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HVAC NEC 2023

HVAC Controls Practice Questions(sensor)-1

Free HVAC Controls Practice Test 1. 25 questions on pneumatic control systems, normally closed vs normally open control devices, controller-controlled device relationships, 4-20 mA current loops, primary control variables, thermocouple high-temperature applications, thermistor sensitivity, RTD positive temperature coefficient, wire length resistance effects, pitot tube air velocity measurement, PID control theory, DDC system advantages, control loop terminology, proportional band, and actuator types.

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Frequently Asked Questions

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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.

HVAC controls fundamentals: A 4-20 mA signal represents 0-100% of a controlled variable. The 4 mA "live zero" allows wire break detection (0 mA = fault, not zero signal). Thermocouples generate a small voltage from the temperature difference between two junctions — ideal for high-temperature measurement. Thermistors (NTC type) decrease resistance with temperature increase — very sensitive but non-linear. RTDs (Resistance Temperature Detectors) increase resistance linearly with temperature — more accurate and stable.

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HVAC Controls Practice Questions(sensor)-1 — Question List

Question 1: In which of the following controls the signal transmission is air pressure?

Self powered controls
Pneumatic controls
Electric controls
Hydraulic controls

Question 2: If the valve opens when the control signal is applied, this valve is called as ………….

Normally open type
Normally closed type

Question 3: The controlled device receives the signals from …………….

Sensor
Controller
Thermocouple
Thermostat

Question 4: Which of the following is the suitable current signal range?

0 mA to 20 mA
4mA to 20 mA
1mA to 20 mA
6 mA to 10 mA

Question 5: Which of the following is the primary controlled variable in HVAC applications?

Temperature
Humidity
Air velocity
Radiant temperature

Question 6: In HVAC applications for higher temperature measurement which of the following sensors is generally used?

Thermocouples
RTD
Bimetal
Thermostat

Question 7: Which of the following temperature sensor has high sensitivity?

Thermister
Thermocouple
RTD
Bimetal

Question 8: The RTD temperature sensor have a …………….. temperature coefficient.

Positive
Negative
Zero

Question 9: When the length of the wire has increased, the resistance of the wire ……………………

Increases
Decreases
Remains same

Question 10: Pitot tube is used to measure ………………..

Air flow velocity
Temperature
Pressure
Humidity

Question 11: What are the three modes of a PID controller and what role does each play in HVAC control?

PID stands for: Position, Intensity, and Duration — the three phases of the HVAC control cycle
PID = Proportional (output proportional to error), Integral (eliminates offset by integrating over time), Derivative (dampens overshoot by responding to rate of change). Most HVAC uses PI without D to avoid noise amplification
PID control is only used for cooling systems — heating systems use only on/off control
The D (derivative) mode is the most important PID component in HVAC temperature control

Question 12: What are the main advantages of Direct Digital Control (DDC) over pneumatic or electric controls in commercial HVAC?

DDC systems are less reliable than pneumatic controls because they require electricity at every control point
DDC advantages include software-based control logic (easily changed), remote monitoring, diagnostic trending, energy optimization strategies, multi-system integration, and remote sensor calibration
DDC and pneumatic controls have the same capabilities — DDC is just a newer packaging of the same technology
DDC is only used for lighting control — HVAC always uses pneumatic or electric controls

Question 13: In HVAC control loop terminology, what is the difference between the "controlled variable," "manipulated variable," and "controlled medium"?

The controlled variable is the valve position, and the manipulated variable is the room temperature
Controlled variable = what is maintained at setpoint (e.g., room temp); manipulated variable = what the controller adjusts (e.g., valve position); controlled medium = the substance through which the manipulation acts (e.g., chilled water)
The controlled variable and manipulated variable are always the same — they refer to the thermostat setpoint
Controlled medium and controlled variable are the same — both refer to the supply air temperature

Question 14: What is "proportional band" in a proportional HVAC controller and how does its setting affect system performance?

Proportional band is the time it takes a controller to respond to a change in setpoint
Proportional band is the range of the controlled variable over which the controller output moves 0-100%; narrow PB gives fast but oscillatory response; wide PB gives stable but offset-prone response
Proportional band should always be set to 0 for the fastest possible response time
Proportional band only applies to digital DDC controllers — pneumatic controllers do not have this setting

Question 15: What are the main types of valve and damper actuators used in HVAC control systems?

All HVAC actuators use 120V AC as the control signal regardless of type or size
Main actuator types are pneumatic (3-15 PSI air), electric floating (3-wire on/off pulses), and electronic modulating (2-10V or 4-20mA proportional); fail-safe spring return holds the safe position on power loss
Actuators are only used for dampers — valves use manual handwheel operation in HVAC systems
Pneumatic actuators are no longer used in HVAC — all modern systems use only electronic actuators

Question 16: What is the difference between the "setpoint," "control point," and "offset" in an HVAC control loop?

