HVAC For Beginners

Air Conditioner Charging

With this air conditioner charging guide, you get the knowledge to check the refrigerant level in an air conditioner and add refrigerant if required. This guide is for central air conditioning systems that use R-22 refrigerant which is common in older units.

AC Charging

**NOTE** In the U.S.A., you must have an EPA REFRIGERANT USAGE CERTIFICATION to purchase refrigerant and repair systems containing ozone depleting refrigerants (CFC or HCFC).


Types Of Refrigerant

There are several types of refrigerants and they are classified by chemical makeup and their ozone depleting potential. Older units will probably contain R-22 which is classified as a CFC which is considered an ozone depleting chemical. R-22 is scheduled to be phased out of use in new units but will be available for repairs for many years. Newer units may contain R-410a which is classified as an HFC which is not considered to be an ozone depleting chemical. Other refrigerants are available but those two are the most common. Some substitutes are available to replace R-22 in existing units but that is normally a job for a professional.

Before checking the charge

Prior to air conditioner charging, it is very important that the routine maintenance has been performed on the system. The air filter, blower wheel, evaporator coil, and condenser coil must be clean. If any of the above are dirty, it can cause symptoms similar to a unit that is low on refrigerant. If refrigerant is added under these conditions, the unit may be damaged. Air conditioner charging should not be performed when outdoor temperatures are below 55F.

Air Conditioner Charging, Tools Required

Refrigerant Charging Guages

Some tools are required in the process of air conditioner charging. A set of gauges is attached to the system to measure operating pressures. They consist of a manifold with a high and low pressure gauge as well as hoses to connect to the system's ports. The gauge sets are manufactured for specific refrigerants therefore you must ensure that you have the proper set for the refrigerant in your system. They contain a scale with pressure and temperature values on it. The temperature at a given pressure mark is referred to as the saturation temperature.

You will also need a device to measure temperatures on the pipes that contain refrigerant as well as a thermometer to measure air temperatures. To measure the temperatures on the pipes you can use a thermocouple based meter or an infrared thermometer.

Infrared Thermometer

Charging procedure

To check the charge in your unit, you should begin by shutting the unit off. The hoses from the gauge manifold can then be attached to the pressure ports on the system. The low pressure hose is normally blue and is connected to the suction line which is the larger of the two pipes. The high pressure hose is typically red and is connected to the liquid line which is the smaller of the two pipes.

The next step is to turn the unit on and allow it to run for at least 15 minutes for the system to reach steady state operation. Then, you should measure the outdoor air temperature, the return air temperature at the furnace/air handler, the suction line temperature, and the liquid line temperature.

Most newer units have a label on the inside of the air conditioner electrical compartment cover. This will give you specific instructions for your unit. It will typically tell you to measure superheat or subcooling. It will also give you a chart that has values for superheat or subcooling for a given outdoor temperature.

Superheat is heat that is added to the refrigerant that causes the temperature of the refrigerant to rise above its saturation temperature. It is found by measuring the suction line temperature and subtracting the low pressure gauge temperature reading from it. To increase superheat in system, refrigerant is removed. To decrease superheat, refrigerant must be added.

Subcooling is heat that is removed from the refrigerant that causes the temperature of the refrigerant to go below its saturation temperature. It is normally found by measuring the liquid line temperature and subtracting it from the value shown on the high pressure gauge reading. To increase subcooling, refrigerant is added to the system. To decrease subcooling, refrigerant must be removed.

If you do not have a charging procedure/chart in your unit, you will need to determine what type of metering device your system uses. This will either be a thermostatic expansion valve or a restrictor orifice.

If your system uses a thermostatic expansion valve (TXV), you can use the following values: The system superheat should normally be about 18 F. The system subcooling should typically be about 25 F.

If your system uses a restrictor, you can use the following table for your air conditioner charging. It contains recommended superheat values for given indoor and outdoor air temperatures.

Superheat VS Indoor Temp
55 60 65 70 75 80 85 90 95 100 105 110 115
70 21 17 14 10 6 5 5 5 5 5 5 5 5
75 25 22 19 16 13 11 8 5 5 5 5 5 5
80 29 27 24 22 19 17 15 12 10 7 5 5 5
85 33 31 29 27 25 23 20 18 16 14 12 9 7
90 38 36 34 32 31 29 27 25 23 22 20 18 16
95 42 40 39 37 36 34 33 31 29 28 26 24 23

The numbers on the top correspond to the outdoor temperature that you measure. The numbers on the far left correspond to the return air temperature. (All temperatures are dry bulb values)

To find the required superheat, find the return air temperature on the left. Then, follow the line to the right and stop on the column at the outside air temperature measurement. The number where they meet is the required superheat.

Why do you have to add refrigerant?

If your unit requires adding refrigerant, you should check for evidence of leaks and repair them. Some typical places to find leaks are the unit pressure ports, weld/braze connections, or places where rubbing has produced a hole in the tubing.

**HOT TIP** If an ac technician tells you that an air conditioner recharge is normal after a certain amount of time, ask them to leave. They do not know what they are talking about and you do not want them doing any ac repair.

The method of air conditioner charging you use will depend on the type of refrigerant in your system. R-22 is normally added in the vapor state (with the container right side up) to the suction line of the system. Blended refrigerants, such as R-410a, are normally added in the liquid state by turning the container upside down.

In either case, the refrigerant should be added slowly in small amounts. Sufficient time (typically 5 minutes) should be allowed between adding amounts to allow the system temperatures and pressures to stabilize. Temperature and pressure readings should then be checked to decide if more refrigerant is required.

You should complete your air conditioner charging by observing a complete cooling cycle. Then, the gauges should be removed with the unit not running.