Free Red Seal Refrigeration Mechanic (313A) Practice Test

Answer: C. Refrigerant exposed to a flame or hot surface can decompose into toxic, corrosive products such as hydrogen fluoride

Halocarbon refrigerants break down when heated by a flame or very hot metal, producing acids such as hydrogen fluoride and hydrogen chloride, and older CFCs can form phosgene. Recover the charge and flow dry nitrogen through the pipe before applying heat.

Check it: RSOS task A-1 (safety-related functions)

Answer: A. Most halocarbon vapour is heavier than air and can displace oxygen in low areas, causing asphyxiation

Safety group A1 means low toxicity and no flame propagation, not harmless. Vapour sinks and collects in pits, basements and confined spaces, displacing oxygen without warning. Ventilate, use refrigerant and oxygen monitoring, and follow the machinery room requirements of the refrigeration code.

Check it: CSA B52 (refrigeration safety)

Answer: D. An electronic micron gauge connected as close to the system as practical

A compound gauge cannot resolve pressures in the micron range, so it cannot show whether moisture and non-condensables have been removed. An electronic micron gauge reads absolute pressure in microns and also lets you do a decay (standing) test after isolating the pump.

Check it: RSOS task A-2 (tools and equipment)

Answer: B. 6°C

Superheat is the actual vapour temperature minus the saturation temperature at the same pressure: 9°C − 3°C = 6°C. Adding the two (12°C) or using either reading alone does not describe how far the vapour is above its boiling point.

Check it: RSOS task B-6 (trade activities – calculations)

Answer: A. 7°C

Subcooling is saturated condensing temperature minus actual liquid line temperature: 46°C − 39°C = 7°C. It shows how much the liquid has been cooled below its condensing point, which helps confirm a full column of liquid reaches the metering device.

Check it: RSOS task B-6 (trade activities – calculations)

Answer: B. About 3.5:1

Compression ratio uses absolute pressures: (1,480 + 101) ÷ (350 + 101) = 1,581 ÷ 451 ≈ 3.5. Dividing gauge pressures (1,480 ÷ 350 ≈ 4.2) overstates the ratio, which is a common error.

Check it: RSOS task B-6 (trade activities – calculations)

Answer: B. To displace oxygen inside the tube so oxide scale does not form and later clog strainers and metering devices

Heating copper in the presence of air forms black oxide scale on the inside wall. Refrigerant and oil later loosen it, plugging filter driers, screens and expansion valves. A low nitrogen purge flow keeps oxygen out during brazing.

Check it: RSOS task B-6 (trade activities – brazing)

Answer: D. 80% of liquid capacity, checked by scale or the cylinder's float or overfill sensor

Liquid refrigerant expands as it warms. Leaving at least 20% vapour space prevents a full liquid cylinder from building hydrostatic pressure that can rupture it. Weigh the cylinder against its marked tare and maximum fill or use an overfill shutoff.

Check it: RSOS task B-6 (refrigerant recovery)

Answer: A. An oil trap (P-trap) so oil collects and is carried up the riser by refrigerant vapour

Oil returns to the compressor by being swept along by vapour. At the base of a vertical suction riser, a short trap lets oil accumulate until vapour velocity carries it up. Line sizing must also keep riser velocity high enough for oil return.

Check it: RSOS task C-7 (plans installation of HVAC/R systems)

Answer: B. Choose a coil with a small design TD between box air and refrigerant, to reduce moisture removal

The colder the coil compared with the box air, the more moisture condenses on it and the drier the box becomes. A lower TD, which requires a larger coil, keeps relative humidity high and reduces product dehydration.

Check it: RSOS task C-7 (load and equipment selection)

Answer: C. 28,800 kJ

Sensible heat Q = m × c × ΔT = 500 kg × 3.6 kJ/kg·°C × (20 − 4)°C = 500 × 3.6 × 16 = 28,800 kJ. Using 20°C alone as the temperature change gives 36,000 kJ, which overstates the load.

Check it: RSOS task C-7 (load calculation)

Answer: C. Low voltage cuts shock and fire risk in field wiring and allows smaller, cheaper thermostat wire and controls

A Class 2 transformer supplies limited-energy 24 V power, so thermostat wiring can be smaller and is safer to handle. The low-voltage circuit then energizes contactor and relay coils that switch the line-voltage loads.

Check it: RSOS task C-8 (plans installation of control systems)

Answer: A. Remove it from the cylinder as liquid and meter it into the system so it does not slug the compressor

The components of a zeotropic blend boil at different temperatures, so drawing vapour off the top changes the mixture left in the cylinder and the composition charged. Taking liquid keeps the blend at its design ratio; meter it slowly into the suction side or charge into the liquid line with the system off.

Check it: RSOS task D-9 (installs HVAC/R systems – charging)

Answer: C. On the suction line near the evaporator outlet, in good contact and insulated from ambient air

The TXV regulates refrigerant flow to hold a set superheat at the evaporator outlet, so its bulb must sense the suction line temperature there. It is clamped to a clean, straight, horizontal section and insulated so ambient air does not affect the reading.

