Superheat and Subcooling Measurements

Superheat and subcooling are two phenomenon that will occur to a medium that boils and condenses, respectively.

Air conditioning technician will use measurement of these properties during troubleshooting and maintenance, to gauge performance of an air conditioning system.


Superheat is defined as

The temperature rise beyond boiling temperature of a medium.


Whereas subcooling is

The temperature fall beyond the condensing temperature of a medium.


Significance of these two properties:

It is important for refrigerant leaving the condenser to be in saturated liquid state.

Hence, cooling beyond saturation point is required to ensure refrigerant to be in saturated liquid state.

A good subcooling is about 5.5 oC (10 oF).

This measurement is used for capillary tube fitted systems.

Most air conditioner evaporators designed for residential, office building, and vehicles will require that all refrigerant returning to the compressor, is in vapour state.

Therefore, superheat factor is important. Amount of this property is similar to that of subcooling, and measured for thermostatic expansion valve (TXV) fitted systems.


1. Measuring refrigerant superheat:

Steps for measuring this begins with,

  1. know the handled refrigerant in your air conditioning system,

  2. operating the air conditioning unit for and wait a few minutes, then,

  3. attaching the compound pressure gauge (in a combination pressure gauge set) to the suction line service valve,

  4. followed by purging the connected line,

  5. use a thermometer to measure the line temperature at the feeler bulb of thermostatic expansion valve,

  6. read off the suction line pressure from the gauge, and add about 2 psi (~ 0.14 bar) to the reading to compensate for pressure drop from the evaporator to the service valve.

    Please note that this addition is only applicable for small systems

  7. read off the corresponding saturation temperature on the compound gauge at the pressure in (6).

    Most pressure gauge sets for air conditioner servicing will have a set of saturation temperature values for different refrigerants.

    You can, however, use p-H (pressure and enthalpy) chart for refrigerants to read off the corresponding saturation temperature

  8. then, subtract (7) from (5), and finally,

  9. superheat can be increased by adjusting TXV’s adjusting knob towards closed position, and decreased by turning the knob in the opposite direction

Setup for checking superheat of an air conditioning system. Condenser picture courtesy of Bryant Air Conditioners
Courtesy for condenser picture: Bryant Air Conditioners


2. Measuring refrigerant subcooling:

Begins with,

  1. know the handled refrigerant in your air conditioning system,

  2. operating the air conditioning unit and wait for a few minutes, followed by,

  3. attaching the plain pressure gauge (in a combination of pressure gauge set) to the liquid line service valve,

  4. followed by purging the connected line,

  5. then use a thermometer for measuring the tube temperature near the service valve,

  6. read off the discharge line pressure from the gauge,

  7. read off the corresponding refrigerant saturation temperature on the gauge,

  8. subtract (5) from (7), and you’ll have the subcooling figure

Setup for checking subcooling of an air conditioning system. Condenser picture courtesy of Bryant Air Conditioners
Courtesy for condenser picture: Bryant Air Conditioners

Digital measuring equipments are widely used nowadays for measuring these values.


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