313A start-up, charging and control setup practice questions.

Five free practice questions on start-up, charging and control setup: block E of the Red Seal 313A Refrigeration & AC Systems Mechanic exam, about 21 of its 125 questions. They are in the exam's format: four options, one correct answer. Answer each one before opening the explanation. No account, no email.

01The questions

1. Procedural · Performs start-up of HVAC/R systems

   At start-up on a 35 °C (95 °F) day a rooftop unit's compressor draws 21 A. The nameplate shows RLA 19 A, LRA 110 A and the condenser fan FLA 2.4 A. How should the mechanic interpret the compressor current?

   1. It is normal; running current is compared to LRA and 21 A is far below 110 A
   2. It is normal; a 10 % overcurrent is within the same ±10 % tolerance as voltage
   3. It is normal; RLA plus the condenser fan FLA is 21.4 A
   4. It exceeds RLA; check head pressure, condenser airflow, charge and voltage before accepting it
   Show the answer

   D. Correct. RLA is the rated-load current the compressor should not exceed under design conditions. Running 10 % over it points at high head pressure (blocked condenser, overcharge, non-condensables), low supply voltage or a mechanical problem, and the condenser fan's FLA is not part of the compressor's figure. Compare to RLA, find the cause, and record the corrected reading as the baseline.

   Why not the others

   A. LRA is the locked-rotor inrush drawn for a fraction of a second at start. Comparing running current to LRA would pass a compressor drawing five times its rating. Running current is compared to RLA.

   B. The ±10 % band is a supply-voltage tolerance. RLA is a ceiling, not a nominal; the compressor's overload will eventually trip above it and the windings run hot until it does.

   C. The condenser fan is a separate motor with its own FLA. Unless the clamp is on a conductor shared by both motors, its current is not added to the compressor's, and the nameplate RLA is for the compressor alone.

2. Procedural · Verifies/sets operating parameters

   Checking the oil failure control (OFC) on a semi-hermetic rack compressor: the oil pump outlet gauge reads 62 psig (427 kPa) and the crankcase (suction) gauge reads 22 psig (152 kPa). What net oil pressure is the OFC sensing?

   1. 84 psi
   2. 62 psi
   3. 40 psi
   4. 22 psi
   Show the answer

   C. Correct. Net oil pressure = oil pump outlet − crankcase pressure = 62 − 22 = 40 psi (276 kPa), the differential the OFC's two bellows compare. A typical OFC cuts out at a manufacturer-specified net pressure (often in the 7 to 12 psi region) after its 90 to 120 s time delay; a 40 psi net figure is a healthy margin. Test the control by making the net pressure fall below cut-out and confirming it trips after the delay and requires manual reset.

   Why not the others

   A. This adds the two readings. The OFC measures a differential, so the crankcase pressure is subtracted from the pump pressure, not added.

   B. 62 psig is the gross pump-outlet pressure. Because the oil pump inlet sits at crankcase pressure, the useful lubricating pressure is the difference, not the gauge reading.

   D. 22 psig is the crankcase pressure, the reference the oil pump works against, not the net oil pressure. The OFC senses the difference between pump outlet and crankcase.

3. Procedural · Completes HVAC/R system charge

   A mechanic is adding R-407C to a running walk-in cooler system to complete the charge. How must the refrigerant be taken from the cylinder and introduced?

   1. As liquid into the discharge line while the compressor runs
   2. As liquid directly into the suction service valve with the manifold wide open
   3. As vapour from the cylinder into the suction side, the standard method for any refrigerant
   4. As liquid from the cylinder, throttled into the suction side so it flashes before the compressor
   Show the answer

   D. Correct. Zeotropic blends must leave the cylinder as liquid so the composition stays as blended; drawing vapour off the top removes the more volatile components first and leaves both cylinder and system off-composition. Liquid is metered slowly through the manifold or a throttling device into the low side so it vaporizes before reaching the compressor. Liquid can also go directly into the high side, but only with the system off.

   Why not the others

   A. Discharge pressure is higher than the cylinder pressure, so nothing flows, and connecting a cylinder to the discharge side of a running compressor risks overpressurizing the cylinder. High-side liquid charging is done with the system off.

   B. Unthrottled liquid into the suction line reaches the compressor as liquid and slugs it. Liquid into the low side of a running system must be throttled so it flashes to vapour first.

   C. Vapour charging is acceptable for pure fluids and azeotropes (R-134a, R-507A). Drawing vapour from a zeotropic blend fractionates it, so R-407C and the other R-4xx blends are charged as liquid.

