Vapour compression system analysis

Turn field readings into a clear diagnostic direction.

Enter whatever reliable measurements are available. CycleCheck derives compatible missing values, compares the cycle against the selected application, and ranks faults worth investigating.

Use stable operating data Confirm airflow, clean coils, correct fan operation, and at least 10-15 minutes of run time before judging charge.

Call setup

Expected ranges and approach calculations change with the application and equipment.

Live analysis

Application Metering device Condenser type Temperature unit System condition Job / site reference Equipment ID Refrigerant

GUIDE

A repeatable diagnostic process

Use patterns across the air side and refrigerant side instead of making a decision from one number.

1. Stabilize before judging charge

Confirm clean heat exchangers, correct fan rotation and speed, open dampers and valves, and a representative load. Allow pressures and temperatures to settle. Pull-down and near-setpoint operation are valid conditions, but their expected ranges are different.

2. Match pressure and pipe temperatures

Superheat and subcooling are only meaningful when pressure and line temperature represent the same circuit location and operating moment. Select the correct refrigerant and use dew/bubble values correctly for blends.

3. Use wet-bulb where moisture matters

Return and supply wet-bulb readings show total cooling response, including latent load. Outdoor wet-bulb is the correct ambient reference for an evaporative condenser; air-cooled equipment normally uses entering dry-bulb.

4. Separate look-alike faults

High superheat can result from low charge, a restriction, low airflow, or high load. Subcooling, coil temperature differences, current, and non-invasive temperature drops help separate those causes before the sealed system is disturbed.

Field readings

Blank fields are allowed. Blue values are inferred from compatible measurements.

Air sideSensible and latent load at the evaporator

Outdoor dry-bulb °F Shade the probe from sun; use air entering the condenser. Outdoor wet-bulb °F Ambient moisture load; approach reference for evaporative condensers. Return dry-bulb °F Mixed air entering the evaporator, away from radiant surfaces. Return wet-bulb °F Indoor latent load; useful for fixed-orifice charging checks. Supply dry-bulb °F Average leaving air after the coil, before duct heat gain if possible. Supply wet-bulb °F Use with return wet-bulb to confirm total cooling response. Dry-bulb split °F Return DB minus supply DB. Interpret with airflow and humidity. Wet-bulb split °F Return WB minus supply WB; a broad total-cooling indicator.

Air-side measurement notes

Take return and supply readings at representative points, not directly in the coil's coldest discharge stream. A high dry-bulb split alone does not prove good airflow: restricted airflow can increase split while reducing capacity. Wet-bulb readings help identify when a small dry-bulb split is explained by heavy latent load.

Low sideEvaporator, superheat, and suction-line heat gain

Saturated suction temp °F Convert suction pressure using the correct refrigerant dew value. Evaporator outlet suction °F Clamp on clean pipe near the evaporator outlet; insulate the probe. Evaporator superheat °F Evaporator outlet line temperature minus SST. Evaporator TD °F Return dry-bulb minus SST; a load and coil-temperature indicator. Compressor inlet suction °F Suction-line temperature near the compressor service valve. Suction-line heat gain °F Compressor inlet minus evaporator outlet temperature.

Low-side measurement notes

Evaporator superheat describes the coil outlet; total superheat at the compressor helps assess floodback risk and suction-line heat gain. Excessive line gain can point to missing insulation, a hot routing, or a long line set and should not be mistaken for evaporator superheat.

High sideCondenser approach, heat rejection, and liquid condition

Saturated condensing temp °F Convert high-side pressure using the correct refrigerant bubble value. Liquid line temp °F Measure near the liquid pressure reference; insulate the probe. Liquid subcooling °F SCT minus liquid temperature at the same pressure reference. Condensing split °F SCT minus condenser entering dry-bulb. Condenser leaving air DB °F Average air leaving the coil; avoid the fan motor heat plume. Condenser air rise °F Leaving air DB minus entering outdoor DB.

Condenser approach and heat-rejection notes

Air-cooled: SCT minus entering dry-bulb is commonly called condensing split or CTOA. Evaporative: SCT minus entering wet-bulb is the useful approach. Water-cooled: SCT minus leaving condenser-water temperature is the refrigerant-to-water approach. Keep the condenser type correct so the app uses the right reference.

Supporting checksOptional non-invasive evidence

Discharge line temp °F Near the compressor discharge; use the same location each visit. Liquid temp before drier °F Clamp immediately upstream of the filter-drier. Liquid temp after drier °F Clamp immediately downstream with a matched probe. Filter-drier temp drop °F Upstream minus downstream; a meaningful drop supports restriction. Compressor current A Measured operating amps under the recorded load. Rated load amps A Compressor nameplate RLA; not a target operating current. Current vs RLA % Operating amps divided by RLA.

Supporting non-invasive check notes

Use matched, insulated pipe probes and allow them to settle. A filter-drier temperature drop is supporting evidence, not proof by itself; confirm pressure drop or flash gas where practical. Compare compressor current with expected load, voltage, and manufacturer performance data rather than treating RLA as a target amperage.

Technician observations

Diagnostic direction

Enter readings to begin.

+ Waiting for field data

Start with ambient, saturation temperatures, and matching line temperatures when available.

Diagnostic aid, not a charging specification. Broad field ranges overlap between equipment designs. Use manufacturer data when available. Do not adjust charge until airflow, load, coil condition, measurement accuracy, and condenser heat rejection are confirmed.