313A refrigerant piping and control wiring practice questions.

Five free practice questions on refrigerant piping and control wiring: block D of the Red Seal 313A Refrigeration & AC Systems Mechanic exam, about 26 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. Recall · Places HVAC/R equipment, components and accessories

   Which type of refrigerant pressure regulator uses a small pilot valve to position the main valve, giving large capacity from a small adjusting force?

   1. Direct-acting regulator
   2. Pilot-operated regulator
   3. Hand expansion valve
   4. Reverse-acting regulator
   Show the answer

   B. Correct. The pilot is a small direct-acting valve that meters pressure onto the main valve's piston or diaphragm, so system pressure does the work of moving a large port. That is why large EPRs, hold-back valves and industrial ammonia regulators are pilot-operated, and why they need a minimum pressure drop to function. Direct-acting is the small-capacity relative.

   Why not the others

   A. In a direct-acting regulator the sensed pressure works straight against the adjusting spring to move the port. Simple and fine for small capacities, but the spring and diaphragm limit the size.

   C. A hand expansion valve is a manually set needle valve with no automatic action at all. It is a metering device for flooded systems and pump-down bypass, not a regulator.

   D. Reverse-acting describes which way the valve responds to a pressure change (closing on a rise instead of opening), not how the main port is driven. It can be direct- or pilot-operated.

2. Procedural · Connects control systems

   The ladder diagram for a walk-in cooler shows a pump-down control circuit. On a call for cooling, what does the box thermostat directly switch?

   1. The liquid-line solenoid; the low-pressure control then cycles the contactor.
   2. The compressor contactor coil directly, with the LLSV in parallel.
   3. The compressor contactor, with the LLSV opening on discharge pressure.
   4. The condenser fan; the compressor follows the fan.
   Show the answer

   A. Correct. In pump-down the thermostat's only load is the LLSV coil. When the stat closes, the solenoid opens, liquid feeds the coil, suction pressure rises above the low-pressure control's cut-in and the LPS contacts close the compressor contactor. When the stat opens, the solenoid shuts, the compressor pumps the evaporator down to the cut-out and the LPS stops it, leaving the low side empty so refrigerant cannot migrate into the crankcase during the off-cycle. The HPS and other safeties stay in series with the contactor coil.

   Why not the others

   B. Both switched together is a plain thermostat-cycled circuit: the compressor stops with the coil full of liquid and there is no pump-down at all.

   C. The LLSV is an electrical solenoid switched by the thermostat, not a pressure-operated valve. Discharge pressure plays no part in pump-down control.

   D. The condenser fan is controlled by the head-pressure control or runs with the compressor; it does not start the compressor. The stat's rung holds the solenoid.

3. Recall · Installs fasteners, brackets and hangers

   Which base material must not carry pipe or equipment hangers on its own?

   1. Solid sawn wood joist
   2. Corrugated metal roof deck
   3. Structural steel beam
   4. Cast-in-place concrete slab
   Show the answer

   B. Correct. The thin corrugated deck spans between joists and is designed to carry the roof, not point loads hung from below; screws or toggles into the deck tear out and can compromise the roofing above. Hangers are attached to the joists or beams with beam clamps or engineered deck-hanger brackets that transfer the load to structure, and the spec or engineer sets the allowable load.

   Why not the others

   A. A wood joist takes lag screws or through-bolts into its centre and is a normal hanger base in wood-frame construction, subject to load and edge-distance rules.

   C. Beam clamps and welded or bolted brackets on structural steel are the preferred hanger attachment in steel buildings. Structural steel is exactly where loads belong.

   D. A concrete slab is the standard base for drilled anchors; with the correct anchor, embedment and edge distance it carries hanger loads well.

4. Procedural · Places control system components

   Where is the defrost-termination / fan-delay (DTFD) thermostat mounted on a walk-in freezer evaporator?

