A 3-ton system's records give these readings at the same blower tap, with a new filter each time. Last year: below the coil +0.42 in. w.c., above the coil +0.22 in. w.c., return −0.20 in. w.c. Today: below the coil +0.66 in. w.c., above the coil +0.20 in. w.c., return −0.18 in. w.c. What do the readings point to?

A fouled indoor coil restricting the airflow.

A blower that has lost speed from a weak motor.

Closed or blocked supply registers in the house.

A loaded filter at the return side of the furnace.

On a spring maintenance visit, a heat pump's subcooling reads below the maker's target and the homeowner asks you to add refrigerant. The leak-test notice on the unit, from an earlier visit, is eight months old and shows no leaks. What does O. Reg. 463/10 require before you refill it?

A new leak test with a no-leak notice, before refrigerant is added.

Nothing further, since the notice on the unit already shows no leaks.

The owner's signed consent to the refill, kept with the service record.

A report to the Ministry, since the system has lost refrigerant.

A ground-source heat pump's loop freeze limit has two settings: WATER, −1.1 °C (30 °F), and ANTIFREEZE, −9.4 °C (15 °F); the manual allows ANTIFREEZE only with the fluid protected to −12.2 °C (10 °F) or lower. It is on ANTIFREEZE; the refractometer shows protection to −6.1 °C (21 °F); entering fluid is −2.2 °C (28 °F). What keeps the unit heating safely all winter?

Strengthen the antifreeze to the required protection and keep the setting.

Top up the loop with plain water to raise its pressure and volume.

Keep both as found, since the entering fluid is above the −6.1 °C freeze point.

Switch the limit to WATER and leave the antifreeze as it is.

Before a season of superheat and subcooling readings, you check your pipe-clamp thermometers by setting their probes in a well-stirred mix of crushed ice and water. Why is that mix used?

It holds at 0 °C (32 °F) while ice remains, a fixed reference to read against.

Its temperature is close to a suction line's, so the probes are checked at working conditions.

It shows how fast each probe responds, which matters more than its accuracy.

It resets each probe's offset, since the probes zero themselves in ice water.

A fixed-orifice R-410A system's indoor coil was matted with dust and has just been cleaned, and nothing else was changed. Under the same indoor and outdoor conditions as before the cleaning, what should happen to the suction pressure?

It rises, as more air over the coil brings more heat to the refrigerant.

It stays the same, since the charge in the system has not changed.

It falls, since the clean coil takes heat out of the air more quickly.

It rises only on a TXV system, since the valve opens for the larger load.

The jacket of the flexible supply duct running through a house's attic is printed R-8. What does that R-value describe?

Its resistance to heat flow; a higher one lets less heat through.

Its resistance to airflow; a higher one adds more static pressure.

Its resistance to water vapour passing through the jacket's facing.

Its pressure rating, the most static the duct may safely carry.

313D Maintenance Practice Questions

Eight free practice questions on air-side, condensate, ventilation and refrigerant-side maintenance, for skill set E (Maintain) 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

Calculation · Our task E-19

You clean a fouled R-410A condenser coil at 29 °C (84 °F) outdoors. Before cleaning, the liquid pressure was 2 900 kPa (421 psig), bubble point 49.0 °C (120 °F). After cleaning, at the same outdoor temperature, it is 2 385 kPa (346 psig), bubble point 41.0 °C (106 °F), with the liquid line at 36 °C (97 °F). How far above outdoor air is the condensing temperature now?
1. 5.0 K
2. 7.0 K
3. 12.0 K
4. 20.0 K
Show the answer

C. Correct. The condensing temperature is the bubble point at the liquid pressure, 41.0 °C; minus the 29 °C outdoor air, that is 12.0 K. Before cleaning it was 49.0 − 29 = 20.0 K, so the cleaning took 8 K off the condensing temperature; compare the new figure with the maker's data for the unit.

Why not the others

A. 41.0 − 36 = 5.0 K is the bubble point minus the liquid-line temperature, which is the subcooling. Condensing temperature above outdoor air is the bubble point minus the outdoor temperature: 41.0 − 29 = 12.0 K.

B. 36 − 29 = 7.0 K uses the liquid-line temperature in place of the saturated condensing temperature. The condensing temperature is the bubble point at the liquid pressure, 41.0 °C, so the answer is 41.0 − 29 = 12.0 K.

D. 49.0 − 29 = 20.0 K uses the pressure from before the cleaning, the reading the cleaning was meant to change. After cleaning the bubble point is 41.0 °C, so the answer is 41.0 − 29 = 12.0 K.

