Discharge temperature
The temperature of refrigerant vapour leaving the compressor, measured on the discharge line. It is the hottest point in the circuit because it carries both the heat absorbed indoors and the work the compressor motor added, and manufacturers use it as a protective limit.
Also called discharge line temperature, compressor discharge temp
What is compressor discharge temperature?
Discharge temperature is measured on the line leaving the compressor and is the hottest point in a refrigeration circuit. It rises whenever returning refrigerant flow falls, which makes it a sensitive protective limit.
Discharge temperature: how it works
Returning vapour cools the compressor motor as it passes through, so the heat the motor sheds is carried away in the flow. Reduce that flow, by losing charge or restricting the circuit, and less heat leaves with it while the same electrical work is still being added. The result appears at the discharge line as a temperature climbing toward the limit the manufacturer set.
Where you meet the term
- In a protection code that stops a heat pump after it has been running rather than at startup.
- In a service report where the discharge reading is quoted alongside suction pressure to explain a charge decision.
- In cold-weather heating, when a system working at its widest temperature lift approaches the limit it clears easily in mild conditions.
How to check it
- Record how long the unit ran before it stopped. A limit reached after twenty minutes and one reached after ninety seconds point at different conditions, and only somebody present can supply that timing. Owner check
- Note the outdoor conditions at each stop. A limit reached only at weather extremes indicates a margin that has shrunk, while one reached in mild conditions indicates something more definite. Owner check
- Compare the sensor against the measured circuit. The discharge thermistor reading is checked against pressures and temperatures elsewhere on the circuit, which is how a drifted sensor is separated from a genuinely hot line. Technician
Discharge temperature: what it is not
- A high discharge temperature does not mean the compressor is at fault. It is a consequence of what the circuit around the compressor is doing, and the compressor is more often the victim than the cause.
- Adding refrigerant to bring a discharge temperature down treats the reading rather than the loss. The charge left the circuit somewhere, and it will leave again.
Discharge temperature questions
Why does discharge temperature rise when charge is low?
Less refrigerant returning means less mass to carry motor heat away, and suction vapour arriving warmer than designed enters compression hotter. Both push the temperature at the discharge line upward.
Is discharge temperature the same as head pressure?
They are related but distinct. Head pressure describes the condensing side of the circuit, while discharge temperature describes the state of the vapour at the compressor outlet and responds to flow as well as pressure.
Can a heat pump run close to its discharge limit safely?
Equipment is designed with margin between ordinary operation and the limit. A system habitually running near it has lost some of that margin, which is a condition to investigate rather than to accept.
Where this definition comes from
Written from service manual, manufacturer support article material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- Simple Self-Diagnosis by Malfunction Code (SM-TS3)Daikin After Sales Service Division chart assigning each malfunction-code letter division to the indoor unit, the outdoor unit, or the system.
- Mitsubishi Electric U2 diagnostic referenceMitsubishi Electric fault-code reference assigning U2 to an abnormally high compressor discharge temperature or a refrigerant shortage detection on the outdoor unit, listing the discharge thermistor, the refrigerant condition, and the expansion device among the checked items.