ITI/NCVT RAC Mechanic (AITT Theory) common mistakes
ITI/NCVT RAC Mechanic (AITT Theory) · Understand the traps and how to avoid them.
Memorising pressure–enthalpy and psychrometric chart readings as isolated numbers (e.g. 'discharge is always 15 bar') instead of reading the diagram from the given state point; when the question changes evaporating or condensing temperature, the candidate reuses the memorised value and gets the whole cycle calculation wrong.
Chart-based topics are learned by copying worked examples rather than practising the construction: drawing the saturation dome, locating the four cycle points, and reading enthalpy along constant-pressure lines. Under exam time pressure the candidate skips the sketch and guesses from memory.
What to do instead
Before every numerical question, draw a quick labelled sketch (four points, arrows for compression, condensation, expansion, evaporation) and mark the given temperatures and pressures on it. Practise 10–15 cycles at different evaporating/condensing temperatures until reading h1, h2, h3, h4 from the chart becomes mechanical. Always state the formula (COP = (h1 − h4)/(h2 − h1)) before substituting.
Confusing the direction of heat flow and the roles of the four components: calling the condenser the 'cooling' part, saying the evaporator 'rejects heat', or claiming the compressor 'produces cold'. Answers then invert the high/low side, the hot-gas line and the liquid line.
Everyday language ('fridge makes things cold') conflicts with the thermodynamic model where cooling is heat absorption. Candidates learn component names by position on a diagram rather than by function and pressure side.
What to do instead
Build a one-page function map: evaporator absorbs heat at low pressure (refrigerant boils), compressor raises pressure and temperature, condenser rejects heat to ambient (refrigerant condenses), expansion device drops pressure and temperature. For each component write the refrigerant state on entry and exit (subcooled liquid, saturated mixture, superheated vapour). Test yourself by tracing a finger around the circuit and naming pressure side and state at each arrow.
Mixing up electrical control components and their terminals when reading or answering about wiring: swapping the start and run windings, misidentifying the common terminal of a compressor, wiring the potential relay or PTC starter incorrectly, or confusing overload protector, thermostat and capacitor functions.
Control circuits are learned by copying a wiring diagram visually rather than by tracing current paths. Candidates do not practise the resistance test (highest resistance across start–run, common is the remaining terminal) or the meaning of each contact.
What to do instead
Learn the terminal identification method cold: measure resistance between all three pins, the pair with the highest reading is start–run, the third pin is common, and the lower of the two remaining readings identifies the run winding. Redraw from memory the wiring for a PSC compressor, a capacitor-start motor and a potential-relay start circuit, labelling L, N, C, S, R, overload and capacitor. Explain aloud what happens when the thermostat opens.
Treating refrigerant handling and environmental rules as trivia: quoting the wrong cylinder colour or connection thread, mixing refrigerants during recovery, or stating incorrect facts about CFC/HCFC phase-out, ODP and GWP, and safe disposal of recovered refrigerant.
This domain is memorisation-heavy and candidates skim it, assuming it is 'only theory'. They also carry over habits from unregulated field practice where topping up and venting were common.
What to do instead
Make a single comparison table for the refrigerants in the syllabus (R-12, R-22, R-134a, R-404A, R-407C, R-410A, R-32, ammonia, CO2) with columns for chemical family, ODP, GWP, oil type, typical application, cylinder colour and service pressure class. Add a short list of handling rules: never mix refrigerants, recover into labelled cylinders, never vent, use PPE and ventilation for ammonia. Revise the table daily for a week.
Applying a service diagnosis as a single symptom-to-part jump ('not cooling means gas leak, so top up') without a systematic check of the whole system: no superheat/subcooling measurement, no current draw check, no airflow check, no look at frost pattern or condenser cleanliness.
Field shortcuts and 'experience stories' replace the diagnostic sequence. Candidates answer troubleshooting questions with the first plausible cause instead of the evidence-gathering order taught in the trade.
