Nevada (NV) C-21B Air Conditioning Contractor Practice
Exam Questions with Verified Answers & Detailed
Rationales
Question 1: Under the Nevada State Contractors Board (NSCB) regulations (NAC 624), what is the
maximum monetary limit assigned to a C-21B license, and what requirement must a contractor fulfill if
a single contract exceeds their assigned monetary limit?*
A. The monetary limit is always capped at $500,000; the contractor must subcontract the excess portion.
B. Monetary limits are determined based on financial standing and experience; a contractor must apply
for and receive an permanent or temporary limit increase prior to bidding or accepting a project
exceeding their limit.
C. There are no monetary limits for C-21 classifications as long as a bid bond is submitted.
D. The limit is determined solely by the gross floor area of the HVAC equipment space being serviced.
CORRECT ANSWER: B. Monetary limits are determined based on financial standing and experience; a
contractor must apply for and receive an permanent or temporary limit increase prior to bidding or
accepting a project exceeding their limit.
Rationale: Under NAC 624.150 and Nevada Revised Statutes (NRS) 624, the NSCB sets a specific
monetary limit on each license based on the applicant’s net worth, working capital, and financial
documentation. A contractor cannot bid on, enter into, or execute a contract that exceeds this limit
without securing a limit adjustment or temporary limit increase from the Board prior to submitting the
bid.
Question 2: According to the International Mechanical Code (IMC) as adopted in Nevada jurisdictions,
what is the minimum required net free ventilation area for a mechanical equipment room housing
fuel-burning HVAC equipment taking combustion air entirely from the outdoors through two
permanent openings?*
A. 1 square inch per 1,000 Btu/h total input rating of all appliances in the space
B. 1 square inch per 4,000 Btu/h total input rating for each opening
C. 1 square inch per 2,000 Btu/h total input rating divided equally between upper and lower openings
D. A fixed standard of 144 square inches regardless of equipment thermal input
CORRECT ANSWER: B. 1 square inch per 4,000 Btu/h total input rating for each opening
,Rationale: Per IMC Section 304.6.1 (Two Permanent Openings Method), when communicating directly
with the outdoors or through vertical ducts, each opening must have a minimum net free area of 1
square inch per 4,000 Btu/h of total input rating of all fuel-burning equipment located inside the
mechanical space.
Question 3: In an air-handling unit utilizing a draw-through configuration, how is the minimum trap
height for a condensate drain pan drain calculated to prevent air blowback or condensate hold-up?*
A. Minimum trap depth must equal the total system static pressure plus 1 inch.
B. Trap depth must be equal to or greater than the maximum negative static pressure (in inches of water
gauge) present in the drain pan section plus at least 1 inch for safety margin.
C. Drain traps are not required if the line vents directly into an open floor sink.
D. The trap seal must strictly equal one-half the diameter of the PVC drain line.
CORRECT ANSWER: B. Trap depth must be equal to or greater than the maximum negative static pressure
(in inches of water gauge) present in the drain pan section plus at least 1 inch for safety margin.
Rationale: In draw-through systems, the cooling coil condensate pan operates under negative static
pressure relative to atmospheric pressure. If the water seal height in the P-trap does not exceed the
negative static suction pressure (plus a 1-inch operating clearance), air drawing inward through the drain
pipe will prevent condensate from draining, causing water to overflow the drain pan.
Question 4: What thermodynamic property change occurs when saturated vapor refrigerant is
compressed in an ideal, perfectly reversible (isentropic) reciprocating compressor?*
A. Enthalpy decreases while pressure remains static
B. Pressure and temperature increase along a line of constant entropy
C. Specific volume increases as density drops
D. Relative humidity inside the cylinder approaches 100%
CORRECT ANSWER: B. Pressure and temperature increase along a line of constant entropy
Rationale: An ideal compression stroke is modeled as an isentropic process ($s_1 = s_2$). As mechanical
work is performed on the refrigerant vapor to elevate its pressure, both temperature and enthalpy
increase along a line of constant entropy on a pressure-enthalpy ($P-h$) diagram, moving the gas deep
into the superheated vapor region.
Question 1: According to standard mechanical code practices for air-conditioning systems, what is the
minimum clearance typically required between a gas-fired furnace flue and combustible materials
when using single-wall metal vent pipe?
,A. 1 inch
B. 3 inches
C. 6 inches
D. 18 inches
CORRECT ANSWER: C. 6 inches
Rationale: Single-wall metal vent connectors for gas appliances generally require a minimum 6-inch
clearance to combustibles to prevent fire hazards, as specified in fuel-gas and mechanical codes.
Question 2: In a vapor-compression refrigeration cycle used in air-conditioning systems, which
component is responsible for rejecting heat from the refrigerant to the outdoor air or water?
A. Evaporator
B. Compressor
C. Condenser
D. Metering device
CORRECT ANSWER: C. Condenser
Rationale: The condenser is the heat-rejection component where high-pressure refrigerant vapor
releases heat and condenses into liquid, enabling continuous cooling.
