The useful approach to the C&G 6187-02 Level 3 Diploma in Refrigeration, Air Conditioning and Heat Pump Systems is diagnostic reasoning rather than memorised component lists: learn to interpret pressure and temperature measurements so you can tell a charge problem from a metering problem, a control fault from a component fault, and a mode fault from a defrost fault. Work through the two paper scenarios, the decision table, the four-week sequence and the rubric below. Treat rubric scores as learning milestones for your own revision — they are study tools, not predictions of any assessment result.
Separating Superheat from Subcooling When Both Readings Look Wrong
Superheat is gas temperature above saturation at the evaporator outlet; subcooling is liquid temperature below saturation at the condenser outlet. They report on opposite sides of the metering device, so one gauge reading cannot diagnose both.
Superheat is measured at the evaporator outlet: subtract the saturation temperature your pressure-temperature table gives for the measured suction pressure from the actual pipe temperature at the sensing point. Subcooling is the mirror image at the condenser outlet: subtract the actual liquid line temperature from the saturation temperature at condensing pressure. Because the two sit on opposite sides of the metering device, you need both values before naming a fault; a single suction reading can support several contradictory explanations.
Worked scenario 1 (illustrative figures): a fixed-orifice system shows a suction saturation value of 5 °C while the evaporator outlet pipe reads 8 °C — 3 K superheat, unusually low; the liquid line reads 38 °C against a 45 °C table value — 7 K subcooling, unremarkable. The plausible mistake is treating the complaint as undercharge and adding refrigerant, even though low superheat points to an overfeeding metering device. Added charge raises head pressure and the risk of liquid floodback to the compressor. The better decision is to record both calculations first, then investigate metering and sensor placement before adjusting charge.
Tracing a Reversing Valve Fault in a Heat Pump Stuck in One Mode
A reversing valve redirects discharge gas between the indoor and outdoor coils. A valve that will not shift can look identical to a low-capacity compressor, so test mode-dependent behaviour before condemning either component.
A reversing valve has one rest position and one energised position, and it moves on a pressure differential, not on the compressor's health. Diagnostic evidence comes from two sources: the temperature difference across the valve's main lines while running, and whether the coil actually receives its control voltage in the required mode. A valve sitting in the wrong position with no coil voltage points to the control circuit; full voltage with no temperature change across the valve points to the valve itself.
Worked scenario 2: a heat pump responds to a heating demand by cooling the indoor air, and the indoor coil behaves exactly as it does in cooling. The plausible mistake is condemning the compressor for 'not heating', even though the unit is clearly running and moving refrigerant. The better decision is to note that the system operates but in the wrong mode, verify coil voltage during the heating call, and compare line temperatures across the reversing valve. It matters because a compressor replacement would be costly and would leave the original fault in place.
Reading Saturation Tables Without Confusing Refrigerants or Columns
Saturation pressure-temperature data is refrigerant-specific, and zeotropic blends add glide with separate dew and bubble values. Using the wrong fluid page or column corrupts every calculation built on it.
Pure refrigerants have one saturation pressure-temperature line. Zeotropic blends condense and evaporate across a temperature range called glide, so tables list bubble values for the start of condensing and dew values for the end of evaporation. Practice convention pairs superheat with dew values and subcooling with bubble values. Reading the wrong column shifts every calculated result by roughly the glide amount — large enough to move a system from 'normal' to 'suspect' on paper without any real change in the plant.
Exercise: from one blend's table, write three flashcards at a low, mid and high pressure, labelling each card with which calculation the value supports; then contrast a pure refrigerant's table, where a single column serves both. Expected observation: on the blend cards you must name dew or bubble explicitly, while the pure-fluid cards need no qualifier. If you catch yourself quoting a blend value without naming the column, that is precisely the habit to correct before any real measurement work.
Tracing a Defrost Sequence on Paper Before Any Live Testing
Defrost is a controlled reversal with defined start and termination conditions. Tracing the sequence on paper — sensors, electrical path, termination logic — exposes gaps in understanding before any energised equipment is involved.
A defrost sequence has identifiable stages: a start condition (coil sensor temperature combined with accumulated runtime, or an adaptive algorithm), reversal of the refrigerant flow, a change in outdoor fan state, and a termination condition (coil temperature or a maximum time). Tracing on paper means writing each stage with its triggering sensor and its electrical path. Gaps surface quickly: an unknown sensor location, an unlabelled relay contact, a termination setting nobody can state from the schematic.
