Eight free practice questions on start-up, control set-up, airflow and charge checks and commissioning, for skill set C (Commission) 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
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At commissioning the air entering the indoor coil reads 23.5 °C and the air leaving it 11.0 °C. The maker's temperature-split table is printed in °F. What split is compared with the table?
- 6.9 °F
- 12.5 °F
- 22.5 °F
- 54.5 °F
Show the answer
C. Correct. The split is 23.5 − 11.0 = 12.5 K, and a temperature difference converts by 1.8 alone: 12.5 × 1.8 = 22.5 °F. That is the figure read against the maker's split range.
Why not the others
A. This divides the 12.5 K difference by 1.8. Kelvin are the larger step, so a difference in °F is bigger: 12.5 × 1.8 = 22.5 °F.
B. This is the difference in kelvin with the unit relabelled. A temperature difference of 1 K equals 1.8 °F, so 12.5 K is 22.5 °F.
D. This applies the full temperature conversion (× 1.8 + 32) to a difference. The 32 belongs only to converting a temperature reading; a difference is 12.5 × 1.8 = 22.5 °F.
Source: STO 313D counselling sheet, skill set C: Commission (our sub-task C-12.01)
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A mechanic who charges both R-32 and R-454B systems notices that the R-32 pressure-temperature chart lists one saturation temperature at each pressure, while the R-454B chart lists a dew point and a bubble point. Why?
- R-32 is an A2L, and A2L charts print a single column.
- R-32 is charged as vapour, so only the dew column is needed.
- R-32's glide is too small to be worth printing.
- R-32 is a single-component refrigerant with no glide.
Show the answer
D. Correct. A pure refrigerant boils and condenses at one temperature for a given pressure, so dew and bubble are the same. R-454B is a zeotropic blend of R-32 and R-1234yf, so it boils over a small range: dew for superheat, bubble for subcooling.
Why not the others
A. R-454B is an A2L as well, and its chart has two columns. The column count follows whether the refrigerant is a blend, not its safety class.
B. How a refrigerant is charged does not change its chart. R-32 has one column because it is a pure fluid whose dew and bubble points coincide.
C. That describes a near-azeotropic blend such as R-410A, whose glide is a fraction of a kelvin. R-32 has no glide at all, because it is one substance.
Source: STO 313D counselling sheet, skill set C: Commission (our sub-task C-12.02)
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Commissioning on a dry spring day: return air 24.0 °C dry bulb and 14.0 °C wet bulb; supply air 11.0 °C. The maker's split table, by return wet bulb: 14 °C, 12 to 14 K; 17 °C, 9 to 11 K; 19 °C, 7 to 9 K. What does the split show?
- Airflow is low, since the split is above the 9 to 11 K range.
- Airflow is high, since dry air should give a smaller split.
- The charge is low, since a split this wide means a starved coil.
- Airflow is normal, since the split fits this wet bulb's range.
Show the answer
D. Correct. The split is 24.0 − 11.0 = 13.0 K. At a 14.0 °C return wet bulb the table allows 12 to 14 K, so the airflow is where the maker wants it. Dry return air means little latent load, so more of the coil's work shows as sensible cooling and a wider split.
Why not the others
A. That row is for a 17 °C return wet bulb, a typical summer figure, not today's 14 °C. Read against the 14 °C row, the 13.0 K split is inside the 12 to 14 K range.
B. Dry air works the other way: with little moisture to condense, more of the coil's capacity cools the air, so the split is larger. The table's 14 °C row reflects that with 12 to 14 K.
C. A coil starved of refrigerant cools less and gives a smaller split, not a larger one. Here the 13.0 K split matches the table for the measured wet bulb.
Source: STO 313D counselling sheet, skill set C: Commission (our sub-task C-12.01)
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A new air handler's set-up menu offers cooling airflow of 350 or 400 CFM per ton. For a humid lakeside house, what does the 350 setting change at the coil?
- More sensible capacity, with less moisture removal.
- More moisture removal, with less sensible capacity.
- More of both, because the coil runs colder.
- Less of both, because less air crosses the coil.
Show the answer
B. Correct. With less air per ton the coil runs colder, further below the air's dew point, so it condenses more moisture, while the smaller airflow carries away less sensible heat. Humid houses often benefit, within the maker's limits.