4. Recall · Performs start-up checks for control systems

   A pneumatic control system on a retrofitted air handler is being commissioned. What must be confirmed about the main air supply before the thermostats and actuators are set up?

   1. It comes from the building's shop-air line without further treatment
   2. It is shut off until every electrical control has been tested
   3. It is at least 100 psig so the actuators have reserve force
   4. Clean, dry, oil-free and regulated to the manufacturer's main-air pressure
   Show the answer

   D. Correct. Pneumatic controls fail on moisture, oil and dirt from the compressor, so the supply passes through a dryer and filter and is regulated to the manufacturer's main-air pressure (commonly around 15 to 25 psig / 103 to 172 kPa). The branch-line range that the controller produces (often 3 to 15 psig) must match the spring ranges of the actuators, or dampers and valves never reach full stroke. The RSOS names pneumatic connections among those verified at control start-up.

   Why not the others

   A. Shop air carries oil and water that clog restrictors and corrode controllers. Control air is dried and filtered, and usually supplied by a dedicated control-air compressor.

   B. The pneumatic and electrical controls are commissioned as one system; the pneumatic side cannot be verified with the air off. Apply main air once it is proven clean, dry and regulated.

   C. Pneumatic actuators are built for low pressures; 100 psig would rupture diaphragms and bellows. Main air is regulated down to the manufacturer's figure, typically under 25 psig.

5. Procedural · Performs pre-start-up checks for HVAC/R systems

   A replacement condensing unit arrives with a nameplate reading 208-230 V, 1 Ø, 60 Hz. The building's disconnect for the old unit is fed from a 600 V, 3 Ø panel. What is the correct pre-start action?

   1. Connect it and check the running voltage; the ±10 % rule covers the difference
   2. Do not connect it; a step-down transformer or a unit for the available supply is required
   3. Reconnect the compressor for 460 V on its terminal block
   4. Connect two of the three legs; a single-phase motor only uses two conductors
   Show the answer

   B. Correct. The RSOS pre-start check verifies that the energy source and the equipment are compatible. A 208-230 V single-phase unit on a 600 V three-phase feed is a mismatch in both voltage and phase; connecting two of the three 600 V legs would destroy the motors. The fix is a transformer sized for the load, arranged through the electrician, or a unit ordered for the available supply.

   Why not the others

   A. The ±10 % tolerance applies to the nameplate voltage: 230 V allows up to 253 V. 600 V is nowhere near that band. Tolerances do not bridge a wrong supply.

   C. A single-phase 208-230 V compressor has no 460 V or 600 V connection. Dual-voltage reconnection exists on some three-phase motors, not on this equipment, and 600 V is not 460 V in any case.

   D. Two legs of a 600 V three-phase system deliver 600 V, not 230 V. The nameplate range is 208-230 V; 600 V would burn out the compressor and fan motors on the first start.

02Answer key

03What block E covers

This block covers bringing a new system into service. It includes pre-start checks such as crankcase heaters, holding charges, valve positions and megohm tests, voltage imbalance and three-phase rotation, superheat and subcooling, charging blends and CO2, EPR and CPR valves, head-pressure control, water flow and airflow, and setting and proving controls: low-pressure differentials, safety switches, defrost, economizers, transducers and alarms.

The Red Seal Occupational Standard names this block “Performs commissioning”. It carries 16.8% of the exam, about 21 of the 125 questions, split across two tasks. The share column is each task's weight within the block, as the standard publishes it.

04What the questions turn on

Superheat or subcooling

A TXV holds its superheat whatever the charge, so subcooling is the reading used to fine-tune a TXV system. On a fixed orifice, charge to the manufacturer's target superheat for the outdoor dry bulb and indoor wet bulb.

Voltage imbalance before start

Imbalance is the largest deviation from the average voltage, divided by the average. Most compressor manufacturers allow 2 %. Above that, the compressor is not started; the cause is upstream of it and is corrected first.

Scroll rotation on first start

A correctly phased scroll pulls suction down and pushes discharge up within seconds. Loud, low amps and no pressure change means reverse rotation. Swap two leads at the compressor, or at the line side if every three-phase motor runs backward.

Setting a low-pressure control

On a low-pressure control the cut-out is the cut-in minus the differential, so 40 psig with a 25 psi differential cuts out at 15 psig. A pump-down that restarts within a minute points to a differential set too narrow.

05The other blocks

The 313A exam has six blocks. Each has its own page of free questions:

The 313A Refrigeration & AC Systems Mechanic practice exam page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.