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HVAC NEC 2023

HVAC Controls and Sensors Practice Test 5

Free HVAC Controls and Sensors Practice Test 5. 25 questions on RTD linear characteristics, capacitance fluid level sensing, RTD resistance behavior, chilled mirror hygrometers, dew point measurement, quartz crystal humidity sensors, magnetic flow meters, thermocouple selection, control valve flow characteristics, freeze stats, high-limit controls, AHU sequences, optimum start, zoning strategies, and chilled water pump differential pressure control.

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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.

Key humidity measurement review: Chilled mirror hygrometer is the gold standard for dew point measurement. A mirror is cooled until moisture condenses; the mirror temperature at condensation equals the dew point. Used for lab calibration of humidity sensors. Capacitive polymer sensors are the workhorse of HVAC humidity measurement (accuracy +/-2-3% RH). Quartz crystal microbalance measures absorbed moisture mass through frequency change -- used in precision applications.

25

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45 min

Time

70%

Pass

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HVAC Controls and Sensors Practice Test 5 — Question List

Question 1: The RTD temperature-resistance characteristic is linear.

True
False

Question 2: When the capacitance sensor is used to measure fluid level in tank, the capacitance valve will ………….. with increase in fluid level.

Increase
Decrease

Question 3: The resistance of RTD ………………. With increase in temperature.

Increases
Decreases

Question 4: Which of the following sensors are used to measure dew point?

Chilled Mirror Hygrometers
Pitot-static tube
Thermister
Magnetic meter

Question 5: …………………………………………….. helps in measuring dew point.

Pressure and temperature
Flow and pressure
Temperature and humidity
Humidity and pressure

Question 6: Which of the following sensor uses frequency to measure relative humidity?

Quartz crystal
Magnetic meter
Pitot-static tube
Ultrasonic

Question 7: When liquid velocity increases the induced voltage in the magnetic flow meter ……………..

Increases
Decreases

Question 8: Which of the following temperature sensor is most suitable to measure 1000 <sup>0</sup> F?

RTD
Thermister
Thermocouple
Thermostat

Question 9: In low temperature measurements the error in the RTD measurement will be high.

True
False

Question 10: When the length of the wire has increased, the resistance of the wire ……………………

Increases
Decreases
Remains same

Question 11: What is the difference between a "linear" and "equal percentage" control valve flow characteristic, and which is preferred for HVAC coil applications?

Linear valves are preferred for all HVAC coil applications because they provide predictable and stable control response
Equal percentage valves are preferred for HVAC coil applications because they compensate for coil non-linearity and low valve authority effects, providing approximately linear heat output versus valve travel
Quick-opening valves are the standard for HVAC modulating coil applications because they respond instantly to control signals
Linear and equal percentage valves are identical for HVAC coil applications -- only the price is different

Question 12: What is a freeze stat (low-limit thermostat) in an AHU, and what safety actions does it trigger?

A freeze stat is a high-pressure safety switch that monitors refrigerant pressure to prevent freezing of the condensing unit
A freeze stat is a low-limit temperature switch with a full-duct-span capillary element; it closes the OA damper, opens the heating valve, and alarms the BAS when any point in the duct drops to 35-38 degrees F
Freeze stats are only needed in northern states -- buildings in warm climates do not require freeze protection
A freeze stat works by measuring the electrical resistance of the cooling coil metal to detect ice formation

Question 13: Describe the basic control sequence for an AHU in cooling mode with an air-side economizer.

In cooling mode an AHU always runs 100% outdoor air -- return air is not used when cooling
AHU cooling sequence: first use economizer if OA conditions permit; if insufficient, close OA to minimum and modulate CHW valve; freeze stat and smoke detector are always highest-priority safety overrides
The economizer activates when outdoor temperature exceeds 75 F to add more outdoor air for cooling
The AHU cooling sequence only involves the chilled water valve -- outdoor air dampers are always fixed at minimum during cooling

Question 14: What is morning warm-up mode in a commercial AHU, and why does it run with 100% return air?

Morning warm-up mode opens the OA damper to 100% to flush stale overnight air from the building before occupants arrive
Morning warm-up closes the OA damper (100% return air recirculation) to efficiently heat the building from setback to comfort setpoint before occupants arrive -- heating cold outdoor air wastes energy and slows warm-up
Morning warm-up mode is the same as occupied cooling mode -- the only difference is the time of day it activates
Morning warm-up is prohibited by ASHRAE 62.1 because running the AHU without outdoor air violates minimum ventilation requirements at all times

Question 15: How is chilled water pump speed controlled in a variable-flow chilled water system?

Chilled water pump speed is controlled by measuring supply air temperature at each AHU and adjusting pump speed to the warmest coil reading
Chilled water pump VFD speed is controlled to maintain differential pressure setpoint at the critical remote AHU; as coil valves close at part load the pump slows down, saving energy proportional to speed cubed
Chilled water pumps always run at full speed -- VFDs damage the chiller by creating variable flow conditions
Chilled water pump speed is controlled by the chiller onboard controller -- the BAS has no input into pump speed

Question 16: What is a high-limit control in a forced-air heating system, and why is it required?

