSOP/TSOP) where:

NSOP = normal system supply operating pressure (at supply plenum)

TSOP = tested system operating pressure (with flow grid or balometer in place)

More detailed information on airflow measurements is 

provided in Attachment C. Note that newer furnaces 

and air handlers may be equipped with a variable 

speed motor that will attempt to overcome high static 

pressures to deliver a constant cfm. It is not necessary 

to calculate corrected airflow for these systems.

Figure 12. The Energy Conservatory 

TrueFlow flow grid.
23 

Before Starting Tests

Conduct a general review of the system to identify the location and general condition of the 

furnace or air handler, condensing unit, thermostats, return air grille(s) and filter(s), and other 

characteristics such as the number of controlled zones and whether a bypass damper is installed. 

Use Table A-1 to record this information. If possible, interview the owner to determine the age 

and service history of the system and to obtain information on any comfort problems 

experienced. If any electrical or other safety hazards are observed, they should be corrected 

before beginning. 

Preliminary Diagnostic Steps

1. Since duct leakage affects airflow and system pressures, ideally ducts should be sealed to 

a maximum leakage of 15% of total airflow or 10 cfm per 100 ft2 of conditioned floor 

area before any testing is initiated. If duct test and seal (DTS) was completed, or if 

leakage was measured without sealing, record the duct leakage in Table A-1. 

2. Determine the condensing unit size from the nameplate. Model numbers of most systems 

include a 24, 30, 36, etc. to indicate the size of the system in kBtuh. Divide by 12 to 

obtain the size in tons. 

3. Check the condenser coil (outdoor unit) for fouling. If the coil is clearly plugged, pull the 

fuse, remove panels from the unit to gain access to the inside surface of the coils and use 

biodegradable coil cleaning solution and a hose with a nozzle to clean the coils. Refer to 

Attachment C for detailed instructions on coil cleaning methods. 

4. Check the condition of the insulation on the suction line and note 

its location.

5. If the system has a TXV, verify that the sensing bulb is securely 

fastened to a horizontal section of the suction line with a copper 

strap, in contact over its entire length, and is insulated with a single 

wrap of insulation with 50% overlap (see Figure 13).

6. Determine the filter velocity at nominal conditions (400 cfm/ton) 

using: 

• Velocity = Tons × 57,600/(LxW) where L and W 

are the filter dimensions in inches

• If the velocity exceeds 300 fpm or the filter is obviously fouled, remove it for the 

duration of the tests and note that a larger filter grille or slot must be provided.

7. Drill 5/16 in. holes at points R (return) and S (supply) as shown in Figure 14. 

Alternate Bulb

Locations

Suction Line

Figure 13. Proper position of TXV 

sensing bulb. 
24

Measure supply vs. return

plenum pressure

> 190 Pa?

Remove filter, measure

supply vs. return plenum

pressure

> 140 Pa?

return plenum pressure

Measure ambient vs.

> 40 Pa?

> 40 Pa?

ambient plenum pressure

Measure supply plenum vs.

System o.k.

Filter / grille undersized

Undersized or kinked

return ducts

Undersized or kinked

supply ducts

yes no

yes no

no yes

Probable blocked no yes

evap coil

Figure 14. Temperature and pressure test points.

8. Start the system fan by removing the condensing unit power fuse and setting the 

thermostat(s) to call for cooling. If the system has multiple zones, manually close the 

bypass damper (if present), activate all zone thermostats, and verify that there is airflow 

at registers in each zone.

9. Measure airflow and make corrections as needed using supply plenum pressures (NSOP 

and TSOP). Enter results in Table A-1. If the airflow is less than 300 cfm per ton,

proceed to the next step, otherwise jump to Step 11. For detailed instructions on airflow 

measurements, see Attachment C.

10. Using a manometer, complete 

the pressure loss diagnostic as 

shown in Figure 15. The first 

measurement point is 

connected to “input” and the 

second to “ref” of the 

manometer. For example, for 

the first test connect the supply 

probe to “input” and the return 

probe to “ref”.

The “ambient probe can be 

located in the attic or indoor 

space. If the equipment is 

located in an unconditioned 

attic or basement, the access 

hatch or door should be left 

open to equalize pressures 

between the house and the 

equipment space.

The static pressure probes 

should be inserted 

perpendicular to the airflow to 

a depth that produces the 

highest pressure.

The pressure drop of the 

Figure 15. Pressure loss diagnostic flow chart.
25

evaporator coil could be directly measured by drilling another hole between the blower 

unit and the coil, but this is not recommended as it would risk causing permanent damage 

to the coil and releasing refrigerant.

It is possible that problems identified during this test can be corrected on the spot, for 

example, straightening a kinked duct. If this is the case, repeat airflow measurements to 

verify that the corrective action was successful. 

If the system includes a variable speed fan motor and high pressure losses are suspected 

even though the airflow exceeds 300 cfm, the pressure loss diagnostic can be used to 

identify problems that, if corrected, will reduce fan energy use and noise.

11. Measure temperature split. Re-insert the condensing unit fuse and wait 15 minutes or 

more for the system to stabilize. Then use the digital thermometer to measure and record 

the dry bulb air temperature at points R and S. Also measure the wet bulb temperature at 

R or at the return grille. The probes should be centered in the air stream. Take 

measurements after two minutes, or after the readings have stabilized. Enter all three 

temperatures in Table A-1. Subtract the supply temperature (S) from the return air dry 

bulb temperature (R) to calculate the temperature split, and enter it in Table A-1.

Over the range of indoor dry bulb temperatures of 76oF–80°F, corresponding wet bulb 

temperatures must be 50oF–60°F in order to obtain target temperature split values (see 

Table 2). If indoor wet bulb temperature is too low, it may be increased by opening 

windows or running the heating system; otherwise the test will have to be delayed until 

weather conditions are more favorable.

12. Refer to Table 2 and select the Target Temperature Split that corresponds to the return air 

dry bulb and wet bulb measurements, and enter the target temperature in Table A-1. The 

measured temperature split should be within ±3°F of values listed in the table. If the 

temperature split is more than 3°F below the target value, one or more defects are causing 

a reduction in capacity. Discontinue testing and refer the system to a qualified technician 

for further diagnostics and repairs. A high temperature split can indicate low airflow.

If the system fails the airflow measurement tests or the temperature split tests, or if other 

significant problems are uncovered during the PD process, provide the HVAC technician with a 

completed PD Form (Table A-1). This information will allow him/her to focus on the critical 

problems and avoid duplication of effort. Work to improve airflow may include installing a 

larger filter grille, long radius elbows, turning vanes, or larger ducting. The HVAC technician 

should re-check airflow and temperature split before proceeding with refrigerant charge tests.
26

Table 2. Target Temperature Split (Return Dry Bulb – Supply Dry Bulb)

Source: California Energy Commission (CEC 2008).
27

3.2 The Comprehensive Diagnostic (CD) Method

Objectives and Scope

The Comprehensive Diagnostic (CD) Method provides guidance to the HVAC technician and is 

designed to detect and remedy most HVAC performance deficiencies by applying accurate 

measurements and repairs in the correct sequence. This method will allow the technician to zero 

in on problems efficiently so as to maximize energy savings impact and minimize time on the 

job. If a preliminary diagnostic (PD) was not previously completed, the scope of the CD Method 

includes a PD.

Skills and Tools Required

The CD Method assumes that the technician performing the work has proper training and 

experience and well-maintained equipment. Skills in diagnosing refrigerant system defects 

require a mix of art and science and improve with experience. In addition to commonly used 

tools and materials, the technician should have the following:

• Manifold set (digital preferred)

• Digital thermometer, preferably with two probes that can simultaneously measure supply 

and return temperatures 

• Digital relative humidity device that can also report wet bulb temperature, or a 

psychrometric chart or tool (available as smart phone “app”)

• Digital thermometer with clamp-on thermocouples for measuring line temperatures (if 

not included with manifold set)

• Vacuum pump (well maintained) and micron pressure gauge

• Scale for weighing refrigerant

• Refrigerant recovery system

• R-22 and R-410a refrigerant

• Digital manometer with hoses and static pressure probes

• Flow grid, fan powered flow hood , or balometer (if airflow was not previously measured 

and verified to be greater than 300 cfm per ton)

• Clamp-on amp meter

• Condenser air exit restrictor (for cold weather testing)

If evacuation of the system is necessary, a four-valve gauge manifold with one 3/8 in. hose for 

connection to the vacuum pump and a separate hose to the refrigerant cylinder is recommended 

to speed evacuation. The valve core depressors should be removed to minimize restrictions.

Measurement inaccuracies have a compounding effect that can result in incorrect refrigerant 

charge or misdiagnosis. Temperature measurement equipment should meet the specifications 

listed in Attachment D. 
28

Need a Refrigerant Refresher?

__________________________________________________________________ 

To review information on safe refrigerant charging 

and recovery techniques, see: 

www.epatest.com/608/manual/Manual.htm

Refrigerant Charge Test Overview

This procedure6 is designed to identify and correct 

refrigerant under/overcharge, liquid line 

restrictions, defective expansion devices, and the 

presence of non-condensables. Charging 

procedures typically only require measurement of 

superheat for fixed orifice expansion devices and 

subcooling for systems with TXVs. However, 

identification of defects other than under/overcharge requires measurement of both subcooling 

and superheat, as well as condensing temperature over ambient temperature (COA). Methods for 

obtaining these measurements are described, as well as methods for using these data to diagnose 

defects.

