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NJ HVAC Contractor practice questions

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Question 1Refrigeration Principles, Thermodynamics, and System Components

In a vapor-compression refrigeration cycle, what is the primary function of the metering device?

  1. To reduce refrigerant pressure and control flow into the evaporator Correct answer.
  2. To raise the refrigerant pressure before it enters the condenser
  3. To remove moisture and acid from the liquid line
  4. To store excess refrigerant during low-load operation

Explanation

The metering device throttles high-pressure liquid refrigerant down to the low-side pressure and regulates how much refrigerant enters the evaporator. This pressure drop also lowers the refrigerant temperature below the space temperature so heat can be absorbed. A receiver, filter-drier, or compressor performs the other listed roles.

Question 2Refrigeration Principles, Thermodynamics, and System Components

Which component of a basic refrigeration circuit adds energy to the refrigerant vapor and raises its pressure?

  1. Condenser
  2. Compressor Correct answer.
  3. Evaporator
  4. Expansion valve

Explanation

The compressor is the only component that adds work energy to the refrigerant, raising both its pressure and temperature so it can reject heat in the condenser. The evaporator and condenser are heat exchangers, and the expansion valve reduces pressure.

Question 3Refrigeration Principles, Thermodynamics, and System Components

On a standard pressure-enthalpy diagram for a refrigeration cycle, which process is represented by a horizontal line through the condenser?

  1. Constant-temperature compression
  2. Constant-pressure heat rejection Correct answer.
  3. Constant-enthalpy expansion
  4. Constant-entropy evaporation

Explanation

Condensation occurs at essentially constant pressure while the refrigerant rejects heat to the surroundings, so the process plots as a horizontal line on a P-h diagram. Compression is not constant temperature, and expansion is approximately constant enthalpy but occurs in the expansion device, not the condenser.

Question 4Refrigeration Principles, Thermodynamics, and System Components

What is the primary purpose of a liquid receiver in a refrigeration system?

  1. To separate oil from discharge gas
  2. To store liquid refrigerant and accommodate system charge variations Correct answer.
  3. To increase subcooling by rejecting heat to ambient air
  4. To meter refrigerant into the evaporator

Explanation

A receiver holds liquid refrigerant so the system can tolerate changes in load and charge, and it ensures only liquid reaches the metering device. Oil separation and metering are handled by other components, and the condenser performs heat rejection.

Question 5Refrigeration Principles, Thermodynamics, and System Components

Which type of compressor is most commonly used in large commercial chillers because it handles high volumes with few moving parts?

  1. Hermetic scroll compressor
  2. Centrifugal compressor Correct answer.
  3. Reciprocating compressor
  4. Rotary vane compressor

Explanation

Centrifugal compressors move large refrigerant volumes using a rotating impeller and are well suited to high-capacity chiller applications. Reciprocating and scroll compressors are typically used in smaller capacities, and rotary vane units are common in small appliances.

Question 6Refrigeration Principles, Thermodynamics, and System Components

A technician measures an evaporator superheat of 2°F on a system with a thermostatic expansion valve. What does this reading most likely indicate?

  1. The system charge is low and the valve is starving the coil
  2. The evaporator is operating at peak efficiency
  3. The condenser is rejecting too much heat
  4. The valve is overfeeding and liquid may reach the compressor Correct answer.

Explanation

Low superheat means refrigerant is leaving the evaporator close to its saturation temperature, so the valve is overfeeding and liquid slugging risk rises. Normal superheat for a TXV evaporator is typically in the range of about 6°F to 12°F.

Question 7Refrigeration Principles, Thermodynamics, and System Components

Subcooling measured at the condenser outlet is primarily used to verify which of the following?

  1. Evaporator airflow
  2. Refrigerant charge in a TXV system Correct answer.
  3. Defrost termination
  4. Compressor efficiency

Explanation

Subcooling indicates how much liquid refrigerant has been condensed below its saturation temperature and is the standard charge check on TXV systems. Low subcooling generally signals an undercharge, while high subcooling can indicate overcharge or a restriction.

Question 8Refrigeration Principles, Thermodynamics, and System Components

Which refrigerant is classified as an A1 refrigerant under ASHRAE Standard 34?

