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NC HVAC practice questions

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Question 1Heating, Ventilation, and Air Conditioning Systems and Equipment

In a forced-air gas furnace, what is the primary function of the heat exchanger?

  1. It separates combustion gases from the supply air stream while transferring heat Correct answer.
  2. It mixes combustion air with the supply air to raise the delivered air temperature
  3. It regulates the manifold gas pressure delivered to the burners
  4. It condenses flue gas moisture to improve combustion efficiency

Explanation

The heat exchanger is a metal barrier that keeps flue products out of the conditioned air while conducting heat from combustion into the supply air. Combustion gases must never mix with supply air because that would introduce carbon monoxide into the space.

Question 2Heating, Ventilation, and Air Conditioning Systems and Equipment

A technician measures a temperature rise across a furnace that is well above the data plate range. Which condition is the most likely cause?

  1. A manifold gas pressure set below the nameplate value
  2. A cracked heat exchanger leaking flue gas into the supply
  3. Restricted airflow through a clogged filter or dirty evaporator coil Correct answer.
  4. Excessive supply airflow from an oversized blower

Explanation

Temperature rise equals heat input divided by airflow, so reduced airflow raises the rise above the rated range. A clogged filter or fouled coil is the most common cause of reduced airflow in a residential system.

Question 3Heating, Ventilation, and Air Conditioning Systems and Equipment

Which type of gas furnace is designed to extract latent heat by condensing water vapor from the flue products?

  1. Induced draft mid-efficiency furnace
  2. Natural draft atmospheric furnace
  3. Direct-fired make-up air unit
  4. Condensing furnace with a secondary heat exchanger Correct answer.

Explanation

A condensing furnace adds a secondary heat exchanger that cools flue products below the dew point, recovering latent heat and reaching annual fuel utilization efficiencies above roughly 90 percent. The condensate must be drained and the flue often vented with PVC.

Question 4Heating, Ventilation, and Air Conditioning Systems and Equipment

A gas furnace ignites briefly and then the flame drops out shortly after the main burner lights. Which component is the first to evaluate when the flame sensor is suspected?

  1. The draft inducer capacitor, checking for capacitance
  2. The rollout limit switch, checking for continuity
  3. The gas valve regulator, checking for outlet pressure
  4. The flame sensor probe, checking for contamination and microamp signal Correct answer.

Explanation

A flame sensor that is coated with oxide cannot rectify enough current, so the control shuts the gas valve off after a short trial period. Cleaning the probe and verifying the microamp reading confirms whether the sensor is the cause.

Question 5Heating, Ventilation, and Air Conditioning Systems and Equipment

What is the purpose of the pressure switch in an induced draft furnace?

  1. It regulates the gas manifold pressure to the burners
  2. It monitors the supply air static pressure across the blower
  3. It limits the flue gas temperature leaving the heat exchanger
  4. It proves that combustion air is moving before allowing ignition Correct answer.

Explanation

The pressure switch senses the negative pressure created by the draft inducer and closes only when adequate combustion airflow is proven. The ignition control then permits the gas valve to open, providing a safety interlock.

Question 6Heating, Ventilation, and Air Conditioning Systems and Equipment

During a heating cycle, a technician finds the draft inducer running, the pressure switch closed, and no ignition after the trial-for-ignition period. The spark is visible at the igniter. What is the most useful next measurement?

  1. Flue gas carbon dioxide concentration
  2. Supply air temperature rise across the heat exchanger
  3. Gas manifold pressure at the valve outlet during the trial for ignition Correct answer.
  4. Blower motor amp draw during the heating cycle

Explanation

With proven draft and visible spark, the remaining requirement for light-off is gas at the burners. Measuring manifold pressure during the trial period confirms whether the valve is opening and delivering gas.

Question 7Heating, Ventilation, and Air Conditioning Systems and Equipment

Which safety device is designed to shut off the gas valve if flame rolls out of the burner compartment?

