Reading Superheat and Subcooling

Why it matters

Superheat and subcooling tell you whether the refrigerant charge and expansion device are behaving correctly. They are the two most useful numbers you can pull from a system.

Superheat

Superheat is the temperature added to refrigerant above its boiling point. It tells you whether the evaporator is being fed correctly.

• Low superheat → too much refrigerant entering the evaporator → risk of floodback to the compressor.
• High superheat → too little refrigerant → starving the evaporator, poor capacity.

How to measure

1. Measure suction line surface temperature near the compressor.
2. Read suction pressure and convert to saturation temperature for your refrigerant.
3. Superheat = line temp − saturation temp.

Subcooling

Subcooling is the temperature removed from refrigerant below its condensing temperature. It confirms the condenser is doing its job and liquid is reaching the metering device.

• Low subcooling → likely undercharged or restricted condenser.
• High subcooling → overcharged or a restriction downstream.

How to measure

1. Measure liquid line temperature at the outlet of the condenser.
2. Read discharge pressure and convert to saturation temperature.
3. Subcooling = saturation temp − line temp.

Rule of thumb

Always measure both together. A single number can mislead you — the two readings cross-check each other and point you at the real fault.