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HVAC NEC 2023

HVAC Certification Practice Test 17

This test covers furnace and air-handler safety switches — the sail (airflow proving) switch and the high-temperature limit switch — and how they fit into the control circuit as safety interlocks.

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HVAC Practice tests

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HVAC electrical exams cover electrical theory (Ohm's Law, circuits), transformers, single-phase and three-phase motors, control wiring, safety, and troubleshooting.

Requirements vary by state. HVAC technicians typically need EPA 608 certification and may need a separate electrical license for line voltage work.

Exam Tip: Both the sail switch and limit switch are safety interlocks — like the refrigerant-side pressure switches, an open safety switch removes power from the circuit it protects.

25

Questions

25 min

Time

70%

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HVAC Certification Practice Test 17 — Question List

Question 1: What does a sail switch (airflow proving switch) confirm before allowing a gas furnace to continue its heating cycle?

That adequate airflow is actually present
That the refrigerant charge is correct
That the thermostat batteries are charged
That the outdoor temperature is above freezing

Question 2: How does a sail switch physically detect airflow?

A small paddle (sail) is pushed by moving air, mechanically closing the switch contacts
It measures refrigerant pressure directly
It uses a photoelectric beam to count air molecules
It relies on outdoor temperature readings only

Question 3: What does a furnace high-temperature limit switch do?

It opens and shuts off the heating call if the plenum/furnace temperature gets too high
It controls the outdoor condenser fan cycling
It measures refrigerant subcooling
It only operates during the cooling season

Question 4: What is the normal resting state of a furnace high-temperature limit switch, similar to the refrigerant-side pressure switches?

Normally closed, opening only when temperature exceeds a safe threshold
Normally open, closing only when temperature exceeds a safe threshold
It has no defined resting state
It is always open regardless of temperature

Question 5: What is a common cause of a sail switch failing to prove airflow, preventing a furnace from firing?

A dirty air filter or blocked return restricting airflow
A fully charged refrigerant system
A properly sized control transformer
Correct thermostat wire colors

Question 6: What is a common cause of a furnace limit switch tripping repeatedly?

Restricted airflow, such as a dirty filter, blocked ducts, or a failed blower motor
A properly sized capacitor
Normal outdoor humidity levels
Correct refrigerant subcooling

Question 7: Like the refrigerant-side pressure switches, how are the sail switch and limit switch typically wired relative to the circuit they protect?

In series, so either one opening removes power from the heating call circuit
In parallel, so both must open together to have any effect
They are not electrically connected to the heating circuit at all
Only one of the two is ever wired into the circuit

Question 8: Why must airflow generally be proven before (or very early in) the heating cycle, rather than only being checked after the burner has been running a while?

Heat produced without adequate airflow to carry it away can quickly cause dangerous overheating
There is no safety reason, it is purely a manufacturing convention
Airflow timing only matters for the cooling cycle, never heating
It is only relevant to electric heat strips, never gas furnaces

Question 9: On many furnaces, what does the limit switch do to the blower in addition to shutting off the burner if it trips?

It can also energize the blower to run and dissipate residual heat, even if the thermostat is not calling for fan
It always shuts off the blower completely with no exceptions
It has no interaction with the blower whatsoever
It only affects the outdoor condenser fan

Question 10: How do the sail switch and limit switch differ in what triggers them?

The sail switch responds to a lack of airflow; the limit switch responds to excess temperature
Both switches respond only to refrigerant pressure
The sail switch responds to temperature; the limit switch responds to airflow
They are two names for the exact same device

Question 11: A furnace runs for several minutes, then shuts off the burner while the blower keeps running briefly afterward, then the cycle attempts to restart. What safety device's behavior does this most closely match?

The limit switch tripping due to excess temperature, then resetting once temperature drops
The condenser fan cycling pressure switch
The reversing valve energizing for defrost
The thermostat's C wire disconnecting

Question 12: A furnace's blower runs, but the burner never fires at all, and a very dirty filter is found during inspection. What safety device is the most likely explanation?

The sail switch is not proving airflow due to restricted flow from the dirty filter
The condenser fan motor has failed
The crankcase heater has failed
The reversing valve is stuck

Question 13: What generally happens to most furnace limit switches once the excessive temperature condition clears?

