Furnace Installation and Maintenance Certification (FC) — Questions and Answers

Question 1: Heat recovery ventilators (HRVs) contribute to furnace system efficiency in tight, well-insulated homes by:

Reducing the furnace's gas input rate
Increasing the AFUE rating stamped on the furnace nameplate
Transferring heat from outgoing stale air to incoming fresh air, reducing ventilation heating losses (Correct answer)
Replacing the need for a furnace in mild climates

Correct answer: Transferring heat from outgoing stale air to incoming fresh air, reducing ventilation heating losses

HRVs pre-condition incoming fresh air using the energy in exhaust air, recovering 70–85% of the heat that would otherwise be lost during ventilation.

Question 2: What does a rollout switch (also called a flame rollout limit) protect against?

Excessive blower motor current draw
Excessive return air temperature
Flames escaping outside the burner box due to blocked heat exchanger or flue (Correct answer)
Overpressure in the gas supply line

Correct answer: Flames escaping outside the burner box due to blocked heat exchanger or flue

A rollout switch detects flames rolling out of the burner compartment, which indicates a blocked heat exchanger or venting problem, and shuts down the furnace.

Question 3: What is the function of the blower door interlock switch on a furnace?

Monitors filter restriction
Controls blower speed
Senses blower motor temperature
Cuts power to the furnace if the blower compartment door is removed (Correct answer)

Correct answer: Cuts power to the furnace if the blower compartment door is removed

The interlock switch de-energizes the furnace when the blower door is open, preventing operation with exposed moving parts and electrical components.

Question 4: What is the typical supply gas pressure (upstream of the gas valve) for a natural gas furnace?

1–2 inches W.C.
5–7 inches W.C. (Correct answer)
10–14 inches W.C.
3.5–5 inches W.C.

Correct answer: 5–7 inches W.C.

Natural gas utility supply pressure is typically 5–7 inches W.C. at the meter and furnace inlet.

Question 5: Which document must be left with the homeowner upon completing a furnace installation?

The manufacturer's installation and operation manual (Correct answer)
The technician's refrigerant handling certificate
The original factory test report
The local utility company's billing schedule

Correct answer: The manufacturer's installation and operation manual

Codes and manufacturer requirements specify that the installation and user operation manual must be left with the owner or affixed near the appliance for future reference.

Question 6: How far must a direct-vent (concentric) termination kit terminate above the ground in a US snow region?

6 inches
18 inches (Correct answer)
12 inches
24 inches

Correct answer: 18 inches

In areas subject to snow accumulation, direct-vent terminations must be at least 18 inches above grade to prevent obstruction by snow.

Question 7: What is the purpose of the dielectric union required when connecting a gas furnace to the gas supply piping that contains dissimilar metals?

To provide a shutoff point for servicing
To prevent galvanic corrosion between dissimilar metals (Correct answer)
To reduce gas pressure at the appliance connection
To allow for thermal expansion of the piping

Correct answer: To prevent galvanic corrosion between dissimilar metals

Dielectric unions electrically isolate dissimilar metals in piping to prevent galvanic corrosion that could deteriorate connections.

Question 8: What is the stoichiometric air-to-fuel ratio for natural gas combustion?

12:1
10:1
20:1
15.4:1 (Correct answer)

Correct answer: 15.4:1

Natural gas requires approximately 15.4 parts air to 1 part fuel for complete stoichiometric combustion.

Question 9: Indoor air quality accessories such as UV lights and whole-house humidifiers affect system efficiency by:

Having no measurable effect on efficiency
Always reducing efficiency by adding electrical load and static pressure
Increasing furnace AFUE by improving combustion air quality
Potentially improving efficiency if they reduce the need to ventilate with unconditioned outdoor air (Correct answer)

Correct answer: Potentially improving efficiency if they reduce the need to ventilate with unconditioned outdoor air

By controlling indoor contaminants and humidity without excessive outdoor air exchange, whole-house accessories can reduce the energy cost of treating outside air.

Question 10: What does the 'B' in a Type B gas vent designation indicate?

Single-wall vent for all gas appliances
Double-wall vent for natural-draft appliances only (Correct answer)
Vent approved for oil-fired appliances
Vent approved for Category IV appliances

Correct answer: Double-wall vent for natural-draft appliances only

Type B vents are double-wall, factory-built vents listed for use with natural-draft (Category I) gas appliances only.

