Gas Safe ACS - Accredited Certification Scheme Combustion Principles Questions and Answers — Questions and Answers
Question 1: Which of the following are the three essential components required for combustion, often referred to as the 'combustion triangle'?
- Fuel, Heat, and Oxygen (Correct answer)
- Gas, Flame, and Ventilation
- Carbon Dioxide, Water Vapour, and Nitrogen
- Methane, Propane, and Butane
Correct answer: Fuel, Heat, and Oxygen
The combustion triangle illustrates the three elements needed for fire to occur: a fuel source to burn, sufficient heat to raise the fuel to its ignition temperature, and an oxidising agent, which is usually oxygen from the air. Removing any one of these components will prevent or extinguish a fire.
Question 2: An engineer is commissioning a new gas boiler. They observe a lazy, yellow-tipped flame on the main burner. What is the most likely cause of this?
- Excessive gas pressure at the appliance
- Incomplete combustion due to insufficient primary air (Correct answer)
- A blocked flue terminal, causing recirculation of products
- High water content in the natural gas supply
Correct answer: Incomplete combustion due to insufficient primary air
A yellow or orange, lazy flame is a classic visual sign of incomplete combustion. This occurs when there is not enough oxygen mixing with the gas, leading to the production of soot (which glows yellow) and dangerous carbon monoxide. Insufficient primary air entrainment at the burner is a primary cause of this condition.
Question 3: According to BS 7967, when using a portable flue gas analyser, what is the primary ratio used to assess the combustion safety of a gas appliance?
- O2 / N2 ratio
- CO2 / O2 ratio
- CO / CO2 ratio (Correct answer)
- CH4 / O2 ratio
Correct answer: CO / CO2 ratio
British Standard BS 7967 provides guidance on the use of electronic portable combustion gas analysers. It specifies action levels based on the ratio of Carbon Monoxide (CO) to Carbon Dioxide (CO2) in the flue gases. This ratio is a key indicator of how completely the fuel is being burned and is used to determine if an appliance is safe or requires remedial action.
Question 4: What are the main products of the complete combustion of natural gas (methane)?
- Carbon Monoxide (CO) and Water (H2O)
- Carbon (C) and Water (H2O)
- Sulphur Dioxide (SO2) and Nitrogen (N2)
- Carbon Dioxide (CO2) and Water Vapour (H2O) (Correct answer)
Correct answer: Carbon Dioxide (CO2) and Water Vapour (H2O)
When natural gas (which is primarily methane, CH4) burns completely with a sufficient supply of oxygen, the hydrocarbon fuel is fully oxidized. This chemical reaction produces harmless Carbon Dioxide (CO2) and Water Vapour (H2O), releasing heat energy in the process.
Question 5: A gas engineer is investigating a report of an intermittent smell of fumes from an open-flued gas fire. A spillage test is conducted. According to standard procedures, when should this test be performed?
- Immediately after lighting the appliance from cold
- After the appliance has been running at maximum rate for 5 minutes (Correct answer)
- Only when a flue gas analyser indicates high CO readings
- With all windows and doors in the property wide open
Correct answer: After the appliance has been running at maximum rate for 5 minutes
Standard UK gas safety procedures require a spillage test (or flue draw test) to be performed on open-flued appliances to ensure products of combustion are being safely removed. The test should be carried out within five minutes of the appliance being lit, and if spillage is detected, the appliance should be left to run for a further 10 minutes before re-testing to allow the flue to warm up and establish a proper draw.
Question 6: Under the Gas Safety (Installation and Use) Regulations 1998, what is the fundamental requirement regarding the air supply for a combustion appliance?
- It must be mechanically supplied by a fan at all times.
- It must be ducted directly from the appliance to outside.
- There must be an adequate supply for combustion and efficient working of the flue. (Correct answer)
- The air supply must be filtered to remove dust particles.
Correct answer: There must be an adequate supply for combustion and efficient working of the flue.
The Gas Safety (Installation and Use) Regulations 1998, and supporting guidance like Approved Document J of the Building Regulations, mandate that any combustion appliance must have an adequate supply of air. This is essential for ensuring complete combustion, preventing the appliance from overheating, and allowing the flue to work efficiently to remove the products of combustion.
Gas Safe ACS - Accredited Certification Scheme Combustion Principles Questions and Answers
Which of the following are the three essential components required for combustion, often referred to as the 'combustion triangle'?