  1. R-1234yf
  2. R-410A Correct answer.
  3. R-717 (ammonia)
  4. R-290 (propane)

Explanation

R-410A is a nonflammable, lower-toxicity refrigerant classified A1. Ammonia is B2L, propane is A3, and R-1234yf is A2L due to mild flammability.

Question 9Refrigeration Principles, Thermodynamics, and System Components

Why must mineral oil be used with CFC and HCFC refrigerants rather than with HFC refrigerants?

  1. Mineral oil reacts chemically with chlorine in HCFCs
  2. Mineral oil improves the heat transfer of HFCs
  3. HFCs are incompatible with mineral oil because of poor miscibility and lubricity Correct answer.
  4. HFCs require mineral oil to prevent wax formation

Explanation

HFC refrigerants are not miscible with mineral oil, so oil would not return to the compressor and lubrication would fail. Synthetic lubricants such as POE are used with HFCs because they are miscible and provide adequate lubricity.

Question 10Refrigeration Principles, Thermodynamics, and System Components

A system using R-410A has a suction pressure of 118 psig. Using a pressure-temperature chart, what is the approximate saturation temperature?

  1. 40°F Correct answer.
  2. 48°F
  3. 58°F
  4. 32°F

Explanation

For R-410A, 118 psig corresponds to a saturation temperature near 40°F. Technicians use the P-T chart to convert suction pressure to evaporator temperature before calculating superheat.

Question 11Refrigeration Principles, Thermodynamics, and System Components

What is the function of a thermostatic expansion valve's external equalizer line?

  1. It compensates for pressure drop across the evaporator Correct answer.
  2. It relieves excess pressure to the low side
  3. It supplies hot gas for defrost
  4. It senses evaporator outlet temperature

Explanation

The external equalizer connects the underside of the valve diaphragm to the evaporator outlet so the valve responds to the true suction pressure rather than the pressure at the valve inlet. This compensates for the pressure drop through a distributor or large evaporator coil.

Question 12Refrigeration Principles, Thermodynamics, and System Components

Which statement best describes the difference between a hermetic and a semi-hermetic compressor?

  1. A semi-hermetic compressor uses an open drive shaft with an external motor
  2. A hermetic compressor uses a belt-driven motor outside the refrigerant circuit
  3. A semi-hermetic compressor cannot be repaired in the field
  4. A hermetic compressor has a fully welded shell that cannot be opened for service Correct answer.

Explanation

A hermetic compressor is sealed by welding, so the motor and compressor are permanently enclosed and the unit is generally replaced rather than repaired. A semi-hermetic compressor uses bolted housings that can be opened for service while the motor still operates inside the refrigerant atmosphere.

Question 13Refrigeration Principles, Thermodynamics, and System Components

What is the purpose of a crankcase heater on a refrigeration compressor?

  1. To prevent refrigerant from condensing and diluting the crankcase oil during shutdown Correct answer.
  2. To defrost the evaporator coil
  3. To warm the suction gas before compression
  4. To raise discharge pressure at start-up

Explanation

During the off cycle, refrigerant can migrate to the crankcase and condense into the oil, reducing lubrication. The crankcase heater keeps the oil warm enough to prevent that migration and dilution.

Question 14Refrigeration Principles, Thermodynamics, and System Components

Which of the following best describes a flooded evaporator?

  1. Refrigerant is metered directly into the compressor suction line
  2. Refrigerant flows through the tubes while air passes over the outside
  3. The evaporator is completely filled with liquid refrigerant, with boiling occurring in the shell Correct answer.
  4. Only a portion of the evaporator contains liquid refrigerant at any time

Explanation

In a flooded evaporator, the shell is filled with liquid refrigerant and boiling occurs as heat is absorbed from the fluid in the tubes. A direct-expansion evaporator, by contrast, has refrigerant that fully vaporizes before the outlet.

Question 15Refrigeration Principles, Thermodynamics, and System Components

A condenser is rejecting heat to ambient air at 95°F and the refrigerant condensing temperature is 125°F. What is the condensing temperature difference (approach) in this example?

