Expo HVAC Technician Academy
Learn how to evaluate compressor pumping performance using absolute pressures, compression ratio, suction and discharge behavior, amp draw, temperature, airflow, refrigerant conditions, and delivered system capacity.
Compressor pumping ability cannot be proven from suction pressure, head pressure, amp draw, or compression ratio alone. First verify airflow, refrigerant charge, metering-device operation, system load, operating stage, and condenser conditions. Then evaluate the compressor using the complete operating pattern.
Compression ratio compares the compressor discharge pressure with suction pressure using absolute pressure, not gauge pressure.
Compression Ratio = Discharge Absolute Pressure ÷ Suction Absolute Pressure
HVAC gauges normally display pressure in psig. Compression-ratio calculations require absolute pressure, so atmospheric pressure must be added to the gauge reading.
Approximate field conversion:
PSIA = PSIG + local atmospheric pressure
Atmospheric pressure changes with elevation and weather, so use appropriate local atmospheric pressure when precision matters.
Gauge pressure references atmospheric pressure. A compressor, however, responds to the actual pressure relationship between the suction and discharge sides.
Dividing one gauge pressure directly by another can produce a misleading compression ratio.
There is no single compression-ratio number that proves every HVAC compressor is good or bad.
Compression ratio changes with:
Use the ratio as one diagnostic measurement inside a complete system evaluation.
A compressor that has lost pumping efficiency may produce a pattern such as:
But that pattern alone does not prove compressor failure.
Similar symptoms can result from airflow, refrigerant, staging, metering-device, load, or heat-transfer problems.
Before condemning the compressor, verify the rest of the system.
High suction pressure can occur with a weak compressor, but it can also result from:
Suction pressure must be interpreted with saturation temperature, superheat, airflow, load, and equipment data.
Lower-than-expected discharge pressure can support a weak-pumping diagnosis, but only after evaluating:
Never diagnose compressor pumping ability using head pressure alone.
Compressor current is another supporting measurement.
Current changes with:
A compressor that is doing less work may draw less current than expected, but low amperage alone does not prove a weak compressor.
Discharge temperature provides additional information about compressor loading and refrigeration conditions.
Elevated discharge temperature may be associated with conditions such as:
Compare temperature measurements with manufacturer limits and the complete system condition.
Compressor diagnosis should ultimately connect to system performance.
If pressures appear unusual, determine whether the system is actually delivering expected cooling or heating capacity.
Useful measurements may include:
| Observation | Diagnostic Direction |
|---|---|
| Higher suction + lower discharge | May support reduced compressor pumping after system causes are eliminated |
| High suction + normal/high head | Evaluate load, airflow, refrigerant flow, and metering behavior |
| Low suction + high head | Investigate restrictions, airflow, charge, and heat rejection |
| Low amps | Evaluate actual compressor loading; not proof of failure |
| Poor capacity with reduced pressure differential | Compressor pumping becomes a stronger possibility after other causes are ruled out |
1. Verify complaint and operating mode.
2. Verify compressor stage or speed.
3. Verify indoor airflow.
4. Verify condenser airflow and heat rejection.
5. Measure suction and discharge pressures.
6. Convert pressures to saturation temperatures.
7. Measure superheat and subcooling.
8. Verify refrigerant charge and metering-device behavior.
9. Calculate compression ratio using absolute pressure.
10. Measure voltage and compressor current.
11. Evaluate compressor temperature.
12. Verify delivered system capacity.
13. Condemn the compressor only when the complete evidence supports it.
Complaint: AC runs continuously and cannot maintain indoor temperature.
Initial observation: Suction pressure appears high and discharge pressure appears lower than expected.
Initial suspicion: Weak compressor.
Further testing:
Lesson: A weak compressor diagnosis becomes defensible only after system conditions that can imitate weak pumping have been tested and eliminated.
Never condemn a compressor because one pressure, one amp reading, or one compression-ratio calculation looks wrong. Prove the system first.
1. Should compression ratio be calculated using gauge pressure?
No. Compression ratio should be calculated using absolute suction and discharge pressures.
2. Does high suction pressure prove a compressor is weak?
No. High suction pressure can result from several system and load conditions.
3. Is RLA the compressor's normal target amperage?
No. RLA is a nameplate reference value and should not be treated as a universal operating target.
4. What should be checked before diagnosing weak compressor pumping?
Verify airflow, charge, metering-device behavior, heat rejection, operating stage, system load, and other system conditions.
5. Why is delivered capacity useful in compressor diagnosis?
It helps determine whether unusual pressures are actually associated with reduced system performance.
Compressor diagnostics can involve energized high-voltage circuits, pressurized refrigerant systems, hot discharge piping, and rotating equipment. Qualified technicians should use appropriate PPE, properly rated instruments, manufacturer service procedures, and approved refrigerant-handling practices.
The Expo HVAC Technician Academy teaches technicians to combine electrical, refrigeration, airflow, and performance measurements before making expensive compressor replacement decisions.
Compressor Diagnostics Training HVAC Technician Academy