  1. Flame rectification sensor
  2. High-limit switch
  3. Flame rollout switch Correct answer.
  4. Draft proving switch

Explanation

Rollout switches are temperature-sensitive devices mounted near the burners that open on abnormal flame rollout and interrupt the gas valve circuit. They are typically manual-reset devices that must be investigated before resetting.

Question 8Heating, Ventilation, and Air Conditioning Systems and Equipment

A technician is checking a hot surface igniter. Which measurement best determines whether the igniter is functioning properly?

  1. Temperature of the igniter tip with an infrared thermometer
  2. Current draw of the igniter during the warm-up period Correct answer.
  3. Voltage at the igniter terminals while the control is powered
  4. Resistance across the igniter element compared with the manufacturer specification

Explanation

Hot surface igniters draw a characteristic current when healthy, and the ignition control often monitors this current to prove the igniter. Measuring current during warm-up verifies both the element and the control's sensing circuit.

Question 9Heating, Ventilation, and Air Conditioning Systems and Equipment

What does a standing pilot with a thermocouple provide in a conventional gas furnace?

  1. A pressure signal that proves the draft through the heat exchanger
  2. A high-voltage spark that lights the main burners on a call for heat
  3. A millivolt signal that holds the gas valve open while the pilot flame is proven Correct answer.
  4. A resistance signal that modulates the gas valve to maintain temperature

Explanation

The thermocouple generates a small millivolt signal when heated by the pilot flame, and this signal energizes the safety magnet in the gas valve. Loss of the pilot flame removes the signal and the valve closes.

Question 10Heating, Ventilation, and Air Conditioning Systems and Equipment

A technician finds a gas furnace with a tripped rollout switch. Which step is the correct first action?

  1. Bypass the switch temporarily to verify burner operation
  2. Investigate the cause of flame rollout before resetting the switch Correct answer.
  3. Reset the switch and observe the next heating cycle
  4. Replace the switch with a higher-temperature-rated device

Explanation

A rollout switch trips because flame has escaped the burner area, often from a blocked flue, cracked heat exchanger, or improper gas pressure. The cause must be found and corrected before the switch is reset, since resetting without diagnosis risks a hazardous condition.

Question 11Heating, Ventilation, and Air Conditioning Systems and Equipment

In a hydronic heating system, what is the function of the circulator pump?

  1. It controls the boiler firing rate based on outdoor temperature
  2. It vents air from the loop during initial fill
  3. It maintains system pressure by adding makeup water
  4. It moves heated water from the boiler through the distribution loop Correct answer.

Explanation

The circulator provides the pressure differential that moves hot water through the piping and terminal units back to the boiler. Flow rate and head determine pump selection for the loop.

Question 12Heating, Ventilation, and Air Conditioning Systems and Equipment

A hot water boiler is short-cycling on high limit. Which condition most directly explains this behavior?

  1. The circulator is undersized for the loop
  2. The expansion tank is waterlogged
  3. The boiler aquastat is set below the high-limit setting with low load Correct answer.
  4. The air separator is fouled with debris

Explanation

Short cycling on high limit occurs when the boiler reaches its limit before the aquastat setpoint is satisfied, which happens with low load or a narrow differential between settings. Adjusting the aquastat differential or addressing the low-load condition resolves it.

Question 13Heating, Ventilation, and Air Conditioning Systems and Equipment

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

  1. It stores hot water for domestic use during peak demand
  2. It removes dissolved oxygen from the loop water
  3. It provides a reservoir for makeup water during leaks
  4. It absorbs the volume change of water as it is heated Correct answer.

Explanation

Water expands as it is heated, and in a closed loop the expansion tank provides an air cushion or diaphragm volume to absorb that expansion. Without it, system pressure would rise excessively and relieve through the relief valve.

Question 14Heating, Ventilation, and Air Conditioning Systems and Equipment

A technician is commissioning a steam boiler and finds the low water cut-off not functioning. What is the correct action?