All measurements and calculations should be recorded in Table A-2 so they can be used for later 

diagnosis. Whenever adjustments to refrigerant charge or other corrections are made, the system 

should be allowed to run 15 minutes before tests are repeated. Systems should not be tested when 

the outdoor temperature is below 55°F, and ideally should be tested at temperature and humidity 

conditions that are close to operating conditions.

Before Starting Tests

Identifying obvious problems at the outset will minimize the number of diagnostic variables and 

save time. Conduct a general review of the system to identify the location and general condition 

of the furnace or air handler, condensing unit, thermostats, return air grille(s) and filter(s), the 

number of controlled zones, the type of refrigerant and expansion valve used (fixed orifice or 

TXV), and the length and location of the refrigerant lines. If a TXV is used, insure the bulb is 

properly secured to the suction line and insulated. This information should be entered in Table 

A-1. If any wiring or electrical hazards are observed, they must be corrected before continuing. If 

a bypass damper is installed, lock it in the fully closed position. Inspect the condenser coil and, if 

necessary, clean it using the procedure described in Attachment C.

Review prior testing or maintenance completed and fill in any missing information in Table A-1. 

Ducts should be pressure tested and sealed to 15% of system airflow or 10 cfm per 1,000 ft2 of 

conditioned floor area before proceeding.

Test Procedures

1. Measure airflow (if not previously measured) using the method described in PD Step 9, 

and follow the airflow diagnostic described in Figure 16 if necessary to identify problem 

areas. Refer to Attachment C for detailed measurement methods. For proper refrigerant 

diagnostics, airflow should be no less than 300 cfm/ton, and can be as high as 500 

cfm/ton in dry climates. It may be necessary to remove the filter to achieve the required 

airflow during testing. 

2. Measure temperature split (if not previously measured) using the method described in 

PD step 11.

 6 Adapted from the method described by the California Energy Commission (CEC 2008) and modified with the assistance of Jim Phillips of The 

Energy Savers.
29

Airflow Too Low? __________________________________________________________________ 

First, go to the next higher blower motor tap 

setting or dip switch setting and retest. 

Next, remove the filter and test again. If airflow is 

still less than 300 cfm/ton, then repairs will be 

needed before refrigerant charge can be tested. 

If removing the filter provides sufficient airflow for 

tests to proceed, be sure to include the addition of 

a second return or larger filter grille to the repair 

list. The velocity at the filter(s) should be no more 

than 300 fpm. 

3. If the indoor temperature is less than 65oF, use 

the heating system or open windows as needed 

to insure that the return air dry bulb temperature 

will be not less than 70oF when measurements 

are taken. Ideally, the indoor temperature 

should be no greater than 80°F.

4. Apply a clamp-on amp meter to one leg of the 

power to the compressor.

5. Connect the refrigerant gauges to the service 

ports, taking proper precautions to avoid 

introducing air into the system.

6. Turn the cooling system on and let it run for 15 

minutes to stabilize temperatures and pressures 

before taking any measurements. While the system is stabilizing, proceed with 

attachment of the temperature sensors.

7. Securely attach one clamp-on temperature sensor to the suction line near the suction line 

service valve and attach another clamp-on temperature sensor to the liquid line near the 

liquid line service valve. Attachment points on lines should be thoroughly cleaned to 

insure good thermal contact between the thermocouple and the line. If the refrigerant line 

set is long and runs through a hot attic, consider measuring the suction line temperature at 

the indoor coil instead of at the condensing unit. 

8. Provide a temperature sensor to measure the condenser entering air dry-bulb temperature. 

Position the sensor so that it records the average condenser air entering temperature and 

is shaded from direct sun.

Measurements (Enter all values into Table A-2)

9. Measure the return air dry bulb (Treturn, db), return air wet bulb (Treturn, wb) and the 

supply air dry bulb temperature (Tsupply, db).

10. Measure the evaporator saturation temperature (Tevap, sat) from the low side gauge.

11. Measure the condenser saturation temperature (Tcond, sat) from the high side gauge.

12. Measure the suction line temperature (Tsuction) from the clamp-on thermocouple.

13. Measure the liquid line temperature (Tliquid) from the clamp-on thermocouple. 

14. Measure the condenser entering air dry-bulb temperature (Tcond, db) by averaging the 

coil entering air temperatures at the bottom, center, and top of coil.

15. Measure compressor amps.

Cold and Hot Weather Testing

The ideal conditions at which to evaluate refrigerant charge is at AHRI test conditions of 80°F 

indoor dry bulb, 67°F indoor wet bulb, and 95°F outdoor dry bulb, but it is rare that these 

conditions will exist when testing is being performed. For non-TXV systems, lower indoor 

temperatures will generally produce a high temperature split and low subcooling. Lower outdoor 

temperatures will result in higher superheat (that can disguise an overcharge) and higher outdoor 
30

temperatures may reduce subcooling to zero (suggestive of an undercharge). These factors must 

be considered when evaluating test results.

Refrigerant charge tests should not be attempted at outdoor temperatures below 55°F and indoor 

temperatures below 70°F. Operation below 55°F outdoor temperature can damage the 

compressor due to flooding and inadequate lubrication. For non-TXV systems, target superheat 

values below 5°F do not exist, and for TXV systems it may be difficult to achieve 

manufacturers’ subcooling targets. It is recommended that tests be delayed at least until weather 

conditions result in indoor and outdoor temperatures over 70°F. 

If the outdoor temperature is below 80°F and superheat is too high (because of too much 

refrigerant in the condenser), the discharge from the condensing unit may be partially blocked to 

simulate an outdoor temperature of 95°F7

. For R-22 systems, raising the high side pressure to 

278 psi using this method would correspond to a condensing temperature of 126°F, which would 

be reasonable for 95°F outdoor air temperature. However, COA must be measured and recorded 

first because the higher condensing temperature will produce a false COA reading. This method 

should never be used at outdoor temperatures below 55°F or if not recommended by the 

manufacturer.

Low indoor dry bulb and wet bulb temperatures can be increased by running the heating system 

and/or by using plug-in electric resistance heaters. The indoor temperature must be maintained 

above 70°F for at least 15 minutes to allow useful charge readings to be taken.

3.3 Calculations 

Temperature Split 

Calculate the temperature split using:

Temperature Split = Treturn, db – Tsupply, db

Find the target temperature split from Table 2 using Treturn, db and Treturn, wb. The measured 

temperature split should not deviate from the target value by more than 3°F. 

Condensing Temperature Over Ambient (COA)

Calculate the difference between the condensing saturation temperature (Tcond, sat) and the 

temperature of the air entering the condenser (Tcond, db). 

COA = Tcond, sat – Tcond, db

A high COA (for example 30°F or higher) can indicate an overcharge, liquid line restriction, or 

non-condensables in the refrigerant. These defects can be distinguished by evaluating subcooling 

and superheat results. 

Superheat 

Calculate Actual Superheat as the suction line temperature minus the evaporator saturation 

temperature:

Actual Superheat = Tsuction – Tevap, sat

 7 This method is documented in CEC 2011
31

For fixed orifice expansion devices: Find the Target Superheat in Table 4 that corresponds to 

the measured return air wet-bulb temperature (Treturn, wb) and condenser air dry-bulb 

temperature (Tcond, db). If a dash mark is read from Table 4, the target superheat is less than 

5°F and it is difficult to perform a refrigerant charge verification test under these conditions.

Calculate the difference between actual superheat and target superheat (Actual Superheat –

Target Superheat). The difference should not vary by more than ±5°F. Refrigerant is normally 

removed if actual superheat is more than 5°F below target (negative difference) and added if 

actual superheat is more than 5°F above target (positive difference).

For TXV’s: If available, determine the acceptable superheat range specified by the 

manufacturer. If not available, superheat values between 4°F and 25°F may be within an 

acceptable range. Since the purpose of a TXV is to maintain constant superheat, measuring 

superheat can be used to determine if the TXV is working properly as indicated in Section 3.4.

Subcooling 

Calculate Actual Subcooling as the condenser saturation temperature minus the liquid line 

temperature: 

Actual Subcooling = Tcond, sat – Tliquid

Refer to manufacturer tables typically provided with the condensing unit to obtain Target 

Subcooling for systems with TXVs. Tests completed for the California Energy Commission 

suggest that subcooling as low as 2°F, and as high as 8°F over the manufacturer’s target value, 

will not reduce normalized sensible EER by more than 5% (CEC 2011).

Systems with fixed orifice expansion devices do not typically provide subcooling targets, but 

values of 10°F ±5° are typical. Subcooling decreases with increasing outdoor temperature and 

may fall to zero above 105°F. Conversely, at an outdoor temperature of 70°F, 25° of subcooling 

may be expected.

Calculate the difference between actual subcooling and target subcooling (Actual Subcooling –

Target Subcooling). For systems with TXVs, the difference should not vary by more than ±3°F, 

but greater variations are seen with fixed orifice systems. For TXV systems, refrigerant is 

normally removed if actual subcooling is more than 3°F above target (positive difference) and 

added if actual subcooling is more than 3°F below target (negative difference).

