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HVAC Excellence Gas Heat study materials
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HVAC Excellence Gas Heat Certification practice questions

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Question 1Gas Heating Fundamentals and Combustion

A natural gas appliance is rated for complete combustion. What are the two primary products of complete combustion of methane?

  1. Methane and nitrogen
  2. Carbon and oxygen
  3. Carbon monoxide and hydrogen
  4. Carbon dioxide and water vapor Correct answer.

Explanation

Complete combustion of a hydrocarbon fuel such as methane combines the fuel with sufficient oxygen to produce carbon dioxide and water vapor, releasing heat. Carbon monoxide appears only when oxygen is insufficient.

Question 2Gas Heating Fundamentals and Combustion

Which condition produces carbon monoxide in a gas-fired furnace?

  1. A flue gas temperature above the dew point
  2. A manifold pressure set to the nameplate value
  3. Insufficient oxygen for complete combustion Correct answer.
  4. Excess primary air at the burner

Explanation

Carbon monoxide forms when the fuel burns with too little oxygen, so carbon is only partially oxidized. Adequate primary air and correct manifold pressure support complete combustion and keep CO low.

Question 3Gas Heating Fundamentals and Combustion

What is the approximate stoichiometric air requirement for burning one cubic foot of natural gas?

  1. About 1 cubic foot of air
  2. About 10 cubic feet of air Correct answer.
  3. About 100 cubic feet of air
  4. About 1,000 cubic feet of air

Explanation

Natural gas needs roughly 10 cubic feet of air per cubic foot of gas for theoretically complete combustion. Real appliances supply excess air above this theoretical amount to ensure clean burning.

Question 4Gas Heating Fundamentals and Combustion

A technician measures 9 percent CO2 in the flue gas of a natural gas furnace. What does this reading most directly indicate?

  1. The combustion air supply is excessive
  2. The heat exchanger is leaking
  3. The fuel-air mixture is richer than ideal Correct answer.
  4. The burners are producing no heat

Explanation

For natural gas, flue CO2 near 8.5 to 9.5 percent suggests a mixture close to ideal, while markedly lower readings point to excess air diluting the products. A high CO2 combined with high CO can signal too little air.

Question 5Gas Heating Fundamentals and Combustion

During a combustion test, what does a steady 0 ppm CO reading in the flue with proper CO2 and stack temperature indicate?

  1. A blocked flue
  2. A failed gas valve
  3. Complete combustion with adequate air Correct answer.
  4. An overfired burner

Explanation

Near-zero CO with expected CO2 and stack temperature shows the fuel-air mixture is burning cleanly. A blocked flue, failed valve, or overfiring would produce abnormal readings or no flame.

Question 6Gas Heating Fundamentals and Combustion

What is the primary purpose of primary air at a gas burner?

  1. To dilute the flue gases after combustion
  2. To mix with gas before ignition for proper flame shape Correct answer.
  3. To cool the heat exchanger
  4. To pressurize the manifold

Explanation

Primary air mixes with the gas before it reaches the flame, establishing the air-gas mixture that determines flame shape and stability. Secondary air is drawn around the flame to complete combustion.

Question 7Gas Heating Fundamentals and Combustion

A burner flame lifts off the burner ports and appears to blow out. Which condition most likely causes this?

  1. Too little primary air
  2. Excessive gas manifold pressure or excess primary air Correct answer.
  3. A cracked heat exchanger
  4. A clogged condensate trap

Explanation

Flame lift occurs when the gas-air mixture velocity exceeds the flame speed, often from excessive manifold pressure or too much primary air. Reducing gas pressure or air restores a stable flame.

Question 8Gas Heating Fundamentals and Combustion

A natural draft furnace produces a yellow, sooting flame with a tall lazy pattern. What is the most likely cause?

  1. Low flue gas temperature
  2. Insufficient primary air Correct answer.
  3. Excess primary air
  4. High supply air temperature

Explanation

A yellow, sooting flame indicates incomplete combustion from too little primary air, causing carbon to form. Increasing primary air until the flame is blue and stable corrects the condition.

Question 9Gas Heating Fundamentals and Combustion

What is the heating (calorific) value of natural gas commonly used for furnace sizing in the United States?