Question 11: What type of duct leakage test measures the total leakage of a duct system at a reference pressure of 25 Pa?

Pitot tube traverse
Duct blaster (duct pressurization) test (Correct answer)
Anemometer test
Blower door test

Correct answer: Duct blaster (duct pressurization) test

A duct blaster test pressurizes the duct system to 25 Pa and measures airflow required to maintain pressure, quantifying total duct leakage.

Question 12: Which type of ignition system uses a resistance element heated to approximately 1,800°F to ignite the gas?

Standing pilot
Intermittent pilot
Spark igniter
Hot surface igniter (Correct answer)

Correct answer: Hot surface igniter

A hot surface igniter (HSI) uses silicon carbide or silicon nitride that glows red-hot to directly ignite the gas-air mixture.

Question 13: When testing a furnace capacitor with a digital multimeter set to capacitance, a reading significantly below the rated MFD value indicates:

The capacitor has failed and should be replaced (Correct answer)
The capacitor is functioning normally
The capacitor is overcharged
The motor needs lubrication

Correct answer: The capacitor has failed and should be replaced

A capacitor reading more than 10% below its rated microfarad value indicates internal degradation and the component should be replaced.

Question 14: A technician measures manifold gas pressure on a natural gas furnace and finds it is 2.0 in. W.C. The manufacturer specifies 3.5 in. W.C. What symptom would this cause?

Burner over-fire and overheating
Weak flames, insufficient heat output, and possible pilot or ignition failure (Correct answer)
Elevated carbon monoxide production
Pressure switch nuisance trips

Correct answer: Weak flames, insufficient heat output, and possible pilot or ignition failure

Low manifold pressure produces weak flames that may not sustain ignition, resulting in reduced heat output and potential ignition lockouts.

Question 15: What is the purpose of a furnace drip leg (dirt pocket) installed on the gas supply line?

To regulate gas pressure before the valve
To trap condensate and debris before they reach the gas valve (Correct answer)
To measure gas consumption for billing purposes
To prevent gas backflow during burner shutdown

Correct answer: To trap condensate and debris before they reach the gas valve

A drip leg is a short capped vertical drop installed upstream of the gas valve to catch pipe scale, moisture, and debris that could damage the valve.

Question 16: The Manual D procedure is used in HVAC for which purpose?

Testing duct leakage
Designing and sizing duct systems for proper airflow (Correct answer)
Calculating heating and cooling loads
Selecting equipment capacity

Correct answer: Designing and sizing duct systems for proper airflow

ACCA Manual D is the industry standard procedure for residential duct system design, sizing trunks and branches for proper airflow distribution.

Question 17: What is the typical manifold gas pressure for a natural gas furnace?

3.5 inches W.C. (Correct answer)
11 inches W.C.
7 inches W.C.
0.5 inches W.C.

Correct answer: 3.5 inches W.C.

Standard natural gas manifold pressure is 3.5 inches water column as specified by most furnace manufacturers.

Question 18: What is the minimum slope per foot required for horizontal PVC vent pipe on a condensing furnace to allow condensate to drain back to the furnace?

1/16 inch per foot
No slope required
1/2 inch per foot away from the furnace
1/4 inch per foot toward the furnace (Correct answer)

Correct answer: 1/4 inch per foot toward the furnace

Horizontal PVC vent sections must slope at least 1/4 inch per foot back toward the furnace so condensate drains to the trap rather than pooling in the pipe.

Question 19: What is the maximum total external static pressure (TESP) typically allowed for a residential furnace?

0.1 inches W.C.
0.5 inches W.C. (Correct answer)
1.0 inches W.C.
2.0 inches W.C.

Correct answer: 0.5 inches W.C.

Most residential furnaces are rated for a maximum TESP of 0.5 inches W.C.; exceeding this restricts airflow and causes overheating.

Question 20: A technician discovers that the induced-draft motor on a furnace is drawing 20% more current than the nameplate rating. What is the MOST likely cause?

Faulty thermostat anticipator
Undersized supply voltage
Oversize gas supply pressure
Bearing wear or partial blockage in the venting system causing the motor to work harder (Correct answer)

Correct answer: Bearing wear or partial blockage in the venting system causing the motor to work harder

Worn bearings or a venting restriction increases load on the inducer motor, causing it to draw more current than its rated amperage.

Question 21: What device on a gas furnace controls the flow of gas to the main burner?

