4.9   Refrigerant Charge    

4.9.1    Refrigerant Charge Verification    

This section provides a summary of the procedures for verifying refrigerant charge for air conditioning systems. RA3.2 of the Reference Residential Appendix describes the procedures in detail. Refrigeration technicians and HERS raters who do the testing should refer to these and other technical documents. This section is intended to provide an overview and explanation of these procedures.

A.   Overview

A split system air conditioner undergoes its final assembly at the time of installation. This installation must be verified to ensure proper performance. Important factors that affect performance include the amount of refrigerant in the system (the charge) and the proper functioning of the metering device. Air conditioner energy efficiency suffers if the refrigerant charge is either too low or too high and if the metering device (TXV or EXV) is not functioning properly. In 'addition to a loss of efficiency and capacity, errors in these areas can lead to premature compressor failure.

To help avoid these problems, the prescriptive standards require that systems be correctly installed.  The prescriptive standards also require that they be field verified in Climate Zones 2, and 8 through 15.  Refrigerant charge verification is also required in any Climate Zone when chosen as a compliance feature using the performance approach.

The requirement to verify the refrigerant charge after installation does not apply to new packaged systems where the manufacturer certifies the charge performed in the factory, however airflow and other requirements must still be verified.  The prescriptive standards regarding verification of refrigerant charge do apply to altered package systems in Climate Zones 2 and 8 through 15.

This section describes the measurements and tests required to verify proper refrigerant charge and to verify that the system’s refrigerant metering device is working as designed. An alternative to the testing requirement is the installation of a charge indicator display that continuously monitors the function of the unit.

The verification of proper refrigerant charge must occur after the HVAC contractor has installed and charged the system in accordance with the manufacturer’s specifications. The procedure requires properly calibrated digital refrigerant gauges, thermocouples, and digital thermometers. When multiple systems in the same home require testing, each must be tested individually.

In a typical residential cooling system, there are two important performance criteria that can be checked relatively easy to verify that there is neither too much nor too little refrigerant in the system.  In systems with a fixed orifice device in the evaporator coil the number to check is called its superheat.  In a system with a variable metering device, the number to check is called its subcooling.

Superheat refers to the number of degrees the refrigerant is raised after it evaporates into a gas.  This occurs inside the evaporator coil (aka, indoor coil).  The correct superheat for a system will vary depending on certain operating conditions.  The target superheat for a system must be obtained from a table provided in the RA3.2 protocols or the manufacturer’s superheat table.  There is an allowed range of several degrees between the measured superheat and the target superheat for a system to pass.

Subcooling refers to the number of degrees the refrigerant is lowered after it condenses into a liquid.  This occurs inside the condenser coil (aka, outdoor coil).  The manufacturer specifies the correct subcooling for a system.  It may vary depending on operating conditions.  Like superheat, there is an allowed range of several degrees between the measured subcooling and the target subcooling for a system to pass.

The temperature at which a refrigerant condenses or evaporates is called its saturation temperature.  Above its saturation temperature, a refrigerant is always a gas.  Below its saturation temperature, a refrigerant is always a liquid. 

Saturation is when a refrigerant exists as both a liquid and a gas.  It always occurs at the same temperature depending on what the pressure of the refrigerant happens to be.  At higher pressures, the saturation temperature goes up and visa-versa.  This convenient property is what makes refrigeration work.

The saturation temperature can be determined by simply measuring the pressure of a refrigerant and referring to a table, known as a pressure-temperature (PT) table, for that specific refrigerant. Saturation temperatures are well documented for all common refrigerants. 

Because variable refrigerant metering devices are prone to failure and even more so to improper installation, it is important that their operation be checked.  The purpose of a metering device is to maintain a relatively constant superheat over a wide range of operating conditions, therefore checking the superheat, in 'addition to the other tests performed, will indicate if the metering device is operating correctly.

Unfortunately, checking superheat and subcooling can only be done under certain indoor and outdoor conditions.  This verification procedure, called the Standard Charge Verification Method, is very weather dependent.

