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HVAC NEC 2023

HVAC Controls & Sensors Practice Test 2

Free HVAC Controls & Sensors Practice Test 2. 25 questions covering thermistors, magnetic flow meters, Bourdon tube pressure sensors, ASHRAE humidity limits, thermopiles, self-powered VAV diffusers, capacitive humidity sensors, CO2 NDIR sensors, differential pressure transmitters, RTD wiring configurations, PIR occupancy sensors, velocity/static/total pressure relationships, duct pressure sensor placement, sensor calibration, and CO safety detection.

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HVAC Practice tests

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Frequently Asked Questions

Yes — completely free with no signup required.

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.

Key sensor review: Thermistor (NTC) — resistance decreases as temperature rises, very sensitive but non-linear. RTD (PT100) — resistance increases linearly with temperature; 100Ω at 0°C. Thermocouple — self-powered, millivolt output, best for high temps. Bourdon tube — mechanical pressure sensor, curls/uncurls with pressure changes. Thermopile — multiple thermocouples in series, self-powered, higher output voltage than a single thermocouple.

14

Questions

45 min

Time

70%

Pass

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HVAC Controls & Sensors Practice Test 2 — Question List

Question 1: Themisters are used to measure which of the following quantity?

Temperature
Resistance
Pressure
Humidity

Question 2: Which of the following is the magnetic flow meter output signal?

Voltage signal
Pneumatic signal
Hydraulic signal
Pressure signal

Question 3: The amount of water vapor that moisture contains increases rapidly with increase in temperature.

True
False

Question 4: What is the maximum allowed relative humidity to maintain indoor air quality as per ASHRAE?

65 %
85 %
90 %
70 %

Question 5: A bourdon tube sensor is used to measure ………..

Temperature
Pressure
Flow velocity
Humidity

Question 6: Magnetic flow measures the flow rate by ………………………. Caused by the moving fluid when exposed to strong magnetic field?

Magnetic induction
Electric induction
Acceleration of diaphragm
Movement of the coil

Question 7: The aim of HVAC control system

Lower energy cost
Increase flexibility
Lower operating cost
All the options are correct

Question 8: Which of the following device is self powered control device?

Thermopile
Thermocouple
Thermister
Thermostat

Question 9: What may be maximum pressure that self powered VAV diffuser can operate?

60 kpa
20 kpa
50 kpa
90 kpa

Question 10: Which of the following is not a self powered control device?

Thermopile
VAV diffuser
Hot Water Control Valve
Pneumatic

Question 11: How does a capacitive polymer humidity sensor measure relative humidity?

A capacitive humidity sensor measures humidity by changing the resistance of a wire exposed to moisture in the air
A capacitive polymer sensor absorbs water molecules which change the polymer dielectric constant and thus capacitance; electronics convert the capacitance change to an RH output signal (0-10V or 4-20mA)
Capacitive sensors cannot measure humidity — they are only used to measure pressure in HVAC systems
A capacitive humidity sensor works by measuring the dew point temperature using a chilled mirror

Question 12: What type of CO2 sensor is most commonly used in HVAC demand-controlled ventilation (DCV), and how does it work?

Chemical absorption sensors are the standard for CO2 measurement — they change color when CO2 is present
NDIR (Non-Dispersive Infrared) sensors are the standard — CO2 absorbs IR at 4.26 µm; less IR at the detector = higher CO2 concentration. Output: 4-20 mA proportional to CO2 ppm
Electrochemical CO2 sensors are used in HVAC because they are cheaper than infrared sensors
CO2 is measured with thermocouples — heat generated by CO2 combustion indicates concentration

Question 13: How does a differential pressure (DP) sensor measure airflow in a VAV box, and what is the relationship between velocity pressure and airflow?

A DP sensor in a VAV box measures the air temperature difference between supply and return to calculate airflow
A DP sensor measures velocity pressure (total − static); velocity = 4005 × √VP and CFM = velocity × duct area; the VAV controller modulates the damper to hit the CFM setpoint
VAV airflow is measured with a turbine flow meter installed in each branch duct
Differential pressure sensors in VAV boxes measure the refrigerant pressure, not airflow

Question 14: Why are 3-wire RTDs preferred over 2-wire RTDs in HVAC applications, and when would you use a 4-wire RTD?

3-wire RTDs are preferred because they require less copper wire and reduce installation cost
3-wire RTDs compensate for lead wire resistance using a third wire to measure and subtract lead resistance; 4-wire RTDs provide the highest accuracy by using separate current and sense wires for complete lead resistance elimination
2-wire RTDs are more accurate than 3-wire RTDs because they have fewer connections that could introduce resistance
RTD wiring type does not affect accuracy — the wire length only matters for signal loss in voltage-type sensors

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

Questions14

Time limit45 min

Pass score70%

Code editionNEC 2023

DifficultyIntermediate

Last reviewedJun 2026

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Source: NEC 2026 Art. 440 & electrical theory