313D exam, skill set B: Assemble and Install practice questions.

Eight free practice questions on piping, pressure testing, evacuation, charging and wiring, for skill set B (Assemble and Install) of Ontario's 313D Residential Air Conditioning Systems Mechanic Certificate of Qualification exam, which Skilled Trades Ontario (STO) sets and delivers. 313D is an Ontario trade with its own exam, separate from the 313A Refrigeration and Air Conditioning Systems Mechanic. These are original questions written by TicketPrep from public sources, Ontario regulations and the trade's working practice, not questions from the exam. Each has four options and one correct answer, as every question in our bank does. Answer each one before opening the explanation. No account, no email.

01The questions

1. Critical · Our task B-7

   During a nitrogen standing test on a new ductless line set, the regulator is set at 2 760 kPa (400 psig) and the cylinder valve and the manifold are left open. The gauge holds at 2 760 kPa for 30 minutes and the test is signed off. Next morning a flare nut at the head foams with bubble solution. Why did the gauge not show the leak?

   1. Nitrogen kept flowing in from the supply to replace the loss.
   2. The overnight temperature drop hid the leak by lowering the pressure.
   3. Nitrogen leaks too slowly to show until a higher pressure is used.
   4. Flare leaks seal under steady pressure and open only as they cool.
   Show the answer

   A. Correct. With the cylinder still connected and open, the regulator keeps feeding nitrogen to hold its set pressure, so a leak shows no drop. For a standing test the cylinder is shut off and disconnected or valved out, leaving the system alone with the gauge.

   Why not the others

   B. A temperature drop makes the pressure fall, so it adds to a leak's drop rather than hiding it. The gauge held because the regulator was still feeding gas.

   C. Nitrogen escapes through a flare leak readily, and the bubble solution found it at the same pressure. The gauge held only because the open cylinder made up the loss.

   D. A leaking flare leaks under steady pressure too, which is why bubble solution finds it. The reading held because the regulator kept making up the loss from the open cylinder.

   Source: STO 313D counselling sheet, skill set B: Assemble and Install (our sub-task B-7.01)

2. Critical · Our task B-7

   Ductless install at a cottage. The manual: evacuate below 300 µm, isolate, and the gauge must not rise above 700 µm within 15 minutes. You isolate at 280 µm and read: 2 minutes 450 µm, 5 minutes 520 µm, 10 minutes 545 µm, 15 minutes 550 µm. What do these readings show?

   1. It fails: the quick early rise means the pump oil is wet.
   2. It fails: a stall in the rise means moisture is still present.
   3. It is not finished: the decay must run 30 minutes.
   4. It passes: the rise has levelled off well under the limit.
   Show the answer

   D. Correct. A quick early rise that flattens out, here at about 550 µm, is the normal settling of gas from the surfaces and the oil, and it stays under the manual's 700 µm. The system is tight and dry enough; the service valves can be opened.

   Why not the others

   A. The pump is isolated during the decay test, so its oil cannot affect the reading. The rise levelled under the limit, so the system passes.

   B. A stall above the limit points to moisture, but this rise flattens near 550 µm, under the 700 µm limit, so it meets the manual. More pumping is not needed.

   C. The manual sets this test: isolate and watch for 15 minutes against a 700 µm limit. The readings meet it and have levelled off well under the limit; a longer hold is not what this manual asks.

   Source: STO 313D counselling sheet, skill set B: Assemble and Install (our sub-task B-7.02)

3. Calculation · Our task B-7

   A new 2-ton R-410A split system's nameplate charge covers a 4.6 m (15 ft) line set. The outdoor unit is 9.1 m (30 ft) from the furnace along the route, and the line set installed is 13.7 m (45 ft) of tubing, the 4.6 m (15 ft) the route does not need left in place as the manual allows. The manual calls for 0.6 oz of R-410A per foot of line set beyond 15 ft. How much refrigerant is weighed in on top of the factory charge?

