HVAC Technician Academy
Learn how to evaluate whether an HVAC compressor is actually moving refrigerant properly using suction pressure, discharge pressure, compression ratio, amp draw, temperature, and full-system operating conditions.
A compressor should not be condemned from amp draw or one pressure reading alone. A weak-pumping compressor is diagnosed by evaluating whether it can maintain the expected pressure differential and refrigerant mass flow under the actual operating load, while also ruling out charge problems, airflow problems, metering-device faults, restrictions, and control issues.
The compressor draws low-pressure refrigerant vapor from the evaporator and raises it to a higher pressure for the condenser.
In simple terms, a healthy compressor should be able to maintain an appropriate pressure relationship between the low side and high side under the current load.
Compression ratio compares absolute discharge pressure to absolute suction pressure.
Gauge pressure must be converted to absolute pressure before calculating the ratio.
At approximately sea level:
Atmospheric pressure varies with elevation and weather, so use the proper value when accuracy matters.
Suction gauge pressure: 120 psig
Discharge gauge pressure: 360 psig
Approximate suction absolute: 134.7 psia
Approximate discharge absolute: 374.7 psia
Compression ratio: 374.7 ÷ 134.7 ≈ 2.78:1
The number itself is not a universal pass/fail value. It must be interpreted for the refrigerant, equipment, load, and operating condition.
Higher compression ratio generally increases compressor workload and discharge temperature.
Excessive ratio can result from conditions such as:
The compressor may be healthy but operating under a damaging system condition.
A compressor with damaged internal valves, scrolls, or pumping elements may lose its ability to create the expected pressure differential.
Possible signs can include:
Before condemning the compressor, rule out:
Compressor current changes with:
A compressor drawing below nameplate RLA is not automatically weak.
RLA is a nameplate reference value, not a universal target for normal operating current.
Continue: Compressor Amp Draw: RLA vs Actual Amps
High amperage can result from high compressor load rather than internal compressor damage.
Investigate:
Low amp draw can occur with:
Current must be interpreted with pressures, temperatures, load, and system configuration.
Excessive compression ratio and low suction conditions can drive discharge temperature higher.
Abnormally high discharge temperature may contribute to:
Compare temperatures with manufacturer guidance and the complete operating condition.
Compressor diagnosis should not stop at the gauges.
Evaluate:
Some equipment and compressor designs should not be subjected to arbitrary pump-down or deep-vacuum tests while operating.
Never force a compressor into an operating condition outside manufacturer procedures simply to "see how low it will pull."
Use approved diagnostic methods for the specific equipment.
Technician Case File
Suction: higher than expected.
Head: lower than expected.
Initial diagnosis: weak compressor.
Further testing: indoor blower is running at an incorrectly high airflow setting and system load is unusually low.
After airflow and operating conditions are corrected: pressure relationship changes significantly.
Lesson: A pressure pattern must be interpreted in the actual load context.
Airflow: verified.
Charge: verified.
Metering device: operating appropriately.
Suction: remains abnormally high for the load.
Discharge: fails to build as expected.
Delivered capacity: very low.
With other major causes eliminated, weak compressor pumping becomes a much stronger diagnosis.
Compressor amps: high.
Head pressure: extremely high.
Outdoor coil: heavily restricted.
Condenser fan: underperforming.
The compressor is heavily loaded because the condenser cannot reject heat. High current is a symptom of the operating condition, not proof of internal compressor failure.
1. Identify equipment, compressor type, refrigerant, and operating stage.
2. Verify indoor airflow.
3. Verify outdoor coil and condenser airflow.
4. Verify refrigerant charge and metering-device operation.
5. Measure suction and discharge pressures accurately.
6. Convert to saturation temperatures.
7. Calculate superheat and subcooling.
8. Evaluate compressor amp draw and supply voltage.
9. Calculate compression ratio when useful.
10. Evaluate temperature change and delivered capacity.
11. Rule out heat-pump reversing-valve bypass where applicable.
12. Condemn the compressor only when the complete evidence shows inadequate pumping performance.
| Pattern | Possible Direction | Next Check |
|---|---|---|
| High suction + low head + poor capacity | Possible weak pumping | Rule out load, airflow, valve, charge, and stage issues |
| Low suction + high head | High compression ratio / system restriction | Airflow, condenser, metering device, charge |
| High amps + high head | Heavy compressor load | Condenser airflow and refrigerant conditions |
| Low amps + low pressures | Low load or low mass flow | Charge, restriction, operating stage |
| Normal pressures + poor room comfort | May not be compressor problem | Airflow, ducts, psychrometrics, delivered capacity |
A running compressor is not automatically a good compressor — but unusual pressures do not automatically make it bad. Prove pumping performance after the rest of the system is verified.
1. Why must absolute pressure be used for compression-ratio calculations?
2. Does low compressor amp draw automatically prove weak pumping?
3. What pressure pattern can support investigation toward weak compressor pumping?
4. Why should airflow and charge be verified before condemning a compressor?
5. What is the strongest general rule for compressor condemnation?
1. Compression ratio compares absolute pressures, not gauge pressures.
2. No. Low current can result from low load, low refrigerant mass flow, undercharge, restriction, or low-capacity operation.
3. Higher-than-expected suction with lower-than-expected discharge and poor delivered capacity can support that diagnosis after other causes are ruled out.
4. Airflow, charge, load, and metering faults can create pressure patterns that imitate compressor problems.
5. Condemn the compressor only after proving that the complete system is correctly configured and the compressor still cannot produce expected pumping performance.
Return to the compressor training hub.
Compressor Amp Draw →Interpret RLA and actual current correctly.
Amp Draw Field Guide →Review the existing technical compressor amp-draw resource.
Compressor testing can involve high refrigerant pressure, hot discharge lines, rotating fans, energized line-voltage circuits, and high starting current. Use properly rated instruments and PPE, follow manufacturer procedures, and do not force compressors into abnormal pump-down or deep-vacuum operating conditions unless explicitly approved by the equipment manufacturer.
Pressure readings, amp draw, refrigerant charge, airflow, metering devices, and delivered capacity all need to be evaluated before a compressor is condemned.
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