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HVAC NEC 2023

HVAC Controls & Sensors Practice Test 3

Free HVAC Controls & Sensors Practice Test 3. 25 questions on air pressure principles, on/off vs modulating control, HVAC control system benefits, controller-sensor relationships, analog vs digital sensors, non-contact sensing, voltage sensors, RTD types, sensor-transducer-transmitter assemblies, open-loop vs closed-loop control, BAS hierarchy levels, pneumatic thermostat operation, VAV box control sequences, dual-duct systems, and control valve authority.

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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.

Control signal types review: Analog input — variable continuous signal, represents a measured quantity (e.g., temperature, pressure, humidity). Examples: 0-10V DC, 4-20mA. Digital (binary) input — two-position on/off signal from a switch, flow switch, pressure switch, or status contact. The controller reads digital inputs as TRUE/FALSE or 1/0. Most HVAC control systems work with both types simultaneously.

25

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45 min

Time

70%

Pass

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HVAC Controls & Sensors Practice Test 3 — Question List

Question 1: Normally the air moves from areas of …………………. to ………………. With the available openings

High pressure, Low pressure
Low pressure, High pressure
High temperature, Low temperature
Low temperature, High temperature

Question 2: Which is the simplest HVAC control?

Cycling
Modulating

Question 3: Which of the following are benefits of HVAC control system?

Maintain thermal comfort conditions
Monitoring the system performance
Increases the efficiency of system
All the options are correct

Question 4: In HVAC control system, the controller gets the signal from

Sensor
Controlled device
Thermopile
Fluid

Question 5: Which of the following sensor is used to provide two position (open or close) signals?

Analog sensor
Digital sensor

Question 6: Which of the following sensor works without contact with measuring quantity?

RTD
Ultrasonic
Thermocouple
Thermister

Question 7: Current sensors are used to measure ………….

Temperature and pressure only
Humidity and temperature and velocity
Pressure and humidity
Temperature, humidity and pressure

Question 8: What is the typical input voltage range to voltage sensor?

0 to 10 Vdc
4 to 20 Vdc
10 to 20 Vdc
0 to 50 Vdc

Question 9: Which of the following is not RTD?

BALCO
10K themister
Bimetallic
Copper

Question 10: …………………………. assembly is called transmitter in control system.

Sensor-transducer
Sensor-controller
Sensor-signal conditioner
Sensor-controlled device

Question 11: What is the difference between open-loop and closed-loop control in HVAC systems, and give an example of each?

Open-loop control is more accurate than closed-loop because it does not require a feedback sensor that could be wrong
Closed-loop (feedback) control continuously measures the controlled variable and adjusts output to minimize error (example: thermostat). Open-loop control acts on a predetermined schedule without measuring the result (example: on/off timer)
Open-loop and closed-loop are terms for two-position and modulating control respectively — they describe the output type, not feedback
Closed-loop control is only used in cooling systems — heating systems always use open-loop (timer-based) control

Question 12: What are the three levels of a modern Building Automation System (BAS) hierarchy?

BAS has only one level — all functions (sensing, control, monitoring) are performed by a single central computer
BAS has three levels: (1) field/device level (DDC controllers, sensors, actuators); (2) supervisory level (plant coordination, optimization); (3) enterprise/management level (operator workstations, trending, reporting, remote access)
The BAS hierarchy levels refer to security access levels — level 1 is administrator, level 2 is technician, level 3 is read-only
BAS hierarchy only applies to chiller plants — single-AHU systems have no hierarchy

Question 13: How does a pneumatic room thermostat work, and what supply air pressure is typically used in a pneumatic control system?

Pneumatic thermostats operate on 120V AC power and send an electrical signal through the air tubing to control valves
A pneumatic thermostat bimetallic element bends with temperature and modulates a nozzle-bleed port; output is 3-15 PSI on the branch line (from 15-25 PSI main air) which positions pneumatic valves and dampers
Pneumatic thermostats use water pressure (hydraulic) from the building domestic water supply as the control medium
Pneumatic systems operate at 40-80 PSI — the same as typical building water pressure

Question 14: What is "trending" in a BAS and how is it used to diagnose HVAC problems?

BAS trending is a method of raising and lowering setpoints on a seasonal schedule — not related to data recording
Trending automatically records BAS point values over time; trend graphs reveal valve hunting, sensor drift, control offset, equipment short-cycling, and other patterns invisible in real-time monitoring
Trending requires shutting down the HVAC system for data collection — systems cannot trend while equipment is running
BAS trending only applies to electrical consumption — it cannot record temperature or pressure data

Question 15: Describe the typical control sequence for a Variable Air Volume (VAV) box with hot water reheat during the cooling season.

In cooling season, a VAV box with reheat keeps the damper fully open at all times — airflow is constant and only the reheat valve changes
VAV reheat sequence: cooling demand → increase airflow to maximum; zone satisfied → minimum airflow, reheat closed; below heating setpoint → hold minimum airflow, open reheat valve proportionally to prevent over-cooling while satisfying heating
A VAV box with reheat closes the damper completely during the heating season and supplies 100% heated air from the reheat coil
VAV boxes with reheat are prohibited by ASHRAE 90.1 as they waste energy — only cooling-only VAV boxes are code-compliant

Question 16: How does a dual-duct (DD) HVAC system differ from a single-duct VAV system, and what is the mixing box in a DD system?

