313A practice questions with the reasoning behind every answer.

Answer: B. Low refrigerant charge

• A: An overcharge raises subcooling and head pressure.
• B (correct): With too little refrigerant the condenser holds little liquid (low subcooling) and the evaporator is starved (high superheat, low suction pressure).
• C: A restriction also starves the evaporator, but refrigerant backs up in the condenser, so subcooling is normal or high.
• D: A dirty condenser raises head pressure; it does not produce low subcooling with a starved evaporator.

Subcooling separates the two classic starved-evaporator faults: low subcooling points to undercharge, high subcooling points to restriction. An undercharge means a leak; find and repair it before recharging.

Sources: Examen Studio explanation (draft) (Diagnosing charge problems); Federal Halocarbon Regulations, 2022 (SOR/2022-110) (s. 10 (leak test and repair before charging))

Answer: C. Terminal 3

• A: Terminal 1 is part of the highest reading (1 to 2), so it is start or run, not common.
• B: Terminal 2 is also part of the highest reading.
• C (correct): The highest reading is across start and run in series (1 to 2), so the terminal left out is common. From common, the higher reading (3 to 1, 3.0 Ω) is the start winding and the lower (3 to 2, 0.5 Ω) is run.
• D: Resistance readings identify the terminals, as long as the windings are intact.

Check: common to start plus common to run equals start to run (3.0 + 0.5 = 3.5). Also test each terminal to the compressor shell for a ground with a meter suited to insulation testing.

Sources: Examen Studio explanation (draft) (Testing compressor motors); Refrigeration and Air Conditioning Mechanic: Examination Weightings and Task Matrix (Task F-15, sub-task F-15.03)

Answer: B. Y and G

• A: W is the heating call.
• B (correct): Y energizes the compressor contactor and G energizes the indoor blower relay, so both run during a cooling call.
• C: C is the transformer common; it provides the return path, not the call.
• D: O is the reversing valve terminal on many heat pumps, and W is heat.

Common conventions: R is 24 V hot, C is common, Y is cooling (compressor), G is fan, W is heat, O or B is the reversing valve on heat pumps. Conventions vary by manufacturer, so always confirm against the equipment wiring diagram.

Sources: Examen Studio explanation (draft) (Low-voltage control wiring); Refrigeration and Air Conditioning Mechanic: Examination Weightings and Task Matrix (Task D-12, sub-task D-12.02)

Answer: A. Slightly downward in the direction of refrigerant flow, toward the compressor.

• A (correct): Oil travels with the refrigerant. A slight fall toward the compressor lets gravity help oil return rather than letting it pool.
• B: A rise against the flow makes oil run back toward the evaporator and collect in the line.
• C: Pitch matters because oil moves along the bottom of the pipe at low velocities.
• D: Oil in the evaporator reduces heat transfer and starves the compressor of lubricant.

Horizontal suction and discharge lines are pitched in the direction of flow; vertical suction risers are sized for enough velocity to carry oil upward, with a trap at the base where the design calls for one. Always check the equipment manufacturer's piping instructions for the specific system.

Sources: Examen Studio explanation (draft) (Refrigerant piping practice); Refrigeration and Air Conditioning Mechanic: Examination Weightings and Task Matrix (Task D-11, sub-task D-11.07)

Answer: B. 8 K

• A: That adds the two temperatures. Superheat is a difference.
• B (correct): Superheat is the actual vapour temperature minus the saturation temperature at the same pressure: 12 °C minus 4 °C is 8 K (8 degrees Celsius of difference).
• C: 4 °C is the saturation temperature, not the superheat.
• D: 12 °C is the measured line temperature, not the superheat.

Superheat equals measured suction line temperature minus saturated suction temperature. Measure the line temperature where the TXV bulb sits (evaporator superheat) or near the compressor (total superheat), and state which one you mean.

Sources: Examen Studio explanation (draft) (Superheat and subcooling); Refrigeration and Air Conditioning Mechanic: Examination Weightings and Task Matrix (Task E-14, sub-task E-14.01)

Answer: B. The compressor is running backwards because of the supply phase sequence.

• A: Low charge lowers suction pressure but a scroll turning the right way still builds a pressure difference.
• B (correct): A three-phase scroll turning in reverse does not compress, so pressures stay equal, current is low and it is noisy. Stop it quickly, lock out, and swap two supply legs.
• C: Three-phase compressors do not use run capacitors.
• D: A metering fault changes pressures, but the compressor would still create a difference and sound normal.