  1. About 1,000 Btu per cubic foot Correct answer.
  2. About 500 Btu per cubic foot
  3. About 5,000 Btu per cubic foot
  4. About 2,500 Btu per cubic foot

Explanation

Natural gas is typically rated near 1,000 Btu per cubic foot, though the exact value varies by locality and gas composition. This figure is used with clocked gas meter readings to determine input rate.

Question 10Gas Heating Fundamentals and Combustion

A technician clocks a gas meter and finds the appliance uses 1 cubic foot in 30 seconds. Using 1,000 Btu per cubic foot, what is the approximate input rate?

  1. 30,000 Btu/h
  2. 180,000 Btu/h
  3. 120,000 Btu/h Correct answer.
  4. 60,000 Btu/h

Explanation

One cubic foot in 30 seconds equals 2 cubic feet per minute, or 120 cubic feet per hour. At 1,000 Btu per cubic foot, the input rate is about 120,000 Btu/h.

Question 11Gas Heating Fundamentals and Combustion

What does the term excess air mean in gas combustion?

  1. Air trapped in the gas line
  2. Air used to cool the burners
  3. Air that leaks into the flue after the draft hood
  4. Air supplied beyond the theoretical amount needed Correct answer.

Explanation

Excess air is the amount supplied above the stoichiometric requirement to ensure all fuel burns. It is expressed as a percentage and typically ranges from about 25 to 50 percent in furnaces.

Question 12Gas Heating Fundamentals and Combustion

Which instrument is used to measure the carbon monoxide concentration in flue gases during a combustion analysis?

  1. Megohmmeter
  2. Anemometer
  3. Manometer
  4. Combustion analyzer Correct answer.

Explanation

A combustion analyzer samples flue gas and measures CO, CO2, and oxygen while calculating efficiency. Manometers measure pressure, anemometers measure air velocity, and megohmmeters test insulation.

Question 13Gas Heating Fundamentals and Combustion

A technician observes a burner flame that is short, sharp, and noisy with a roaring sound. What does this indicate?

  1. A cracked heat exchanger
  2. Too little primary air
  3. Excessive primary air or gas pressure Correct answer.
  4. A blocked condensate drain

Explanation

A short, roaring, hard flame indicates the mixture is too lean, often from excess primary air or high gas pressure. The flame speed cannot keep up with the mixture velocity.

Question 14Gas Heating Fundamentals and Combustion

What is the purpose of a draft hood on a natural draft gas appliance?

  1. To preheat combustion air
  2. To stabilize draft and vent flue gases while preventing backdrafts Correct answer.
  3. To increase manifold gas pressure
  4. To mix gas and air

Explanation

A draft hood relieves draft variations and prevents backdrafts from spilling flue products into the space. It also dilutes flue gases with room air before they enter the vent.

Question 15Gas Heating Fundamentals and Combustion

A technician finds the flue gas temperature of a condensing furnace is about 100°F. What does this suggest?

  1. The heat exchanger is leaking
  2. The burners are overfired
  3. The furnace is condensing and recovering latent heat Correct answer.
  4. The gas valve is undersized

Explanation

Condensing furnaces cool flue gases below the dew point, recovering latent heat and producing low stack temperatures. Non-condensing furnaces typically show stack temperatures well above 300°F.

Question 16Gas Heating Fundamentals and Combustion

What is the dew point of flue gases from natural gas combustion, above which a condensing furnace will not condense?

  1. About 250°F
  2. About 400°F
  3. About 130°F Correct answer.
  4. About 60°F

Explanation

Water vapor in natural gas flue products begins condensing near 130°F, so condensing furnaces cool the flue gas below that point. Non-condensing appliances keep flue gases above the dew point to avoid corrosion.

Question 17Gas Heating Fundamentals and Combustion

A technician measures 7.5 inches water column manifold pressure on a natural gas furnace rated for 3.5 inches. What is the most likely effect?

  1. No change in operation
  2. Reduced input and possible incomplete combustion
  3. Overfiring with higher input and possible flame lift Correct answer.
  4. Immediate lockout of the ignition control

Explanation

Manifold pressure roughly double the nameplate value overfires the burner, raising input and risking flame lift and overheating. The regulator must be adjusted to the nameplate pressure.

Question 18Gas Heating Fundamentals and Combustion

Which gas is added to natural gas to give it a detectable odor?

  1. Propane
  2. Carbon dioxide
  3. Nitrogen
  4. Mercaptan Correct answer.

Explanation

Mercaptan is the odorant added so leaks can be detected by smell. Propane, nitrogen, and carbon dioxide may be present or added for other reasons but are not the odorant.

