What is the best method of properly charging an air conditioning system? Technicians need to know all of the methods that can be used. Each method has a particular use depending on the type of equipment and the make and model. No one method is right for all circumstances. If you are ever in doubt, consult the manufacturer’s installation instructions. Remember, overcharging and undercharging can be detrimental to the equipment, a disadvantage for the customer economically, and can create callbacks for the contractor.

1) What is the most accurate method for charging an air conditioning system?

A) Superheat

B) Subcooling C) Weigh-in D) Frost-line

2) When using the weigh-in method of system charging, what must be known?

A) Total line set capacity

B) Liquid line capacity only C) Suction line capacity only D) Hot gas line capacity only

3) When charging a system by the superheat method, what two measurements are required?

A) Liquid line temperature and liquid line pressure

B) Suction line temperature and evaporator leaving air temperature C) Suction line temperature and suction line pressure D) Evaporator entering air temperature and leaving air temperature

4) Air conditioning systems that use thermostatic expansion valves can use which of the following methods for accurate charging?

A) Superheat

B) Subcooling C) Weigh-in D) Frost-line

5) What is the sub-cooling of an R-22 air conditioning system operating with a liquid line temperature of 110°F and a high side pressure of 260 psig?

A) 0°F

B) 6°F C) 10°F D) 150°F

6) What is the superheat of an R-22 air conditioning system operating with suction line temperature of 52°F and a low side pressure of 62 psig?

A) 10°F

B) 12°F C) 14°F D) 16°F


2) A Weighing in a charge is the most accurate method of system charging when the required full charge is known. Installation literature provides instructions on the amounts of refrigerants to be added or removed from an installation depending on the length of the line set. These calculations are based on the volume internally of the tubing. The manufacturers provide this information on proper charging so that the equipment will operate at the optimum level.

3) C The superheat method of charging requires knowing the evaporating refrigerant temperature determined from the suction pressure and comparing it to the suction line temperature. The amount of sensible heat added to the refrigerant after the saturation is called superheat. Manufacturers determine the proper superheat for the particular system under changing load and ambient operation. The installer is furnished this information in a graph or table listing the various ambient operation. The installer is furnished this information in a graph or table listing the various inputs (suction pressure, wetbulb temperature, drybulb temperature, ambient temperature, head pressure.) The technician locates this information on the supplied materials to determine the proper charge.

4) B The Subcooling method of charging is similar to superheat but uses the temperature of the liquid refrigerant from the condenser and compares it to the temperature of the refrigerant at at the condensing pressure. Again, the manufacturers supply charts and graphs particular to their piece of equipment. The technician will follow a procedure similar to that used in superheat charging.

5) C

6) D Knowing how to calculate both superheat and subcooling is a critical function for the technician. He should always consult a temperature-pressure chart of the particular refrigerant and the manufacturer’s literature to determine the proper superheat or subcooling that will result in proper system charge.

Publication date: 01/29/2001