Why not the others
A. That is the effect of more airflow per ton: a warmer coil that cools more air but wrings out less water. Less airflow runs the coil colder and removes more moisture.
C. A colder coil removes more moisture, but less air across it carries less sensible heat, so sensible capacity falls. The trade is moisture for sensible capacity, not a gain in both.
D. Sensible capacity falls with less air, but moisture removal rises because the coil runs colder. The lower setting trades sensible capacity for dehumidification.
Source: STO 313D counselling sheet, skill set C: Commission (our sub-task C-12.01)
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An R-454B split is running in cooling. At the outdoor unit: suction 850 kPa (123 psig), where the P/T chart gives a dew point of 8.4 °C and a bubble point of 6.9 °C; suction line 14.0 °C; liquid 2 600 kPa (377 psig), bubble point 46.6 °C; liquid line 38.0 °C. What is the superheat?
- 5.6 K
- 7.1 K
- 8.6 K
- 29.6 K
Show the answer
A. Correct. Superheat is the suction-line temperature minus the saturated suction temperature, taken from the dew column for a blend: 14.0 − 8.4 = 5.6 K. At the dew point the last liquid has boiled, so anything above it is superheat.
Why not the others
B. This uses the bubble point at the suction pressure: 14.0 − 6.9 = 7.1 K. For a zeotropic blend such as R-454B superheat is read from the dew column, giving 5.6 K.
C. This is the liquid side's subcooling, 46.6 − 38.0 = 8.6 K, not superheat. Superheat comes from the suction side: 14.0 − 8.4 = 5.6 K.
D. This subtracts the suction dew point from the liquid-line temperature: 38.0 − 8.4 = 29.6 K. Superheat is measured on the suction line: 14.0 − 8.4 = 5.6 K.
Source: STO 313D counselling sheet, skill set C: Commission (our sub-task C-12.02)
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A split heat pump is running in heating. What does a gauge on the vapour-line (large line) service valve at the outdoor unit read?
- Compressor suction pressure, the low side.
- Compressor discharge pressure, the high side.
- Liquid pressure ahead of the outdoor metering device.
- Indoor coil pressure below its metering device.
Show the answer
B. Correct. In heating the reversing valve sends the compressor's hot discharge gas out the vapour line to the indoor coil, which is the condenser. That port reads high-side pressure; suction is read where the low side returns to the compressor.
Why not the others
A. That is true in cooling, when the vapour line returns gas from the indoor evaporator. In heating the same line carries discharge gas to the indoor coil, so its port reads the high side.
C. Liquid from the indoor coil returns through the small liquid line, not the vapour line. The vapour line carries the compressor's discharge gas in heating.
D. In heating the indoor coil is the condenser and its metering device is bypassed; the vapour line connects to the coil's hot-gas end. Its port reads compressor discharge pressure.
Source: STO 313D counselling sheet, skill set C: Commission (our sub-task C-12.02)
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An R-454B TXV split's label calls for 6 K (11 °F) subcooling. Liquid is 2 400 kPa (348 psig) and the liquid line 37.3 °C (99.1 °F). At 2 400 kPa the R-454B P/T chart gives a bubble point of 43.3 °C (109.9 °F); the R-410A chart gives 41.3 °C (106.3 °F). The digital manifold, still set to R-410A, shows 4.0 K subcooling. What should the mechanic do?
- Set the manifold to R-454B and leave the charge as it is.
- Recover refrigerant, since the subcooling reads high.
- Add refrigerant until the manifold reads 6 K.
- Recover and weigh in the whole charge, since the reading is in doubt.
Show the answer
A. Correct. With R-454B's bubble point, subcooling is 43.3 − 37.3 = 6.0 K, the label's target. The 4.0 K on the display comes from R-410A's chart (41.3 − 37.3); set the right refrigerant and the reading agrees.
Why not the others
B. Neither figure is high: the display shows 4.0 K and the true value is 6.0 K, on target. Recovering would undercharge a system that is right.
C. The manifold is converting pressure with R-410A's table, which gives a bubble point 2 K too low. Charging to its 6 K would push true subcooling to about 8 K and overcharge the system.