Question 19Gas Heating Fundamentals and Combustion

A technician must determine whether a furnace is operating within its rated input. Which measurement is required?

  1. Supply air temperature rise only
  2. Clocked gas meter reading and heating value Correct answer.
  3. Flue draft only
  4. Manifold pressure only

Explanation

Input rate is calculated from the volume of gas consumed per unit time multiplied by the heating value. Temperature rise and manifold pressure are useful but do not alone determine input.

Question 20Gas Heating Fundamentals and Combustion

What is the role of secondary air in a gas burner?

  1. It mixes with gas before the burner ports
  2. It dilutes the gas at the manifold
  3. It cools the ignition control
  4. It surrounds the flame to complete combustion Correct answer.

Explanation

Secondary air is drawn from the surrounding space around the flame to finish combustion of any remaining fuel. Primary air, by contrast, mixes with gas before ignition.

Question 21Gas Heating Fundamentals and Combustion

A technician observes a burner flame that is stable but the appliance trips on the high-limit switch repeatedly. Which factor is most likely?

  1. A shorted flame sensor
  2. A lean gas-air mixture
  3. Low manifold pressure
  4. Reduced airflow across the heat exchanger Correct answer.

Explanation

A high-limit trip with a stable flame usually indicates heat is not being carried away, often from restricted airflow such as a dirty filter or blower fault. Correcting airflow resolves the limit trip.

Question 22Gas Heating Fundamentals and Combustion

What does a flame rectification sensor rely on to prove flame?

  1. A thermocouple voltage
  2. Ionization current through the flame Correct answer.
  3. A pressure switch closure
  4. A bimetallic strip

Explanation

A flame rectification sensor uses the flame as a conductor to pass a small DC current, proving flame to the control. Thermocouples and pressure switches are separate devices.

Question 23Gas Heating Fundamentals and Combustion

During a combustion analysis, the oxygen reading in the flue is 9 percent. What does this indicate?

  1. A leaking heat exchanger
  2. A lean mixture with substantial excess air Correct answer.
  3. A rich mixture with insufficient air
  4. Complete combustion with no excess air

Explanation

High flue oxygen indicates excess air diluting the products, meaning the mixture is lean. Ideal natural gas combustion typically shows about 4 to 6 percent oxygen in the flue.

Question 24Gas Heating Fundamentals and Combustion

Which factor most affects the amount of excess air a natural draft burner draws?

  1. The supply air blower speed
  2. The thermostat setpoint
  3. The color of the burner flame
  4. The vent connector diameter and draft Correct answer.

Explanation

Draft and venting geometry influence how much air is drawn into the burner and dilution air inlet. Blower speed and thermostat settings affect heat delivery, not combustion air.

Question 25Gas Heating Fundamentals and Combustion

What is the primary safety concern when flue gases spill from a draft hood into the living space?

  1. Increased efficiency
  2. Reduced condensate
  3. Higher gas pressure
  4. Carbon monoxide exposure Correct answer.

Explanation

Spillage carries carbon monoxide and other combustion products into the occupied space, creating a health hazard. Spillage is often caused by blocked flues, backdrafts, or inadequate draft.

Question 26Gas Heating Fundamentals and Combustion

Which three elements must be present at the burner for combustion of natural gas to occur and be sustained?

  1. Fuel gas, oxygen, and an ignition source Correct answer.
  2. Fuel gas, water vapor, and a catalyst
  3. Fuel gas, nitrogen, and a spark
  4. Fuel gas, carbon dioxide, and heat

Explanation

Combustion requires a combustible fuel, an oxidizer (oxygen from air), and enough heat to reach the fuel's ignition temperature. Removing any leg of this triangle stops combustion, which is why gas valves, air openings, and ignition controls are all safety-critical. Nitrogen and carbon dioxide are not oxidizers and will not support burning of the gas.

Question 27Gas Heating Fundamentals and Combustion

A technician needs to verify that a gas appliance's input rate matches its rating plate. Which combination of measurements is required?

  1. Static pressure and blower amp draw
  2. Manifold pressure and flue gas temperature
  3. Draft pressure and ambient temperature
  4. Gas meter timing and the gas heating value Correct answer.

Explanation

Input rate is determined by measuring the volume of gas consumed per unit time at the meter and multiplying by the gas's heating value in Btu per cubic foot. Input (Btu/h) equals cubic feet per hour times heating value. Flue temperature, draft, and blower electrical data describe performance but do not establish the firing rate.