There is another way to verify proper refrigerant charge that is not weather dependent and that is by weighing the refrigerant.  Called the Weigh-in Charge Verification Method, this approach can only be performed by the installer.  It can be verified by the HERS rater either by simultaneous observation or by using the Standard Method when conditions permit.

B.   Minimum System Airflow Verification for Refrigerant Charge Verification

To have a valid charge test, the system airflow must be verified to be at least 300 cfm/ton for altered systems and 350 cfm/ton for new systems. The procedures for measuring total system airflow are found in RA3.3.  They include plenum pressure matching using a fan flow meter, a flow grid, a powered flow hood and the traditional (non-powered flow hood).  The airflow verification procedures for refrigerant charge verification no longer include the temperature split method.

If a system does not meet the minimum airflow requirements, remedial steps may be required to increase the system airflow. More airflow is generally better for systems with air conditioning.  Not only does this allow proper refrigerant charge to be verified, but it also improves the overall performance of the system.  When able to be performed on a system, regardless of the refrigerant charge verification procedure, minimum system airflow must always be verified.  Note that Section 150.2(b)1F states that systems must be installed with “all applicable procedures”.  This includes the minimum system airflow requirements.

In some cases, improving airflow may be cost prohibitive and there is a process for documenting this (RA3.  When this option is used, verification by sample groups is not allowed.

Minimum airflow is critical to proper air conditioner operation.  Reducing airflow reduces cooling capacity and efficiency.  Many systems in California have oversized equipment and undersized ducts.  In newly installed duct systems the minimum airflow requirement is higher because the opportunity is there to design and install a better system.  In altered systems, the installer may be required to modify the ducts system to meet the minimum.  It should be noted that the minimums of 300 and 350 cfm/ton are far lower than the desired airflow for most systems, which is usually 400 cfm/ton and up.

C.   Standard Charge Verification Procedure (RA3.2.2)

The first step is to turn on the air conditioning system and let it run for at least 15 minutes in order to stabilize temperatures and pressures. While the system is stabilizing, the HERS rater or the installer may attach the instruments needed to take the measurements.

Figure 4-38– Measurements for Refrigerant Charge and Airflow Tests
Source: California Energy Commission


The following measurements shall be taken by the technician or HERS rater when applicable.

1.   The return air wet bulb and dry bulb temperatures are measured in the return plenum before the blower at the location labeled "Title 24 – Return Plenum Measurement Access Hole".  This hole must be provided by the installer, not the rater. (see point 2 in Figure 4-38). See figure RA 3.2-1 for more information on the placement of the measurement access hole (MAH).

2.   Additionally, the outdoor air dry bulb temperature is measured at the point where the air enters the outdoor condensing coil (see point 4 in Figure 4-38). It is important that this outdoor temperature sensor be shaded from direct sun during the verification procedure.

In 'addition to the air temperature measurements, four refrigerant properties need to be measured. Two of these measurements are taken near the suction line service valve before the line enters the outdoor unit ) and are used to check the superheat.

1.   The first measurement is the temperature of the refrigerant in the suction line, which is taken by a clamp-on thermocouple or other suitable device insulated from the outdoor air. (see point 5 in Figure 4-38)

2.   The second measurement determines the saturation temperature of the refrigerant in the evaporator coil. (see point 6 in Figure 4-38).The saturation temperature can be determined from the low-side (suction line) pressure and a saturation temperature table for the applicable refrigerant.

To check the subcooling, two more refrigerant properties are required, and may be measured near the liquid line service valve at the point where the line exits the outdoor unit (see points 7 and 8 in Figure 4-38):

1.   The liquid refrigerant temperature in the liquid line is measured by a clamp-on thermocouple insulated from the outdoor air.

2.   The condenser saturation temperature can be determined from the liquid line pressure and a saturation temperature table for the applicable refrigerant.

Note: determination of the condenser saturation temperature and the liquid line temperature is used only for the subcooling verification method on systems with TXV or EXV metering devices.