Source: STO 313D counselling sheet, skill set E: Maintain (our sub-task E-19.02)
Critical · Our task E-19

A 3-ton system's records give these readings at the same blower tap, with a new filter each time. Last year: below the coil +0.42 in. w.c., above the coil +0.22 in. w.c., return −0.20 in. w.c. Today: below the coil +0.66 in. w.c., above the coil +0.20 in. w.c., return −0.18 in. w.c. What do the readings point to?
1. A blower that has lost speed from a weak motor.
2. Closed or blocked supply registers in the house.
3. A loaded filter at the return side of the furnace.
4. A fouled indoor coil restricting the airflow.
Show the answer

D. Correct. The coil's pressure drop is the reading below it minus the reading above it: 0.42 − 0.22 = 0.20 in. w.c. last year and 0.66 − 0.20 = 0.46 in. w.c. today, while the duct readings on both sides fell a little, as they do with less airflow. Only the coil became more restrictive: clean it and re-measure.

Why not the others

A. A weaker blower moves less air and every reading falls, including the one below the coil; here that reading rose from +0.42 to +0.66 in. w.c. Something downstream of the blower got more restrictive: the coil.

B. Restriction in the supply ducts would raise the reading above the coil; it fell from +0.22 to +0.20 in. w.c. The added restriction is between the two coil readings, in the coil itself.

C. A new filter was fitted each time, and a loaded one would make the return reading more negative; it went from −0.20 to −0.18 in. w.c. The rise is across the coil: 0.66 − 0.20 = 0.46 in. w.c. against 0.20.

Source: STO 313D counselling sheet, skill set E: Maintain (our sub-task E-19.04)
Procedural · Our task E-21

On a spring maintenance visit, a heat pump's subcooling reads below the maker's target and the homeowner asks you to add refrigerant. The leak-test notice on the unit, from an earlier visit, is eight months old and shows no leaks. What does O. Reg. 463/10 require before you refill it?
1. Nothing further, since the notice on the unit already shows no leaks.
2. A new leak test with a no-leak notice, before refrigerant is added.
3. The owner's signed consent to the refill, kept with the service record.
4. A report to the Ministry, since the system has lost refrigerant.
Show the answer

B. Correct. Section 25(1)(b) lets a certified person refill only if a leak-test notice no more than six months old shows no leaks; this one is eight months old. Test the system for leaks, affix the new notice under s. 23(3), and refill only if it shows no leaks; a leak found is repaired first.

Why not the others

A. The notice's age matters: s. 25(1)(b) accepts a no-leak notice only if it is no more than six months old. At eight months it no longer supports a refill.

C. The owner's agreement does not replace any refill condition; s. 25(1) needs a certified person, a no-leak notice no more than six months old and no apparent damage.

D. A report to the Ministry under s. 19(2) is for a discharge of 100 kg or more; a heat pump's charge is a small fraction of that. What the refill needs is a current no-leak notice.

Source: STO 313D counselling sheet, skill set E: Maintain (our sub-task E-21.01); O. Reg. 463/10
Critical · Our task E-21

A ground-source heat pump's loop freeze limit has two settings: WATER, −1.1 °C (30 °F), and ANTIFREEZE, −9.4 °C (15 °F); the manual allows ANTIFREEZE only with the fluid protected to −12.2 °C (10 °F) or lower. It is on ANTIFREEZE; the refractometer shows protection to −6.1 °C (21 °F); entering fluid is −2.2 °C (28 °F). What keeps the unit heating safely all winter?
1. Top up the loop with plain water to raise its pressure and volume.
2. Keep both as found, since the entering fluid is above the −6.1 °C freeze point.
3. Switch the limit to WATER and leave the antifreeze as it is.
4. Strengthen the antifreeze to the required protection and keep the setting.
Show the answer

D. Correct. With the limit at −9.4 °C (15 °F) and the fluid able to freeze at −6.1 °C (21 °F), fluid in the heat exchanger could freeze before the limit trips and split it. Raising the concentration to protect to −12.2 °C (10 °F) or lower meets the manual's condition, so the setting is safe and the unit keeps running on cold days.

Why not the others

A. Plain water dilutes the antifreeze and raises its freeze point further, the wrong direction. Any top-up uses antifreeze solution at a strength that restores the manual's protection.

B. The fluid leaving the heat exchanger is colder than the −2.2 °C (28 °F) entering it, and it falls further as the ground cools. The −9.4 °C trip sits below the −6.1 °C freeze point, so the exchanger is not protected.