  1. Add a manual reset high-limit as a substitute
  2. Operate the boiler at reduced pressure until repair
  3. Lock out the boiler until the low water cut-off is repaired Correct answer.
  4. Increase the feedwater flow to prevent low water

Explanation

The low water cut-off is a critical safety control that prevents the boiler from firing without adequate water, which could cause a dangerous dry-fire condition. A boiler with a nonfunctional low water cut-off must be taken out of service until it is repaired.

Question 15Heating, Ventilation, and Air Conditioning Systems and Equipment

Which boiler type uses a burner that fires into a combustion chamber surrounded by water and is commonly used for larger commercial heating loads?

  1. Cast iron sectional boiler
  2. Condensing wall-hung boiler
  3. Fire-tube boiler
  4. Water-tube boiler Correct answer.

Explanation

Water-tube boilers circulate water through tubes that are heated by combustion gases, allowing high capacity and rapid response for larger commercial loads. Fire-tube and sectional boilers are used in other capacity ranges and configurations.

Question 16Heating, Ventilation, and Air Conditioning Systems and Equipment

In a heat pump operating in heating mode, what is the function of the reversing valve?

  1. It controls the compressor unloading during low load
  2. It modulates refrigerant flow to maintain superheat
  3. It bypasses the outdoor coil during defrost
  4. It switches refrigerant flow direction to change between heating and cooling Correct answer.

Explanation

The reversing valve changes the direction of refrigerant flow so the indoor coil becomes the condenser in heating and the evaporator in cooling. Its position is controlled by the mode signal from the thermostat or defrost control.

Question 17Heating, Ventilation, and Air Conditioning Systems and Equipment

A heat pump in heating mode is operating with a low outdoor temperature and the outdoor coil begins to frost. Which control sequence is correct?

  1. The reversing valve shifts to cooling mode and the indoor blower stops
  2. The outdoor fan speeds up to remove frost by increased airflow
  3. The low-pressure switch opens and shuts down the compressor until frost clears
  4. The defrost control initiates a defrost cycle and may energize auxiliary heat to temper supply air Correct answer.

Explanation

When frost accumulates, the defrost control initiates a defrost cycle, typically by shifting to cooling mode and/or stopping the outdoor fan while energizing auxiliary heat to prevent cold supply air. Defrost terminates on time or temperature.

Question 18Heating, Ventilation, and Air Conditioning Systems and Equipment

Which statement correctly describes the balance point of a heat pump system?

  1. The point at which the compressor reaches its maximum current draw
  2. The outdoor temperature at which heat pump capacity equals the building heat loss Correct answer.
  3. The refrigerant pressure at which the reversing valve changes position
  4. The indoor temperature at which auxiliary heat is locked out

Explanation

The balance point is the outdoor temperature where heat pump capacity matches the structure's heat loss, so no supplemental heat is needed above that temperature. Below it, auxiliary heat makes up the difference.

Question 19Heating, Ventilation, and Air Conditioning Systems and Equipment

During a heating call, a heat pump's outdoor unit is running but the indoor coil is not getting warm and the suction line is cold. Which condition is most consistent with these readings?

  1. The reversing valve is stuck in the cooling position Correct answer.
  2. The indoor blower is set to the wrong speed
  3. The outdoor fan motor has failed
  4. The auxiliary heat is energized continuously

Explanation

If the reversing valve fails to shift, the system continues to operate in cooling mode, so the indoor coil stays cold and the outdoor coil rejects heat. This matches a cold indoor coil and cold suction line during a heating call.

Question 20Heating, Ventilation, and Air Conditioning Systems and Equipment

Which component in a gas-fired unit heater provides the combustion air and vents flue products through a single wall penetration?

  1. A draft hood with a vertical flue
  2. A power venter with a combined intake and exhaust termination Correct answer.
  3. A condensing secondary heat exchanger
  4. A barometric draft regulator

Explanation

Power-vented unit heaters use a fan to draw combustion air and discharge flue products through a combined sidewall termination. This allows installation without a vertical chimney.