   1. 9 oz
   2. 18 oz
   3. 27 oz
   4. 36 oz
   Show the answer

   B. Correct. The adjustment follows the tubing installed, spare length included, beyond what the nameplate charge covers: 45 - 15 = 30 ft, and 30 ft x 0.6 oz/ft = 18 oz (1 lb 2 oz), weighed in as liquid. Superheat or subcooling then fine-tunes the charge to the maker's target.

   Why not the others

   A. This uses the 30 ft route distance instead of the tubing installed: (30 - 15) x 0.6 = 9 oz. The refrigerant fills the whole tubing, spare length included, so the length is 45 ft: (45 - 15) x 0.6 = 18 oz.

   C. This applies the rate to the whole line set: 45 x 0.6 = 27 oz. The nameplate charge already covers the first 15 ft, so only (45 - 15) x 0.6 = 18 oz is added.

   D. This counts the liquid and suction tubes as two separate lengths: 2 x 30 x 0.6 = 36 oz. The rate is per foot of line set, the pair of tubes together, so the addition is 30 x 0.6 = 18 oz.

   Source: STO 313D counselling sheet, skill set B: Assemble and Install (our sub-task B-7.03)

4. Calculation · Our task B-8

   A house's heat loss at its −25 °C (−13 °F) design temperature is 15.8 kW. At that temperature the maker's data give the heat pump a COP (heat delivered ÷ electrical input) of 1.9 while it draws 3.1 kW. The air handler's heat strips are to cover the heat pump's shortfall at design. To the nearest 0.1 kW, what strip capacity is needed?

   1. 5.9 kW
   2. 9.9 kW
   3. 12.7 kW
   4. 14.2 kW
   Show the answer

   B. Correct. Heat-pump output = COP × input = 1.9 × 3.1 = 5.89 kW. Shortfall = 15.8 − 5.89 = 9.91 kW, which rounds to 9.9 kW. The smallest kit the air handler lists at or above that is chosen; the strips also temper the supply air during defrost.

   Why not the others

   A. 1.9 × 3.1 = 5.9 kW is the heat pump's own output at design, not the shortfall. The strips supply what the heat pump cannot: 15.8 − 5.89 = 9.9 kW.

   C. 15.8 − 3.1 = 12.7 kW subtracts the electrical input as if it were the heat delivered. At a COP of 1.9 the heat pump delivers 1.9 × 3.1 = 5.89 kW, leaving 15.8 − 5.89 = 9.9 kW.

   D. 15.8 − (3.1 ÷ 1.9) = 15.8 − 1.63 = 14.2 kW divides by the COP instead of multiplying. Output = COP × input = 5.89 kW, so the shortfall is 15.8 − 5.89 = 9.9 kW.

   Source: STO 313D counselling sheet, skill set B: Assemble and Install (our sub-task B-8.01)

5. Critical · Our task B-8

   A two-zone ductless outdoor unit with no automatic port-matching feature serves a den head and a guest-room head. Ports A and B each have their own refrigerant connections, expansion valve and interconnect terminals. At start-up only the den is set to cool: the den head runs its fan but blows room-temperature air, while the guest-room head, switched off, has a sweating coil and cold lines. What was installed wrong?

   1. Each head's line set and cable go to different ports.
   2. The den head's line set is short of refrigerant charge.
   3. The den head's coil sensor has failed open circuit.
   4. The outdoor reversing valve is stuck in heating.
   Show the answer

   A. Correct. The den head is wired to one port's terminals but piped to the other port. When it calls, the outdoor unit opens the expansion valve for the port it is wired to, which feeds the guest-room head's lines, so that coil goes cold and the calling head gets nothing. Each head's line set and cable must land on the same port letter.

   Why not the others

   B. Low charge would leave the den coil partly cold and starved; it would not send cold refrigerant into the idle guest-room head. Refrigerant is reaching the wrong head.

   C. A failed coil sensor would raise a fault code or stop the den head; it cannot route refrigerant into the other head's lines.

   D. Stuck in heating, the guest-room lines would run hot, not cold and sweating. The system is cooling; it is cooling the wrong head.

