HVAC Technician Academy
Learn how to diagnose refrigeration systems using superheat, subcooling, suction pressure, discharge pressure, saturation temperatures, TXV behavior, restrictions, charge patterns, airflow, and compressor performance.
Do not diagnose from one gauge pressure. First verify airflow and system operating conditions. Then measure suction and liquid-side pressures, convert them to saturation temperatures, measure line temperatures, calculate superheat and subcooling, identify the metering device, and interpret the complete pattern using manufacturer data.
Low suction pressure does not automatically mean low refrigerant.
High head pressure does not automatically mean overcharge.
High superheat does not identify one specific failure.
Refrigeration diagnostics becomes accurate when several measurements agree with the same failure pattern.
Step 1: Understand saturation temperature and pressure-temperature relationships
Step 2: Learn superheat and subcooling
Step 3: Learn suction-pressure patterns
Step 4: Learn head-pressure patterns
Step 5: Learn high and low superheat/subcooling combinations
Step 6: Separate charge problems from restrictions
Step 7: Understand TXV and fixed-orifice behavior
Step 8: Evaluate compressor pumping only after airflow and charge are verified
Learn how overfeeding, low airflow, charge, and metering-device behavior can create this pattern.
High Superheat + High Subcooling →Separate restrictions and starved evaporator conditions from simple refrigerant undercharge.
High Superheat + Low Subcooling →Study the classic low-refrigerant-inventory pattern and the faults that can imitate it.
High Head Pressure →Diagnose condenser airflow, overcharge, non-condensables, restrictions, and high outdoor load.
High Suction Pressure →Separate high evaporator load, overfeeding, compressor pumping problems, and airflow effects.
Low Suction Pressure →Diagnose low airflow, restrictions, low load, low charge, and metering problems without guessing.
Learn why fixed-orifice and TXV systems use different charging methods and why airflow comes first.
TXV Problems & Symptoms →Review TXV starvation, overfeeding, sensing-bulb problems, and restriction symptoms.
Refrigerant Restriction Symptoms →Recognize restriction patterns and avoid adding refrigerant to a starved evaporator.
Overcharged AC Symptoms →Understand elevated subcooling, head pressure, compressor load, and efficiency loss.
Possible direction: Low refrigerant inventory or evaporator starvation.
Must also check: Airflow, leaks, load, and metering device.
Possible direction: Restriction with refrigerant backed up upstream.
Must also check: TXV, filter-drier, liquid line, and temperature drop.
Possible direction: Overfeeding, low airflow, or possible overcharge.
Must also check: Airflow and metering-device control.
Possible direction: Low evaporator load or starvation.
Must also check: Airflow before refrigerant.
Possible direction: High load, low compression differential, or overfeeding.
Must also check: Compressor, TXV, airflow, and operating stage.
The evaporator refrigerant circuit reacts directly to the amount of heat transferred from indoor air.
If airflow is low:
This is why charge should never be adjusted until airflow is known to be acceptable.
Continue: Airflow & Static Pressure Training
Superheat helps reveal how much the refrigerant vapor has been heated above saturation after boiling.
High superheat can support evidence of a starved evaporator.
Low superheat can support evidence of an evaporator being heavily fed.
But the value must be interpreted according to metering-device type and system conditions.
Subcooling measures how far liquid refrigerant has been cooled below condensing saturation temperature.
It can help evaluate how much liquid refrigerant is being held in the condenser and liquid line.
On many TXV systems, manufacturer target subcooling is the primary field charge reference.
Both can starve the evaporator.
But their high-side behavior often differs.
A restriction may cause refrigerant to accumulate upstream, producing elevated subcooling, while an undercharged system may show low refrigerant inventory and lower subcooling.
High-side pressure reacts strongly to heat rejection.
Check:
Do not condemn a compressor because suction and discharge pressures look unusual until:
Continue: Compressor Compression Ratio & Pumping Diagnostics
Field reference for elevated suction pressure diagnosis.
High Head Pressure Guide →Field reference for condenser and high-side problems.
Low Refrigerant Symptoms →Separate true low charge from similar failure patterns.
Restriction Symptoms →Diagnose starved evaporators without mischarging the system.
Refrigeration diagnosis can involve high refrigerant pressure, hot discharge lines, rotating fans, energized electrical components, and refrigerant handling requirements. Use appropriate PPE and rated instruments, verify refrigerant type, and follow manufacturer procedures and applicable refrigerant regulations.
Airflow restrictions, metering-device faults, dirty coils, refrigerant charge, compressor performance, and load conditions can produce overlapping pressure symptoms. Diagnose the complete system before adding refrigerant.
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