Question 21Heating, Ventilation, and Air Conditioning Systems and Equipment

A technician is checking a ducted electric resistance heater. The heater has power but no heat output. Which measurement is most appropriate to isolate the fault?

  1. Refrigerant superheat at the indoor coil
  2. Gas manifold pressure at the burner
  3. Voltage across the heating element terminals and at the sequencer contacts Correct answer.
  4. Outdoor ambient temperature and humidity

Explanation

Electric resistance heaters rely on contactors or sequencers to apply voltage to the elements, so checking voltage at the element and at the sequencer contacts identifies whether the control or the element is at fault. There is no refrigerant or gas circuit in a resistance heater.

Question 22Heating, Ventilation, and Air Conditioning Systems and Equipment

In a warm-air furnace, what is the function of the fan limit switch?

  1. It starts the blower when the heat exchanger reaches temperature and stops it when the exchanger cools Correct answer.
  2. It limits the gas pressure supplied to the burners
  3. It proves the draft through the flue
  4. It controls the ignition sequence of the burners

Explanation

The fan limit switch has a fan-on setting that starts the blower after the heat exchanger warms and a fan-off setting that continues blower operation to remove residual heat. It also includes a high-limit setting that interrupts the burner circuit on overtemperature.

Question 23Heating, Ventilation, and Air Conditioning Systems and Equipment

A technician measures a supply air temperature rise of 55 degrees F on a furnace rated for 35 to 65 degrees F, but the customer reports cold rooms far from the furnace. Which conclusion is best supported?

  1. The furnace is overfiring and the gas valve must be derated
  2. The furnace is operating within its rated temperature rise and the complaint likely involves duct distribution Correct answer.
  3. The blower is oversized and must be replaced with a smaller motor
  4. The heat exchanger is restricted and must be replaced

Explanation

A rise within the nameplate range indicates the furnace is delivering its rated heat output at the measured airflow. Comfort complaints in distant rooms then point to duct design, balancing, or insulation rather than furnace capacity.

Question 24Heating, Ventilation, and Air Conditioning Systems and Equipment

Which practice is required when venting a category IV condensing furnace?

  1. Venting into a shared chimney with a natural draft water heater
  2. Venting with double-wall Type B vent terminating above the roofline
  3. Venting with single-wall galvanized pipe through an existing masonry chimney
  4. Venting with PVC or other approved plastic pipe sloped back to the furnace for condensate drainage Correct answer.

Explanation

Category IV appliances produce acidic condensate and low flue temperatures, so they are vented with approved plastic pipe and sloped to drain condensate back to the appliance or trap. Common venting with natural draft appliances is not permitted.

Question 25Heating, Ventilation, and Air Conditioning Systems and Equipment

A technician is diagnosing a hydronic system where some radiators are hot and others remain cold. Which cause is most consistent with this pattern?

  1. The aquastat is set too high
  2. The expansion tank is undersized
  3. The boiler relief valve is leaking
  4. The system is air-bound or improperly balanced Correct answer.

Explanation

Uneven heating in a hydronic loop typically results from trapped air or unbalanced flow, where some terminal units receive insufficient circulation. Air vents and balancing valves are used to correct the distribution.

Question 26Heating, Ventilation, and Air Conditioning Systems and Equipment

A technician is preparing to leak-test a newly installed gas piping system that will operate at a pressure of 5 psig during the test. Which medium is appropriate for this test?

  1. Dry nitrogen or compressed air Correct answer.
  2. Acetylene from a torch set
  3. Liquid propane from a supply tank
  4. Oxygen from a service cylinder

Explanation

Gas piping is pressure-tested with a dry, inert, nonflammable medium such as nitrogen or compressed air. Using fuel gas or oxygen for testing creates fire and explosion hazards.

Question 27Heating, Ventilation, and Air Conditioning Systems and Equipment

Which value most closely represents the heating value of natural gas used for sizing gas appliances in the field?