They automatically reset (re-close) once temperature drops back to a safe level
They permanently destroy themselves after tripping once
They require the entire furnace to be replaced after every trip
They have no reset behavior of any kind

Question 14: A limit switch keeps tripping and auto-resetting repeatedly over a single heating cycle. Is this acceptable to leave as-is since it "resets itself"?

Yes, since it resets automatically there is nothing to worry about
No — repeated tripping means the underlying cause (like restricted airflow) needs to be diagnosed and fixed
The switch should simply be removed from the circuit permanently
This behavior only matters in summer

Question 15: How is the sail switch conceptually similar to the refrigerant-side low-pressure switch discussed elsewhere in this guide?

Both are safety interlocks that must confirm a safe operating condition before allowing equipment to run
They are electrically identical devices used interchangeably
They have no functional similarity at all
Only the sail switch is a real safety device

Question 16: How might a technician verify a sail switch is closing properly during normal blower operation?

Check for continuity or the expected voltage/logic state across the switch while the blower is running with adequate airflow
The switch can only be checked by disassembling the entire furnace
Sail switches cannot be tested with any electrical meter
Only a refrigerant gauge set can check a sail switch

Question 17: Where is a furnace limit switch typically positioned to best sense a dangerous temperature rise?

In or near the furnace plenum, where it can directly sense the heated air/furnace temperature
Outdoors, near the condensing unit
Inside the thermostat housing only
On the liquid line, near the metering device

Question 18: True or False: A sail switch's only job is to shut things down after a problem has already occurred, similar to how a limit switch reacts after overheating starts.

True
False
Only true on electric furnaces
Only true on very old equipment

Question 19: Why might a larger furnace have more than one limit switch installed?

To monitor temperature at multiple points, providing more complete protection across the unit
Extra limit switches are purely decorative and serve no function
This is done only to increase manufacturing cost with no benefit
Multiple limit switches are never used on any real equipment

Question 20: If a sail switch is open (not proving airflow) while a limit switch is closed (temperature normal), what happens to the heating call circuit if both are wired in series as expected?

The circuit stays open (de-energized) because of the open sail switch, regardless of the limit switch state
The circuit energizes anyway because the limit switch is closed
Both switches must be open at the same time to have any effect
Series wiring means only the limit switch state matters

Question 21: What would be the safety concern if a sail switch failed in a permanently closed position, always reading as if airflow were present?

The furnace could be allowed to fire even during an actual airflow blockage, risking overheating
There is no safety concern in this failure mode
It would only affect the cooling cycle, not heating
It would automatically shut off the entire electrical service

Question 22: What would be the safety concern if a limit switch failed in a permanently closed (welded shut) position?

It would no longer be able to open and shut off the heating call during a genuine overheating event
This failure mode has no practical consequence
It would immediately stop the blower motor permanently
It would only affect the condenser fan

Question 23: Match the safety device to what it senses: which one directly senses air movement rather than temperature?

The limit switch
The sail switch
The high-pressure switch
The low-pressure switch

Question 24: Match the safety device to what it senses: which one directly senses excess heat rather than airflow?

The limit switch
The sail switch
The crankcase heater
The reversing valve solenoid

Question 25: A furnace fires normally, but shuts down and locks out after tripping its limit switch three times in one heating cycle, even though the filter was recently changed. What should the technician check next?

Nothing further, the problem must be solved since the filter was changed
The blower motor and duct system for another airflow restriction, since a fresh filter rules out that specific cause
Only the outdoor condenser fan needs inspection
Only the refrigerant charge needs inspection

Key NEC References for This Test

Article	What It Covers
Art. 440	Air Conditioning Equipment — use nameplate MCA and MOCP, not FLA alone
Art. 430	Motors — general motor rules that Art. 440 modifies

Study Tip: Practice reading ladder diagrams. Trace each rung from L1 to the load: contacts open → circuit dead. Contacts closed → current flows. This is how HVAC electrical troubleshooting works.

EXAM CARD HVAC

Questions25

Time limit25 min

Pass score70%

Code editionNEC 2023

DifficultyIntermediate

Last reviewedJun 2026

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Source: NEC 2026 Art. 440 & electrical theory