3.4 Diagnosis

Evaluating the Measurements

To determine the nature of the defect requires a full diagnosis using COA, EST, superheat, and 

subcooling measurements. It is critical to first determine whether the system uses a TXV or fixed 

orifice metering device because different diagnostic procedures apply. All of the measurements 

listed below must be obtained before attempting to complete a diagnosis. Once a defect is 

identified and corrected, measurements should be repeated. For example, if the system is 

overcharged, the resulting high COA can make it difficult to detect other problems such as noncondensables or a liquid line restriction that would only be identified through retesting.
32

Temperature Split. Assuming that adequate airflow has been verified using direct flow 

measurements, a low temperature split (relative to the target value) indicates low capacity, which 

can be caused by one or more defects. Ideally, proper temperature split (within +/-3oF) should be 

further validated by checking refrigerant charge since proper temperature split can be found in 

systems that are overcharged or that contain non-condensables. Relatively low indoor wet 

bulb/dry bulb temperatures tend to elevate the temperature split and can conceal other problems, 

so measurements should ideally be made when temperatures are close to the AHRI rating points 

of 80°F dry bulb and 67°F wet bulb.

Condensing Temperature Over Ambient (COA). A high COA (over 30°F for 13 SEER and 

over 40°F for 10 SEER and lower) can indicate an overcharge condition, non-condensables in 

the refrigerant, or a liquid line restriction (typically a plugged filter-dryer). A high COA caused 

by non-condensables and/or a liquid line restriction will be accompanied by high subcooling. If 

subcooling is not too high, a drop in temperature may be observed across the restriction due to 

the drop in pressure. A low COA suggests an undercharge, or an expansion device overfeed 

(check for an improperly installed TXV bulb).

Evaporator Saturation Temperature (EST or Tevap, sat). A low EST (less than 28°F) 

indicates either an undercharge or liquid line restriction or both. A low EST may also indicate a 

fouled evaporator coil. Excessive fouling would be detected during airflow tests, but a low EST 

may also be seen with minimal air blockage if dirt on the coil causes non-laminar airflow 

through the coil. A high EST (>45) will be seen at high outdoor temperatures (>100), with an 

overcharged system, and could also indicate an expansion device overfeed.

Superheat and Subcooling. Typically, low superheat and high subcooling indicate an 

overcharge, and refrigerant should be removed. Conversely, high superheat and low subcooling 

indicate an undercharge, and refrigerant should be added. High superheat coincident with high 

subcooling, and low superheat coincident with low subcooling are indicators of other problems.

Using Diagnostic Flow Charts

Figures 17 and 18 and Table 3 can be used to isolate which possible system defects are 

responsible for test results that deviate from targets. Use the chart that corresponds to the 

expansion device type and the appropriate values for X and Y from Table 3. Both the individual 

measurements and the charts should be used to guide the diagnostic process.

Table 3. Recommended COA and EST Values to be Used in Diagnostics.

System Maximum Indoor Wet Minimum

SEER COA (X) Bulb Temp. EST (Y)

>12 25 62-63 35

10-12 30 64-65 40

<10 35 66-67 45
33

Figure 17. Refrigerant diagnostic flowchart: systems with fixed orifice expansion devices.

Figure 18. Refrigerant diagnostic flowchart: systems with thermostatic expansion devices.

Correct diagnosis of refrigerant systems can require creative thinking. Since values of COA and 

EST vary with system efficiency and indoor wet bulb temperature, the values in Table 3 should 

be used as starting points for the quantities of X and Y in the flow chart, but it may be necessary 
34 

Vacuum Pump Maintenance is 

Important!

__________________________________________________________________ 

If vacuum pump oil isn’t frequently changed, as 

after each use, it will lose the ability to reach the 

levels of vacuum that are necessary to remove noncondensable gases from the system.

Don’t Mix Lubricants!

__________________________________________________________________ 

Lubricants used for R-22 systems and R-410A 

systems are not compatible. When upgrading 

systems be to R-410A be careful to purge systems 

thoroughly. Using manifold gauges that are 

dedicated for R-22 and R-410A use will help avoid 

contamination and will have pressure ranges that 

are appropriate to the refrigerant type.

to modify these values. For example, an oversized evaporator coil will increase the load on the 

compressor and will tend to increase COA. A low outdoor temperature will lower COA, and a 

low indoor temperature will lower the EST.

Several of the defects listed in the flow diagrams can be checked by simple inspection, for 

example the TXV bulb attachment and coil fouling. If, after making these corrections and 

adjusting charge, the tested values continue to disagree with target values, defects including bad 

or out-of-adjustment TXV, non-condensables, and a liquid line restriction must be investigated. 

Diagnosis may require multiple passes through the flow chart and multiple corrections. 

Measurements and flow chart checks should always be repeated after correcting any faults or 

adjusting refrigerant charge until the measured values approach target values. 

If a site glass is installed, this can be used as another diagnostic tool. Bubbles in the refrigerant 

are an indication of an undercharge, expansion device overfeed, or very high outdoor air 

temperature. 

Liquid line restrictions are usually the result of a dirty filter-dryer or a plugged capillary tube, but 

the liquid line should also be inspected for kinks if this is suspected. Solid contaminants or 

moisture can cause a restriction in the filter-dryer.

 

A further test can be used to detect non-condensables. With the system running in a steady state 

condition, measure the temperature at a return bend about halfway up the condenser coil and 

convert this temperature to a pressure using the appropriate refrigerant table. If the measured 

high side pressure on the manifold gauge is significantly greater than the pressure corresponding 

to the condensing temperature, it is likely the system contains non-condensables and will need to 

be evacuated and recharged. 

Removing and Replacing Refrigerant

If non-condensables are detected or it is necessary to 

open the system to correct a defect such as a plugged 

filter-dryer, it is extremely important to use proper 

procedures for evacuation and recharging to avoid 

venting refrigerant into the atmosphere and to prevent 

the introduction of non-condensables and other 

contaminants into the refrigerant system. Detailed 

procedures for evacuation and recharging are 

provided in Attachment C. General procedures are as 

follows: 

• Recover old refrigerant.

• Purge the system with nitrogen to remove 

moisture and check for leaks by holding the 

system at 150 psi for 10 minutes. If the system 

will not hold 150 psig, then locate and repair 

leaks.
35 

• Evacuate the system to 240 microns. If pressure does not rise above 500 microns after 10 

minutes, then the evacuation has been completed and the system is ready for charging 

with new refrigerant.

• Recharge using the weigh-in method. Follow up with superheat, subcooling, and final 

temperature split tests. 

Compressor Amps and Condenser Fan Amps (or Watts). Measured amps (or watts) that are 

significantly higher than nameplate can be used to detect a failing compressor or condenser fan 

motor (motor winding or bearing failure). As a more detailed check, measure the current draw 

from the compressor only and compare to the manufacturer’s compressor performance data to 

obtain the target amperage that corresponds to the conditions under which the current draw is 

measured. Excessive current draw can indicate an overcharge or non-condensables. When 

measured amps are lower than the manufacturer’s listed value, causes could be an undercharge, 

liquid line restriction, or expansion valve failure. 

Equipment Replacement

If the compressor or other components are found to be defective or ready for replacement by 

virtue of age, low efficiency or capacity, oversizing, or other reasons, manufacturers’ installation 

instructions should be strictly followed for refrigerant charging and other installation procedures. 

Most likely, the indoor coil will require replacement because of the higher pressure ratings 

needed for R-410A. 

Condensing units are typically provided with sufficient charge to accommodate 15–25 ft of 

refrigerant tubing. The line sets and indoor coil must be purged with nitrogen and evacuated to 

240 microns of mercury and held for 10 minutes prior to opening service valves. Be sure to 

replace vacuum pump oil after each evacuation to eliminate contamination, which will damage 

the pump and prevent it from achieving the required 240 micron vacuum. When not in use, cover 

the vacuum pump exhaust port. 
36

Table 4. Target Superheat.
37

Table 4 (continued). Target Superheat.
38

References

ACCA (1995). “Manual S – Residential Equipment Selection.” Air Conditioning Contractors of 

America. Washington, DC. 

Aldrich, R., S. Puttagunta. (2011). “Measure Guideline: Sealing and Insulating Ducts in Existing 

Homes.” Consortium for Advanced Residential Buildings, Steven Winter Associates. 

http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/meas_guide_seal_duc

ts.pdf 

ANSI (2008). ANSI/AHRI Standard 210/240. “2008 Standard for Performance Rating of Unitary 

Air-Conditioning & Air-Source Heat Pump Equipment.” American National Standards Institute. 

Arlington, VA.

ASHRAE (2010). 2010 ASHRAE Handbook – Refrigeration. Page 8.2. Table 1. Typical Factory 

Dehydration and Moisture-Measuring Methods for Refrigeration Systems. American Society of 

Heating, Refrigeration, and Air-Conditioning Engineers, Inc., 1791 Tullie Circle, N.E., Atlanta, 

GA 30329. http://www.ashrae.org

Blasnik, M.; Downey, T.; Proctor, J.; Peterson, G. (1996). “Assessment of HVAC Installations in 

New Homes in APS Service Territory.” Proctor Engineering Group Report for Arizona Public 

Service Company.