Question 28Gas Heating Fundamentals and Combustion

A technician measures gas flow of 2 cubic feet in 60 seconds with a heating value of 1,000 Btu per cubic foot. What is the approximate input rate?

  1. 30,000 Btu/h
  2. 60,000 Btu/h
  3. 2,000 Btu/h
  4. 120,000 Btu/h Correct answer.

Explanation

Two cubic feet in 60 seconds equals 2 cubic feet per minute, or 120 cubic feet per hour. Multiplying 120 cubic feet per hour by 1,000 Btu per cubic foot gives 120,000 Btu/h. This metering method is the standard field check of appliance input.

Question 29Gas Heating Fundamentals and Combustion

A technician observes a burner flame that is stable at the ports but the appliance repeatedly trips the high-limit switch. Which condition most likely explains this?

  1. A faulty flame sensor
  2. Insufficient gas manifold pressure
  3. Reduced airflow across the heat exchanger Correct answer.
  4. A cracked heat exchanger

Explanation

A stable flame with repeated high-limit trips points to heat that is not being carried away, typically from restricted airflow caused by a dirty filter, dirty blower wheel, or blocked coil. The burner is producing normal heat, but the airside cannot remove it, so the limit opens. Gas pressure problems and flame-sensor faults would disturb the flame or cause lockout rather than a limit trip.

Question 30Gas Heating Fundamentals and Combustion

Which gas is intentionally added to natural gas so that leaks can be detected by smell?

  1. Propane
  2. Mercaptan Correct answer.
  3. Carbon monoxide
  4. Hydrogen sulfide

Explanation

Mercaptan, an odorant, is injected into natural gas so that a concentration well below the flammable range can be smelled. It gives the familiar rotten-egg odor associated with gas leaks. Hydrogen sulfide also smells, but it is toxic and corrosive and is not the standard odorant used.

Question 31Gas Heating Fundamentals and Combustion

A technician reads 9 percent oxygen in the flue gas of a natural draft furnace. What does this indicate?

  1. The heat exchanger is leaking
  2. The appliance is starved for combustion air
  3. The gas valve is overfiring the burner
  4. The appliance is operating with excessive excess air and dilution Correct answer.

Explanation

Higher flue oxygen means more air is passing through than is needed to burn the gas, which is common on natural draft equipment because of draft hood dilution and infiltration. Excess air lowers efficiency by carrying heat up the flue. Low oxygen, not high, would suggest air starvation or overfiring.

Question 32Gas Heating Fundamentals and Combustion

Which condition most directly determines the amount of excess air drawn by a natural draft burner?

  1. The gas heating value
  2. The vent connector height and the draft hood relief opening Correct answer.
  3. The manifold gas pressure setting
  4. The blower motor speed

Explanation

On a natural draft appliance, the draft created by the vent and the open area of the draft hood relief determine how much room air is pulled through the combustion zone. That dilution air is the main source of excess air. Gas pressure affects flame shape and input, not dilution airflow.

Question 33Gas Heating Fundamentals and Combustion

A flame rectification sensor proves flame by measuring which electrical property?

  1. The capacitance of the sensor to ground
  2. The current that flows when the flame conducts ions between the sensor and burner Correct answer.
  3. The resistance of the igniter element
  4. The voltage induced in the gas valve coil

Explanation

In flame rectification, the flame acts as a conductor that allows a small direct current to flow between the sensor and the grounded burner, typically in the microamp range. The control recognizes this current as proof of flame. The signal is a rectified current, not igniter resistance or valve coil voltage.

Question 34Gas Heating Fundamentals and Combustion

A technician is checking a natural gas furnace and measures a manifold pressure of 3.5 inches water column, but the rating plate specifies 3.5 inches water column for natural gas. What does this indicate?

  1. The manifold pressure is too low and must be increased.
  2. The manifold pressure is correct for natural gas operation. Correct answer.
  3. The manifold pressure is too high and must be reduced.
  4. The manifold pressure reading is irrelevant; only the supply pressure matters.

Explanation

The rating plate specifies the required manifold pressure for the fuel type; for natural gas, 3.5 inches water column is a common value. A measured pressure matching the rating plate indicates the gas valve is set correctly. Manifold pressure must be verified against the rating plate, not assumed.