Verify phase sequence at start-up of three-phase equipment, especially scrolls. Many units include a phase monitor. Prolonged reverse running can damage the compressor.

Sources: Examen Studio explanation (draft) (Start-up checks); Refrigeration and Air Conditioning Mechanic: Examination Weightings and Task Matrix (Task E-13, sub-task E-13.02)

Answer: B. The reading rises steadily and keeps climbing towards atmospheric pressure without levelling off.

• A: A rise that levels off is typical of moisture or oil outgassing: the pressure climbs until it reaches the vapour pressure of what is boiling off, then stops.
• B (correct): Air entering through a leak has no equilibrium point short of atmospheric pressure, so the rise continues. That steady climb is the signature of a leak.
• C: That is a passed decay test: tight and dry.
• D: Pressure in an isolated system cannot fall on its own. A falling reading suggests a gauge or connection problem.

A decay test separates the two reasons a system will not hold vacuum. Moisture boils off until the pressure reaches its vapour pressure, then the curve flattens. A leak lets air in continuously, so the curve keeps rising. If you see a plateau, keep evacuating; if you see a steady climb, find the leak.

Sources: Examen Studio explanation (draft) (Evacuation and decay testing); Refrigeration and Air Conditioning Mechanic: Examination Weightings and Task Matrix (Task B-7, sub-task B-7.03)

Answer: A. Dry nitrogen, through a regulator, at or below the equipment's rated test pressure.

• A (correct): Dry nitrogen is inert and contains no moisture. A regulator keeps the test pressure within what the equipment nameplate and the manufacturer allow.
• B: Oxygen in contact with compressor oil under pressure can cause an explosion. Never pressurise a refrigeration system with oxygen.
• C: Compressed air carries moisture and oxygen into the system, which you then have to remove, and it contaminates the oil.
• D: Charging a system with a halocarbon for the purpose of leak testing is prohibited under the Federal Halocarbon Regulations, 2022 for the systems they cover. It also releases refrigerant when you find the leak.

Pressure test with dry nitrogen and a regulator, respect the test pressure on the nameplate, and let the pressure stabilise before judging a drop (temperature changes move the reading). Leak test before charging: the federal regulations prohibit charging for leak testing (s. 9) and require a leak test and repair before charging (s. 10).

Sources: Federal Halocarbon Regulations, 2022 (SOR/2022-110) (ss. 9 and 10); Refrigeration and Air Conditioning Mechanic: Examination Weightings and Task Matrix (Task B-7, sub-task B-7.02)

Answer: B. Short cycling, poor moisture removal and uneven comfort.

• A: Oversizing tends to do the opposite for humidity, and short cycling raises cost per unit of cooling.
• B (correct): An oversized unit satisfies the thermostat before the coil has run long enough to remove much moisture. Frequent starts add wear and the house feels cool but clammy.
• C: The thermostat controls when it stops, which is exactly why run cycles become short.
• D: Capacity mismatch does not directly flood the compressor. The typical symptoms are short cycles and humidity complaints.

Size equipment from a load calculation rather than the old unit's nameplate or a rule of thumb. In Canada, residential heating and cooling loads are commonly calculated to CSA F280. An accurately sized unit runs longer cycles and removes more moisture.

Sources: Examen Studio explanation (draft) (Equipment selection); Refrigeration and Air Conditioning Mechanic: Examination Weightings and Task Matrix (Task C-8, sub-task C-8.02)

Answer: A. Try the unit's controls to confirm nothing starts, then test for voltage with a meter you have just proven on a live source.

• A (correct): Isolation is only proven by checking it. Operating the controls shows the equipment will not start, and a voltage test with a meter proven before and after use confirms there is no energy at the point of work, including from a second feed or a mislabelled disconnect.
• B: A lock shows the switch position, not the state of the conductors. Welded disconnect blades, a second supply or a wrong disconnect can all leave the panel live.
• C: Removing fuses is not a substitute for a personal lock, and removing your lock gives up your control of the energy source.
• D: A thermostat only interrupts the control circuit. Line voltage is still present at the contactor, capacitor and compressor terminals.

Lockout is a sequence: shut down, isolate, lock and tag, release stored energy, then verify isolation before starting work. The verification step is the one people skip. Test the controls, then test for absence of voltage with a meter you have proven works. Capacitors can hold a charge after power is removed, so discharge and check them as well.

Sources: OSH Answers: Lockout/Tagout (Steps of a lockout procedure: step 7, verify isolation); Refrigeration and Air Conditioning Mechanic: Examination Weightings and Task Matrix (Task A-1, sub-task A-1.02)