Braun, J.E.; Cheung, H.;, Yuill, D. (2011). “Purdue University Update to the Western HVAC

Performance Alliance AFDD Subcommittee –July 2011”. Presentation relating to the project, 

Test Methods for Evaluating Fault Detection and Diagnostic 

Systems. www.performancealliance.org/Portals/4/Documents/CommitteeWorkspace/AFDD/Pur

due%20FDD%20evaluation%20update.pdf.

Carrier (1995). “HVAC Servicing Procedures” Carrier catalog number 020-040. 

Carrier (2010). “Installation Instructions, 24ABB, 24ABC BaseTM Series Air Conditioners with 

Puron Refrigerant.” Carrier catalog number 24ABB-C-6SI. July 2010.

CEC (2008). “Residential Field Verification and Diagnostic Procedures.” Appendix RA3 to the 

California 2008 Title 24 Residential Energy Standards. California Energy Commission.

CEC (2011). “Measure Information Template – Residential Refrigerant Charge Testing and 

Related Issues.” Prepared for the 2013 California Building Energy Efficiency Standards under 

the California Utilities Statewide Codes and Standards Program. California Energy Commission.

DOE( 2010). “Buildings Energy Data Book.” U.S. Department of Energy, Energy Efficiency and 

Renewable Energy. http://buildingsdatabook.eren.doe.gov/.

Downey, T.; Proctor, J.. (2002). “What Can 13,000 Air Conditioners Tell Us?” Proceedings of 

the 2002 ACEEE Summer Study on Energy Efficiency in Buildings. Washington, DC.

Hunt, M.; Heinemeyer,K.; Hoeschele, M. (2010). “HVAC Energy Efficiency Maintenance 

Study.” Project report prepared by Davis Energy Group for Southern California Edison.
39

JB Industries (2007). “Deep Vacuum: Its Principle and Application.” JB Industries, PO Box 

1180, Aurora, IL 60507-1180. www.jbind.com

Kuonen (2011). “Air Conditioning Best Practices.” Home Energy Magazine. May/June 2011.

Maier, A.; Scancarello, M. (2009). “Polyolester Oils: Handling the New Lubricant in R-410A 

Systems.” Air Conditioning Heating Refrigeration News, August 2009. 

http://www.achrnews.com/articles/polyolester-oils-handling-the-new-lubricant-in-r-410asystems.

Means (2011). “RS Means Residential Cost Data.”.30th Edition. Reed Construction Data. 

Norwell, MA.

Messenger, M. (2008). “Strategic Plan to Reduce the Energy Impact of Air Conditioners.” CEC400-2008-010. California Energy Commission. June 2008.

Mowris, R., Blankenship, A., Jones, E. (2004). "Field Measurements of Air Conditioners with 

and without TXVs." ACEEE Summer Study on Energy Efficiency in Buildings. August 2004.

Mowris, R., Jones, E., Esham, R. (2012). “Laboratory Measurement of HVAC Installation and 

Maintenance Faults.” ASHRAE Transactions v. 118, pt. 2.

Neme, C.; Proctor, J.;, Nadel, S. (1999). “Energy Savings Potential from Addressing Residential 

Air Conditioner and Heat Pump Installation Problems”. Report Number A992, American 

Council for an Energy Efficient Economy.

Parker, D.; Sherwin, J.; Ranstad, R.; Shirey, J. (1997). “Impact of Evaporator Coil Airflow in 

Residential Air-Conditioning Systems”, ASHRAE BN-97-2-1, ASHRAE Transactions 1997, v. 

103, pt. 2.

Parker, D.;Sherwin, J.; Hibbs, B. (2005). “Development of High Efficiency Air Conditioner 

Condenser Fans.” Publication Number FSEC-CR-1674-05. Florida Solar Energy Center. 

Cocoa, Florida.

PG&E (2001). “Influence of the Expansion Device on the Performance of a Residential SplitSystem Air Conditioner.” PG&E Report No 491-01.1. Pacific Gas & Electric Company. 

January 2001.

PNNL (2011). “Energy Renovations: HVAC-A Guide for Contractors to Share with 

Homeowners”. Volume 14, Building America Best Practices Series, Building Technologies 

Program. Pacific Northwest National Laboratory and Oak Ridge National Laboratory. August 

2011. http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/hvac_guide.pdf

Proctor, J.; Chitwood, R.; Wilcox, B.A. (2011). “Efficiency Characteristics and Opportunities of 

New California Homes.” California Energy Commission. Sacramento, CA.
40

Proctor, J. (1998), “Monitored in-situ Performance of Residential Air-Conditioning Systems”, 

ASHRAE Transactions, 104(1): 1833-1840. Atlanta: American Society of Heating, Refrigerating 

and Air Conditioning Engineers, Inc.

Proctor, J. (1997). “Field Measurements of New Residential Air Conditioners in Phoenix, 

Arizona”, ASHRAE Transactions ,103(1): 406-415. Atlanta: American Society of Heating, 

Refrigerating and Air-Conditioning Engineers, Inc.

RASS (2009). “2009 California Residential Appliance Saturation Study”. CEC-200-2010-004.

California Energy Commission. October 2010.

Siegel, J.; Wray, W. (2001). “An Evaluation of Superheat-Based Refrigerant Charge Diagnostics 

for Residential Cooling Systems.” LBNL-47476. Lawrence Berkeley Laboratory.

Springer, D. (2009). “Is There a Downside to High MERV Filters?” Home Energy Magazine, 

Nov/Dec 2009.

Temple, K.; Rossi, T. (2006). “Enhanced Refrigeration Diagnostics for an Improved Air 

Conditioning Tune-Up Program.” 2006 ACEEE Summer Study on Energy Efficiency in 

Buildings. American Council for an Energy Efficient Econonomy. 

Wilcox, B. (2007). “Revisions to the Residential Standards and ACM Calculations.” Presentation 

to the California Energy Commission. June 13, 2007.

Wilcox, B.; Chitwood, R.( 2010). “2010 California New Home Energy Survey.” Presentation at 

the 2010 EEBA Conference, Austin, Texas. April 22, 2010.

Wray, C.; Walker, I.; Sherman, M. (2002). “Accuracy of Flow Hoods in Residential 

Applications.” 2002 ACEEE Summer Study on Energy Efficiency in Buildings. American 

Council for an Energy Efficient Economy.
41

Appendix A: Test Forms and Checklists

Table A-1. Preliminary Diagnostics Form.

Site Address ________________________ Checked by _________________ Date __________

TEST/OBSERVATION CRITERIA RESULTS/NOTES

Maintenance Problems Yes or No, describe if yes

Comfort Problems Yes or No, describe if yes

System Age / Condition Years / good, fair, poor

Condensing Unit Make/Model From nameplate

Condenser Size kBtuh/12, from nameplate Tons

Condenser Coil Condition Good, or Fouled and Cleaned

Number of Returns 1 or ___

Number of Zones 1 or ___

Bypass Damper Installed If system is zoned, yes or no

Filter Location In furnace / at return grille / other

Filter Condition New / Fair / Clogged / Missing

Suction Line Insulation/Location Condition / Attic, Garage, Other

TXV bulb secured and insulated No, Yes, or Repaired

Minimum Required Airflow = 300 x Tons CFM

Filter Size = L ____” x W____” /144 ft2

Filter Velocity = Minimum Airflow / Filter Size FPM (<300)

Normal System Op. Pres. (NSOP) □with filter □without filter Pa

Airflow at All Registers? Yes, or No and specify

Measured Duct Leakage If DTS performed (optional) CFM

Measured Airflow Method: CFM

Supply air dry bulb temperature Measured at supply plenum (S) Deg. F

Return air dry bulb temperature Measured at return plenum (R) Deg. F

Return air wet bulb temperature Measured at return air intake Deg. F

Temperature Split. = R – S Deg. F

Target Temperature Split From Table 2 Deg. F

Measured Airflow with Filter Must be greater than Min. Required CFM

Measured Airflow without Filter If first measurement <300 CFM CFM

Test Pressures. Refer to PD Step 

10 for instructions.

 

Supply – Return with filter (<190 Pa)

Supply – Return w/o filter (<140 Pa) 

Supply – Ambient (< 40 Pa)

Supply – Ambient (< 40 Pa)

______Pa

______Pa

______Pa

______Pa 

Results and Actions

□ Temperature split within 3oF of target □ Problem(s) identified: (describe)

□ Airflow adequate with filter in place □ Referred to HVAC tech

□ Airflow adequate only with filter removed □ Repairs made: (describe)
42

Table A-2. Comprehensive Diagnostics Form (supplement to Table A-1).