  1. About 1,000 Btu per cubic foot Correct answer.
  2. About 10,000 Btu per cubic foot
  3. About 25,000 Btu per cubic foot
  4. About 100 Btu per cubic foot

Explanation

Natural gas is commonly rated near 1,000 Btu per cubic foot for sizing and clocking calculations. The other values are off by an order of magnitude or more.

Question 28Heating, Ventilation, and Air Conditioning Systems and Equipment

A technician needs to clock the gas meter to verify input rate. Which measurement is required in addition to the volume of gas used?

  1. The indoor blower's static pressure
  2. The temperature rise across the heat exchanger
  3. The flue gas temperature at the vent connector
  4. The time interval over which that volume was consumed Correct answer.

Explanation

Input rate is found from the volume of gas consumed divided by the time it took to consume it, then multiplied by the heating value. The other readings describe airflow and combustion efficiency, not gas input.

Question 29Heating, Ventilation, and Air Conditioning Systems and Equipment

A technician measures 24 volts across the gas valve terminals but hears no click and gets no gas flow. Which conclusion is best supported?

  1. The pressure switch is stuck open
  2. The transformer secondary is open
  3. The thermostat is not calling for heat
  4. The valve operator or coil has failed internally Correct answer.

Explanation

Voltage present at the valve terminals with no response indicates the operator itself is not functioning. An open transformer or open pressure switch would remove voltage from the valve.

Question 30Heating, Ventilation, and Air Conditioning Systems and Equipment

A natural gas flame at the burners is lifting off the ports and appears to be roaring with a short, blue, noisy pattern. Which condition most likely causes this?

  1. Excessive primary air or overfiring gas pressure Correct answer.
  2. A cracked heat exchanger
  3. Insufficient primary air at the burner
  4. A blocked flue passage

Explanation

Flame lift and a noisy, roaring flame indicate too much primary air or excessive manifold pressure. Insufficient primary air produces a soft yellow-tipped flame rather than lift.

Question 31Heating, Ventilation, and Air Conditioning Systems and Equipment

What is the function of the primary air supplied at a gas burner?

  1. It mixes with gas before combustion at the burner ports Correct answer.
  2. It provides the draft that carries flue gases up the vent
  3. It dilutes the flue products after they leave the heat exchanger
  4. It cools the heat exchanger during the off cycle

Explanation

Primary air mixes with gas at or before the burner ports to establish the flame. Secondary air is drawn around the flame, and dilution air is added at the draft hood.

Question 32Heating, Ventilation, and Air Conditioning Systems and Equipment

A technician observes the burners ignite and burn steadily, but flame rolls out around the draft hood when the indoor blower starts. Which condition should be investigated first?

  1. A low thermostat setpoint
  2. A restricted flue or blocked heat exchanger passage Correct answer.
  3. A dirty indoor blower wheel
  4. A failed hot surface igniter

Explanation

Spillage when the blower starts points to a venting or heat exchanger restriction that the blower's pressure disrupts. Ignition and thermostat issues would not produce spillage at the draft hood.

Question 33Heating, Ventilation, and Air Conditioning Systems and Equipment

Which material is typically required for venting a category I natural gas furnace in good condition?

  1. Single-wall PVC pipe
  2. Schedule 40 galvanized water pipe
  3. Type L copper tubing
  4. Type B double-wall vent Correct answer.

Explanation

Category I appliances are non-condensing and are vented with listed double-wall vent such as Type B. Plastic venting is reserved for condensing appliances, and water pipe is not a vent material.

Question 34Heating, Ventilation, and Air Conditioning Systems and Equipment

A technician is checking combustion with a flue gas analyzer on a natural gas furnace. Which carbon monoxide reading in the flue products is most consistent with clean, complete combustion?

  1. About two percent by volume
  2. A few parts per million Correct answer.
  3. About ten percent by volume
  4. Several thousand parts per million

Explanation

Properly adjusted natural gas appliances produce only trace CO, typically in the low parts per million. High CO readings indicate incomplete combustion from air, gas, or venting problems.