A dual-duct system distributes refrigerant through two separate copper pipes to individual zone fan coil units
A dual-duct system has separate cold deck and hot deck distribution ducts; a zone mixing box blends proportional amounts of cold and hot air (through opposing dampers) to hit the zone supply temperature setpoint
Dual-duct systems are the most energy-efficient HVAC system type because they eliminate the need for a central cooling plant
The mixing box in a dual-duct system contains a small compressor that cools or heats air independently at each zone

Question 17: What is "valve authority" in a hydronic control system, and why must it be above 0.5 (50%) for good control?

Valve authority refers to the electrical power rating of the valve actuator motor — higher authority means a more powerful motor
Valve authority N = ΔP_valve ÷ ΔP_total_circuit; must be ≥0.5 so the valve controls at least half the circuit pressure drop, giving it adequate flow control authority. Low authority causes poor, unstable control
Valve authority must always equal 1.0 (100%) in HVAC — any value below 1.0 indicates an undersized valve
Valve authority only applies to pneumatic valves — electronic modulating valves have unlimited authority

Question 18: What is the difference between a dry-bulb temperature economizer and an enthalpy (wet-bulb or calculated enthalpy) economizer?

Dry-bulb and enthalpy economizers are identical — they both measure outdoor temperature to control the outside air damper
Dry-bulb compares OA temperature only; enthalpy compares total heat content (sensible + latent). Enthalpy economizers are better in humid climates where cool but humid OA may have MORE total heat than the return air
Enthalpy economizers use refrigerant cooling to pre-cool outdoor air before it enters the building
ASHRAE 90.1 prohibits enthalpy economizers because humidity sensors are unreliable and cause false economizer lockouts

Question 19: How does a chiller plant supervisory controller sequence multiple chillers for energy efficiency?

Chiller sequencing always runs all installed chillers at the same time — part-load efficiency is not a consideration
The supervisory controller monitors total cooling load and stages chillers to run near their most efficient range (75-90% load); a second chiller starts at ~85% load and stages off when load drops to avoid short-cycling
Chiller sequencing is based only on the outdoor temperature — all chillers start when OAT exceeds 85°F
Multiple chillers are never staged — building codes require all installed chillers to run simultaneously for redundancy

Question 20: What is "alarm management" in a BAS and what is "alarm flooding"?

Alarm flooding is a feature that alerts every building occupant simultaneously when any HVAC sensor reads out of range
Alarm management prioritizes and routes BAS alarms to appropriate personnel; alarm flooding occurs when a single fault cascades into hundreds of secondary alarms, overwhelming operators. Solutions include alarm suppression and root-cause prioritization
BAS alarms should all have the same priority to ensure operators respond to each one equally
Alarm management is only applicable to fire alarm systems — HVAC BAS systems do not generate alarms

Question 21: How is condenser water (cooling tower leaving water) temperature controlled in a chiller plant?

Condenser water temperature is controlled by adding chemicals to the cooling tower water to change its specific heat
Cooling tower fan VFD modulates fan speed to maintain condenser water temperature setpoint; modern plants use optimal reset to find the condenser water temperature that minimizes combined chiller + fan energy
Condenser water temperature is always fixed at 85°F and cannot be adjusted — the cooling tower fans run at fixed speed
Condenser water temperature is controlled by the chiller itself — the cooling tower has no control system

Question 22: What is Modbus and how is it used in HVAC controls systems?

Modbus is a wireless communication standard used only for thermostat setpoint adjustment in residential HVAC
Modbus is a serial (RS-485) master-slave protocol used to communicate with chillers, VFDs, boilers, and meters in HVAC; a BAS master polls Modbus registers to read sensor values and write setpoints
Modbus and BACnet are the same protocol — they use identical register maps and communication methods
Modbus is prohibited in commercial HVAC because it does not encrypt data — only BACnet/IP with TLS encryption is permitted

Question 23: Where should a supply air temperature (SAT) sensor be installed in an air handling unit, and what does it control?

The supply air temperature sensor should be mounted directly on the chilled water coil face to detect the coldest air temperature
The SAT sensor is installed at least 3 duct diameters downstream of the coil for thorough air mixing; it controls the cooling/heating coil valve to maintain the SAT setpoint and serves as the reference for VAV zone control
Supply air temperature sensors are installed in the return air duct so the sensor is protected from cold supply air temperatures
There is no supply air temperature sensor in a VAV system — zone temperature sensors do all the temperature control

Question 24: What is mixed air temperature control in an AHU, and what failure mode must the control system prevent?

Mixed air temperature control only applies when a chiller is running — during heating season there is no mixed air temperature control
Mixed air temperature control blends OA and RA dampers to maintain MAT setpoint; the critical failure to prevent is freezing — freeze stats (set at 35-38°F) must override economizer and close OA dampers to prevent coil ice damage
Mixed air temperature should always be maintained at 72°F regardless of outdoor conditions
Mixed air temperature sensors are only used in residential split systems — commercial AHUs do not have mixed air sections

Question 25: What is Demand-Controlled Ventilation (DCV) and which ASHRAE standard makes it mandatory for certain occupancies?

DCV is voluntary and is never required by energy codes — building owners choose whether to install it
DCV modulates OA ventilation based on CO2 measurement (occupancy proxy); ASHRAE 90.1 requires it in spaces with ≥40 persons/1,000 sq ft and OA supply >3,000 CFM, saving 10-30% ventilation energy in variable-occupancy spaces
DCV reduces ventilation to zero when a space is unoccupied — this is required by ASHRAE 62.1 for energy efficiency
DCV uses temperature sensors, not CO2 — it modulates ventilation based on how warm the space is getting

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