Site Address ________________________ Technician_________________ Date __________

TEST/OBSERVATION CRITERIA RESULTS/NOTES

Return air dry bulb temperature Treturn, db oF

Return air web bulb temperature Treturn, wb oF

Supply air dry bulb temperature Tsupply, db oF

Evaporator saturation temperature Tevap, sat oF

Condenser saturation temperature Tcond, sat oF

Suction line temperature Tsuction oF

Liquid line temperature Tliquid oF

Condenser entering air dry bulb Tcond, db (measure average) oF

Compressor amps, measured Amps

Compressor amps, rated From condensing unit nameplate Amps

Temperature Split From Table A-1 oF

Condensing over Ambient = Tcond, sat – Tcond, db

Superheat Measured Tsh, meas = Tsuction – Tevap, sat

Superheat Target Tsh, target (from Table 4)

Superheat Deviation = Tsh, meas – Tsh, target

Subcooling Measured Tsc, meas = Tevap, sat – Tliquid

Subcooling Target Tsc, target (from manufacturer 

tables, or use 10°F ±5°)

Subcooling Deviation = Tsc, meas – Tsc, target

Diagnosis and Actions □ Undercharge

□ Overcharge

□ Liquid line restriction

□ Non-condensables

□ Faulty TXV or bulb installation

□ Compressor amps excessive

□ Repairs made: (describe)
43

Appendix B: Supporting Data and Calculations 

B-1: Energy Savings for Replacement with High Efficiency Air Conditioners

This section describes modeling and calculations that were completed to estimate the energy 

savings and incremental costs for replacement of existing air conditioners with systems that have 

performance ratings higher than the current standard of SEER 13. BEopt was used to model 

existing homes in five U.S. climate zones. The foundation was assumed to be slab-on-grade for 

all zones. Building geometry and configuration were determined based on research and review of 

the 2010 DOE Buildings Energy Data Book (BEDB) (DOE, 2011) and the 2009 California 

Residential Appliance Saturation Study (RASS) (KEMA, 2010) and include the following:

• Single story 1,800 square ft home with three bedrooms. This is consistent with RASS and 

BEDB averages. RASS average for single family homes is 1,882 ft2 for CA building 

stock. BEDB estimates characteristics of a typical single-family home from the mid 

1970s with a heated average floor space of 1,934 ft2 and a cooled average floor space of 

1,495 ft2. Average conditioned floor area of homes constructed between 1980 and 1989 is 

1,806 ft2.

• 16% glass as a percentage of conditioned floor area, based on RASS averages. The 

BEDB reports window areas for a typical 1970s-era home of between 11% and 15% of 

conditioned floor area.

• Glass is equally distributed on all four sides to eliminate orientation effects

• Wall area and slab perimeter based on an aspect ratio of 2.0. This is consistent with a 

sampling of actual homes. To maintain equal wall and glazing areas distribution on all 

orientations, an L-shape building model was used.

The home and its existing air conditioner were assumed to be from the 1980’s era. Building 

characteristic assumptions are based on the Building America House Simulation Protocols (HSP)

as well as California’s Title 24 vintage assumptions, where appropriate. U-factor and solar heat 

gain coefficient (SHGC) values for the single metal pane windows were taken from the default 

values in Tables 116-A and 116-B in California’s 2008 Building Energy Efficiency Standards

(CEC 2010).

Table B-1. Existing home assumptions by building vintage

1980-1989

Wall Insulation R-11

Window U-value/SHGC Single metal

1.28/0.80

Slab Insulation R-Value 

and Depth

R-0

Ceiling Insulation R-9

Title-24 default SEER and EER assumptions were used for the base case air conditioner for units 

installed between 1984 and 1991; SEER 8.9, EER 7.8. Fan watt draw was estimated at 0.58 

W/cfm. Ductwork was assumed to be located in the attic with R-2.1 insulation and 30% leakage.

Results of simulations for each climate zone/city are listed in Table B-2. These values were used 
44

to calculate energy savings. A flat rate of $0.12 per kWh was used to calculate utility bill 

savings. 

Table B-2. Annual Energy Use Determined by BEopt.

Climate Zone Sample City

Annual Cooling Energy Use (kWh)

Base 

Case 

SEER 13, 

EER 11.1

SEER 15, 

EER 12.7

SEER 18, 

EER 13.4

Hot-Dry Fresno, CA 7,435 5,304 4,619 4,866

Hot-Humid Houston, TX 8,356 6,247 5,456 5,833

Mixed-Humid Atlanta, GA 5,343 3,823 3,332 3,547

Marine Santa Rosa, CA 1,009 684 593 645

Cold Denver, CO 2,677 1,885 1,640 1,850

Air conditioner sizes for the existing and replacement systems that were used for cost estimating 

are listed in Table B-3. Average installed costs of $130, $140, and $160 per kBtuh for the 13, 15, 

and 18 SEER systems, respectively, were obtained from the National Residential Energy 

Efficiency Measures Database at: http://www.nrel.gov/ap/retrofits/index.cfm. These were used 

to calculate the incremental costs for 15 and 18 SEER replacements that are listed in Table 1.

The higher energy use for the SEER 18 system reported in Table B-2 is a result of the 

performance curve utilized by BEopt for two-speed condensing units. Some high SEER products 

perform better on average, but when operated only at high speed, perform only marginally better 

than SEER 13 systems.

Table B-3. Assumed Sizes of Replacement Air Conditioners.

Climate Zone A/C Size

Hot-Dry 3.0

Hot-Humid 3.0

Mixed-Humid 2.5

Marine 1.5

Cold 2.0

Table B-4. Replacement Cost vs. SEER Rating.

Replacement Cost 13 15 18

 Hot-Dry $4,680 $5,040 $5,760

 Hot-Humid $4,680 $5,040 $5,760

 Mixed-Humid $3,900 $4,200 $4,800

 Marine $2,340 $2,520 $2,880

 Cold $3,120 $3,360 $3,840

It should be noted that the energy use value of 7,435 kWh/year determined using BEopt for the 

Fresno climate is more than six times higher than California Appliance Saturation Survey data 
45

(RASS 2009), which cites a unit energy consumption value of 1,189 kWh/year. Further research 

is needed to determine the source of this discrepancy.

B-2: Replacement Blower Motor Energy Savings Estimates

To measure fan energy savings for an ECM motor replacement, a 1970’s vintage GE Model 

BLU100C942A1 furnace was tested first with its stock motor and subsequently with an 

Evergreen 1/3-1/2 HP replacement motor. To simulate different supply duct static pressure 

conditions, each motor was tested using three different “equivalent” supply restrictions (A, B, 

and C) that yielded corresponding plenum pressures of 100, 150, and 200 Pa at the ECM motor’s 

highest tap setting. A TrueFlow grid was used to measure airflow and served as a proxy for the 

filter and return plenum/grille airflow restriction. The airflow produced by this furnace is suitable 

for a 3-ton air conditioner if the ducts are of adequate size, but since typical supply plenum 

pressures are close to 0.5” (125 Pa), the furnace is more appropriate for a 2-1/2 ton system. Test 

results are shown in Table B-5. Of the three PSC motor tap settings, the Medium setting 

provided the closest correspondence to the ECM’s High setting, and the PSC’s Medium setting

corresponded to the ECM’s Med-Low tap setting, so these pairs were used to estimate energy 

savings.

Curve fits of supply plenum pressure vs. power were used to develop equations that allow 

calculation of energy savings as a function of heating and cooling supply plenum pressures 

(representing a range of duct systems) and annual operating hours. The curve fits were used to 

develop a spreadsheet that can be used to test different operating assumptions. 

Table B-5. Results of Comparative Tests of a PSC and ECM Motor.

Target “Duct” Measured Airflow Power

Pressure Setting Pressure, Pa CFM Watts

PSC, "Cooling Mode" 100 A 100 1255 830

High tap setting 150 B 150 1037 762

200 C 200 819 705

PSC, "Cooling Mode" A 90 1182 683

Medium tap setting B 139 995 591

C 194 788 524

PSC, "Heating Mode" A 57 950 580

Low tap setting B 106 865 524

C 162 719 458

ECM, "Cooling Mode" A 87 1152 492

Evergreen, "High" tap settings B 143 991 483

C 194 803 437

ECM, "Heating Mode" A 61 960 284

Evergreen, "Med-Low" tap setting B 107 852 305

C 163 729 324
46

For the example provided in Table B-6, run times were obtained from an eQuest simulation of a 

typical house located in Fresno (hot-dry climate). At $0.17 per kWh, annual savings were 

calculated to be about $30. If financed under a 15-year loan at 5%, these savings would offset an 

expenditure of $130, which would not cover the cost of the motor and labor for replacement 

unless the original motor was defective. Once a contractor has become familiar with the 

replacement process, the time to complete the work is one-half to one hour, depending on site 

conditions.

Motor operating hours are the most critical factor in determining savings, and are a function of 

the house heating and cooling loads and system capacities. When houses are tightened up and 

insulation is improved, run times will be reduced, diminishing savings from fan motor 

replacement.

Table B-6. Annual Cost Savings Calculation Example.

Measured Plenum Pressure, Heating 100 Pa

Annual Heating Hours 397 Assume 25% run time, Nov-Feb

Watt Savings-Heating 230.3 Watts

Annual Heating Savings 91.4 kWh

Measured Plenum Pressure, Cooling 150 Pa

Annual Cooling Hours 798 Assume 10% run time, June-Sept

Watt Savings-Cooling 104.1 Watts

Annual Cooling Savings 83.0 kWh

Total Savings 174.5 kWh

Mean Tariff $0.17 per kWh

Cost Savings $ 29.66 Annual Savings

Genteq provides an Energy Savings Calculator 

at: http://www.genteqcomfort.com/ShowMeTheMoney/EnergySavingsCalculator.aspx.

Inputs are location (city/state) and motor size. Entering Stockton, California, and 1⁄2 hp yielded 

savings of $92 and much higher run times of 1158 hours (cooling) and 1834 hours (heating).