Check it: RSOS task D-9 (installs metering devices)

Answer: A. So oil carried by the refrigerant drains toward the compressor instead of pooling in the line

Oil leaves the compressor with the discharge gas and must return through the suction line. A slight pitch in the direction of flow helps gravity move oil to the compressor, especially at low loads when vapour velocity drops.

Check it: RSOS task D-9 (installs piping)

Answer: B. Dry nitrogen, regulated to a pressure no higher than the system's rated test pressure

Oxygen can react violently with compressor oil and cause an explosion, and compressed air adds moisture. Dry nitrogen is inert and dry. Always use a regulator and relief device and do not exceed the test pressure on the equipment nameplate.

Check it: RSOS task D-9 (installs HVAC/R systems – pressure testing)

Answer: D. The terminal that reads 5 Ω to common and is part of the highest (7 Ω) reading

The highest reading (7 Ω) is across both windings, R to S, so the third terminal is common. From common, the lower resistance (2 Ω) is the run winding and the higher (5 Ω) is the start winding. Note 2 + 5 = 7, which confirms the windings are intact.

Check it: RSOS task D-10 (installs control systems – motors)

Answer: B. The cabinet is under negative pressure, so the trap seals the drain, stopping air being drawn in and holding water in the pan

With the fan pulling air through the coil, the drain pan sits under suction. Without a water seal, air rushes up the drain and condensate cannot leave, which can overflow the pan or blow water into the duct. A properly sized trap and slope prevent this.

Check it: RSOS task D-9 (installs HVAC/R systems – condensate)

Answer: B. The compressor is running backward because the phase rotation is reversed

A scroll compressor only pumps in one direction. With two supply phases reversed it runs backward, makes abnormal noise and builds no pressure difference. Shut it down quickly and swap two line leads, then confirm normal pressures.

Check it: RSOS task E-11 (commissions HVAC/R systems)

Answer: B. About 1.9%

Average = (208 + 212 + 204) ÷ 3 = 208 V. The largest deviation from average is 4 V. Imbalance = 4 ÷ 208 × 100 ≈ 1.9%. Even small voltage imbalance causes much larger current imbalance and motor heating.

Check it: RSOS task E-11 (commissioning – electrical checks)

Answer: D. A steady rise that levels off indicates moisture still boiling off, while a continuous rise to atmosphere indicates a leak

After isolating the pump, watch the micron gauge. If pressure rises and then stabilizes, moisture is still evaporating and more evacuation is needed. If it keeps rising steadily, air is leaking in and the leak must be found and fixed.

Check it: RSOS task E-11 (commissioning – evacuation)

Answer: B. 100 kPa

Differential is the difference between the cut-in and cut-out settings: 170 − 70 = 100 kPa. Too small a differential causes short cycling; too large allows the box temperature to drift.

Check it: RSOS task E-12 (commissions control systems)

Answer: A. That it shifts properly between heating and cooling, with its solenoid energized in the mode the manufacturer specifies

Manufacturers differ: some energize the reversing valve in cooling, others in heating. Confirm the thermostat setup matches the equipment, then check that the valve shifts and refrigerant temperatures change correctly in each mode.

Check it: RSOS task E-12 (commissions control systems)

Answer: C. Low refrigerant charge, likely from a leak

An undercharge starves the evaporator (high superheat, low suction pressure) and leaves too little liquid backed up in the condenser (low subcooling, bubbles in the glass). Find and repair the leak before weighing in the correct charge.

Check it: RSOS task F-14 (services HVAC/R systems)

Answer: D. The system is overcharged

Excess refrigerant backs up as liquid in the condenser, reducing the surface left for condensing. That raises head pressure and subcooling. Recover refrigerant until superheat and subcooling match the manufacturer's charging data.

Check it: RSOS task F-14 (services HVAC/R systems)

Answer: A. The drier is partially restricted and should be replaced

There should be little or no temperature drop across a liquid line drier. A noticeable drop means pressure is falling inside it, so some liquid flashes to vapour. Replace the drier after recovering or isolating the charge.

Check it: RSOS task F-14 (services HVAC/R systems)

Answer: C. 34 µF

6% of 35 µF is 2.1 µF, so the acceptable range is 32.9 to 37.1 µF. Only 34 µF falls within that range. Always discharge a capacitor safely before testing it.

Check it: RSOS task F-15 (maintains and services control systems)

Answer: B. Cleaning the condenser coil and checking that the condenser fans run correctly

A dirty condenser or a failed condenser fan reduces heat rejection, so condensing temperature and head pressure rise. That raises power draw and compressor discharge temperature. Coil cleaning and fan checks are core maintenance items.

Check it: RSOS task F-13 (maintains HVAC/R systems)

Answer: C. The contactor coil is open or the contactor is mechanically stuck

Correct voltage across the coil proves the control circuit has delivered power. If the contactor still does not close, the coil itself (check its resistance with power off) or the armature is at fault. A failed transformer or open safety switch would leave no voltage at the coil.

Check it: RSOS task F-15 (maintains and services control systems)

Answer: D. To check the insulation resistance between the windings and ground for breakdown before it causes a short

A megohmmeter applies a high DC test voltage and reads insulation resistance in megohms. Falling readings over time warn of moisture, acid or insulation damage. A standard ohmmeter uses too low a voltage to find weak insulation.

Check it: RSOS task F-15 (motor diagnostics)