BTEB Refrigeration & AC Exam common mistakes
BTEB Refrigeration & AC Exam · Understand the traps and how to avoid them.
Using the pressure-enthalpy chart to read enthalpy values straight off the saturation dome instead of following constant-entropy or constant-enthalpy lines to the actual state point, so the calculated refrigerating effect comes out wrong.
Candidates memorize the shape of the cycle diagram but never practise locating subcooled liquid, superheated vapour, and two-phase states on the chart, so they confuse the saturation region with the superheat region.
What to do instead
Before the exam, draw five cycles from scratch on a blank P-h chart and label every point with its pressure, temperature, enthalpy, and phase. Check each enthalpy value against the nearest axis gridline and confirm the state lies on the correct side of the dome.
Treating the expansion valve process as isenthalpic in words but then calculating work or heat transfer across it, or placing the evaporator outlet inside the two-phase dome when the problem states a superheat value.
The throttle is the least intuitive component; candidates apply the first law incorrectly and forget that throttling conserves enthalpy while dropping both pressure and temperature.
What to do instead
Write the energy balance h3 = h4 explicitly for every throttle problem, and mark the evaporator outlet at the stated superheat temperature rather than on the saturation line. Do three worked examples with different superheat values.
Mixing up absolute and gauge pressure when selecting saturation temperatures from tables, or ignoring the pressure drop across the evaporator and suction line.
Gauge readings are what technicians see in the field, and candidates carry that habit into numerical questions where tables require absolute pressure, producing a large systematic error.
What to do instead
Adopt a fixed habit: convert every given pressure to absolute before opening a table, and write the conversion line on the answer sheet. Practise ten conversions alternating between gauge-to-absolute and absolute-to-gauge.
Diagnosing a low superheat or high superheat condition by adjusting refrigerant charge first, without checking airflow, metering device restriction, or suction line insulation.
Charge is the most familiar lever, so candidates jump to it. They also fail to link superheat to evaporator loading rather than to charge alone.
What to do instead
Work through a fixed diagnostic order for every superheat scenario: airflow and coil condition, then metering device, then line insulation, then charge. Write the order out and apply it to at least five case descriptions.
Calculating sensible and latent loads with the wrong airflow, the wrong specific heat or latent heat value, or by adding sensible and latent fractions incorrectly when converting to total capacity.
Candidates plug numbers into formulas they half-remember and do not track units, so a load in Btu/h gets mixed with watts or a mass flow in kg/s with a volume flow in m3/s.
What to do instead
Always write the formula, then the units of every term, before substituting. Do a set of six load problems where the answer must be stated in both SI and imperial units and cross-checked for plausibility.
Sizing equipment from floor area alone, ignoring orientation, glazing, occupancy, internal gains, and ventilation, or forgetting to add outdoor air load to the space load.
Rough rules of thumb are widely repeated and easy to recall, so candidates skip the component-by-component estimate that the exam expects.
What to do instead
Build a one-page checklist of load components and run it against three sample spaces with different glazing and occupancy. Explicitly separate space load from ventilation load and add them at the end.
Confusing the psychrometric terms: reading dry-bulb where wet-bulb is asked, mixing up relative humidity with humidity ratio, or misplacing the apparatus dew point and bypass factor.
The chart has many overlapping lines and candidates read the nearest scale instead of the correct family of lines for the property requested.
What to do instead
Practise plotting ten state points and reading all seven properties at each. For coil problems, draw the room condition, the apparatus dew point, and the leaving condition, and compute the bypass factor from the line lengths.
Describing commissioning steps out of order, such as charging before evacuation, or skipping leak testing, triple evacuation, and micron-level verification.
Candidates learn the steps as a list of words rather than a sequence with reasons, so they cannot tell which step must precede which.
What to do instead
Write the commissioning sequence as a numbered flow with the acceptance criterion for each step, for example evacuation held below 500 microns with the pump isolated. Recite the sequence and justify each ordering decision.
Answering electrical troubleshooting questions without isolating power, or describing a motor or compressor check without mentioning winding resistance, insulation resistance, and supply voltage under load.
Candidates focus on the refrigeration side and treat electrical checks as an afterthought, so their answers omit the safety and measurement steps examiners look for.
What to do instead
For every electrical fault scenario, structure the answer as isolate and lock out, then verify supply, then measure winding and insulation resistance, then interpret. Practise this template on five fault descriptions.
Giving safety answers that are generic, such as 'wear PPE', instead of naming the specific hazard, the control measure, and the regulation-linked limit, for example refrigerant exposure limits, oxygen deficiency in plant rooms, or recovery rather than venting.
Safety questions are read as common sense, so candidates do not prepare specific figures, procedures, or equipment names and lose marks on precision.
What to do instead
Build a short reference card of refrigerant safety groups, exposure limits, recovery and evacuation procedures, and confined-space entry controls, and practise writing answers that name the hazard, the control, and the verification step.