Question 35Heating, Ventilation, and Air Conditioning Systems and Equipment

A technician is setting up a furnace for a temperature rise check. Which pair of measurements is needed?

  1. Outdoor temperature and return air temperature
  2. Flue gas temperature and supply air temperature
  3. Static pressure and outdoor temperature
  4. Return air temperature and supply air temperature Correct answer.

Explanation

Temperature rise is the difference between supply and return air temperatures at the unit. Outdoor and flue temperatures do not enter that calculation.

Question 36Heating, Ventilation, and Air Conditioning Systems and Equipment

A furnace has a temperature rise well above the nameplate range. Which condition is the most likely cause?

  1. A shorted fan limit switch
  2. A thermostat with a low differential
  3. Low airflow through the heat exchanger Correct answer.
  4. Excessive gas manifold pressure

Explanation

When airflow is low, the same heat output raises the air temperature more, producing an excessive rise. High manifold pressure would raise capacity but not as directly as restricted airflow.

Question 37Heating, Ventilation, and Air Conditioning Systems and Equipment

Which device in a forced-air furnace is designed to open and shut off the burners if the heat exchanger overheats?

  1. The fan-on switch
  2. The high-limit switch Correct answer.
  3. The flame sensor
  4. The pressure switch

Explanation

The high-limit switch opens on excessive temperature to protect the heat exchanger and shut off the burners. The fan switch controls blower operation, and the pressure switch proves draft.

Question 38Heating, Ventilation, and Air Conditioning Systems and Equipment

A technician finds a furnace with a limit switch that has opened and the burners are off, but the indoor blower continues to run. Which explanation best fits?

  1. The flame sensor is shorted to ground
  2. The fan limit is holding the blower on to cool the heat exchanger Correct answer.
  3. The thermostat is stuck in the cooling mode
  4. The gas valve is stuck open

Explanation

On a limit trip, the fan control keeps the blower running to remove residual heat. A stuck gas valve would continue supplying gas, and a cooling call would not relate to the limit.

Question 39Heating, Ventilation, and Air Conditioning Systems and Equipment

Which type of electric heating element is commonly used in a duct heater because it can be formed into a compact coil?

  1. Ceramic radiant panel
  2. Induction coil
  3. Cast iron boiler section
  4. Open coil resistance wire Correct answer.

Explanation

Open coil resistance wire is the standard element in electric duct heaters and is formed into coils to concentrate heat in a compact space. Cast iron sections and induction coils are unrelated to duct heating.

Question 40Heating, Ventilation, and Air Conditioning Systems and Equipment

A technician is checking an electric duct heater with a sequencer. Only one element stage heats even though the thermostat calls for full capacity. Which component should be checked first?

  1. The blower motor capacitor
  2. The room thermostat calibration
  3. The air filter
  4. The sequencer or contactor for the unenergized stages Correct answer.

Explanation

A sequencer or contactor energizes each stage; if a stage does not heat, that control or its element circuit is the first suspect. Blower and thermostat issues would affect all stages.

Question 41Heating, Ventilation, and Air Conditioning Systems and Equipment

What is the purpose of a thermal cutout on an electric resistance heater?

  1. To reduce voltage to the element during part-load operation
  2. To sequence the blower after the element is energized
  3. To increase the heating capacity at low outdoor temperatures
  4. To interrupt power if airflow is lost and the element overheats Correct answer.

Explanation

A thermal cutout is a safety device that opens on excessive temperature, typically from loss of airflow. Capacity modulation and blower sequencing are handled by other controls.

Question 42Heating, Ventilation, and Air Conditioning Systems and Equipment

A technician is converting a natural gas furnace to propane. Which change is normally required?

  1. Installing the manufacturer's propane orifices and adjusting manifold pressure Correct answer.
  2. Replacing the heat exchanger
  3. Removing the draft inducer
  4. Increasing the vent diameter

Explanation

Propane has a different heating value and density, so the burner orifices and manifold pressure must be changed per the manufacturer's instructions. Venting and heat exchanger changes are not part of a fuel conversion.

