313A maintenance, service and troubleshooting practice questions.

Five free practice questions on maintenance, service and troubleshooting: block F of the Red Seal 313A Refrigeration & AC Systems Mechanic exam, about 28 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 · Repairs HVAC/R systems

   An older R-22 reach-in cooler is to be retrofitted to an HFC blend that the compressor manufacturer approves with POE oil. Which sequence is correct?

   1. Recover the R-22, charge the blend as vapour into the high side, keep the mineral oil
   2. Top the R-22 system off with the new blend and let the oils mix
   3. Charge the blend to the original R-22 weight and superheat, no other changes
   4. Recover, change the oil to POE, replace the drier, evacuate, charge liquid by weight, relabel
   Show the answer

   D. Correct. HFC blends need POE, so the mineral oil is drained and flushed until residual mineral oil is below the manufacturer's limit. The drier is replaced, the system is evacuated and the blend is weighed in as liquid (zeotropes fractionate if charged as vapour). The equipment is relabelled with the new refrigerant and oil.

   Why not the others

   A. Mineral oil is not miscible with HFC blends and will log in the evaporator. Zeotropic blends are charged as liquid, not vapour. Change the oil and drier.

   B. Mixing refrigerants creates an unknown blend with no P/T chart and contaminates any recovery cylinder. Mineral oil does not return with HFCs. Recover fully and change the oil.

   C. Retrofit charges differ from the original refrigerant's, blends use dew and bubble points, and the TXV may need review. Follow the manufacturer's retrofit guide, oil change included.

2. Procedural · Performs predictive and scheduled maintenance on HVAC/R systems

   The scheduled maintenance for an ice-machine condensing unit includes checking and tightening the electrical connections. Which procedure is correct?

   1. Tighten each lug while the unit runs so a change in amps confirms a bad joint
   2. Spray the terminals with contact cleaner and skip tightening if none look burnt
   3. Lock out, verify zero voltage, inspect for discolouration and torque the terminals
   4. Tighten every terminal as hard as possible to prevent future loosening
   Show the answer

   C. Correct. Loose connections heat, discolour and eventually burn open. The check is done de-energized with LOTO and zero-energy verification, and terminals are torqued to the specified value rather than simply cranked tight.

   Why not the others

   A. Working on live terminals is an arc-flash and shock hazard, and the amp change is not a useful confirmation. Lock out and verify zero voltage first.

   B. A loose connection can look fine until it fails. The RSOS calls for visually checking and tightening connections to prevent electrical failure.

   D. Over-torquing strips threads, cracks lugs and damages conductors. Use the manufacturer's torque value.

3. Procedural · Calibrates operating and safety controls

   A walk-in cooler's pump-down low-pressure control has an adjustable range and differential. The manufacturer calls for a cut-in of 275 kPa (40 psig) and a differential of 172 kPa (25 psi). At what pressure should the control cut out?

   1. 65 psig
   2. 40 psig
   3. 15 psig
   4. 25 psig
   Show the answer

   C. Correct. For a low-pressure control the cut-out is the cut-in minus the differential: 40 − 25 = 15 psig (about 103 kPa). The range dial sets the cut-in, the differential dial sets how far below it the compressor stops. Verify with a gauge while pumping down.

   Why not the others

   A. Adding the differential to the cut-in is the high-pressure control convention (cut-in = cut-out minus differential). On a low-pressure control the cut-out is below the cut-in.

   B. Forty psig is the cut-in, the pressure at which the compressor restarts. The cut-out sits one differential below it.

   D. Twenty-five psi is the differential, the gap between cut-in and cut-out, not the cut-out pressure itself. Subtract it from the cut-in.

4. Procedural · Troubleshoots HVAC/R systems

   A CSR compressor on an ice machine will not start. The mechanic removes the potential relay and, with it de-energized, measures the contacts between terminals 1 and 2 as open (OL). What does this indicate?

   1. The relay is normal; potential-relay contacts are open until the motor reaches speed
   2. The coil has burned open, so the reading is meaningless
   3. The start capacitor is shorted, which has welded the contacts
   4. The relay has failed; its contacts should be closed at rest to put the start capacitor in circuit
   Show the answer

   D. Correct. A potential relay's 1–2 contacts are normally closed. At start, the start capacitor is in circuit through them; as the motor comes to speed the back EMF across the start winding energizes the coil (terminals 2–5) and opens the contacts. Contacts open at rest mean the motor never gets its start capacitor. Replace the relay with the specified part.

   Why not the others

   A. That is backwards: the contacts are closed at rest and open as the motor accelerates. Open at rest is a failed relay.

   B. The coil is between terminals 2 and 5; the 1–2 reading is the contact set. An open coil would leave the contacts closed and the start capacitor never dropping out, which is a different failure.

   C. A shorted start capacitor would tend to weld the contacts closed, not open. Open contacts are a burned-out or broken relay.

5. Procedural · Tests HVAC/R system components and accessories

   An acid test on an oil sample drawn from a scroll compressor on a rooftop unit gives a positive (acidic) result, although the unit is running normally. What does this indicate and what should be done?

   1. Refrigerant overcharge; recover some refrigerant
   2. A normal result for POE oil; no action is needed
   3. The compressor is already burnt out; replace it immediately
   4. Contamination; change the oil and filter-driers, then retest
   Show the answer

   D. Correct. Acid forms from moisture and heat breaking down the refrigerant and oil. Caught early, an oil change plus new liquid-line and, if needed, suction-line driers removes it and prevents the winding insulation attack that ends in a burnout. Retest after a run period.

   Why not the others

   A. Charge level has no bearing on acid formation. Acid comes from moisture and heat; treat the contamination.

   B. POE is neutral when clean. A positive acid test means the oil is already degrading and the system is contaminated.

   C. A running compressor with acidic oil is not yet a burnout. Cleaning the system now is the way to avoid the replacement.

02Answer key

03What block F covers

This block covers keeping systems running and finding faults. It includes inspections and run checks, coil cleaning, filters, oil acid tests, glycol checks, defrost and compressor valve tests, diagnosing charge, airflow, restriction and non-condensable problems from superheat and subcooling, compressor windings, capacitors and relays, reversing valves, burnout clean-up, retrofits, and troubleshooting sensors, transducers, networks and control circuits.

The Red Seal Occupational Standard names this block “Performs maintenance and service”. It carries 22.4% of the exam, about 28 of the 125 questions, split across three tasks. The share column is each task's weight within the block, as the standard publishes it.

04What the questions turn on

Superheat and subcooling together

High superheat with low subcooling and low amps is an undercharge. Normal superheat with high subcooling and high head on a clean condenser is an overcharge. High superheat with normal or high subcooling and a temperature drop across the drier is a restriction.

Standing pressure finds air

A system at rest sits at the P/T saturation pressure for its temperature. A standing pressure well above the chart value means non-condensables, which collect in the condenser and raise head pressure. Recover, evacuate and recharge.

Finding Common, Run and Start

On a single-phase compressor, the highest resistance reading is Start to Run, so the terminal left out of it is Common. The lowest reading is Run to Common. The remaining terminal is Start, and the start winding reads higher.

Walking a series control circuit

In an energized series circuit, an open switch has the full control voltage across it and a closed one reads near zero. Placing the meter across each safety in turn finds the open device, which then points to the refrigeration problem.

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.