B-3: Verification of Temperature Split Method for Checking Refrigerant Charge

The California Energy Commission’s Title 24 energy standards include an alternate method for 

verifying airflow using the temperature difference between the supply and return plenums. This 

“temperature split” method is used to confirm the validity of superheat or subcooling refrigerant 

charge tests, specifically to insure that low airflows are not producing erroneous charge 

measurements.

Most home performance contractors (HPCs) carry duct blasters and can use them to accurately 

measure airflow but are not equipped to measure or correct refrigerant charge. Consequently, the 

PD method described in this guideline provides for HPCs to verify airflow and use temperature 

split to detect significant refrigerant-side problems. Low refrigerant charge, liquid line 

restrictions, expansion valve defects, and the presence of excessive non-condensable gases can 

all affect cooling capacity, which in turn will proportionally reduce the temperature split.

The Target Temperature Split table (Table 2) in the Implementation section of this guideline is 

borrowed from Title 24 compliance documents (CEC 2008). To verify the values in the 
47

temperature split table, Robert Mowris of Verified Inc. commissioned Intertek to test the method 

using one properly charged air conditioner operated under a variety of conditions. Table B-5 

compares the actual measured temperature split (TS) with values in Table 2 (Required TS) using 

fixed airflow and return air dry bulb (RDB) temperatures, and varying return air wet bulb 

(RWB) and condenser entering air temperatures. The greatest difference between target and 

measured splits was 3.0°F (Run 418), which is the same deviation allowed under the Title 24 

tests. These results suggest that a temperature split greater than 3°F provides sufficient evidence 

that the capacity may be compromised by a refrigerant side defect.

Table B-5. Laboratory Test Results Comparing Measured and Target Temperature Splits.
48

Appendix C: Detailed Methods and Procedures

C-1: Air Conditioner Replacement Decision Tree

The following diagram can be used to guide decisions on when to consider replacing an older air 

conditioner (PNNL 2011). This diagram does not address changes in sizing caused by 

improvements to the building enclosure or air conditioners that were incorrectly sized when 

originally installed. Manual J and S calculations should be completed prior to selecting 

equipment for replacement.

Figure C-1. Air conditioner replacement decision tree.
49

C-2: Detailed Airflow Measurement Methods

The following methods were excerpted from Appendix RA-3 of the California Title 24 

Residential Energy Standards (CEC 2008). Modifications were made to the method to allow 

measurement of airflow with and without the filter in place.

General Instructions

Begin tests with the filter installed. If the measured airflow is less than 300 cfm per ton of AC, 

repeat the measurements with the filter removed. Use either Table A-1 or Table A-2 to record the 

results

.

Airflow tests must be conducted with the fan operating at the speed used for AC (cooling speed). 

The fan switch on newer systems provides an airflow lower than that used for cooling, in which 

case the system must be set to operate in cooling mode by lowering the thermostat setting. The 

condenser may be disabled by pulling the fuses if necessary to avoid over-cooling the space. 

For multizone systems, all testing must be performed with all zone dampers open and bypass 

dampers locked in the closed position. In all cases, refer to manufacture instructions for the 

particular test equipment used for additional information.

Powered Flow Hood (Fan Flowmeter) 

This fan flow measurement shall be performed using the following procedures:

1. With the system fan on at the maximum speed (the cooling speed), measure the pressure 

difference (in Pa) between the supply plenum and the conditioned space (Psp). Psp is the 

target pressure to be maintained during the fan flow tests. Place the pressure probe in the 

Supply Pressure Measurement Location marked “S” in Figure B-1. Adjust the probe to 

achieve the highest pressure and then firmly attach the probe to ensure that it does not 

move during the fan flow test.

Figure C-2. Pressure measurement locations.

2. Attach the fan flowmeter to the duct system at the main (largest) return air grille. If there 

are two or more returns, seal off the other return grilles. 

3. Turn on the system fan and the fan flowmeter, adjusting the fan flowmeter until the 

pressure between supply plenum and conditioned space matches Psp.

4. Record the flow through the flowmeter (Qah, cfm); this is the diagnostic fan flow. In 

some systems, system fan and fan flowmeter combinations may not be able to produce 

enough flow to reach Psp. In this case, record the flow (Qah, cfm) and pressure (Pmax) 
50

between the supply plenum and the conditioned space. Use Pmax and Psp with the 

following equation to correct measured system flow (Qahcorrected).

Qahcorrected = Qah x (Psp/Pmax)0.5

Flow Grid 

The fan flow measurement shall be performed using the following procedures:

1. With the system fan on at the maximum speed (the cooling speed), measure the pressure 

difference (in Pascal) between the supply plenum and the conditioned space (Psp). Place 

the pressure probe in the Supply Pressure Measurement Location marked “S” in Figure 

B-1. Adjust the probe to achieve the highest pressure and then firmly attach the probe to 

ensure that it does not move during the fan flow test.

2. Remove the filter and replace it with a flow grid that corresponds to the filter size.

3. Re-measure the system operating pressure with the flow grid in place.

4. Measure the airflow through the flow grid (Qah) and the test pressure (Ptest).

5. Use the following equation to correct flow through the flow grid and pressure (Qah and 

Ptest) to operating condition at operating pressure (Psp).

Qahcorrected = Qah x (Psp/Ptest)0.5

Flow Capture Hood

If neither a powered flow hood or a flow grid are available, a calibrated flow capture hood 

(balometer) may be used, though accuracy will vary greatly depending on the type and 

orientation of the device used. With the system fan on at the maximum speed (the cooling 

speed), use the flow hood to measure airflow at each return grille. Sum the measured return 

airflows to determine the system fan flow (Qah, cfm). To improve accuracy of balometer 

measurements, the same correction may be applied as used with the flow grid.
51

C-3: Detailed Condenser Coil Cleaning Instructions

Accumulated dust, lint, leaves, grass clippings, and other debris can be sucked into the fins of a 

condensing unit coil as air is drawn through them by the fan. These obstructions will lower the 

effectiveness of the coil, elevate condensing temperature, and reduce cooling efficiency. A visual 

inspection is generally adequate to determine whether the coil must be cleaned, though cleaning 

a relatively unobstructed coil will likely improve efficiency.

1. If a digital thermometer with a surface probe is available, measuring the temperature of 

the liquid line (the small uninsulated line) before and after the cleaning process will 

provide some indication of the effectiveness of the cleaning process. 

2. Biodegradable coil cleaners can be obtained from HVAC supply houses and will reduce 

the cleaning time and improve the effectiveness of the cleaning process. A hose with a 

spray nozzle or a pressure washer are also required. If using a pressure washer, care must 

be exercised to insure the water stream is not so powerful as to bend or damage the fins 

of the coil. Follow the instructions listed below:

3. Shut off the cooling system at the thermostat and pull the fuse from the panel that 

powers the outdoor unit. 

4. If the exterior of the coil is conspicuously covered with litter, use a shop vacuum to 

remove it, being careful not to bend the 

fins.

5. Carefully remove the top of the cabinet. 

As shown in Figure B-1, the fan motor is 

sometimes attached to the top. In this 

case, set the fan-motor assembly aside and 

prop it up as needed to avoid putting strain 

on the wires. 

6. Spray the coil from the inside and outside 

first using water, then using the coil 

cleaner and allow it to sit for 5-10 

minutes, or as directed by the cleaning 

solution instructions.

7. Use the spray nozzle or pressure washer to 

flush the coil until clean.

8. Replace the top and restore the fuse disconnect.

Note that some two-row condenser coils are split and curved in a “U” shape and the fins are 

separate and do not align, causing clogging between the two rows. In this case, it may be 

necessary to separate the coils for proper cleaning, a job that requires the skills of a qualified 

HVAC technician.

Figure C-3. Removing of the top of 

the condensing unit for cleaning 

(Source: the familyhandyman.com, 

reprinted with permission).
52

C-4: Refrigerant Procedures to Prevent Non-Condensables and Restrictions

To reduce the introduction of contaminants while lines are being brazed, it is recommended 

practice to leave one end of the line open and slowly introduce nitrogen into the line, allowing it 

to flow out the other open end. The nitrogen will prevent oxidation and contamination resulting 

from brazing by-products. Most manufacturers recommend installation of a filter drier in “all 

field-connected split-system air conditioners” (Carrier). The filter drier should be installed on the 

liquid line before the expansion device to remove moisture, acid, sludge, and particulate material 

from the refrigerant system and to avoid refrigerant restrictions. Filter driers should not be 

exposed to ambient air for more than a few minutes or else they will be contaminated and will 

lose their effectiveness. A deep vacuum will not remove moisture from a filter drier.

Proper charging procedures (see Section C-8) must be used to clear lines of nitrogen and to avoid 

introducing air into the system. Air contains moisture, which can create oil sludge that reduces 

the lubricating properties of the oil, and plugs screens and oil passages in the compressor. 

Moisture can also damage the synthetic POE oil used in R410A and can freeze in the expansion 

device to cause restrictions. The heat of compression causes moisture to react with refrigerant 

and form hydrochloric and hydrofluoric acid. These acids erode machine surfaces and cause 

copper ions from the tubing to deposit on heated surfaces in the compressor causing compressor 

failure. These contaminates can cause the insulation on the motor windings to break down.