   Source: STO 313D counselling sheet, skill set B: Assemble and Install (our sub-task B-8.02)

6. Critical · Our task B-9

   A two-storey house has two new condensing units side by side, one per floor, each with its own disconnect on the wall beside it. With the disconnect beside unit 1 open and locked, a low-impedance tester proven on a live source reads 240 V across unit 1's contactor line terminals. When the disconnect beside unit 2 is opened, those terminals read 0 V. What is the most likely cause?

   1. Unit 1's disconnect has welded blades that do not open.
   2. Unit 1 is fed through the disconnect set beside unit 2.
   3. Unit 2's 24 V control wiring is back-feeding unit 1.
   4. The tester is reading induced voltage on idle conductors.
   Show the answer

   B. Correct. Unit 1's terminals stay live with its own disconnect locked open and go dead only when unit 2's disconnect opens, so the feeds are crossed between the disconnects and the units. Each disconnect must open the unit it stands beside, or locking it out protects no one: correct the wiring, label both, then prove each unit dead at its own terminals.

   Why not the others

   A. Welded blades would keep unit 1 live whichever other switch was opened. Here opening unit 2's disconnect kills unit 1's terminals, so unit 1's supply passes through that switch.

   C. Control wiring carries 24 V from an indoor transformer, not 240 V across the line terminals, and opening unit 2's disconnect would not cut it. The 240 V follows unit 2's disconnect because that switch feeds unit 1.

   D. A low-impedance tester loads out induced voltage, and induced voltage would not vanish when an unrelated switch opens. The 240 V is a solid supply arriving through unit 2's disconnect.

   Source: STO 313D counselling sheet, skill set B: Assemble and Install (our sub-task B-9.01)

7. Recall · Our task B-7

   A new line set and indoor coil hold a deep vacuum, and the decay reading stays steady. Why does that not replace the nitrogen pressure test?

   1. A vacuum can reveal leaks at the brazed joints but never at the flares.
   2. A joint under vacuum sees one atmosphere at most, not running pressure.
   3. A micron gauge reads gauge pressure, so it cannot see the small leaks.
   4. O. Reg. 463/10 does not accept a vacuum hold as a leak test on its own.
   Show the answer

   B. Correct. Under vacuum the pressure difference across a joint can never exceed atmospheric pressure, about 101 kPa (14.7 psi), a fraction of what the system holds when it runs. A joint that seals at that load can still open under pressure, so the nitrogen test at the manual's pressure comes first.

   Why not the others

   A. A leak at a flare lets air in under vacuum just as a brazed-joint leak does. The limit of a vacuum test is the small pressure difference it puts across every joint.

   C. A micron gauge reads absolute pressure, and it is sensitive enough to show small leaks as a steady rise. The limit is the one-atmosphere load a vacuum puts on the joints.

   D. O. Reg. 463/10 does not deal with vacuum holds; its testing rule bars using refrigerant as a test gas. The reason is physical: a vacuum loads each joint with at most one atmosphere.

   Source: STO 313D counselling sheet, skill set B: Assemble and Install (our sub-task B-7.02); O. Reg. 463/10

8. Recall · Our task B-7

   A digital micron gauge on a new line set reads 450 µm during evacuation. What does a micron of vacuum measure?

   1. Gauge pressure below atmosphere, in thousandths of an inch of mercury.
   2. Vacuum depth below atmosphere, in millionths of a metre of mercury.
   3. Absolute pressure, in thousandths of a millimetre of mercury.
   4. Water vapour left in the system, in parts per million.
   Show the answer

   C. Correct. One micron is 0.001 mm of mercury, counted up from a perfect vacuum, so 450 µm is a very low absolute pressure; standard atmosphere is 760 mm, or 760 000 µm. A compound gauge cannot resolve this range.

   Why not the others

   A. A micron reading is absolute, not gauge, and the unit is metric: a thousandth of a millimetre of mercury. Zero microns is a perfect vacuum.

