5th Class sample questions, explained
6 questions from our free public 5th Class sample, with the answer, a worked explanation, and why each other option is not the answer. They are original exam-style questions written to the public syllabus, not questions from a real exam. Print 10 free 5th Class questions.
1. What is identified as the primary disadvantage of electric boilers when compared to fuel-fired alternatives?
- A. The low on-site energy conversion efficiency of electric heating
- B. The frequent replacement of heating elements due to scale buildup on their surfaces
- C. The more demanding water treatment programs required for the elements
- D. The larger physical footprint needed to house the electrical switchgear
- E. The relatively high operating cost of electricity compared to fuel
Show the answer and explanation
Answer: E. Electricity usually costs more than fuel for the same heat, and that running cost is the main drawback of an electric boiler. The unit itself is compact: it needs no furnace, chimney, or fuel storage, and it turns about 98% of the incoming electricity into heat at the plant. Low efficiency and a large footprint are not fair criticisms. Immersion elements can need closer water treatment, but that is not the main reason to prefer a fuel-fired boiler.
Why the other options are not the answer
- A. On-site conversion of electricity to heat is very efficient (about 98%), so low efficiency is not the drawback.
- B. Element care is a maintenance item, not the main disadvantage the text identifies. That is the cost of the electricity.
- C. Stricter water treatment applies to immersion-type units, but it is not given as the main disadvantage.
- D. Electric boilers are usually more compact than fuel-fired units because they need no furnace, chimney or fuel storage.
Question ID KE-501-C04-Q004 · Unit 5-1: Low Pressure Boiler Components and Operation · Report an error
2. Which type of burner control continuously adjusts the firing rate to keep boiler pressure at a steady value regardless of changing load?
- A. Flame failure control
- B. Modulating control
- C. On-off control
- D. High-low fire control
- E. High limit control
Show the answer and explanation
Answer: B. A modulating control continuously varies the fuel and air supply to match the boiler load, keeping pressure at a constant value. On-off control only switches the burner fully on or off; high-low fire selects between two fixed rates; high limit control shuts down the burner as a safety measure rather than regulating firing rate.
Why the other options are not the answer
- A. Flame failure control shuts off the fuel if the flame is lost; it does not set the firing rate.
- C. On-off control runs the burner fully on or fully off, so pressure swings between the cut-in and cut-out.
- D. High-low fire only switches between two fixed firing rates; it does not vary the rate continuously.
- E. A high limit control is a safety device that shuts the burner off; it does not regulate the firing rate.
Question ID KE-501-C08-Q001 · Unit 5-1: Low Pressure Boiler Components and Operation · Report an error
3. When an automatic gas valve opens too rapidly, a larger-than-normal combustible mixture accumulates in the furnace and ignites all at once, creating a momentarily high pressure. What is this event called?
- A. Puff back
- B. Furnace lockout
- C. Backdraught
- D. Light-off surge
- E. Flashback
Show the answer and explanation
Answer: A. A puff back occurs when an automatic gas valve opens too quickly, flooding the furnace with a larger-than-normal combustible mixture that ignites all at once, building a momentarily high pressure that blows back through air intakes and observation openings. This is why most larger gas valves are designed to open slowly. A flashback is a different event in which flame travels back into the burner body.
Why the other options are not the answer
- B. A lockout is the safety shutdown state a burner management system goes to, not a delayed-ignition event.
- C. Backdraught describes a fire flaring when air reaches an oxygen-starved space; it is not caused by a gas valve opening too fast.
- D. Not the term used for this event; the momentary high pressure from a delayed, over-rich light-off is called a puff back.
- E. A flashback is flame travelling back into the burner body, a different event.
Question ID KE-501-C14-Q007 · Unit 5-1: Low Pressure Boiler Components and Operation · Report an error
4. A trap is installed in a sanitary drainage system primarily to:
- A. Capture hair and solid debris before they enter the waste stack
- B. Slow the flow so grease can settle out before reaching the municipal main
- C. Regulate flow velocity so solids remain in suspension in the pipe
- D. Create a water seal that blocks sewer gases from entering the building
- E. Provide an access point for rodding out blockages in the drain line
Show the answer and explanation
Answer: D. The purpose of a trap is to hold a water seal that prevents sewer gases from backing up through the fixture outlet into the building. Capturing debris is the function of strainers; providing cleaning access is the function of cleanouts; flow velocity is maintained by correct pipe slope and sizing.
Why the other options are not the answer
- A. Catching debris is the job of a strainer, not a trap.
- B. Separating grease is the job of a grease interceptor, not a fixture trap.
- C. Flow velocity is set by correct pipe slope and sizing, not by a trap.
- E. Rodding access is provided by a cleanout, not a trap.
Question ID KE-502-C13-Q001 · Unit 5-2: Elements of Human Comfort in Facility Operation · Report an error
5. While water is being converted to steam at 100°C and atmospheric pressure, a thermometer placed in the water shows no temperature change even though heat is still being added. What term describes this added heat?
- A. Sensible heat of the liquid
- B. Heat of compression
- C. Superheat
- D. Specific heat
- E. Latent heat of evaporation
Show the answer and explanation
Answer: E. Latent heat is heat added or removed that causes a change of state at a constant temperature, so the thermometer shows no change. The specific form during liquid-to-vapour conversion is latent heat of evaporation. Sensible heat would produce a temperature rise; superheat is sensible heat added to steam already above the saturation temperature.
Why the other options are not the answer
- A. Sensible heat changes the temperature, but the thermometer shows no change here.
- B. Heat of compression comes from compressing a gas; nothing is being compressed here.
- C. Superheat is heat added to steam that is already above its saturation temperature.
- D. Specific heat is a property (heat per kilogram per degree), not the name for heat that changes state.
Question ID KE-503-C08-Q004 · Unit 5-3: Basic Physical Science, Safety, and Regulation for Facility Operations · Report an error
6. Which treatment method is most often applied after a settling tank to remove the suspended solids that remain in the water?
- A. Lime-soda chemical softening
- B. Reverse osmosis through a semi-permeable membrane
- C. Filtration
- D. Zeolite ion exchange softening
- E. Deaeration
Show the answer and explanation
Answer: C. Filters are frequently used in conjunction with settling tanks: the settling tank removes the bulk of the suspended impurities by gravity, and the water then passes through a filter to capture any remaining fine particles. Deaeration targets dissolved gases, and zeolite or lime-soda softening targets dissolved solids. Neither addresses the suspended-solids left after settling.
Why the other options are not the answer
- A. Lime-soda softening targets dissolved hardness, not fine suspended particles.
- B. Reverse osmosis targets dissolved solids and needs clean feed water; it is not the step normally used to catch solids left after a settling tank.
- D. Zeolite softening removes dissolved hardness, not the suspended solids left after settling.
- E. Deaeration removes dissolved gases, not suspended solids.
Question ID KE-503-C17-Q009 · Unit 5-3: Basic Physical Science, Safety, and Regulation for Facility Operations · Report an error
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