C-5: Vacuum Pump Maintenance

A common installation practice is to attach the vacuum pump to the system and allow it to run 

for a given time period, typically 4 hours. After that time, there is an assumption that a sufficient 

vacuum has been pulled to remove any non-condensables from the system. No vacuum gauge is 

installed, nor reference to potential vacuum level or vacuum change when the vacuum pump is 

turned off as specified in manufacturers' installation literature. While common, this is a 

problematic approach as vacuum pumps rarely receive recommended maintenance and cannot be 

guaranteed to produce the required system evacuation.

Manufacturers indicate that vacuum pump maintenance is required after each evacuation (JB 

Industries, 2007). Required maintenance includes draining contaminated oil while the vacuum 

pump crankcase is still warm from use, flushing the reservoir, refilling with new oil, and 

recycling old oil. New oil should not be exposed to the atmosphere for longer than two minutes 

because it will adsorb atmospheric moisture, causing loss of lubrication and sealing ability. New 

oil should be run in the pump for two or three minutes and then drained and refilled with fresh 

oil. This will remove contaminates from the pump and the oil. The vacuum pump must be stored 

with the exhaust port plugged or capped to prevent contamination due to air, water vapor, and 

dirt. A slight cut or dirt on the o-ring seals can cause leaks. The mating flare fitting faces must be 

wiped after each use and checked for damage before hookup. Vacuum pump oil creates a fine 

lubricant at these connections. Without this maintenance, the vacuum pump oil can react with 

refrigerants and produce hydrofluoric and hydrochloric acids. These acids can, over time, 

corrode and damage internal surfaces of the pump making it impossible to achieve the required 

vacuum for system installation.

Vacuum pump oil serves three functions: 1) coolant, 2) lubricant, and 3) sealant. If the oil is 

contaminated, then the vapor pressure of the oil will increase which causes the pump to not 

achieve proper evacuation. A new pump with clean oil can achieve a 15 to 50 micron vacuum. 
53

Putting the vacuum gauge on the pump and capping the other fittings will tell whether or not the 

oil is contaminated or the pump is not working. If the pump cannot achieve 50 microns, then 

either the oil is contaminated or the pump is not working properly. It is difficult to visually 

determine if oil is contaminated with air or moisture, so manufacturers recommend draining, 

flushing and refilling with new oil after every use. Some contaminated systems or larger systems 

require from two to five oil changes during the evacuation process.

The vacuum pump is only as good as the oil that can maintain the correct vapor pressure. 

Vacuum pump oil will not introduce contamination to the system. However, contaminated oil 

will prevent the vacuum pump from achieving the 240 micron vacuum. The 2010 ASHRAE 

Handbook - Refrigerant specifies a 240 micron vacuum for AC systems (some manufacturers 

recommend 240 microns for R410A and 500 microns for R22.). In order for the pump to pull a 

240 to 500 micron vacuum, the oil must be clean and moisture-free throughout evacuation. At 

the beginning of the evacuation procedure, water vapor is removed quickly. If the system 

contains moisture, the oil will get contaminated quickly and this will make it difficult to achieve 

the correct vacuum. At the beginning of the evacuation, the gas ballast valve should be opened to 

remove moisture from the system and avoid contaminating the vacuum pump oil. After the 

vacuum pump quiets down and the micron gauge reads 1500 microns or less, the gas ballast 

valve should be closed. 

A slight cut or dirt in the o-ring seals can cause leaks, therefore, the mating flare fittings faces 

should be wiped and checked for damage before hook up. It is important that evacuation always 

be performed from both the low and high sides of the system since this can save as much as 75% 

of the time to perform the evacuation compared to evacuating from only one side.

C-6: Evacuation and Recovery Procedures

Proper recovery and evacuation procedures are required to avoid introducing non-condensables 

or restrictions to the AC system. To speed the evacuation process, it is necessary to remove the 

Schrader valve cores to eliminate the restriction they cause. However, the valve core depressors 

in the gauge hose connectors also cause a high restriction. Either remove the valve core 

depressors from the hose connectors, or use gauge hoses without valve core depressors during 

the evacuation process and change to regular hoses with EPA low loss fittings after the Schrader 

valve cores are replaced. An added restriction is also imposed by three-valve manifolds with 1⁄4” 

house connections to the vacuum pump. For rapid evacuation, it is highly recommended that the 

service tech include (among other tools and equipment) a four-valve gauge manifold with a 3/8

in. hose to the vacuum pump, a separate hose to the refrigerant cylinder, and hoses with either no 

valve core depressors. The four-valve manifold setup can be used exclusively for the evacuation 

process.

Never pull a vacuum through Schrader valves. This practice will increase the evacuation time 

due to the high restriction it will cause. 

The following procedures are recommended to perform refrigerant recovery:

1. The EPA prohibition on venting requires that the following procedures be followed when 

connecting or disconnecting refrigerant hoses (USEPA 1970):
54

• Hoses must be equipped with EPA low-loss fittings.

• Attach center hose from manifold to recovery unit

2. Install a filter-drier in the vapor line inlet of recovery unit to protect compressor and CRO 

valve. Connect another hose from recovery unit to recovery tank. 

3. Open both valves on manifold and the valve on the recovery tank with the hose 

connected to it. Turn on the recovery unit and move valves on the recovery unit to 

recover refrigerant. Wait until the low-side gauge on the manifold reads 0 psig (0 inches 

of Hg) or slightly less than ambient pressure. Then close all valves on manifold and 

recovery unit. Shut off recovery unit.

After a motor burn-out, the refrigerant should never be reused and all recovered refrigerant must 

comply with ARI 700. 

If it is necessary to run the vacuum pump overnight, it is highly recommended to use a full-port 

solenoid valve in-line with the vacuum line to the pump to avoid losing vacuum during a power 

failure. If loss of power occurs during the evacuation, the oil in the pump and non condensables 

will be drawn back into the system. Never base evacuation on time. Always measure the quality 

of the vacuum with a calibrated micron gauge.

The triple evacuation or deep vacuum methods are both acceptable evacuation methods and are 

interchangeable. General evacuation procedures are listed below.

1. To insure that the vacuum pump is capable of pulling an adequate vacuum, first connect 

the micron gauge directly to the pump, cap off other pump ports, and start the pump. Let 

it run until the micron gauge reads 50 microns or lower. Do not attempt to use a vacuum 

pump that does not pass this test. Remove the gauge and cap off the port to which it was 

attached. 

2. After refrigerant is recovered and with atmospheric pressure in the system, remove the 

Schrader valve cores from system access ports, connect the flare tee to the high side 

access port, connect the low and high side hoses (each with no valve core depressors) to 

their appropriate access valve ports. Cap off the open flare tee port.

3. If necessary to check for refrigerant leaks, pressurize the system with dry nitrogen to 150 

psi. When leaks are fixed and the system is returned to atmospheric pressure, remove the 

cap on the flare tee and connect the micron gauge to that port. 

4. Next, connect the 3/8 in. gauge hose from the manifold to the vacuum pump and the 

refrigerant hose to the refrigerant cylinder. Before connecting the cylinder, weigh it with 

a digital scale and record. Purge the refrigerant hose, close the refrigerant valve, start the 

vacuum pump, and open the pump and high and low side manifold valves. Open the 

pump’s ballast valve and close it at 1,500 microns.

5. When the pressure is reduced to the recommended 240 microns, close the vacuum pump 

valve on the manifold and observe that the pressure does not exceed 500 microns after 5 

minutes.
55

To use the Triple Evacuation Method:

1. Perform a preliminary leak test by closing valves and adding dry nitrogen to 150 psig and 

hold for 10 minutes to remove water vapor. If system pressure does not stay at 150 psig 

for 10 minutes, then locate and repair any leaks.

2. Attach hoses from the liquid line and suction line Schrader valves to the vacuum pump 

(pumping from high and low side simultaneously can reduce the evacuation time by up to 

75%). Open valves on manifold and turn on vacuum pump for the first time. At the 

beginning of the evacuation, the gas ballast valve should be opened to remove moisture 

from the system and avoid contaminating the vacuum pump oil. After the vacuum pump 

quiets down and the micron gauge reads 1500 microns or less, the gas ballast valve 

should be closed. Evacuate to 1,000 microns as indicated on the micron gauge.

3. If the indicator shows a pressure rise above 1500 microns and the pressure continues to 

rise without leveling off, a leak exists in the AC system or connecting hoses. Locate and 

repair leaks. If no leaks are detected, then system is leak proof (as indicated above). 

4. If the system pressure rises slowly to between 1,000 and 1500 microns, then there is 

moisture in the system. Close valves and add dry nitrogen to remove water vapor and 

increase system pressure above atmospheric pressure to 15 psig and hold for 10 minutes.

5. Open valves on manifold and turn on vacuum pump a second time and evacuate to 240 

microns and no higher than 500 microns. Immediately break the vacuum by adding dry 

nitrogen to 15 psig and hold for 3 minutes.

6. Open valves on manifold and turn on vacuum pump a third time and evacuate to 240 

microns and no higher than 500 microns, as indicated on the micron gauge and hold for 5 

minutes.

7. If the pressure isn’t rising above 500 microns after 5 minutes, then the triple evacuation 

has been completed and the system is ready for charging with new refrigerant.

To use the Deep Evacuation Method:

1. Perform a preliminary leak test by closing valves and adding dry nitrogen to 150 psig 

and hold for 10 minutes to remove water vapor. If system pressure does not stay at 150 

psig for 10 minutes, then locate and repair any leaks. 