D. The doubt is in the instrument's setting, not the charge. Set the manifold to R-454B and the subcooling reads 6.0 K, on target; there is nothing to recover.
Source: STO 313D counselling sheet, skill set C: Commission (our sub-task C-12.02)
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A heat pump's cooling charging chart is entered with the indoor return-air wet bulb, but its heating chart with the indoor return-air dry bulb. Why does dry bulb suffice in heating?
- Wet bulb cannot be measured in the dry indoor air of winter.
- In winter indoor air, dry bulb and wet bulb are the same.
- The indoor coil only heats, so moisture does not load it.
- Heating charts are less precise, so the simpler reading is used.
Show the answer
C. Correct. In heating the indoor coil is the condenser: it warms the air without condensing any water from it, so its load depends on the air's dry-bulb temperature alone. In cooling the coil also removes moisture, so the chart needs wet bulb.
Why not the others
A. Wet bulb can be read in any air; in dry winter air it simply sits further below dry bulb. Dry bulb is used because the heating coil does nothing with moisture.
B. They are the same only in saturated air. Dry winter indoor air has a wet bulb well below its dry bulb; the heating chart uses dry bulb because moisture does not load the coil in heating.
D. The choice is not about precision. The indoor coil's load in heating is purely sensible, so dry bulb is the reading that fixes it.
Source: STO 313D counselling sheet, skill set C: Commission (our sub-task C-12.02)
02Answer key
| Question | Answer | Our task |
|---|---|---|
| Q1 | C | Task C-12 |
| Q2 | D | Task C-12 |
| Q3 | D | Task C-12 |
| Q4 | B | Task C-12 |
| Q5 | A | Task C-12 |
| Q6 | B | Task C-12 |
| Q7 | A | Task C-12 |
| Q8 | C | Task C-12 |
03What skill set C covers
Skill set C is the start-up and hand-over of a new residential system. Our four tasks for it are getting ready to start the system, setting up the controls, verifying airflow and refrigerant charge, and verifying operation and commissioning the whole system. The questions here lean on the readings a mechanic takes at start-up: the temperature split across the indoor coil, superheat and subcooling on single-component refrigerants and on zeotropic blends such as R-454B, airflow settings per ton, and the gauge readings on a heat pump in each mode. Each one asks what the readings mean against the maker's tables, which is how a commissioning check is judged.
STO's 313D exam counselling sheet lists six weighted skill sets and gives Commission 10 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 13 questions from skill set C is ours, worked out from the sheet's percentages. The four 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.
| Our task | In our bank |
|---|---|
| Task C-10 Get ready to start the system | 9 |
| Task C-11 Set up the controls | 11 |
| Task C-12 Verify airflow and refrigerant charge | 29 |
| Task C-13 Verify operation and commission the whole system | 11 |
04What the questions turn on
Temperature split, read against the maker's table
The split is the return air temperature minus the supply air temperature across the indoor coil. A temperature difference converts by 1.8 alone, with no 32 added, so a 12.5 K split is 22.5 °F. Makers' split tables are entered by the return-air wet bulb: dry return air means a small latent load and a wider split, so a 13 K split can be exactly right on a dry spring day.
Airflow per ton trades sensible for latent
When an air handler offers 350 or 400 CFM per ton, the lower setting runs the coil colder and further below the air's dew point, so it removes more moisture while carrying away less sensible heat. Humid houses often benefit from the lower setting, but only within the maker's limits.
Blends: dew for superheat, bubble for subcooling
A single-component refrigerant such as R-32 boils and condenses at one temperature for each pressure, so its pressure-temperature chart prints one column. A zeotropic blend such as R-454B has glide: superheat is measured from the dew point at suction pressure and subcooling from the bubble point at liquid pressure. A digital manifold left set to R-410A on an R-454B system gives the wrong subcooling, so the fix is the refrigerant setting, not the charge.
Heat pumps turn the gauges around
In heating the reversing valve sends discharge gas out the vapour line to the indoor coil, which becomes the condenser, so the large-line service valve at the outdoor unit reads high-side pressure. Because the indoor coil only heats the air in that mode and condenses no water from it, heating charge charts are entered with the return-air dry bulb, while cooling charts need the wet bulb.
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.