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HVAC NEC 2023

HVAC Controls & Sensors Practice Test 4

Free HVAC Controls & Sensors Practice Test 4. 25 questions covering capillary temperature sensors, resistance-to-cross-sectional area relationships, thermocouple self-powered operation, ultrasonic transit-time flow measurement, electronic vs non-electronic sensors, dew point temperature, thermocouple millivolt output, bi-metallic temperature sensing, psychrometrics, PT100 RTD fundamentals, thermocouple types J and K, sensor dead band, diaphragm pressure sensors, vortex flow meters, and CO2 sensor demand-controlled ventilation.

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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.

Sensor types in HVAC — quick reference: Capillary sensors fill a tube with temperature-sensitive liquid that expands/contracts, typically used in remote bulb thermostats and immersion temperature controls for ducts and tanks. Thermocouple — self-powered (generates millivolt output), no external power needed, best for high temperature measurement. Bimetallic strip — two metals bonded together with different thermal expansion rates, bends with temperature change (used in basic thermostats and freeze stats).

25

Questions

45 min

Time

70%

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HVAC Controls & Sensors Practice Test 4 — Question List

Question 1: ………………………… sensors are used to measure temperature in ducts, piper and tanks.

Capillary sensors
RTD
Bimetallic strips
Thermocouples

Question 2: The resistance of an element ……………….. to the cross sectional area of it.

Inversely proportional
Directly proportional

Question 3: Which of the sensors does not require external power supply?

RTD
Thermistor
Thermocouple
Thermostat

Question 4: In ultrasonic sensors, the fluid flow can be measured as a function of the ……………..

Difference between time travel of upward wave and the downstream wave
Difference between pressure of upward wave and the downstream wave
Pressure of the sound wave
Difference between velocity of upward and downstream wave

Question 5: Which of the following is not an electronic sensor?

Capillary sensor
RTD
Themister
Thermocouple

Question 6: The temperature which causes moisture in the air to condensate is …………………

Saturation temperature
Dew temperature
Super heat temperature
Boiling temperature

Question 7: What is the output voltage range of thermocouple temperature sensor?

Volts
milli-volts
Mega volts
Kilo volts

Question 8: What can be measured with bi-metalic strip sensor?

Temperature
Pressure
Humidity
Flow Velocity

Question 9: Which of the following sensors uses sound wave to measure the fluid velocity and fluid level?

Ultrasonic sensor
Magnetic flow meter
Pitot static tube
Capillary tubes

Question 10: The temperature –resistance characteristic of thermister is ……………

Linear
Non-linear

Question 11: In psychrometrics, if the dry-bulb temperature is 80°F and the wet-bulb temperature is 70°F, what does this indicate about the air condition?

A 10°F wet-bulb depression means the air is completely saturated (100% RH) and no further cooling is possible
The 10°F wet-bulb depression (80°F DB − 70°F WB) indicates moderately dry air at approximately 62% RH; larger wet-bulb depression = drier air. At 100% RH, wet-bulb equals dry-bulb (no evaporative cooling)
Wet-bulb depression is only used for outdoor air measurement — it cannot be measured inside a building
A 10°F wet-bulb depression means the air is dangerously humid and requires immediate dehumidification

Question 12: What does "PT100" mean for an RTD temperature sensor, and what is its approximate resistance at 0°C and at 100°C?

PT100 stands for platinum sensor with a 100-volt supply requirement
PT100 = Platinum RTD with 100 Ω at 0°C; it increases approximately 0.385 Ω per °C (so ~138.5 Ω at 100°C), providing a near-linear, stable, highly accurate temperature measurement
PT100 sensors are made from platinum and output exactly 100 mA of current proportional to temperature
PT100 is interchangeable with a 10K thermistor — they have the same resistance at all temperatures

Question 13: What is the difference between a Type J and Type K thermocouple in HVAC and industrial applications?

Type J and Type K thermocouples are identical — the letters only indicate the manufacturing batch, not the materials
Type J (Iron/Constantan) is good to 750°C but oxidizes above that; Type K (Chromel/Alumel) is the most common general-purpose thermocouple, rated to 1260°C. In HVAC, Type K is preferred for high-temperature flue gas and combustion measurements
Type K thermocouples are only used in residential applications — commercial HVAC always uses Type J
Type J is the most accurate thermocouple type and is required by ASHRAE 15 for refrigerant machinery room temperature monitoring

Question 14: What is "dead band" in an HVAC control system and how does it differ from the controller dead band vs system dead band?

Dead band should always be set to zero for the most responsive temperature control — any dead band is a code violation
Dead band is the range between heating and cooling setpoints where no action is taken; ASHRAE 90.1 requires at least 5°F dead band to prevent simultaneous heating and cooling and reduce equipment cycling
Dead band only applies to pneumatic controls — digital DDC systems do not need dead band because they are more precise
Dead band refers to the time between when an alarm is triggered and when the BAS notifies an operator

Question 15: How does a diaphragm pressure sensor work, and how does it differ from a Bourdon tube pressure gauge?

A diaphragm pressure sensor is identical to a Bourdon tube — both use a mechanical needle to indicate pressure
A diaphragm sensor uses a deflecting metal membrane with strain gauges to produce 4-20mA or 0-10V electronic output (suitable for DDC); a Bourdon tube is a mechanical C-shaped tube that moves a needle — visual-only, not electronic
Diaphragm sensors can only measure pressures above 100 PSI — they cannot measure low duct static pressure
Bourdon tube pressure sensors are the most accurate type available and are required in commercial HVAC systems

Question 16: How does a vortex flow meter work and what are its advantages in HVAC applications?