Question 35Gas Furnace Components and Sequence of Operation

On a standing-pilot gas furnace, which component proves the pilot flame is established before the main gas valve is allowed to open?

  1. The rollout switch
  2. The high-limit switch
  3. The flame sensor or thermocouple circuit Correct answer.
  4. The draft pressure switch

Explanation

The pilot-proving device, either a thermocouple on a standing pilot or a flame sensor/rectification rod on intermittent ignition, confirms a flame is present before the gas valve opens. Without proof, the ignition module keeps the main valve closed to prevent unburned gas accumulation.

Question 36Gas Furnace Components and Sequence of Operation

A hot surface ignition (HSI) furnace calls for heat but the igniter glows and the main burner never lights. Which condition most directly explains this symptom?

  1. The high-limit switch is stuck closed
  2. The thermostat is set to continuous fan
  3. The gas valve is not opening during the ignition trial Correct answer.
  4. The circulating blower is running too fast

Explanation

If the igniter glows but no gas reaches the burner, the module is not energizing the gas valve or the valve is failing to open, so ignition cannot occur. The igniter glowing confirms the ignition sequence reached the trial-for-ignition stage.

Question 37Gas Furnace Components and Sequence of Operation

What is the primary function of the draft inducer assembly on a modern induced-draft gas furnace?

  1. To sense the presence of a flame
  2. To mix gas and air at the manifold
  3. To pull combustion air through the burners and vent flue gases Correct answer.
  4. To circulate conditioned air through the ductwork

Explanation

The draft inducer is a blower that establishes negative pressure in the combustion chamber, drawing combustion air in and pushing flue products out the vent. This induced draft also lets the pressure switch prove airflow before ignition.

Question 38Gas Furnace Components and Sequence of Operation

During a normal heating cycle, when does the circulating blower typically start on a furnace equipped with a fan-on delay?

  1. After the heat exchanger has warmed, following a timed or temperature-based delay Correct answer.
  2. Only after the high-limit switch opens
  3. Before the gas valve opens
  4. Immediately when the thermostat calls for heat

Explanation

The fan-on delay holds the blower off until the heat exchanger reaches a useful temperature, preventing a blast of cold air at startup. The delay is usually timed by the control board or triggered by a temperature sensor.

Question 39Gas Furnace Components and Sequence of Operation

What does the flame rectification sensor rely on to confirm a burner flame?

  1. A rise in resistance across the igniter
  2. A small DC current flowing through the ionized flame to ground Correct answer.
  3. The position of the gas valve diaphragm
  4. The temperature of the flue gases

Explanation

In flame rectification, the module applies an AC voltage to the sensor; the flame conducts and rectifies it, producing a small DC microamp signal the board reads as flame present. Loss of that microamp signal shuts the gas valve.

Question 40Gas Furnace Components and Sequence of Operation

Which safety device is designed to shut off gas if flame rolls out of the burner box toward the front of the furnace?

  1. The draft pressure switch
  2. The flame sensor
  3. The rollout (flame roll-out) switch Correct answer.
  4. The fan limit control

Explanation

Rollout switches are thermal devices mounted near the burners that open when flame spills outward, indicating a blocked flue or cracked heat exchanger. They interrupt the gas valve circuit to stop combustion.

Question 41Gas Furnace Components and Sequence of Operation

A furnace has a standing pilot with a thermocouple. The pilot flame is lit but the main burner will not come on. Which is the most likely cause?

  1. The blower capacitor has failed
  2. The draft inducer motor is running backward
  3. The flame sensor is dirty
  4. The thermocouple is not generating enough millivoltage to hold the valve open Correct answer.

Explanation

In a standing-pilot system the thermocouple generates millivolts that hold the gas valve open; a weak or failing thermocouple drops the signal and the valve closes. Cleaning or replacing the thermocouple restores the circuit.

Question 42Gas Furnace Components and Sequence of Operation

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

  1. To limit the heat exchanger temperature
  2. To prove the inducer is moving combustion air before ignition Correct answer.
  3. To measure supply-air static pressure
  4. To regulate gas manifold pressure

Explanation

The pressure switch senses the negative pressure created by the inducer and closes to allow the ignition sequence to proceed. If the inducer fails or the flue is blocked, the switch stays open and the gas valve is kept from opening.