Question 43Heating, Ventilation, and Air Conditioning Systems and Equipment

Which statement best describes the operation of a modulating gas furnace?

  1. It varies the vent diameter to control draft
  2. It cycles the blower on and off to control capacity
  3. It varies firing rate to match the heating load Correct answer.
  4. It uses a fixed firing rate with a longer off cycle

Explanation

Modulating furnaces vary the gas input rate to match the load, improving comfort and efficiency. Cycling the blower or changing vent size are not modulation methods.

Question 44Heating, Ventilation, and Air Conditioning Systems and Equipment

A technician is diagnosing a furnace with an inducer that runs and then shuts off before any ignition attempt. Which component is most likely preventing the sequence from continuing?

  1. The gas valve
  2. The flame sensor
  3. The blower motor
  4. The pressure switch Correct answer.

Explanation

The sequence requires the pressure switch to prove draft after the inducer starts. If the switch does not close, the control stops the sequence before ignition.

Question 45Heating, Ventilation, and Air Conditioning Systems and Equipment

Which component proves that the inducer is developing sufficient draft before the ignition sequence proceeds?

  1. A high-limit switch
  2. A flame sensor
  3. A pressure switch connected to the inducer housing Correct answer.
  4. A flame rollout switch

Explanation

The pressure switch senses the pressure difference created by the inducer and closes to permit ignition. Rollout and limit switches are temperature safety devices.

Question 46Heating, Ventilation, and Air Conditioning Systems and Equipment

A technician is checking a furnace with a hot surface igniter that glows but the burners do not light. Which condition should be verified next?

  1. The thermostat fan setting
  2. The blower wheel rotation
  3. The condensate trap level
  4. Gas supply and manifold pressure at the valve Correct answer.

Explanation

Once the igniter proves hot, the control opens the gas valve, so gas supply and manifold pressure must be verified. Blower rotation and fan settings do not affect ignition.

Question 47Heating, Ventilation, and Air Conditioning Systems and Equipment

Which statement correctly describes the function of a draft hood on a natural draft gas appliance?

  1. It isolates the appliance from vent pressure changes and admits dilution air Correct answer.
  2. It measures the temperature of the flue products
  3. It boosts the draft by adding a blower
  4. It condenses flue gases before they leave the appliance

Explanation

A draft hood stabilizes draft by breaking the direct connection to the vent and admitting dilution air. It has no blower and does not condense gases or measure temperature.

Question 48Heating, Ventilation, and Air Conditioning Systems and Equipment

Which gas is most commonly used as the fuel for residential heating appliances in areas without natural gas service?

  1. Propane Correct answer.
  2. Carbon monoxide
  3. Methane hydrate
  4. Nitrogen

Explanation

Propane is the common fuel for appliances where natural gas mains are unavailable. Carbon monoxide and nitrogen are not fuels.

Question 49Heating, Ventilation, and Air Conditioning Systems and Equipment

A technician finds a gas furnace with a manifold pressure of 3.5 inches water column on natural gas. Which condition does this most likely indicate?

  1. A severe overfire condition
  2. A propane appliance running on natural gas
  3. A failed gas valve that is fully closed
  4. Normal manifold pressure for many natural gas appliances Correct answer.

Explanation

Many natural gas furnaces are rated near 3.5 inches water column manifold pressure. It is not an overfire condition and would not exist at a fully closed valve.

Question 50Heating, Ventilation, and Air Conditioning Systems and Equipment

A technician is diagnosing a furnace that trips the rollout switch repeatedly after a short period of operation. Which condition is the most likely cause?

  1. A weak hot surface igniter
  2. A thermostat with a high differential
  3. A restricted secondary heat exchanger or flue Correct answer.
  4. A dirty blower wheel

Explanation

Repeated rollout trips indicate combustion products are not escaping properly, often from a restricted flue or heat exchanger. A weak igniter or thermostat issue would not cause rollout.