A vortex flow meter uses a spinning impeller whose rotation speed is proportional to flow velocity
A vortex meter uses a bluff body to create alternating vortex shedding; vortex frequency is proportional to flow velocity. Advantages: no moving parts, handles steam and hot water, 20-30:1 turndown
Vortex flow meters can only measure refrigerant flow and are not suitable for water or steam in HVAC
Vortex meters work by measuring the pressure drop across an orifice plate — similar to a Pitot tube measurement

Question 17: How does an orifice plate measure fluid flow in an HVAC piping system?

An orifice plate measures flow by counting the number of times a turbine wheel rotates per minute
An orifice plate creates a flow restriction; the differential pressure across it is proportional to Q² (Bernoulli); a DP transmitter measures this ΔP and a square-root extraction converts it to flow rate
An orifice plate is used only for gas flow — it cannot measure liquid flow in HVAC piping
The orifice plate generates a flow signal without any separate differential pressure transmitter

Question 18: What is a hot wire anemometer and when is it used in HVAC work?

A hot wire anemometer measures temperature by heating water in a tube and measuring the pressure generated
A hot wire anemometer measures air velocity by sensing how quickly moving air cools a heated wire (more velocity = more cooling); used in HVAC for low-velocity airflow at grilles and diffusers where pitot tubes are not sensitive enough
Hot wire anemometers are only used to measure outdoor wind speed — they cannot be used for duct air velocity measurement
An anemometer measures humidity by detecting moisture on a heated wire — it is the same as a hygrometer

Question 19: What is an "averaging sensor" used in HVAC ductwork, and when is it preferred over a single-point insertion probe?

An averaging sensor is used only in round ducts — rectangular ducts always use single-point probes
An averaging sensor spans the duct cross-section to measure a true average temperature, solving stratification problems in large ducts or after mixing boxes where a single point may read unrepresentatively hot or cold
Averaging sensors are less accurate than single-point probes because they measure many temperatures and may average out important high or low readings
An averaging sensor continuously rotates inside the duct to sample air at all positions over time

Question 20: Where should CO2 sensors be placed for demand-controlled ventilation (DCV), and what location should be avoided?

CO2 sensors for DCV must always be placed in the supply air duct to measure the outdoor air being supplied to the space
CO2 DCV sensors should be placed in the occupied zone (5-6 ft AFF) or in the return duct; avoid supply air ducts where outdoor air dilutes CO2 to near-ambient levels, making occupancy detection impossible
CO2 sensors must be placed directly on the ceiling at the highest point of the room where CO2 accumulates
CO2 sensors for DCV should be installed outdoors to measure the difference between indoor and outdoor CO2 levels

Question 21: What is a condensate overflow switch and how does it protect an HVAC system from water damage?

A condensate overflow switch measures the relative humidity of air leaving the cooling coil to detect low refrigerant
A condensate overflow (float) switch rises with drain pan water level if the drain is clogged; it shuts off the unit or triggers a BAS alarm to prevent overflow and ceiling water damage
Condensate overflow switches are only required for rooftop units — indoor air handlers do not need overflow protection
The condensate overflow switch monitors refrigerant pressure in the liquid line to detect inadequate condensate

Question 22: What is an aquastat and how does it protect a hot water boiler from overheating?

An aquastat measures the water flow rate in a hot water heating system to balance the distribution circuit
An aquastat is an immersion thermostat that controls boiler water temperature: an operating control maintains the desired temperature by cycling the burner, and a high-limit (often manual reset) shuts off the burner if temperature exceeds the safe maximum
Aquastats are only used in swimming pool heating systems — hot water boilers use electronic pressure switches for temperature control
The aquastat measures combustion gas temperature in the flue — it does not sense boiler water temperature

Question 23: What is the difference between a return air CO2 sensor and a zone (space) CO2 sensor for DCV, and what are the trade-offs?

Return air CO2 sensors are more accurate than zone sensors because return air is already mixed and represents a better sample
Zone CO2 sensors respond faster and enable individual zone DCV control; return air sensors are simpler and cheaper (one per AHU) but cannot pinpoint which zone needs ventilation — best for distributed, consistent-occupancy spaces
CO2 sensors must always be in the return air duct — zone (space) sensors are prohibited because occupants might tamper with them
Zone and return air CO2 sensors are identical in function and location — they are different names for the same installation

Question 24: What is the function of a high-pressure safety switch on a refrigeration system, and what typically causes it to trip?

The high-pressure safety switch is installed on the low side (suction) to monitor evaporator pressure
The high-pressure switch on the discharge side shuts the compressor off if head pressure exceeds the setpoint; common causes of tripping include dirty condenser coil, condenser fan failure, refrigerant overcharge, or non-condensables in the system
High-pressure switches only trip during cold weather because low temperatures increase refrigerant pressure
The HP switch automatically recharges refrigerant when it trips — it serves both a safety and charging function

Question 25: What are "low ambient controls" for an air conditioner, and why are they needed when cooling below 50°F outdoor temperatures?

Low ambient controls heat the refrigerant to ensure it stays in liquid form when outdoor temperatures drop below freezing
Low ambient controls maintain adequate head pressure for proper compressor and TXV operation during cold weather; methods include condenser fan cycling, fan VFD speed control, or condenser refrigerant flooding (backing up liquid to reduce effective condensing area)
Low ambient controls are not needed for R-410A systems — R-410A operates at the same efficiency at all outdoor temperatures
Low ambient controls only apply to heat pump systems — cooling-only air conditioners automatically shut off when outdoor temperatures drop below 65°F

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 limit45 min

Pass score70%

Code editionNEC 2023

DifficultyIntermediate

Last reviewedJun 2026

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Source: NEC 2026 Art. 440 & electrical theory