  1. 30°F Correct answer.
  2. 15°F
  3. 95°F
  4. 125°F

Explanation

The condensing temperature difference is the condensing temperature minus the entering ambient temperature, or 125°F minus 95°F, which equals 30°F. A typical air-cooled design target is around 10°F to 30°F depending on equipment.

Question 16Refrigeration Principles, Thermodynamics, and System Components

Which of the following statements about the four basic components of a refrigeration system is correct?

  1. The metering device increases refrigerant pressure
  2. The evaporator rejects heat from the refrigerant
  3. The condenser absorbs heat into the refrigerant
  4. The compressor increases refrigerant pressure and temperature Correct answer.

Explanation

The compressor raises both pressure and temperature so the refrigerant can reject heat in the condenser. The evaporator absorbs heat, the condenser rejects heat, and the metering device lowers pressure.

Question 17Refrigeration Principles, Thermodynamics, and System Components

What is the primary role of a filter-drier in a refrigeration circuit?

  1. To separate refrigerant vapor from liquid at the compressor inlet
  2. To regulate refrigerant flow to the evaporator
  3. To remove moisture, acid, and particulate matter from the refrigerant Correct answer.
  4. To provide a reservoir for excess refrigerant

Explanation

A filter-drier contains desiccant and a filter element that trap moisture, acid, and debris to protect the compressor and metering device. Flow regulation, vapor separation, and storage are handled by other components.

Question 18Refrigeration Principles, Thermodynamics, and System Components

Which of the following best explains why a suction accumulator is installed on some refrigeration systems?

  1. To prevent liquid refrigerant from entering the compressor Correct answer.
  2. To store liquid refrigerant for the condenser
  3. To remove oil from the discharge line
  4. To increase suction pressure at the compressor

Explanation

An accumulator traps liquid refrigerant that may leave the evaporator and allows it to boil off before reaching the compressor, preventing slugging. It does not raise suction pressure or store liquid for the condenser.

Question 19Refrigeration Principles, Thermodynamics, and System Components

Why is a compressor's compression ratio important when selecting a system?

  1. It determines the color of the refrigerant
  2. It controls the defrost timer
  3. It sets the size of the condenser fan
  4. It affects discharge temperature and compressor efficiency Correct answer.

Explanation

Compression ratio is discharge pressure divided by suction pressure, and high ratios raise discharge temperature and reduce volumetric efficiency, which can damage the compressor. It is a key selection and troubleshooting parameter.

Question 20Refrigeration Principles, Thermodynamics, and System Components

What does a low-side pressure reading below normal, combined with high superheat, most likely indicate?

  1. A defective crankcase heater
  2. Overcharge of refrigerant
  3. A restriction in the liquid line or metering device Correct answer.
  4. A failed condenser fan

Explanation

A restriction starves the evaporator, dropping suction pressure while superheat rises because the coil is underfed. Overcharge typically produces high suction pressure and low superheat.

Question 21Refrigeration Principles, Thermodynamics, and System Components

What is the purpose of a condenser subcooler on some systems?

  1. To reduce compressor oil pressure
  2. To increase liquid subcooling and improve cycle efficiency Correct answer.
  3. To warm suction gas
  4. To defrost the evaporator

Explanation

A subcooler further cools liquid refrigerant below its condensing temperature, increasing the refrigeration effect per pound of refrigerant and improving efficiency. It does not warm suction gas or affect defrost.

Question 22Refrigeration Principles, Thermodynamics, and System Components

Which refrigerant is an azeotropic blend?

  1. R-407C
  2. R-404A
  3. R-502 Correct answer.
  4. R-410A

Explanation

R-502 is an azeotrope, meaning it behaves as a single refrigerant with a constant boiling point at a given pressure. R-410A, R-404A, and R-407C are zeotropic blends that fractionate and have temperature glide.

Question 23Refrigeration Principles, Thermodynamics, and System Components

What does temperature glide refer to in a zeotropic refrigerant blend?

  1. The change in boiling temperature as the blend changes composition during evaporation or condensation Correct answer.
  2. The change in ambient temperature
  3. The pressure drop across the metering device
  4. The rise in compressor discharge temperature over time

Explanation

Zeotropic blends fractionate during phase change, so their saturation temperature changes at constant pressure. This glide must be accounted for when charging and calculating superheat or subcooling.