   B. The unit is right, a micrometre of mercury, but the reading is absolute pressure counted up from a perfect vacuum, not depth below atmosphere: 0 µm is a perfect vacuum and the atmosphere is about 760 000 µm.

   D. The gauge measures total absolute pressure, whatever the gas. Moisture shows up only indirectly, as a rise that stalls on the decay test.

   Source: STO 313D counselling sheet, skill set B: Assemble and Install (our sub-task B-7.02)

02Answer key

03What skill set B covers

Skill set B takes a residential system from delivery to a sealed, charged and wired installation. Our five tasks for it are receiving the equipment, preparing the site and setting the units; installing the refrigerant piping; leak testing, evacuating and charging a new system; installing the equipment, air distribution and accessories; and installing the power and control wiring. Split systems, ductless heads, heat pumps with backup heat and their line sets run through most of the questions. Many are about reading what a test result really shows, because a standing test or a vacuum decay can look like a pass when it is not, and a wiring or porting mistake often shows up only at start-up.

STO's 313D exam counselling sheet lists six weighted skill sets and gives Assemble and Install 21 per cent of the exam, with a pass mark of 70 per cent. The sheet's body says “Updated as of December 5, 2016” and follows the older training standard, not the one in force since December 2023, so its weights may not match today's exam. STO publishes no question count for the 313D exam, only that its exams have between 90 and 150 questions. Our mock is 130 questions in 4 hours, and its draw of 27 questions from skill set B is ours, worked out from the sheet's percentages. The five tasks below and the right-hand column are TicketPrep's own grouping: the column is how many questions our bank holds on each task, Standard and Advanced together, not a count from the exam.

04What the questions turn on

A standing test means the supply is off

A nitrogen standing test proves a joint only when the system is left alone with its gauge: if the cylinder valve and regulator stay open, the regulator keeps feeding nitrogen to hold its set pressure and a leaking flare shows no drop. A deep vacuum that holds does not replace the pressure test either, because under vacuum a joint never sees more than one atmosphere, about 101 kPa (14.7 psi), across it, a fraction of its running pressure.

Reading microns and a decay test

A micron is 0.001 mm of mercury counted up from a perfect vacuum, so it is an absolute pressure: standard atmosphere is 760 000 µm, and a compound gauge cannot resolve this range. After isolation, a quick early rise that levels off, here near 550 µm against a manual's 700 µm limit, is gas settling out of the surfaces and the oil, not a failed system. The manual's own target and decay limit decide the result.

Charge adjustments follow the tubing installed

The nameplate charge covers a stated line-set length. The refrigerant added on top of it is the installed tubing beyond that length, spare tubing left in place included, times the manual's rate, weighed in as liquid; superheat or subcooling then fine-tunes the charge to the maker's target. Sizing heat strips for a heat pump works the same way from the maker's data: the heat pump's output at design (COP × input) is subtracted from the design heat loss, and the strip kit covers the rest.

Each head and each disconnect must match its own unit

On a multi-zone ductless outdoor unit without automatic port matching, each head's line set and its interconnect cable must land on the same port letter: crossed ports make the calling head blow room air while the idle head's coil sweats. The same idea applies to power. A disconnect beside a condensing unit must open that unit, and a proven tester that still reads 240 V at the contactor with that disconnect locked open means the feeds are crossed; correct and label them, then prove each unit dead at its own terminals.

05The other skill sets

STO publishes no minimum score for any one skill set, only the 70 per cent pass mark on the whole exam, so every skill set's marks count toward it. Each of the six skill sets has its own page of free questions:

The 313D practice exam page describes the full question bank. Working toward the full-scope trade as well? See the 313A practice exam; the two trades share their first-level training. More free questions for every exam we cover: free practice questions. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it. TicketPrep is not affiliated with Skilled Trades Ontario or CSA Group. CSA B52, CSA F280 and the Ontario Electrical Safety Code are cited by name only and never reproduced; Ontario regulations such as O. Reg. 875/21 and O. Reg. 463/10 are quoted only briefly.