Question 3: When calculating cooling load for a commercial space using ACCA Manual N principles,
which factor is most critical for determining sensible heat gain through exterior walls?
A. Occupant activity level only
B. U-factor, area, and temperature difference
C. Refrigerant type
D. Duct material
CORRECT ANSWER: B. U-factor, area, and temperature difference
Rationale: Sensible heat gain through walls is calculated using the formula Q = U × A × ΔT, where U is the
overall heat-transfer coefficient, A is area, and ΔT is the design temperature difference.
Question 4: What is the primary purpose of vibration isolators installed under air-conditioning
compressors and air handlers?
A. To increase system capacity
B. To reduce transmission of mechanical vibration and noise to the building structure
C. To improve refrigerant flow
D. To control static pressure
CORRECT ANSWER: B. To reduce transmission of mechanical vibration and noise to the building
structure
, Rationale: Vibration isolators (spring or elastomeric) absorb and dampen vibration from rotating
equipment, preventing noise and structural transmission as required by sound and vibration control
standards.
Question 5: In duct construction according to SMACNA standards, what is the recommended
maximum unsupported span for rectangular metal duct of 24-inch width when using standard
hangers?
A. 4 feet
B. 8 feet
C. 10 feet
D. 12 feet
CORRECT ANSWER: B. 8 feet
Rationale: SMACNA HVAC Duct Construction Standards typically limit hanger spacing for medium-sized
rectangular ducts to 8 feet to maintain structural integrity and prevent sagging.
Question 6: Which type of refrigerant metering device maintains a relatively constant superheat at the
evaporator outlet in residential and light-commercial air-conditioning systems?
A. Capillary tube
B. Fixed orifice
C. Thermostatic expansion valve
D. High-side float
CORRECT ANSWER: C. Thermostatic expansion valve
Rationale: A thermostatic expansion valve (TXV) modulates refrigerant flow based on evaporator outlet
superheat sensed by its bulb, providing efficient and stable operation under varying loads.
Question 7: Under EPA Section 608 regulations applicable to air-conditioning contractors, what
certification is required to purchase and handle regulated refrigerants?
A. Nevada contractor license only
B. EPA Section 608 technician certification
C. Local building permit
D. OSHA 10-hour card
CORRECT ANSWER: B. EPA Section 608 technician certification
Rationale: Federal EPA Section 608 requires technicians to hold appropriate Type I, II, III, or Universal
certification to legally purchase, recover, and service regulated refrigerants.
Exam Questions with Verified Answers & Detailed
Rationales
Question 1: Under the Nevada State Contractors Board (NSCB) regulations (NAC 624), what is the
maximum monetary limit assigned to a C-21B license, and what requirement must a contractor fulfill if
a single contract exceeds their assigned monetary limit?*
A. The monetary limit is always capped at $500,000; the contractor must subcontract the excess portion.
B. Monetary limits are determined based on financial standing and experience; a contractor must apply
for and receive an permanent or temporary limit increase prior to bidding or accepting a project
exceeding their limit.
C. There are no monetary limits for C-21 classifications as long as a bid bond is submitted.
D. The limit is determined solely by the gross floor area of the HVAC equipment space being serviced.
CORRECT ANSWER: B. Monetary limits are determined based on financial standing and experience; a
contractor must apply for and receive an permanent or temporary limit increase prior to bidding or
accepting a project exceeding their limit.
Rationale: Under NAC 624.150 and Nevada Revised Statutes (NRS) 624, the NSCB sets a specific
monetary limit on each license based on the applicant’s net worth, working capital, and financial
documentation. A contractor cannot bid on, enter into, or execute a contract that exceeds this limit
without securing a limit adjustment or temporary limit increase from the Board prior to submitting the
bid.
Question 2: According to the International Mechanical Code (IMC) as adopted in Nevada jurisdictions,
what is the minimum required net free ventilation area for a mechanical equipment room housing
fuel-burning HVAC equipment taking combustion air entirely from the outdoors through two
permanent openings?*
A. 1 square inch per 1,000 Btu/h total input rating of all appliances in the space
B. 1 square inch per 4,000 Btu/h total input rating for each opening
C. 1 square inch per 2,000 Btu/h total input rating divided equally between upper and lower openings
D. A fixed standard of 144 square inches regardless of equipment thermal input
CORRECT ANSWER: B. 1 square inch per 4,000 Btu/h total input rating for each opening
,Rationale: Per IMC Section 304.6.1 (Two Permanent Openings Method), when communicating directly
with the outdoors or through vertical ducts, each opening must have a minimum net free area of 1
square inch per 4,000 Btu/h of total input rating of all fuel-burning equipment located inside the
mechanical space.
Question 3: In an air-handling unit utilizing a draw-through configuration, how is the minimum trap
height for a condensate drain pan drain calculated to prevent air blowback or condensate hold-up?*
A. Minimum trap depth must equal the total system static pressure plus 1 inch.
B. Trap depth must be equal to or greater than the maximum negative static pressure (in inches of water
gauge) present in the drain pan section plus at least 1 inch for safety margin.