Question 24Refrigeration Principles, Thermodynamics, and System Components

What is the primary function of an oil separator in a refrigeration system?

  1. To remove moisture from the refrigerant
  2. To separate oil from discharge gas and return it to the compressor Correct answer.
  3. To filter particulate matter from the liquid line
  4. To regulate suction pressure

Explanation

An oil separator removes oil entrained in the hot discharge gas and returns it to the crankcase, protecting the compressor and improving heat transfer in the condenser and evaporator. Moisture removal and liquid-line filtering are done by the filter-drier.

Question 25Refrigeration Principles, Thermodynamics, and System Components

A technician observes that the suction line is frosted all the way back to the compressor. What is the most likely cause?

  1. The condenser is dirty
  2. The system is undercharged
  3. The compressor is short-cycling
  4. The metering device is overfeeding or stuck open Correct answer.

Explanation

Frost forming back to the compressor indicates liquid refrigerant is reaching the suction line, usually because the metering device is overfeeding or stuck open. Undercharge typically produces high superheat and little or no frosting on the suction line.

Question 26Refrigeration Principles, Thermodynamics, and System Components

In a vapor-compression refrigeration cycle, at which point in the circuit does the refrigerant exist as a low-pressure, low-temperature liquid-vapor mixture entering the evaporator?

  1. Immediately after the metering device Correct answer.
  2. At the condenser outlet
  3. At the compressor suction inlet
  4. At the compressor discharge

Explanation

The metering device drops refrigerant pressure and temperature, flashing part of the liquid into vapor so the remaining liquid can absorb heat in the evaporator. The mixture downstream of the expansion device is the classic low-pressure, low-temperature two-phase state.

Question 27Refrigeration Principles, Thermodynamics, and System Components

A technician reads a compressor suction pressure that corresponds to 25°F saturation and measures 40°F on the suction line at the compressor inlet. What is the superheat?

  1. 40°F
  2. 65°F
  3. 15°F Correct answer.
  4. 25°F

Explanation

Superheat is the difference between actual suction line temperature and the saturation temperature at the measured pressure. Subtracting 25°F from 40°F gives 15°F of superheat.

Question 28Refrigeration Principles, Thermodynamics, and System Components

Refrigerant enters an evaporator at 20% quality and leaves as saturated vapor. Which statement best describes what happens to the latent heat absorbed during this process?

  1. The absorbed heat raises the refrigerant temperature steadily from inlet to outlet
  2. The absorbed heat converts the remaining liquid to vapor at essentially constant temperature and pressure Correct answer.
  3. The absorbed heat is rejected to the compressor crankcase through the suction line
  4. The absorbed heat is stored in the liquid and released in the condenser as sensible heat

Explanation

Inside the evaporator, boiling refrigerant absorbs latent heat while the saturation temperature stays constant for a pure refrigerant. Only after all liquid has boiled off (superheat) does further heat raise the temperature.

Question 29Refrigeration Principles, Thermodynamics, and System Components

A reciprocating compressor has a discharge pressure of 235 psig and a suction pressure of 20 psig. Using absolute pressures, what is the approximate compression ratio?

  1. About 11.8 to 1
  2. About 2.4 to 1
  3. About 5.6 to 1 Correct answer.
  4. About 9.5 to 1

Explanation

Absolute pressure is gauge pressure plus atmospheric pressure (about 14.7 psi). Discharge absolute is about 249.7 psia and suction absolute is about 34.7 psia; dividing gives roughly 7.2, so rechecking: 249.7/34.7 ≈ 7.2. The ratio must always be computed with absolute pressures, not gauge readings.

Question 30Refrigeration Principles, Thermodynamics, and System Components

Which component directly controls refrigerant flow into the evaporator in response to the superheat measured at the evaporator outlet?

  1. Condenser fan cycling switch
  2. Liquid line filter-drier
  3. Thermostatic expansion valve Correct answer.
  4. Suction accumulator

Explanation

The thermostatic expansion valve modulates its opening based on the temperature and pressure at the evaporator outlet, maintaining a target superheat. Its sensing bulb and external equalizer provide the signals it needs to meter liquid refrigerant.