Question 43Gas Furnace Components and Sequence of Operation

A technician observes the following: inducer starts, pressure switch closes, igniter glows, burner lights briefly, then the gas valve closes after a few seconds and the sequence retries. Which component should be checked first?

  1. The blower motor windings
  2. The flame sensor signal Correct answer.
  3. The thermostat batteries
  4. The condensate trap level

Explanation

A burner that lights and then drops out shortly after ignition usually indicates the control board is not receiving a reliable flame signal, so it shuts the valve as a safety. Cleaning or checking the flame sensor and its microamp reading is the first step.

Question 44Gas Furnace Components and Sequence of Operation

During the ignition sequence of a direct-spark ignition furnace, what proves the burner flame after the spark stops?

  1. The inducer pressure switch
  2. The spark electrode gap
  3. The gas valve solenoid coil
  4. The flame sensor rod reading rectified current Correct answer.

Explanation

On direct-spark systems the spark ignites the burner, but flame proving is done by a separate flame sensor using rectification. The board continues to monitor the microamp signal and shuts the valve if it is lost.

Question 45Gas Furnace Components and Sequence of Operation

What is the correct sequence of operation for a typical induced-draft hot surface ignition furnace at a call for heat?

  1. Inducer starts, pressure switch closes, igniter heats, gas valve opens, flame proves, blower starts Correct answer.
  2. Gas valve opens, igniter heats, inducer starts, flame proves, blower starts
  3. Blower starts, gas valve opens, igniter heats, inducer starts, flame proves
  4. Igniter heats, blower starts, inducer starts, gas valve opens, flame proves

Explanation

The control board first starts the inducer and verifies airflow through the pressure switch, then energizes the igniter, opens the gas valve for the trial, proves flame, and finally starts the blower after the fan-on delay. This order prevents gas from entering without airflow or ignition.

Question 46Gas Furnace Components and Sequence of Operation

A furnace's high-limit switch repeatedly trips during a heating cycle. Which condition is the most likely cause?

  1. A failed inducer capacitor
  2. Restricted airflow across the heat exchanger Correct answer.
  3. A weak flame sensor signal
  4. A miswired thermostat

Explanation

The high-limit opens when the heat exchanger overheats, most often because insufficient air passes over it due to a dirty filter, blocked coil, or slow blower. Correcting the airflow restores normal temperature rise.

Question 47Gas Furnace Components and Sequence of Operation

What role does the gas valve's redundant operator (redundant solenoid) play in a combination gas valve?

  1. It provides a second safety shutoff so gas cannot flow if the main operator sticks open Correct answer.
  2. It regulates manifold pressure to the burners
  3. It vents excess gas to the flue
  4. It senses the flame during ignition

Explanation

A combination valve contains two operators in series; the redundant operator is a safety shutoff that closes if the main valve fails to seat. This redundancy prevents continuous gas flow on a single failure.

Question 48Gas Furnace Components and Sequence of Operation

A technician measures 24 VAC at the gas valve terminals during a call for heat, but the burner does not light and no gas odor is present. What does this indicate?

  1. The valve is receiving power but may be failing to open internally Correct answer.
  2. The thermostat is not calling for heat
  3. The flame sensor is shorted to ground
  4. The pressure switch is stuck open

Explanation

Voltage at the valve terminals shows the control board is commanding the valve to open; if no gas flows, the valve operator or its internal seats may be defective. The next step is to verify inlet pressure and valve operation.

Question 49Gas Furnace Components and Sequence of Operation

What is the function of the fan/limit control on a conventional furnace?

  1. It regulates the gas manifold pressure
  2. It starts and stops the blower based on plenum temperature and opens on over-temperature Correct answer.
  3. It ignites the pilot flame
  4. It proves the inducer airflow

Explanation

The fan/limit control uses a temperature-responsive element in the plenum: it closes on temperature rise to start the blower and opens at a high limit to cut off the burner. It serves both blower control and over-temperature protection.

Question 50Gas Furnace Components and Sequence of Operation

A two-stage gas furnace is operating on a first-stage call. How does the gas valve typically behave compared with second stage?

  1. It delivers a lower regulated manifold pressure to the burners Correct answer.
  2. It disables the flame sensor
  3. It prevents the inducer from running
  4. It opens only the redundant operator and bypasses the main valve

Explanation

On a two-stage valve, first stage regulates to a lower manifold pressure for reduced firing rate, and second stage raises it for full capacity. The inducer and flame-proving circuits remain active in both stages.