What to do instead
Memorise and apply a fixed diagnostic sequence for each complaint: (1) verify complaint and operating conditions, (2) check power supply, voltage and current, (3) check airflow and condenser/evaporator cleanliness, (4) measure suction and discharge pressures, (5) calculate superheat and subcooling, (6) compare with expected values, (7) only then name the faulty component. Practise writing this sequence for 'unit runs but does not cool', 'unit trips overload' and 'ice on suction line'.
Confusing temperature, heat and pressure concepts and their units: mixing °C with K in expansion-valve or superheat calculations, treating gauge pressure as absolute pressure, or using the wrong unit for heat quantity (kcal vs kW vs kJ).
The basics are taught early and assumed known, so candidates never consolidate them. Pressure gauges read gauge pressure while tables are in absolute pressure, and the +1 atm conversion is forgotten under exam pressure.
What to do instead
Write a units card: temperature conversion (K = °C + 273.15), absolute pressure = gauge + atmospheric (≈1.013 bar), 1 kcal = 4.187 kJ, 1 TR = 3.517 kW, 1 bar = 100 kPa. For every numerical answer, write the unit beside the number and check dimensional consistency. Do five conversion drills before attempting cycle or psychrometry problems.
Misreading measuring instruments and stating results with impossible precision: reading a vernier or micrometer without the zero error correction, reading a manifold gauge on the wrong scale (compound vs high-pressure), or quoting a dimension to 0.001 mm from a rule.
Instrument use is practised physically in the workshop but examined in words, so candidates never rehearse the reading logic. Many have only used one scale of a gauge or one type of vernier.
What to do instead
Rehearse on paper: draw a vernier scale and micrometer thimble at several positions and calculate the reading including least count and zero error. For manifold gauges, state which scale applies to which side and that the compound gauge reads below zero (vacuum). Always record the least count of the instrument and round the answer to it.
Answering installation and commissioning questions with generic steps, omitting the specific checks: no vacuum hold test, no leak test with dry nitrogen, no pipe sizing or bending radius rule, no brazing with nitrogen purge, no insulation and drainage checks, no recording of ambient and operating parameters at handover.
Installation is learned as a sequence of actions on site, not as a documented checklist. Candidates remember 'install and start' but not the verification steps that examiners test.
What to do instead
Build a commissioning checklist in order: site and support check, pipe routing and sizing, brazing with nitrogen purge, pressure test with dry nitrogen, vacuum to specified micron level and hold test, charge by weight, electrical connections and earthing, drainage slope, insulation, then run test recording ambient temperature, suction/discharge pressures, superheat, subcooling and current. Memorise the checklist and practise writing it out in under five minutes.
Skipping trade safety specifics in favour of general statements: answering 'wear PPE' instead of naming the correct PPE for brazing, refrigerant, or electrical work, and failing to identify the specific hazard (asphyxiation from refrigerant in a pit, ammonia toxicity, electric shock from a capacitor, fire from oil and oxygen).
Safety questions look easy, so candidates give the shortest generic answer. They also underestimate hazards they see daily and have normalised.
What to do instead
Make a hazard–control table: task (brazing, refrigerant recovery, electrical testing, working at height, lifting) against specific hazard, correct PPE, and control measure. Include the facts examiners look for: discharge capacitors before touching terminals, ventilate low-lying areas for heavier-than-air refrigerants, never use oxygen as a pressure test gas, use nitrogen only, and keep a fire extinguisher suitable for electrical fires. Learn each entry as a sentence you can write.
Confusing the characteristics of air-conditioning system types: describing a window unit's airflow and fresh-air provision the same as a ductable or central plant, or misstating where the evaporator, condenser and expansion device sit in a split, cassette or package unit.
The systems are visually familiar but their construction is rarely compared side by side. Candidates learn one system well and generalise its layout to all others.
What to do instead
Draw a simple schematic for each system type (window, split, cassette, ductable, package, central chilled-water plant) showing where the evaporator, condenser, compressor, expansion device, filters and drain pan are, and note whether it is unitary or split, air-cooled or water-cooled, and how fresh air is introduced. Compare the six drawings and list two differences between each neighbouring pair.