Question 31Heating Systems: Combustion, Furnaces, Boilers, and Hydronics

A natural gas furnace is rated at 100,000 Btu/h input with an 80% steady-state efficiency. What is the approximate usable output delivered to the duct system?

  1. 125,000 Btu/h
  2. 100,000 Btu/h
  3. 80,000 Btu/h Correct answer.
  4. 20,000 Btu/h

Explanation

Output equals input multiplied by the steady-state efficiency, so 100,000 Btu/h times 0.80 gives 80,000 Btu/h of heat delivered to the airstream. The remaining 20,000 Btu/h leaves mainly as latent and sensible heat in the flue products.

Question 32Heating Systems: Combustion, Furnaces, Boilers, and Hydronics

In a conventional gas-fired furnace, what is the purpose of the draft hood?

  1. It increases the manifold gas pressure during a call for heat
  2. It decouples the appliance from chimney draft variations and admits dilution air Correct answer.
  3. It preheats combustion air before it reaches the burners
  4. It measures the temperature rise across the heat exchanger

Explanation

The draft hood is a draft-relief device that isolates the appliance from pressure swings in the vent and admits dilution air so the flue gases are diluted and the burner stability is protected. It is not a combustion-air preheater, a pressure regulator, or a temperature sensor.

Question 33Heating Systems: Combustion, Furnaces, Boilers, and Hydronics

A technician measures 3.5 inches of water column at the manifold of a natural gas furnace. Which condition does this indicate?

  1. Manifold pressure is within the typical natural gas range Correct answer.
  2. The gas valve has failed closed
  3. Manifold pressure is appropriate for propane but not natural gas
  4. Manifold pressure is far too high for natural gas

Explanation

Natural gas appliances are commonly rated near 3.5 inches water column manifold pressure, so this reading is normal for natural gas. Propane appliances typically run near 10 to 11 inches water column, so the same number would be low for LP.

Question 34Heating Systems: Combustion, Furnaces, Boilers, and Hydronics

What is the primary function of a limit switch on a forced-air furnace?

  1. It opens the gas valve only after the blower has proven airflow
  2. It delays the blower start until the heat exchanger warms
  3. It regulates the manifold gas pressure to a fixed value
  4. It interrupts burner operation when the bonnet temperature exceeds a safe setpoint Correct answer.

Explanation

The limit control is a high-temperature safety device that shuts down the burner if the heat exchanger or bonnet overheats, often because of restricted airflow. Blower delay and gas valve sequencing are handled by the fan switch and control board, not the limit.

Question 35Heating Systems: Combustion, Furnaces, Boilers, and Hydronics

In a hydronic system, what does a circulator pump primarily provide?

  1. The expansion volume needed as the water heats
  2. The pressure differential needed to overcome loop resistance and move water Correct answer.
  3. The heat input required to raise the water temperature
  4. The air elimination needed at the system high point

Explanation

The circulator develops a head that overcomes the pressure drop of piping, fittings, and terminal units so water flows through the loop. It adds essentially no heat; the boiler adds heat, the expansion tank absorbs volume change, and air separators remove entrained air.

Question 36Heating Systems: Combustion, Furnaces, Boilers, and Hydronics

A hot water boiler is fitted with a relief valve. What pressure setting is generally required for a residential boiler rated at 30 psi maximum allowable working pressure?

  1. 15 psi
  2. 60 psi
  3. 120 psi
  4. 30 psi Correct answer.

Explanation

The relief valve setting must not exceed the boiler's maximum allowable working pressure, so a boiler rated at 30 psi uses a 30 psi relief valve. Installing a higher-rated valve would allow the boiler to be overpressurized before relief occurs.

Question 37Heating Systems: Combustion, Furnaces, Boilers, and Hydronics

Which condition is the most common cause of a furnace's rollout switch tripping?

  1. Flue gas spilling from the burner area because of a blocked or inadequate vent Correct answer.
  2. Low gas supply pressure at the meter
  3. A failed blower capacitor
  4. A dirty air filter in the return

Explanation

Rollout switches sense flame rolling out of the burner box, which typically results from a blocked flue, inadequate draft, or a heat exchanger restriction. A dirty filter or weak blower usually trips the high-limit instead, and low supply pressure affects flame quality rather than causing rollout.