Practical exercise: from a paper schematic, annotate the defrost path in one colour and the normal heating path in another, then list the observations you would expect on a gauge and sensor log if defrost ran correctly. Expected observations include a rise on the suction side during the reversal, a warming outdoor coil, and the defrost sensor approaching its termination point. Keep this as a desk exercise; live verification of defrost on energised, pressurised equipment belongs in supervised training, never in unsupervised practice.
Linking F-Gas Duties and Records to the System Work You Would Sign For
In the UK, F-gas work sits inside a regulatory framework linking leak checking, record keeping, recovery and handling to certified competence. For study, treat the logbook as the bridge between theory and practice.
A useful way to study the framework is through what generates a record: charge size, check dates, refrigerant added, and the identity of the certified person handling it. Writing a mock logbook entry after each paper scenario forces the question 'which actions in this job would need documenting?' — a question that is easy to skip while reading but hard to answer under assessment pressure, because it requires connecting the physical task to its administrative consequence.
Keep credentials distinct here. A systems diploma and an F-gas handling certificate are different qualifications with different purposes, and regulatory duties can change, so do not carry thresholds or category rules into your notes from memory or from unverified summaries. The durable study habit is conceptual: for every system task in your notes, name the record it produces and who must be qualified to perform it. If you cannot name both, mark that task as a revision gap and return to it.
Choosing the First Measurement: A Symptom-to-Check Decision Table
Use the table to choose a first measurement, not to jump to a conclusion. Each row starts from an observable symptom on a training rig and states what a deviant reading would help rule out.
The value of a decision table is that it converts a vague complaint into one specific next action, and each action's outcome eliminates at least one branch of the fault tree. Build your own rows as you revise: any symptom you meet in a scenario deserves a row if you cannot immediately state its first measurement. A table you have written yourself, with reasoning you can defend line by line, is far more useful in revision than a generic list copied from elsewhere.
Notice the discipline the table enforces: no row concludes with a component replacement, because no single reading justifies one. Where a row's result is ambiguous, the correct next step is a second measurement from a different part of the system — for example, pairing a superheat reading with a subcooling reading before discussing charge. Practise saying aloud what each result rules out; if you cannot finish that sentence, the row is not finished.
| Symptom | First measurement | What a deviant result suggests | What it helps rule out |
|---|---|---|---|
| Warm supply air during a cooling call | Temperature difference across the indoor coil | Small difference raises airflow or charge questions | Confirms whether the unit is running at all before deeper fault-finding |
| High condensing pressure | Condensing saturation temperature compared with ambient | Large gap suggests condenser airflow or flow restriction | Rules a metering-device explanation in or out |
| Ice on the suction line | Superheat at the evaporator outlet | Low superheat suggests overfeed or floodback | Rules out normal low-load operation |
| Compressor short cycling | Control circuit behaviour against the pressure pattern | Cycling without a matching pressure cause points to controls | Rules out a genuine high-pressure or low-pressure trip |
| Poor or absent defrost | Coil sensor temperature during a defrost call | A sensor that never approaches termination suggests sensor or flow-reversal issues | Rules out correct defrost operation |
A Four-Week Preparation Sequence With a Self-Check Rubric
Sequence the work: saturation-table fluency first, then component function, then paper diagnostics, then timed scenario practice against the rubric. Repeat each cycle with fresh symptom sets until your scores stop improving.
A realistic four-week cycle: week one, flashcards on saturation data and the dew/bubble distinction; week two, one-page function sheets for the reversing valve, metering devices, defrost controls and safety devices; week three, paper scenarios — two symptom sets per session, each answered with a calculation and a named first measurement; week four, timed practice using the decision table, followed by a mock logbook entry for each scenario. Adapt the order to your strengths, but keep calculation fluency ahead of scenario practice.
Run the rubric after each cycle. For every item, score 0 (cannot start), 1 (can do with notes) or 2 (can do unaided). A consistent pattern of 2s across all items is a reasonable study milestone; treat any 0 or 1 as next week's focus. The readiness checks below close each cycle — they are concrete, observable and repeatable, which is what makes them useful. They indicate study readiness only; they measure neither assessment content nor a predicted outcome, because those depend on factors this rubric does not cover.
- Readiness check 1: compute superheat and subcooling from labelled pressures and temperatures in under two minutes, stating explicitly whether each table value is dew or bubble.
- Readiness check 2: trace a defrost sequence from a blank schematic without notes, naming each start and termination condition.
- Readiness check 3: for five different symptoms, name the first measurement you would take and complete the sentence 'this rules out...'.
- Readiness check 4: write a complete mock logbook entry for a paper recovery scenario, naming the record, the task and who must be qualified to perform it.
References and further reading
Use these references to explore the concepts and check the latest information from the relevant organizations.