C. The WATER setting trips at −1.1 °C (30 °F); with the entering fluid already at −2.2 °C (28 °F) the unit would lock out on every cold day and leave the house on backup heat. The fluid needs strengthening.

Source: STO 313D counselling sheet, skill set E: Maintain (our sub-task E-21.02)
Recall · Our task E-22

Before a season of superheat and subcooling readings, you check your pipe-clamp thermometers by setting their probes in a well-stirred mix of crushed ice and water. Why is that mix used?
1. Its temperature is close to a suction line's, so the probes are checked at working conditions.
2. It shows how fast each probe responds, which matters more than its accuracy.
3. It holds at 0 °C (32 °F) while ice remains, a fixed reference to read against.
4. It resets each probe's offset, since the probes zero themselves in ice water.
Show the answer

C. Correct. While ice and water are both present and the mix is stirred, it stays at the ice point, 0 °C (32 °F). A probe that reads 1 K (1.8 °F) off there carries that error into every superheat and subcooling figure, so recalibrate it, correct for it or replace it before relying on it.

Why not the others

A. A suction line in cooling runs well above 0 °C, and a liquid line far above it, so the bath is not chosen to match either. It is chosen because its temperature is fixed and known.

B. Response time is not what the bath checks. The bath holds a known temperature, so any probe that reads off it shows its error directly.

D. The bath does not change the probe; it only reveals its error. The offset is then corrected by the instrument's calibration function or noted, or the probe is replaced.

Source: STO 313D counselling sheet, skill set E: Maintain (our sub-task E-22.01)
Procedural · Our task E-19

A fixed-orifice R-410A system's indoor coil was matted with dust and has just been cleaned, and nothing else was changed. Under the same indoor and outdoor conditions as before the cleaning, what should happen to the suction pressure?
1. It stays the same, since the charge in the system has not changed.
2. It falls, since the clean coil takes heat out of the air more quickly.
3. It rises, as more air over the coil brings more heat to the refrigerant.
4. It rises only on a TXV system, since the valve opens for the larger load.
Show the answer

C. Correct. The matted coil starved the evaporator of air; with the airflow restored, more heat reaches the refrigerant and the saturated suction temperature and pressure rise. On a fixed orifice the superheat rises too.

Why not the others

A. Suction pressure depends on the load on the evaporator as well as the charge. Restoring the airflow raises the load the coil sees, so the suction pressure rises with the same charge.

B. A clean coil does transfer more heat, but that heat goes into the refrigerant and raises its boiling temperature and pressure. Low suction is the symptom of the matted coil, not the clean one.

D. A fixed-orifice system's suction pressure rises with evaporator load as well; on it the superheat rises too, because the orifice cannot open. The rise in suction comes from the airflow, whatever the metering device.

Source: STO 313D counselling sheet, skill set E: Maintain (our sub-task E-19.02)
Calculation · Our task E-19

A supply joint in a two-storey house's attic has come apart, and about 150 CFM of 12 °C (54 °F) supply air pours into the 52 °C (126 °F) attic instead of reaching the rooms, which are held at 23 °C (73 °F). Using 1.08 × CFM × ΔT (°F) for standard air, how much sensible cooling is lost to the rooms?
1. 3 078 Btu/h
2. 8 586 Btu/h
3. 11 664 Btu/h
4. 12 825 Btu/h
Show the answer

A. Correct. The leaking air takes with it the cooling it would have delivered to the rooms: 1.08 × 150 × (73 − 54) = 1.08 × 150 × 19 = 3 078 Btu/h, about a quarter of a ton of sensible capacity lost before it reaches a register. Reconnect and seal the joint, then insulate it.

Why not the others

B. 1.08 × 150 × (126 − 73) = 8 586 uses the difference between the attic and the rooms. The air never reaches the rooms; what they lose is the cooling it would have given them, warming from 54 °F to the 73 °F room air: 1.08 × 150 × 19 = 3 078 Btu/h.

C. 1.08 × 150 × (126 − 54) = 11 664 uses the attic temperature, which measures what the attic air gains, not what the rooms lose. The rooms lose the cooling that air would have given them: 1.08 × 150 × (73 − 54) = 3 078 Btu/h.

D. 4.5 × 150 × 19 = 12 825 uses the total-heat factor, which multiplies an enthalpy difference in Btu/lb, not a temperature difference. Sensible heat of standard air uses 1.08: 1.08 × 150 × 19 = 3 078 Btu/h.