C. Drain traps are not required if the line vents directly into an open floor sink.
D. The trap seal must strictly equal one-half the diameter of the PVC drain line.
CORRECT ANSWER: B. Trap depth must be equal to or greater than the maximum negative static pressure
(in inches of water gauge) present in the drain pan section plus at least 1 inch for safety margin.
Rationale: In draw-through systems, the cooling coil condensate pan operates under negative static
pressure relative to atmospheric pressure. If the water seal height in the P-trap does not exceed the
negative static suction pressure (plus a 1-inch operating clearance), air drawing inward through the drain
pipe will prevent condensate from draining, causing water to overflow the drain pan.
Question 4: What thermodynamic property change occurs when saturated vapor refrigerant is
compressed in an ideal, perfectly reversible (isentropic) reciprocating compressor?*
A. Enthalpy decreases while pressure remains static
B. Pressure and temperature increase along a line of constant entropy
C. Specific volume increases as density drops
D. Relative humidity inside the cylinder approaches 100%
CORRECT ANSWER: B. Pressure and temperature increase along a line of constant entropy
Rationale: An ideal compression stroke is modeled as an isentropic process ($s_1 = s_2$). As mechanical
work is performed on the refrigerant vapor to elevate its pressure, both temperature and enthalpy
increase along a line of constant entropy on a pressure-enthalpy ($P-h$) diagram, moving the gas deep
into the superheated vapor region.
Question 1: According to standard mechanical code practices for air-conditioning systems, what is the
minimum clearance typically required between a gas-fired furnace flue and combustible materials
when using single-wall metal vent pipe?
,A. 1 inch
B. 3 inches
C. 6 inches
D. 18 inches
CORRECT ANSWER: C. 6 inches
Rationale: Single-wall metal vent connectors for gas appliances generally require a minimum 6-inch
clearance to combustibles to prevent fire hazards, as specified in fuel-gas and mechanical codes.
Question 2: In a vapor-compression refrigeration cycle used in air-conditioning systems, which
component is responsible for rejecting heat from the refrigerant to the outdoor air or water?
A. Evaporator
B. Compressor
C. Condenser
D. Metering device
CORRECT ANSWER: C. Condenser
Rationale: The condenser is the heat-rejection component where high-pressure refrigerant vapor
releases heat and condenses into liquid, enabling continuous cooling.
Question 3: When calculating cooling load for a commercial space using ACCA Manual N principles,
which factor is most critical for determining sensible heat gain through exterior walls?
A. Occupant activity level only
B. U-factor, area, and temperature difference
C. Refrigerant type
D. Duct material
CORRECT ANSWER: B. U-factor, area, and temperature difference
Rationale: Sensible heat gain through walls is calculated using the formula Q = U × A × ΔT, where U is the
overall heat-transfer coefficient, A is area, and ΔT is the design temperature difference.
Question 4: What is the primary purpose of vibration isolators installed under air-conditioning
compressors and air handlers?
A. To increase system capacity
B. To reduce transmission of mechanical vibration and noise to the building structure
C. To improve refrigerant flow
D. To control static pressure
CORRECT ANSWER: B. To reduce transmission of mechanical vibration and noise to the building
structure
, Rationale: Vibration isolators (spring or elastomeric) absorb and dampen vibration from rotating
equipment, preventing noise and structural transmission as required by sound and vibration control
standards.
Question 5: In duct construction according to SMACNA standards, what is the recommended
maximum unsupported span for rectangular metal duct of 24-inch width when using standard
hangers?
A. 4 feet
B. 8 feet
C. 10 feet
D. 12 feet
CORRECT ANSWER: B. 8 feet
Rationale: SMACNA HVAC Duct Construction Standards typically limit hanger spacing for medium-sized
rectangular ducts to 8 feet to maintain structural integrity and prevent sagging.
Question 6: Which type of refrigerant metering device maintains a relatively constant superheat at the
evaporator outlet in residential and light-commercial air-conditioning systems?
A. Capillary tube
B. Fixed orifice
C. Thermostatic expansion valve
D. High-side float
CORRECT ANSWER: C. Thermostatic expansion valve
Rationale: A thermostatic expansion valve (TXV) modulates refrigerant flow based on evaporator outlet
superheat sensed by its bulb, providing efficient and stable operation under varying loads.
Question 7: Under EPA Section 608 regulations applicable to air-conditioning contractors, what
certification is required to purchase and handle regulated refrigerants?
A. Nevada contractor license only
B. EPA Section 608 technician certification
C. Local building permit
D. OSHA 10-hour card
CORRECT ANSWER: B. EPA Section 608 technician certification
Rationale: Federal EPA Section 608 requires technicians to hold appropriate Type I, II, III, or Universal
certification to legally purchase, recover, and service regulated refrigerants.