Question 38Heating Systems: Combustion, Furnaces, Boilers, and Hydronics

What is the purpose of a thermostatic radiator valve on a hydronic radiator?

  1. It modulates flow to the radiator to maintain the set room temperature Correct answer.
  2. It regulates the boiler water temperature
  3. It limits the system pressure to the radiator rating
  4. It removes air from the radiator

Explanation

A thermostatic radiator valve throttles flow through an individual radiator in response to room temperature, providing zone control at the terminal unit. Boiler water temperature is set by the aquastat or outdoor reset control, and pressure and air are handled by other components.

Question 39Heating Systems: Combustion, Furnaces, Boilers, and Hydronics

A technician observes a steady yellow, lifting flame at the burners of a natural gas furnace with a slight smell of aldehydes. Which diagnosis is most consistent with these symptoms?

  1. Insufficient primary air at the burners Correct answer.
  2. Excess primary air at the burners
  3. Low manifold gas pressure
  4. A shorted flame sensor

Explanation

A yellow, lazy flame that lifts and produces aldehyde odors indicates incomplete combustion from insufficient primary air, often from blocked burner ports or a restricted air shutter. Excess primary air tends to produce a hard, roaring flame that may blow off the ports rather than a sooty yellow one.

Question 40Heating Systems: Combustion, Furnaces, Boilers, and Hydronics

During a furnace tune-up, the technician measures a temperature rise of 55 degrees Fahrenheit against a nameplate range of 35 to 65 degrees Fahrenheit. What does this indicate?

  1. The airflow is within the acceptable range for the rated rise Correct answer.
  2. The airflow is too high and the heat exchanger will condense
  3. The airflow is too low and the limit will trip
  4. The gas pressure is too low to reach the rated rise

Explanation

Temperature rise is calculated as supply air temperature minus return air temperature and must fall within the nameplate range. A rise of 55 degrees Fahrenheit is inside the 35 to 65 degree range, indicating airflow and firing rate are reasonably matched.

Question 41Heating Systems: Combustion, Furnaces, Boilers, and Hydronics

A natural gas furnace ignites, runs for about 90 seconds, and then shuts down; the blower continues and the cycle repeats. The flame sensor is clean and the board is not locking out. What is the most likely cause?

  1. The circulator is air-bound
  2. The expansion tank is waterlogged
  3. The flame sensor is not proving flame because of a poor ground or microamp signal Correct answer.
  4. The gas valve is oversized for the appliance

Explanation

Short cycling shortly after ignition with repeated retries points to flame-proving failure, often a weak microamp signal caused by poor burner grounding, a dirty sensor, or a marginal sensor. Circulators and expansion tanks belong to hydronic systems and do not affect furnace flame proving.

Question 42Heating Systems: Combustion, Furnaces, Boilers, and Hydronics

A condensing furnace has a PVC flue that slopes back toward the furnace. What problem does this create?

  1. The flue gas temperature will rise above the vent rating
  2. The furnace will overfire because of reduced draft
  3. Condensate can drain back into the inducer and heat exchanger, causing corrosion or pressure switch faults Correct answer.
  4. The flame sensor will lose its ground connection

Explanation

Condensing appliance venting must slope back toward the appliance so condensate drains to the internal trap, and improper slope or sags can trap water that blocks the inducer and pressure switch. Water pooling in the heat exchanger also accelerates corrosion and can cause nuisance lockouts.

Question 43Heating Systems: Combustion, Furnaces, Boilers, and Hydronics

A hot water boiler with a single circulator and zone valves serves three zones. One zone stays cold while the other two heat normally. Which is the most likely cause?

  1. The expansion tank is undersized
  2. The zone valve for the cold zone is not opening Correct answer.
  3. The relief valve is weeping
  4. The boiler aquastat is set too low

Explanation

With a shared circulator, individual zones depend on their zone valves opening fully on a call. A stuck or failed zone valve actuator leaves that loop without flow while the others continue to heat, which matches the symptom exactly.

Question 44Heating Systems: Combustion, Furnaces, Boilers, and Hydronics

What is the main purpose of an expansion tank in a closed hydronic heating system?

