How to Calculate Superheat and Subcooling for HVAC and Refrigeration Systems

What is Superheat

Superheat is the measured value of the difference between the actual temperature of a refrigerant vapor and the saturation temperature of that refrigerant at the same pressure.  Another way to describe it is the temperature of a vapor above it’s boiling temperature at a given pressure.

Ensuring proper superheat in HVAC and refrigeration systems is very important.  Correct superheat ensures that the compressor is not being flooded.  Flooding occurs when liquid refrigerant makes its way back to the compressor.  This can cause lubrication issues and could damage the compressor.

How to Calculate Superheat

Tools Needed

• Refrigeration gauge manifold set with hoses.
• Pipe thermistor
• PT Chart

Obtain Superheat

• Connect your gauge manifold the system and record your low side pressure.
• Connect your pipe thermistor to the suction line as close to the condenser as possible and at least 6 inches from the compressor.
• Using your PT Chart convert your low side pressure from step 1 to temperature.
• The difference between these 2 temperature readings is your calculated superheat.

(Suction line temp – Saturation temp = Superheat)

• Consult manufacturer for desired superheat.

High Superheat = Starved Evaporator

Low Superheat = Flooded Evaporator

What is Subcooling

Subcooling is the measured value of the amount the refrigerant liquid is cooled before exiting the condenser. 

How to Calculate Subcooling

Obtain Subcooling

• Connect your gauge manifold the system and record your high side pressure.
• Connect your pipe thermistor to the liquid line as close to the evaporator as possible and before the metering device.
• Using your PT Chart convert your high side pressure from step 1 to temperature.
• The difference between these 2 temperature readings is your calculated subcooling.  (Saturation temp – Liquid Line temp = Subcooling)

• Consult manufacturer for desired subcooling.

                High Subcooling = Flooded Condenser

                Low Subcooling = Starved condenser

Diagnosing your system using superheat and subcooling

                High Superheat and Low Subcooling

                                Your refrigeration system is undercharged.

                Low Superheat and High Subcooling

                                Your refrigeration system is overcharged.

                High Superheat and High Subcooling

                                You have a possible blockage within the refrigeration system.

                Low Superheat and Low Subcooling

                                Metering device is either too large or not installed.