2. Attach hoses from the liquid line and suction line Schrader valves to the vacuum pump. 

Open valves on manifold and turn on vacuum pump and evacuate to 240 to 500 microns 

as indicated on the micron gauge and hold for ten minutes. 

3. If the indicator shows a pressure rise above 1500 microns and the pressure continues to 

rise without leveling off, a leak exists in the AC system or connecting hoses. Locate and 

repair leaks. If no leaks are detected, then system is leak proof. 

4. If the system pressure rises slowly to between 1,000 and 1500 microns, then there is 

moisture in the system. Close valves and add dry nitrogen to remove water vapor and 

increase system pressure above atmospheric pressure to 15 psig and hold for 10 minutes. 

Then, open valves on manifold and turn on vacuum pump a second time and evacuate to 

240 microns and no higher than 500 microns.
56

5. If the pressure isn’t rising above 500 microns after 5 minutes, then the deep vacuum has 

been completed and the system is ready for charging with new refrigerant.

C-7: Charging Procedures

The following procedures are recommended to properly charge an air conditioner with 

refrigerant. Only add refrigerant charge through the liquid line. Adding charge through the 

suction line can damage the compressor.

1. Perform proper evacuation recovery and deep vacuum procedure noted above.

2. With the evacuation complete, remove the evacuation hoses, install Schrader valve cores, 

and purge the conventional three-valve manifold and hoses with new refrigerant. This can 

be accomplished on the EPA fittings by connecting the fitting to a W male flare fitting, 

purge, remove the flare fitting, and connect EPA fitting to the appropriate Schrader 

access valve port on the system. Repeat on the other hose. 

3. Determine the factory charge from manufacturer’s specifications on the name plate of 

documentation on the manufacturer’s website. For split systems, include any line-set 

length beyond or shorter than the standard line-set length per manufacturer’s 

specifications. 

4. Place a tank of new refrigerant upside down on digital scale to ensure liquid is charged to 

the system (especially important for R410A), attach center hose of manifold to new 

refrigerant tank, locate manifold so as not to affect refrigerant scale readings, and place 

low-side hose to the suction line Schrader valve of the air conditioner (all hoses are 

evacuated to vacuum). Tare the scale to indicate a zero reading.

5. Open the valve on the tank of new refrigerant to fill the air conditioner with new 

refrigerant to as close to the factory charge as possible. Reinstall Schrader valve cores 

when the system is under a slight positive pressure.

6. The air conditioner will gradually fill with new refrigerant close to the factory charge. As 

the air conditioner gets within 10% to 40% of the factory charge, turn ON the air 

conditioner and allow the compressor to draw in the correct factory refrigerant charge. 

7. Check for non condensables and/or restrictions (per diagnostic thresholds of evaporator 

saturation and condenser saturation temperatures). 

8. Check the final charge to achieve proper superheat, subcooling, non condensables, and 

restrictions for TXV- and non-TXV-equipped systems (per manufacturer’s 

specifications).

C-8: Proper Refrigerant Procedures to Avoid Non-condensables and Restrictions

Proper refrigeration procedures are required to avoid introducing non-condensables and 

restrictions to the AC system. The EPA prohibition on venting requires precautions that must be 

followed when connecting or disconnecting refrigerant hoses. Never use contaminated 

refrigerant for recharging a system especially after burnout of a compressor. Beyond these 

guidelines, the following procedures should be followed in the care and use of service 

equipment.

1. All hoses must be equipped with EPA low-loss fittings.
57

2. Follow procedures carefully to insure that the system is fully evacuated and that hoses are 

purged with new refrigerant before recharging.

3. After charging and with the air conditioner operational, open the high-side valve and lowside valves on the manifold. This will cause any refrigerant trapped in the high-side and 

middle hose to be drawn into the low-side of the system. When both manifold gauge 

pressures are equalized, so the high-side is the same pressure as the low-side, then close 

off both high-side and low-side valves on the manifold. Then, remove the low-side 

manifold hose from the suction line Schrader valve of the air conditioner.

C-9: ASHRAE Recommendations for Proper Evacuation to Remove Noncondensable Air and Water Vapor

Removal of non-condensable gases—air, nitrogen and other gases that are left in the system and 

mixed with the refrigerant—can have severe impacts on the performance of the unit. The 2010 

ASHRAE Handbook – Refrigeration recommends that units be evacuated to 240 microns of Hg. 

In general, evacuating to 240 microns Hg should remove non-condensables, but might not 

eliminate all moisture.

Removal of moisture can be much more difficult. The 2010 ASHRAE Handbook – Refrigeration

states the following. “Excess moisture in refrigeration systems may lead to freeze-up of the 

capillary tube or expansion valve. Contaminants can cause valve breakage, motor burnout, and 

bearing and seal failure. Except for freeze-up, these effects are not normally detected by a 

standard factory test. Therefore, it is important to use a dehydration technique that yields a safe 

moisture level without adding foreign elements or solvents. In conjunction with dehydration, an 

accurate method of moisture measurement must be established. Many factors, such as the size of 

the unit, its application, and the type of refrigerant, determine the acceptable moisture content. 

Table 1 shows moisture limits recommended by various manufacturers for particular 

refrigeration system components.”

C-10: Method to Detect Blocked Distributor Tubes

Although it is an infrequent problem, unequal refrigerant flow can be caused by kinked or 

blocked distributor tubes. One way to detect this is, with pressure gauges installed, remove the 

coil cover and cover the opening with cardboard. While monitoring the suction pressure gauge, 

block the return grille enough to reduce the evaporator temperature to about 20oF–22°F. Let the 

system run for awhile so frost will form on the coil return bends. Remove the cardboard every 

few minutes to look for frost. When frost becomes evident, look for return bends with less or no 

frost as a clue to less or no refrigerant flow to an offending circuit. If all return bends have equal 

frost formation, it is a sign that refrigerant flow is uniform. Depending on where the low or no 

frost area is, the unequal frost formation could also be caused by non-laminar air flow, causing 

the TXV (if present) to throttle back, thereby increasing superheat and preventing frost from 

forming in those areas of the coil that have higher airflow.
58

Appendix D: Test Equipment Specifications and Calibration

D-1: Recommended Accuracy of Test Equipment

Temperature: Accuracy: ±(0.1% of reading + 1.3oF)

Resolution: 0.2oF

Response Time: 15 seconds to reach temperature that is 40oF warmer or cooler 

than the ambient temperature

Duct Pressure: Accuracy: ±0.2 Pa (use Dwyer A303 static pressure probes or equal)

Airflow: Accuracy: ±7% of reading or ±5 cfm, whichever is greater

Refrigerant Accuracy: ±3% of reading

Pressure:

D-2: Calibration Method for Digital Thermometer Sensors

The following procedure should be used to check thermometer/temperature sensor calibration:

1. Fill an insulated cup (foam) with crushed ice. The ice must completely fill the cup. Add 

water to the top of the cup.

2. Insert two sensors into the center of the ice bath and attach them to the digital 

thermometer.

3. Let the temperatures stabilize. The temperatures must be 32°F (plus or minus 1°F). If the 

temperature is off by more than 1°F, make corrections according to the manufacturer’s 

instructions. Any sensors that are off by more than 2°F must be replaced.

4. Switch the sensors and ensure that the temperatures read on both channels are still within 

plus or minus 1°F of 32°F.

5. Affix sticker with calibration check date onto sensor.

D-3: Refrigerant Gauge Calibration Procedure

Refrigerant gauges should be checked monthly to ensure that they are reading the correct 

pressures and corresponding temperatures. The following procedure should be used to check 

gauge calibration:

1. Place a refrigerant cylinder in a stable environment and let it sit for 4 hours minimum to 

stabilize to the ambient conditions.

2. Attach a calibrated sensor to the refrigerant cylinder using tape so that there is good 

contact between the cylinder and the sensor.

3. Insulate over the sensor connection to the cylinder.

4. Zero the low side and high side refrigerant gauges with all ports open to atmospheric 

pressure (no hoses attached).
59

5. Re-install the hose, attach the high side gauge to the refrigerant cylinder, and open the 

valves to measure the pressure in the refrigerant cylinder.

6. Read the temperature of the sensor on the refrigerant cylinder.

7. Using a pressure/temperature chart for the refrigerant, look up the pressure that 

corresponds to the temperature measured.

8. If gauge does not read the correct pressure corresponding to the temperature, the gauge is 

out of calibration and needs to be replaced or returned to the manufacturer for calibration.

9. Close the valve to the refrigerant cylinder, and bleed off a small amount of refrigerant to 

lower the high side pressure to give a corresponding temperature to between 45°F and 

55°F.

10. Open the valves between the high side gauge and low side gauge.

11. If the two gauge’s corresponding refrigerant temperatures do not read within 1°F of each 

other, the low side gauge is out of calibration and needs to be replaced or returned to the 

manufacturer for calibration

12. Affix sticker with calibration check date onto refrigerant gauge.

D-4: Other Devices

Airflow measurement devices and manometers should be returned to the manufacturer for 

calibration when the period of the manufacturer’s guaranteed accuracy expires.
DOE/GO-102013-3766 ▪ March 2013

Printed with a renewable-source ink on paper containing at 

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