Fault diagnosis
Refrigerant circuit fault diagnosis (superheat and subcooling)
- where
- superheat minus target (K)
- measured superheat (K)
- target superheat (K)
- subcooling minus target (K)
- measured subcooling (K)
- target subcooling (K)
- evaporator temperature difference (K)
- air temperature entering the evaporator (°C)
- saturated evaporating temperature (°C)
- condenser split (K)
- saturated condensing temperature (°C)
- air temperature entering the condenser (°C)
Explain this equation
Compares your superheat and subcooling with their targets and, if you enter them, the evaporator TD and condenser split with their normal values. Each comes out low, normal or high, and the pattern is matched against the classic refrigerant-circuit faults to rank the most likely causes.
ΔSH and ΔSC show how far the charge and feed are from where the manufacturer wants them. TD is how much colder the evaporating refrigerant is than the air going over the coil: it rises when the coil is starved of refrigerant or air. CS is how much hotter the condensing refrigerant is than the air cooling it: it rises with a dirty condenser, overcharge or air in the system and falls with undercharge or a weak compressor.
The equations are simple differences; the diagnosis comes from the pattern. For example high superheat with low subcooling means too little refrigerant in circulation, whereas high superheat with high subcooling means refrigerant is there but held back by a restriction.
Sense check: an undercharged split system typically shows superheat several kelvin above target, subcooling near zero, a TD above normal and a condenser split below normal. A dirty condenser shows a high split with near-normal subcooling.
Watch out: always confirm airflow over both coils before changing the charge, and confirm the top-ranked fault with the listed checks. Two faults at once can hide each other.
Ranks the likely refrigerant-circuit faults from superheat and subcooling, the type of expansion device and, optionally, the evaporator temperature difference and condenser split. Each likely fault comes with the checks that confirm it and how to put it right.
Work out superheat and subcooling first with the calculator for your refrigerant, which also gives the evaporating and condensing saturation temperatures.
Enter your values to see the result.
Good to know
Bands used: superheat more than 3 K from target is high or low (below 2 K is always treated as low); subcooling more than 2 K from target; evaporator TD and condenser split more than 3 K from the normal figure you enter. Typical normal values: evaporator TD around 15–22 K for comfort cooling and 6–10 K for coldrooms; condenser split about 8–12 K for high-efficiency air-cooled units and 14–17 K for older ones. Use the manufacturer’s figures where you have them. Let the system run at a steady load for at least 15 minutes before taking readings. A TXV holds superheat steady over a wide range, so on TXV systems subcooling and the temperature differences carry more weight; on fixed-orifice systems superheat moves with charge and load.
Common mistake
Adding or removing refrigerant on superheat or subcooling alone without first checking airflow over both coils. Low evaporator airflow and a dirty condenser produce readings that look like charge faults.
Limits
A pattern-matching aid based on the published fault tables, not a substitute for a competent engineer’s diagnosis. It assumes a single fault, a steady load and correct readings; two faults together (for example an undercharged system with a dirty condenser) can mask each other. It does not use compressor current, discharge temperature or compression ratio, which you should also check. Bands are typical, not universal.
Sources
- ACHR News, The Professor (J. Tomczyk): Air conditioning system troubleshooting (inefficient compressor: high suction, low head, high superheat, low subcooling; non-condensables: high head and high subcooling)
- ACHR News, The Professor: Refrigerant undercharge
- ACHR News: Seven telltale signs of a system overcharged with refrigerant
- ACHR News, The Professor: Symptoms of air in a refrigeration system (normal superheat, high subcooling, high condenser split)
- ACHR News: Troubleshooting inefficient compressors
- ACHR News: More refrigerant flow problems (inefficient compressor, non-condensables, condenser entering air temperature)
- ACHR News, The Professor: Systematic troubleshooting of a restricted liquid line (high superheat, normal to high subcooling, low condenser split; 2 psi drier pressure drop)
- ACHR News: Understanding superheat (superheat = measured vapour temperature − saturation temperature at the same point)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the equipment manufacturer’s data, CIBSE guidance, BS EN 378, the F-gas Regulations and your company procedures. Refrigerant work needs the correct F-gas certification.