Superheat Charge Verification Method (RA3.

The Superheat Charge Verification Method is used on units with a fixed refrigerant metering device (not a TXV or EXV).

Airflow verification must be confirmed prior to starting the Superheat Verification Method.

Table 4-22 – Structure of Target Superheat



Return Air Wet-Bulb Temperature (°F)
(T Return, wb)












Condenser Air Dry-Bulb Temperature (°F)
(T condenser, db)


Target Superheat = (Suction Line Temperature minus Evaporator Saturation Temperature) – See Reference Residential Appendix Table RA3.2-2









The Superheat Verification Method involves comparing the actual (measured) superheat temperature to a target value from a table. The actual superheat temperature is the measured suction line temperature (TSuction, db) minus the evaporator saturation temperature (TEvaporator, Saturation). The target superheat value is read from a table (Table RA3.2-2 of the Reference Residential Appendix or the manufacturer’s superheat table).

For illustration purposes, the structure of Table RA3.2-2 is shown above as Table 4-23.

Only an EPA-certified technician may add or remove refrigerant.  Under no circumstances may a HERS rater add or remove refrigerant on systems that they are verifying.

D.   Subcooling Verification Method (RA3.

The Subcooling Verification Method is used on units with a variable refrigerant metering device (a TXV or EXV).

Airflow verification must be confirmed prior to starting the Subcooling Verification Method.

The Subcooling Verification Method involves comparing the actual subcooling temperature to the target value supplied by the manufacturer. The actual subcooling is the condenser saturation temperature (TCondenser, Saturation) minus the liquid line temperature (TLiquid).

E.   Weigh-in Charging Procedure

The weigh-in charging procedure involves charging the system by determining the appropriate weight of refrigerant based on the size of the equipment and refrigerant lines rather than by actual performance of the system. Systems utilizing the weigh-in procedure by the installer for any reason may not be third party verified by using sample groups.

The weigh-in procedure does not relieve the installer from having to ensure proper airflow.

There are two installer options for the weigh-in procedure. One involves the adjustment to the amount of refrigerant in a system by adding or removing a fraction of the refrigerant as specified by the manufacturer (weigh-in charge adjustment). The other involves evacuating the entire system and recharging it with the correct total amount of refrigerant, by weight (weigh-in total charge).

The weigh-in charge adjustment procedure may only be used when a new factory-charged outdoor unit is being installed and the manufacturer provides adjustment specifications based on evaporator coil size and refrigerant line size and length.

The weigh-in total charge may be used for any weigh-in procedure but still requires manufacturer’s adjustment specifications. Only the installer/technician may perform any kind of weigh-in procedure.

F.   Equipment Limitations

The Standards only specifically require verification of refrigerant charge for air cooled air conditioners and air source heat pumps. All other types of systems are not expressly exempt from the refrigerant charge requirements.  Certain portions of the requirements may still apply, such as the minimum system airflow requirement. The installer would have to verify with the manufacturer and confirm with the CEC. The installer must adhere strictly to the manufacturer’s specifications.

Variable refrigerant flow systems and systems such as mini-splits that cannot be verified using the standard approach must demonstrate compliance using the weigh-in method. Verification by the HERS rater can only be accomplished by simultaneous observation of the installer’s weigh-in process.

G.   HERS Rater Verification Procedures

When required by the Certificate of Compliance, HERS raters will perform third party field verification and diagnostic testing of refrigerant charge.  This may include the standard method, simultaneous observation of the weigh-in method, verification of minimum system airflow, and verification of installation of the measurement access hole.

The verification procedures are essentially identical for the rater and the installer except that the tolerances for passing the superheat and subcooling tests are less stringent for the rater’s test.  This is to allow for some variations is measurements due to instrumentation or test conditions (e.g., weather).