Setpoint and control point are always identical — the controller adjusts the system until they match exactly
Setpoint = desired value; control point = actual steady-state value; offset = their difference. Proportional-only control always has offset; adding Integral action brings the control point to match the setpoint
Offset is a desired feature of P-only controllers that prevents the system from hunting around the setpoint
Control point is the temperature at which the thermostat switches from heating to cooling mode

Question 17: What is "two-position" (on/off) control, and what is the "differential" (dead band) associated with it?

Two-position control switches the device between low speed and high speed — never fully on or fully off
Two-position control switches fully ON or OFF; the differential (dead band) is the temperature range between on and off switching points, preventing short cycling while allowing normal temperature fluctuation
The differential in two-position control is a time delay that prevents the system from restarting for a fixed number of minutes
Two-position control is only used for cooling — heating systems always use modulating control

Question 18: What is BACnet and why is it important for HVAC controls interoperability?

BACnet is a type of pneumatic control tubing used for air-pressured control signals in commercial buildings
BACnet (ASHRAE 135) is an open communication standard that allows HVAC controllers from different manufacturers to share data on the same network, enabling true systems integration
BACnet is required only for fire alarm systems — HVAC systems use Modbus exclusively
BACnet is a European standard that is not accepted in the United States

Question 19: What is "reset control" in commercial HVAC and give two examples of how it saves energy?

Reset control manually changes setpoints when operators arrive in the morning and leave in the evening
Reset control automatically adjusts setpoints based on conditions: raising chilled water temp at lower loads (saves chiller energy) and raising supply air temp at part load (reduces chiller and reheat energy)
Reset control only applies to heating systems — cooling setpoints cannot be reset
Reset control increases setpoints uniformly by 5°F every hour regardless of actual building conditions

Question 20: How is relative humidity typically controlled in a commercial building during the cooling season?

Humidity in commercial buildings is controlled only by opening and closing exhaust dampers to let in outdoor air
Primary cooling-season humidity control is by the cooling coil dehumidifying supply air; if insufficient, options include reheat (energy-intensive), dehumidification mode (lower airflow), or supplemental dehumidifiers
Commercial buildings do not control humidity — only temperature setpoints are maintained
Humidity is increased by adding moisture in winter and no special measures are needed in summer

Question 21: What is the difference between a two-way and three-way control valve in a hydronic system?

A two-way valve allows water to flow in two directions simultaneously, while a three-way valve restricts to one direction
A two-way valve varies flow through one circuit (variable flow); a three-way valve maintains constant total pump flow by routing excess through a bypass as the coil valve closes
Three-way valves are more energy-efficient because they handle three separate flow streams simultaneously
Two-way valves are only used for air damper control — hydronic systems always use three-way valves

Question 22: How do occupancy sensors integrate with HVAC controls to save energy in commercial buildings?

Occupancy sensors are only used for lighting — HVAC systems cannot integrate with occupancy detection
Occupancy sensors signal HVAC to setback temperature and reduce ventilation when unoccupied, saving 15-30% HVAC energy in variable-occupancy spaces
Occupancy sensors cause HVAC systems to turn off completely when a space is unoccupied for more than 5 minutes
Occupancy-based HVAC control is prohibited by ASHRAE 62.1 because it may reduce ventilation below minimum requirements

Question 23: What is "controls point-to-point checkout" and why must it be completed before functional testing?

Point-to-point checkout is not necessary if the controls contractor installs their own hardware and software
P2P checkout verifies each sensor input and control output is correctly wired by stimulating inputs and commanding outputs; it must precede functional testing because sequence tests fail for wrong reasons if wiring is incorrect
Point-to-point checkout is done after functional testing to clean up any remaining wiring issues
P2P checkout is only needed when BACnet is used as the communication protocol

Question 24: How is BTU metering accomplished for a chilled water system, and what parameters must be measured?

BTU metering only requires measuring the return water temperature — supply temperature and flow are constants
BTU metering requires supply temperature, return temperature, and flow rate; BTU/hr = GPM × 500 × ΔT (the 500 constant accounts for water density and specific heat)
BTU meters measure electrical consumption of the chiller — not the actual heat removal from the building
BTU meters are only used for heating systems — cooling systems are metered in tons, not BTU/hr

Question 25: What refrigerant detection system does ASHRAE Standard 15 require in mechanical equipment rooms containing refrigeration equipment?

ASHRAE 15 does not require refrigerant detectors — personnel are expected to smell refrigerant leaks before they become dangerous
ASHRAE 15 requires refrigerant detectors with audible and visual alarms that trigger at TLV concentration and activate emergency ventilation — providing warning and automatic dilution before dangerous levels are reached
Refrigerant detectors are only required in rooms with ammonia — HFC refrigerant rooms have no detection requirement
ASHRAE 15 requires refrigerant detectors outside the machinery room only — no detection inside is needed

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

Timer starts immediately. Progress is auto-saved.

Source: NEC 2026 Art. 440 & electrical theory