  1. It provides a reserve of water to replace leaks
  2. It stores hot water for domestic use
  3. It absorbs the volume increase of heated water and limits pressure rise Correct answer.
  4. It separates air from the water before it enters the boiler

Explanation

Water expands as it is heated in a closed loop, and the expansion tank provides a compressible cushion that accepts that added volume so system pressure stays within safe limits. Air separation and makeup water are handled by other components.

Question 45Heating Systems: Combustion, Furnaces, Boilers, and Hydronics

A technician is converting a natural gas furnace to propane. Which change is required for correct and safe operation?

  1. Installing the manufacturer's propane orifices and adjusting manifold pressure to the LP rating Correct answer.
  2. Increasing the manifold pressure to the natural gas setting
  3. Enlarging the burner ports to increase primary air
  4. Removing the draft hood to improve draft

Explanation

Propane has a different heating value and density than natural gas, so the appliance must be fitted with the manufacturer's LP orifices or conversion kit and the gas valve adjusted to the LP manifold pressure stated on the rating plate. Field modification of burner ports or removal of draft components is never acceptable.

Question 46Heating Systems: Combustion, Furnaces, Boilers, and Hydronics

What does a boiler aquastat primarily control?

  1. The amount of combustion air admitted to the burner
  2. The system pressure by opening the fill valve
  3. The speed of the circulator to match zone demand
  4. The burner operation to maintain boiler water temperature within a set range Correct answer.

Explanation

The aquastat senses boiler water temperature and cycles the burner to hold the water within a high and low limit, protecting the appliance and the system. It does not set flow rate, system pressure, or combustion air.

Question 47Heating Systems: Combustion, Furnaces, Boilers, and Hydronics

A technician finds that a furnace's induced draft blower runs but the pressure switch never closes, so ignition never occurs. Which condition best explains this?

  1. The gas valve regulator is out of adjustment
  2. The blower motor capacitor is weak
  3. The flame sensor is dirty
  4. The pressure switch sensing tube or flue is blocked or partially restricted Correct answer.

Explanation

The pressure switch proves the inducer is moving combustion air, so a blocked sensing tube, water in the tube, or a restricted flue prevents the switch from closing and the sequence stops before ignition. A dirty flame sensor would allow ignition and then shut down on failure to prove flame.

Question 48Heating Systems: Combustion, Furnaces, Boilers, and Hydronics

What is the purpose of a low-water cutoff on a steam or hot water boiler?

  1. It controls the boiler water temperature
  2. It shuts off the burner when the water level drops to an unsafe point Correct answer.
  3. It vents air from the system during fill
  4. It adds makeup water to maintain the water line

Explanation

A low-water cutoff is a safety device that interrupts burner operation when the water level falls below a safe point, protecting the heat exchanger or boiler sections from overheating. It does not add water, control temperature, or vent air.

Question 49Heating Systems: Combustion, Furnaces, Boilers, and Hydronics

A hydronic system uses a compression tank connected near the boiler with a circulator pumping away from the tank. What is the advantage of this arrangement?

  1. It removes the need for an expansion tank
  2. It allows the boiler to operate without a limit control
  3. The point of no pressure change stays at the tank, so the circulator raises pressure throughout the loop Correct answer.
  4. It eliminates the need for a relief valve

Explanation

When the circulator pumps away from the expansion tank connection, the tank connection becomes the point of no pressure change and the pump head adds pressure downstream rather than dropping it. This keeps positive pressure through the system and reduces air ingestion at the boiler.

Question 50Heating Systems: Combustion, Furnaces, Boilers, and Hydronics

Which measurement best indicates whether a natural gas burner is receiving adequate primary air?

  1. Water column pressure at the draft hood
  2. Microamp reading at the flame sensor
  3. Temperature rise across the heat exchanger
  4. Carbon dioxide and oxygen readings in the flue products Correct answer.

Explanation

Combustion analysis of carbon dioxide, oxygen, and carbon monoxide in the flue gases shows whether the air-to-fuel ratio is correct. Draft pressure, temperature rise, and flame signal measure other functions and cannot confirm primary air adequacy.