FC Safety Standards & Codes for Furnace Systems 3 — Questions and Answers
Question 1: A Category IV furnace vents with positive pressure and produces condensate. Which vent material is specifically listed for this application?
- Type B double-wall vent
- Single-wall galvanized steel
- Listed PVC or CPVC vent pipe (Correct answer)
- Type L vent connector
Correct answer: Listed PVC or CPVC vent pipe
Category IV appliances require vent systems listed for positive pressure and condensate resistance, such as Schedule 40 PVC or CPVC approved for this use.
Question 2: What is the purpose of the dielectric union required when connecting a gas furnace to the gas supply piping that contains dissimilar metals?
- To allow for thermal expansion of the piping
- To prevent galvanic corrosion between dissimilar metals (Correct answer)
- To provide a shutoff point for servicing
- To reduce gas pressure at the appliance connection
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 3: Under the International Residential Code (IRC), what is the minimum height of a furnace room door that opens into a garage where the furnace contains an ignition source?
- The furnace must be elevated 18 inches above the garage floor (Correct answer)
- A 1-hour fire-rated door is required
- The door must be self-closing and self-latching
- No special door is required if the furnace is sealed combustion
Correct answer: The furnace must be elevated 18 inches above the garage floor
The IRC requires that appliances with ignition sources installed in garages be elevated at least 18 inches above the floor to keep ignition sources above potential fuel vapors.
Question 4: Which safety control is required on all gas furnaces to shut off gas flow if the pilot flame is extinguished?
- Pressure relief valve
- Thermocouple or flame sensor (flame supervision device) (Correct answer)
- Limit switch
- Rollout switch
Correct answer: Thermocouple or flame sensor (flame supervision device)
A flame supervision device such as a thermocouple (standing pilot) or flame sensor (electronic ignition) shuts off gas if the pilot or burner flame is lost.
Question 5: What is the maximum permissible CO concentration in parts per million (ppm) in the flue gas of a properly operating gas furnace, as referenced by most safety standards?
- 200 ppm air-free
- 400 ppm air-free (Correct answer)
- 50 ppm air-free
- 800 ppm air-free
Correct answer: 400 ppm air-free
Most U.S. standards, including ANSI Z21.47, allow a maximum of 400 ppm CO air-free in furnace flue gases under normal operating conditions.
Question 6: According to NFPA 70 (NEC), which wire gauge is the minimum required for a 240-volt, 20-ampere dedicated furnace circuit?
- 14 AWG copper
- 12 AWG copper (Correct answer)
- 10 AWG copper
- 8 AWG copper
Correct answer: 12 AWG copper
A 20-ampere circuit requires 12 AWG copper conductors per NEC Table 310.16 and 310.15 ampacity rules.
Question 7: Under the IMC, what type of opening is required between adjacent rooms to count toward unconfined space combustion air requirements?
- A louvered door with a minimum 1 sq in per 1,000 BTU/hr
- Permanent openings with a combined free area of at least 1 sq in per 1,000 BTU/hr of all appliances (Correct answer)
- Any open doorway regardless of size
- A grille with a minimum free area of 100 sq in
Correct answer: Permanent openings with a combined free area of at least 1 sq in per 1,000 BTU/hr of all appliances
Permanent openings must provide a combined free area of at least 1 square inch per 1,000 BTU/hr of total appliance input to supply adequate combustion air.
FC Safety Standards & Codes for Furnace Systems 3
A Category IV furnace vents with positive pressure and produces condensate.
Which vent material is specifically listed for this application?