   1. In the air stream at the fan discharge.
   2. On the drain-pan heater so it senses when the ice in the pan has melted.
   3. On the coil at the maker's specified return bend, usually where ice clears last.
   4. On the suction line at the compressor.
   Show the answer

   C. Correct. The DTFD ends defrost when the coil, not the air, has warmed enough to shed its ice, and holds the fans off until the coil is cold again so meltwater is not blown onto product. It is clamped to a coil return bend at the spot the maker specifies (typically the last part of the coil to clear, often the top or the far end from the defrost heater), with good metal-to-metal contact or heat-transfer compound. Mounted where the coil clears first, it terminates defrost with ice still on the rest of the coil.

   Why not the others

   A. Air temperature rises long before the ice is gone; an air-sensing DTFD terminates defrost early and leaves ice on the coil. The sensor clamps to the coil metal.

   B. The pan heater is a heated element; a thermostat on it would sense the heater, not the coil. The DTFD reads coil temperature.

   D. The suction line at the compressor is far from the coil and tells the control nothing about the ice. That location is used for other controls, not DTFD.

5. Procedural · Installs HVAC/R piping and tubing

   A horizontal suction line runs 25 m (80 ft) from a walk-in evaporator to a compressor that sits above it, with a 9 m (30 ft) riser at the compressor end. How is the line laid out for oil return?

   1. Pitched toward the compressor, trapped at the riser base and every 6 m (20 ft) of rise.
   2. Level, with one trap at the top of the riser.
   3. Pitched toward the compressor, with no traps so oil is never held up.
   4. Pitched toward the evaporator so oil drains back to the coil on the off-cycle.
   Show the answer

   A. Correct. The horizontal run slopes down toward the compressor (at least about 1 in 240: 12 mm per 3 m, or 1/2 in per 10 ft) so oil drains with the flow; the trap at the base of the riser collects oil so the gas velocity can lift it in slugs, and an intermediate trap about every 6 m (20 ft) of rise keeps the column of oil that must be lifted at one time small. An inverted trap at the top prevents drain-back to the evaporator on the off-cycle.

   Why not the others

   B. A trap at the top only stops drain-back; it does nothing to help oil climb the riser. The working trap belongs at the bottom, with the run pitched toward it.

   C. Without a base trap the oil puddles at the foot of the riser and is carried up only in surges when enough has collected, starving the compressor between surges. The trap makes the return regular.

   D. Oil in the evaporator reduces heat transfer and never reaches the crankcase, which is what fails. Suction lines slope toward the compressor.

02Answer key

03What block D covers

This block covers installing the system once it is planned. It includes checking deliveries against the take-off, setting condensing units, compressors and unit coolers, spring isolators and vibration eliminators, anchors, hangers and riser clamps, suction line pitch, oil traps and double risers, brazing and flaring, driers, sight glasses and TXV bulbs, holding charges, and wiring thermostats, transformers, pump-down circuits and DDC networks.

The Red Seal Occupational Standard names this block “Performs installation”. It carries 20.8% of the exam, about 26 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

Pitch and traps for oil return

Pitch horizontal suction lines toward the compressor, at least 12 mm per 3 m (1/2 in per 10 ft). Trap each suction riser at its base and about every 6 m (20 ft) of rise, so less oil is lifted at once.

TXV bulb and equalizer

On suction lines larger than 22 mm (7/8 in), strap the bulb at 4 or 8 o'clock on a horizontal run and insulate it. Tap the external equalizer just downstream of the bulb. A capped equalizer port starves the coil.

Wiring a pump-down circuit

In pump-down the thermostat switches only the liquid-line solenoid, and the low-pressure control cycles the compressor contactor. Putting the contactor on the thermostat stops the compressor with the coil full of liquid, which then migrates to the crankcase.

Sizing and tapping transformers

Add up the VA of every coil, relay and actuator; 42 VA of load needs a larger transformer than 40 VA. On a 208 V supply, a primary left on the 240 V tap gives about 20.8 V and contactors hum without pulling in.

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.