The following conditions prohibit verification using sample groups:

1.   Weigh-in method; or

2.   When the minimum airflow cannot be met despite reasonable remediation attempts. (See RA3.

As always, to be eligible for sampling, the system must first be verified and passed by the installer.  If sampling is not being used, the rater will perform the verification process only after the installer has charged the system according to manufacturer’s specifications.

H.   Winter Setup Procedures

Reference Appendix RA1 provides for the approval of special case refrigerant charge verification procedures when the equipment is specifically approved by the manufacturer for such procedures.  One such procedure is found in RA1.2.  It provides for a modification to the standard charge procedure when conditions make the standard charge method difficult.

The Standard Charge Verification Procedure (Section RA3.2.2 of the Reference Residential Appendices) calls for the outdoor temperature to be within the manufacturer's specified range.  When outdoor temperatures are below 70°F, the setup for the Standard Charge Verification Procedure must be modified in order to achieve the proper system pressure differential needed for the procedure.  (Note: the Standard Charge Verification procedure is generally allowed to be used down to 55°F without the Winter Setup; however, the 70°F requirement mentioned here is typical of most manufacturers’ requirements for the Winter Setup). The Winter Setup for the Standard Charge Verification Procedure (Winter Charge Setup) allows both installers and HERS Raters to utilize the Standard Charge Verification Procedure of RA3.2.2 in the winter.  Note that the Weigh-in Charging Procedure specified in Section RA3.2.3 may also be used, but only by the installer.

The Winter Charge Setup creates the right conditions at the unit being tested for outdoor temperatures above 37°F and below 71°F that allow the system to operate in the same range of pressure differences between the low side pressure and the high side pressure as occurs during warm outdoor temperatures.

1.   The Winter Charge Setup is used only for units equipped with variable metering devices, which include Thermostatic Expansion Valves (TXV) and Electronic Expansion Valves (EXV) for which the manufacturer specifies subcooling as the means for determining the proper charge for the unit, including units equipped with micro-channel heat exchangers. The Winter Charge Setup achieves an appropriate high side - low side pressure differential to conduct the Standard Charge Verification Procedure, by restricting the airflow at the condenser fan outlet through the use of a Condenser Outlet Air Restrictor. Once this pressure differential is achieved, the Variable Metering Device Calculations are conducted in the same way as the variable metering device procedures described in Reference Residential Appendix RA  All other applicable requirements of Section RA3.2.2 remain the same and must also be completed when using the Winter Charge Setup,


Though not specifically mentioned in the CID protocols of Residential Appendix RA3.4.2, the Winter Set Up Method detailed in RA1.2 may be used when normally allowed.  For purposes of CID verification the Winter Setup Method will be treated the same as the Subcooling Method.


I.    Utilizing Weigh-in Charging Procedure at Low Outdoor Temperatures 

When a new HVAC system in installed, for enforcement agencies to issue an occupancy permit, the HVAC installer must check the refrigerant charge and a HERS rater must verify the correct charge; however, EXCEPTION to Section 150.1(c)7A provides for an alternative third party HERS verification if the weigh-in method is used when the outdoor temperatures are less than 55 degrees F. 

Typically, when the weigh-in method used by the installing contractor to ensure proper refrigerant charge, a HERS rater must perform a charge verification in accordance to the procedures outlined in the Reference Residential Appendix RA3.2, which is the standard charge procedure described above in this chapter.  However, since the standards charge verification procedures (RA3.2) cannot be performed when the outdoor temperatures are less than 55 degrees; the standards provide the installer with two choices:

1.   Utilize the “HERS Rater - Observation of Weigh-In Charging Procedure” as prescribed in Reference Residential Appendix RA3.2.3.2, to demonstrate compliance, AND install an Occupant Controlled Smart Thermostat (OCST), or

2.   Wait for warmer temperatures and perform the standard charge verification procedure, which can delay the project. 

As noted above, when the HVAC installer elects this procedure for verification (RA3.2.3.2), the system thermostat must be an Occupant Controlled Smart Thermostat (OCST) which conforms to the requirements of Reference Joint Appendix JA5.