Compressor Overheating — Abnormal Discharge Temperature

A field reference for compressor faults — what causes it, how to confirm it on the unit, how to repair it, and what fails next if you leave it. Written for working HVAC & refrigeration technicians.

Compressor

From the field — what overheating leaves behind

Abnormal discharge temperature rarely announces itself — it shows up later, baked into the oil. These photos are from a semi-hermetic R-22 compressor on a cold-storage room that "still ran" but had quietly cooked itself.

The unit on site. It still started and ran, but capacity had fallen off and there was a faint metallic note under load — classic signs that discharge temperature had been running high for a long time. We pulled it rather than patch it.

The oil that came out. Healthy refrigeration oil is a clear amber; this is black and grainy. Oil burns this color when discharge temperature stays too high — low charge, poor condenser heat rejection, high superheat, or lost lubrication. Once the oil is cooked, metal is already wearing inside.

Overheating is a slow killer: by the time the oil looks like this, the bearings and valves are already on the clock. The cheap fix is a thermometer on the discharge line and the habit of logging it — catch the upward creep before it bakes the compressor.

How to confirm it on site

Discharge line surface > 120°C abnormal
Compressor casing too hot to touch
Suction superheat > 15°C
Suction/discharge pressure → compression ratio
Ambient temp > 40°C in machine room

Root causes

High suction superheat — low refrigerant or TXV blocked → motor cooling fails
High condensing pressure — fouled condenser, fan stop
Excessive compression ratio
Wrong refrigerant type
High ambient temperature — poor ventilation

If you leave it unrepaired

Discharge temp >120°C → oil degradation (normal 70–100°C)
Oil carbonization → sludge → system contamination
Winding insulation damage → eventual burnout
Internal protection trip → short-cycling → faster burnout
Refrigerant decomposition → acid → further damage

How to fix it

Address root cause: refrigerant/condenser/TXV
Verify condenser fan
Improve machine room ventilation
Test oil TAN → replace oil+drier if acidic
Severe: consider compressor replacement

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FAQ

What causes compressor overheating — abnormal discharge temperature?

High suction superheat — low refrigerant or TXV blocked → motor cooling fails High condensing pressure — fouled condenser, fan stop Excessive compression ratio Wrong refrigerant type High ambient temperature — poor ventilation

How do I diagnose compressor overheating — abnormal discharge temperature on site?

Discharge line surface > 120°C abnormal Compressor casing too hot to touch Suction superheat > 15°C Suction/discharge pressure → compression ratio Ambient temp > 40°C in machine room

How do I fix compressor overheating — abnormal discharge temperature?

Address root cause: refrigerant/condenser/TXV Verify condenser fan Improve machine room ventilation Test oil TAN → replace oil+drier if acidic Severe: consider compressor replacement

What happens if compressor overheating — abnormal discharge temperature is left unrepaired?

Discharge temp >120°C → oil degradation (normal 70–100°C) Oil carbonization → sludge → system contamination Winding insulation damage → eventual burnout Internal protection trip → short-cycling → faster burnout Refrigerant decomposition → acid → further damage