Pressure regulator
Orifice spud
Gas valve (Correct answer)
Manifold

Correct answer: Gas valve

The gas valve opens and closes to control gas flow to the main burner based on thermostat demand.

Question 22: Under NFPA 54, what is the minimum required distance between the gas meter outlet and the furnace appliance connector when using a flexible corrugated metallic connector?

No minimum distance, connector length governs
The connector must not extend through walls, floors, or ceilings (Correct answer)
At least 6 inches from the meter outlet
At least 3 feet from any ignition source

Correct answer: The connector must not extend through walls, floors, or ceilings

NFPA 54 prohibits flexible connectors from passing through walls, floors, or ceilings; they must be used only in exposed locations.

Question 23: What is the correct action when a gas odor is detected inside the building during a furnace startup procedure?

Immediately shut off the gas supply at the meter or main valve, ventilate the building, and evacuate before calling the utility (Correct answer)
Light a match to identify the source of the odor
Turn the furnace thermostat to its highest setting to burn off residual gas
Increase gas pressure to purge the line quickly

Correct answer: Immediately shut off the gas supply at the meter or main valve, ventilate the building, and evacuate before calling the utility

A gas odor indicates a potential leak; the only safe response is to shut off the gas supply, leave the building without operating any switches, ventilate, and call the utility company from outside.

Question 24: What is the primary cause of high static pressure in a residential duct system?

Low gas pressure
Oversized ductwork
Restrictive air filter, undersized ducts, or excessive elbows (Correct answer)
A dirty heat exchanger

Correct answer: Restrictive air filter, undersized ducts, or excessive elbows

Clogged filters, undersized supply or return ducts, and numerous sharp elbows are the most common causes of excessive static pressure.

Question 25: Flex duct installed with excessive sag will cause which problem?

Increased airflow restriction due to liner compression and tight bends (Correct answer)
Overheating of the furnace
Increased duct leakage
Condensation in the duct

Correct answer: Increased airflow restriction due to liner compression and tight bends

Sagging flex duct compresses the inner liner, dramatically increasing airflow resistance and reducing delivered CFM to registers.

Question 26: What is the purpose of a fresh air intake (combustion air duct) brought to the furnace room?

To cool the heat exchanger
To dilute gas leaks
To ensure adequate combustion air supply so negative pressure doesn't cause backdrafting (Correct answer)
To supply air for occupant comfort

Correct answer: To ensure adequate combustion air supply so negative pressure doesn't cause backdrafting

A dedicated combustion air duct prevents depressurization of the mechanical room, which could cause flue gases to backdraft into the building.

Question 27: When converting a furnace from natural gas to propane (LP), what must be replaced?

The heat exchanger
The blower motor
The orifice spud and pressure regulator spring (Correct answer)
The thermocouple

Correct answer: The orifice spud and pressure regulator spring

Propane has higher BTU content and different pressure requirements, so orifices must be downsized and the regulator spring changed.

Question 28: What is the purpose of balancing dampers in a forced-air distribution system?

To adjust airflow to individual zones or rooms to achieve comfort balance (Correct answer)
To prevent duct condensation
To prevent backdrafting in the furnace
To reduce duct static pressure overall

Correct answer: To adjust airflow to individual zones or rooms to achieve comfort balance

Balancing dampers allow technicians to restrict airflow to over-supplied areas and redirect it to under-supplied rooms.

Question 29: Which condition most commonly causes a furnace burner to produce a lifting or floating flame?

Cracked heat exchanger
Faulty thermocouple
Too little gas pressure
Excess primary air at the burner (Correct answer)

Correct answer: Excess primary air at the burner

Excess primary air causes the flame to lift off the burner ports because the mixture velocity exceeds the flame propagation speed.

Question 30: What is the minimum clearance typically required between a gas furnace and combustible materials?

0 inches if listed as zero-clearance
1 inch on the sides, 6 inches on the front (Correct answer)
12 inches on all sides
6 inches on all sides

Correct answer: 1 inch on the sides, 6 inches on the front

Most furnaces require at least 1 inch side clearance and 6 inches front clearance unless listed as zero-clearance by the manufacturer.

Furnace Installation and Maintenance Certification (FC)

The FC Furnace Installation and Maintenance Certification assesses technicians' knowledge and skills in residential and commercial furnace installation, service, and maintenance, covering combustion systems, heat exchangers, ductwork, and blower systems.

Questions

35 min

Time Limit

70%

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