High-limit controls are found only in refrigeration systems to prevent the compressor from overheating
A supply air high-limit shuts off the heating source if supply air exceeds a safe maximum temperature (130-150 F), protecting against fire from blocked airflow, stuck thermostat contacts, or fan failure
High-limit controls are optional in commercial systems -- the operating thermostat alone is sufficient safety protection
A high-limit control in a heating system is the same as a freeze stat -- they are installed in the same location and serve the same purpose

Question 17: What is the typical air velocity range in a residential supply air duct main trunk, and what problems occur when velocity is too high?

Residential duct velocity should be maintained above 3,000 FPM in all branches to ensure good mixing at room registers
Residential supply main duct velocity: 600-900 FPM. Excessive velocity causes ductwork noise, high static pressure loss (proportional to V squared), and uncomfortable drafts at diffusers
Air velocity in HVAC ducts has no effect on noise level -- only the fan type determines system noise
Residential supply ducts should run at 50-100 FPM to minimize friction loss and maximize energy efficiency

Question 18: What happens to HVAC systems when a fire alarm activates in a commercial building?

When a fire alarm activates, all HVAC systems must continue operating at maximum airflow to provide fresh air to evacuating occupants
Fire alarm activation causes AHU shutdown or smoke control mode, smoke dampers close, stairwell pressurization fans start; specific response is per NFPA 90A/92 and AHJ requirements
HVAC systems have no connection to fire alarms -- fire and HVAC are completely independent systems
Fire alarm activation only affects fan speed -- all dampers remain in their normal positions during a fire event

Question 19: What factors determine HVAC zone boundaries, and what are the consequences of poor zoning?

HVAC zones should cover maximum square footage per thermostat regardless of load profile or solar exposure
Zones should group areas with similar load profiles, sun exposure, occupancy schedules, and ventilation needs; poor zoning causes comfort complaints, energy waste, and simultaneous heating and cooling in the same zone
Zone boundaries are determined solely by architectural floor plan -- HVAC loads have no influence on zone layout
All modern commercial buildings should have a single zone with one thermostat -- multiple zones increase controls cost without any benefit

Question 20: How does an occupancy schedule in a BAS save energy, and what is optimum start as an enhancement to scheduling?

Occupancy scheduling in BAS is only used for lighting -- HVAC systems always run at the same setpoints 24 hours per day
Occupancy schedules switch HVAC to setback setpoints during unoccupied hours; optimum start calculates the minimum lead time to recover to comfort setpoint at exactly occupancy time, saving 10-30% over fixed early start
Optimum start always starts the HVAC system 4 hours before occupancy regardless of indoor temperature or outdoor conditions
Occupancy scheduling raises cooling setpoint and increases outdoor air during unoccupied periods to flush stale air at lower energy cost

Question 21: What is an Energy Recovery Ventilator (ERV) and how does it reduce HVAC energy while maintaining fresh air ventilation?

An ERV generates electricity from exhaust air movement to power the fresh air fan -- it does not transfer heat
An ERV pre-conditions incoming outdoor air using energy recovered from exhaust air (70-80% efficiency); in summer it pre-cools and dehumidifies OA, in winter it pre-heats OA, reducing HVAC load while maintaining ventilation
ERVs are only effective in cold climates -- in warm or humid climates they make the system less efficient
An ERV recirculates exhaust air back into the supply stream to reduce energy use -- no heat transfer occurs

Question 22: What is the difference between a fire damper and a smoke damper in an HVAC duct system?

Fire dampers and smoke dampers are identical -- they are just different names for the same device
Fire dampers close at 165 F via fusible link (passive thermal, stops fire spread); smoke dampers close on FACP electrical command (active, stops smoke spread). Combination FSDs do both
Smoke dampers close automatically at 165 F just like fire dampers -- the only difference is they are located in return air ducts
Fire dampers are only required in residential construction -- commercial buildings use smoke dampers for all duct penetrations

Question 23: What are the four basic input/output (I/O) types on a DDC controller, and give an HVAC example of each?

DDC controllers only have two I/O types: on/off inputs and on/off outputs -- all HVAC measurements are binary
DDC controllers have four I/O types: Analog Input (variable sensor reading), Binary Input (on/off status), Analog Output (variable actuator command 0-10V), and Binary Output (on/off relay command to start equipment)
The four DDC I/O types are: high/low input, timed output, pulse input, and digital output
DDC controllers have only analog inputs -- all outputs are binary (on/off) because control devices can only be fully open or fully closed

Question 24: What is outdoor air reset (OAR) control for a hot water heating boiler, and how does it save energy?

Outdoor air reset raises the hot water temperature as outdoor temperature increases, providing more heat on warm days
Outdoor air reset lowers hot water setpoint as outdoor temperature rises (less heating load); saves energy by reducing standby losses and enabling condensing boiler mode at low return temperatures when the weather is mild
Outdoor air reset is only applicable to steam boilers -- hot water boilers always maintain a fixed 180 F setpoint
Outdoor air reset uses outdoor humidity, not temperature, to adjust the hot water setpoint based on perceived comfort

Question 25: What is demand limiting (peak demand management) in a BAS, and why is it economically important for commercial buildings?

Demand limiting permanently reduces HVAC capacity to save energy -- buildings must accept less comfort to reduce utility bills
Demand limiting temporarily sheds HVAC loads before crossing peak demand thresholds, reducing utility demand charges (which can be 30-50% of commercial bills) while minimizing occupant comfort impact
Demand limiting only applies to lighting systems -- HVAC loads are excluded from demand management programs
Peak demand charges are calculated from annual electricity consumption, not from short-interval peak measurements

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