CENWAT Boiler Types 3 — Questions and Answers
Question 1: What is an open-flued boiler?
- A boiler that draws combustion air from the room in which it is installed (Correct answer)
- A boiler with no flue at all
- A boiler installed outdoors
- A boiler that vents through an open window
Correct answer: A boiler that draws combustion air from the room in which it is installed
An open-flued boiler draws combustion air from the room (indoor air) and discharges flue gases via a flue to atmosphere; adequate room ventilation is essential.
Question 2: What is a room-sealed (balanced flue) boiler?
- A boiler that draws combustion air from outside and discharges flue gases externally, sealed from the room (Correct answer)
- A boiler with a sealed heat exchanger
- A boiler installed in a sealed plant room
- A boiler with sealed controls
Correct answer: A boiler that draws combustion air from outside and discharges flue gases externally, sealed from the room
A room-sealed boiler draws all combustion air directly from outside and discharges flue gases externally, completely isolated from the indoor air; no room ventilation is required for combustion.
Question 3: What is a fan-assisted flue (positive pressure flue) on a boiler?
- A flue system where a fan forces flue gases out and draws in combustion air, allowing flexible flue routing (Correct answer)
- A gravity flue that relies on natural draught
- A flue used only for oil boilers
- A flue that draws air from a separate outdoor duct
Correct answer: A flue system where a fan forces flue gases out and draws in combustion air, allowing flexible flue routing
Fan-assisted (positive flue) systems use an integral fan to overcome resistance in longer or complex flue runs, giving greater flexibility in siting the boiler.
Question 4: What is the purpose of the flue gas analyser check during boiler commissioning?
- To verify combustion efficiency and CO2/O2 levels are within manufacturer's specification (Correct answer)
- To measure system water pressure
- To check for gas leaks at joints
- To verify flue gas temperature only
Correct answer: To verify combustion efficiency and CO2/O2 levels are within manufacturer's specification
A flue gas analysis confirms that combustion is efficient and safe, checking CO2, O2 and CO levels against the manufacturer's specification to ensure correct air/gas ratio.
Question 5: What colour is the flame on a correctly burning gas boiler burner?
- Predominantly blue with a small yellow tip (Correct answer)
- Entirely yellow
- Entirely orange
- Bright white
Correct answer: Predominantly blue with a small yellow tip
A correctly burning gas flame is predominantly blue, indicating complete combustion. A yellow or orange flame indicates incomplete combustion and possible carbon monoxide production.
Question 6: What does 'thermal store' refer to in hot water system terminology?
- A large insulated cylinder that stores primary circuit water, heating DHW indirectly via a heat exchanger (Correct answer)
- A cylinder that stores cold water
- A boiler with a built-in buffer tank
- An insulated room used to store heating equipment
Correct answer: A large insulated cylinder that stores primary circuit water, heating DHW indirectly via a heat exchanger
A thermal store is a large primary water storage cylinder that acts as a heat battery; domestic hot water is heated indirectly by passing mains water through an internal coil or heat exchanger.
CENWAT Boiler Types 3
What is an open-flued boiler?