Source: STO 313D counselling sheet, skill set E: Maintain (our sub-task E-19.04)
Recall · Our task E-19

The jacket of the flexible supply duct running through a house's attic is printed R-8. What does that R-value describe?
1. Its resistance to heat flow; a higher one lets less heat through.
2. Its resistance to airflow; a higher one adds more static pressure.
3. Its resistance to water vapour passing through the jacket's facing.
4. Its pressure rating, the most static the duct may safely carry.
Show the answer

A. Correct. R-value is thermal resistance, the inverse of the U-factor (R = 1/U): R-8 means U = 1/8 = 0.125 Btu/h per ft² per °F, half the heat flow of R-4 for the same area and temperature difference. In a hot attic it limits the heat the cooled supply air picks up on its way to the rooms.

Why not the others

B. Airflow resistance comes from the duct's inner core, its length, bends and any sag or kinks, not from the insulation wrapped outside it. The R-value rates the insulation's resistance to heat flow.

C. Water vapour is held back by the jacket's outer vapour retarder, which the R-value does not rate; thicker insulation keeps the jacket warmer, but that is its heat resistance at work. The R-value is the insulation's resistance to heat flow (R = 1/U).

D. A flexible duct's pressure and velocity limits are its maker's separate ratings. The R-value says how well its insulation resists heat flow.

Source: STO 313D counselling sheet, skill set E: Maintain (our sub-task E-19.04)

02Answer key

Question	Answer	Our task
Q1	C	Task E-19
Q2	D	Task E-19
Q3	B	Task E-21
Q4	D	Task E-21
Q5	C	Task E-22
Q6	C	Task E-19
Q7	A	Task E-19
Q8	A	Task E-19

Want the rest of skill set E?

The full TicketPrep 313D bank has 99 questions on air-side, condensate, ventilation and refrigerant-side maintenance (84 Standard and 15 Advanced) and 550 across all six skill sets (400 Standard + 150 Advanced), covering every one of our 28 tasks, with every option explained the way the eight above are. Its timed 130-question mock exams run four hours and score you against the 70 per cent pass mark. Try five free in the quiz, no account, or get the whole bank with a $69 one-time Exam Pass. All-Access ($119) covers 313D with 313A and every other exam on the site.

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03What skill set E covers

Skill set E is scheduled maintenance on residential cooling and heat-pump systems. Our five tasks for it are planning, recording and timing the maintenance; maintaining the air side; maintaining condensate, humidification and ventilation equipment; maintaining the refrigerant side; and working safely and keeping the site clean. The questions ask what a cleaning or a repair should change in the readings, how to read static pressure across a coil, how much capacity a duct leak costs, when O. Reg. 463/10 allows a refill, and how to keep instruments and ground-loop fluid fit for the season.

STO's 313D exam counselling sheet lists six weighted skill sets and gives Maintain 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 E 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.

Our task	In our bank
Task E-18 Plan, record and time the maintenance	15
Task E-19 Maintain the air side	37
Task E-20 Maintain condensate, humidification and ventilation equipment	25
Task E-21 Maintain the refrigerant side	17
Task E-22 Work safely and keep the site clean	5

04What the questions turn on

A clean coil shows in the numbers

On the condenser, the condensing temperature is the bubble point at the liquid pressure, and its rise over outdoor air is the figure to compare with the maker's data before and after cleaning. On the indoor coil of a fixed-orifice system, restoring the airflow brings more heat to the refrigerant, so suction pressure and superheat both rise after a matted coil is cleaned.

Static pressure and ducts

The coil's pressure drop is the reading below it minus the reading above it; when that drop grows year over year at the same blower tap and filter, while the duct readings fall slightly, the coil is fouling. A supply joint open to a hot attic loses the cooling its air would have delivered, 1.08 × CFM × the room-to-supply ΔT in °F, and the R-value printed on a flexible duct is its resistance to heat flow, R = 1/U.

No refill without a current leak-test notice

Low subcooling on a maintenance visit is not permission to add refrigerant. Under O. Reg. 463/10 s. 25(1)(b), a certified person may refill only if a leak-test notice no more than six months old shows no leaks. An eight-month-old notice is not enough: test the system, affix the new notice, and refill only if no leak is found.

Instruments and loop fluid fit for the season

A well-stirred mix of crushed ice and water stays at 0 °C (32 °F) while ice remains, so it is a fixed point to check pipe-clamp probes against before a season of superheat and subcooling readings. On a ground-source heat pump, the loop's freeze limit setting and the fluid's measured freeze protection have to agree: if the antifreeze is weaker than the manual requires for its setting, strengthen the fluid rather than leave the heat exchanger at risk.

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.