CCN1 Gas Appliance Installation and Commissioning 2 — Questions and Answers
Question 1: What is the acceptable CO/CO2 ratio in flue gas for a correctly adjusted natural gas boiler?
- Above 0.010
- Between 0.008 and 0.010
- Between 0.004 and 0.008
- Below 0.004 (Correct answer)
Correct answer: Below 0.004
A CO/CO2 ratio below 0.004 indicates efficient and complete combustion in a correctly adjusted natural gas boiler; a ratio above 0.004 indicates incomplete combustion requiring investigation.
Question 2: What is the correct working gas pressure for natural gas at a domestic appliance inlet, measured while the appliance is operating at full load?
- 17 mbar
- 20 mbar (Correct answer)
- 21 mbar
- 25 mbar
Correct answer: 20 mbar
The working pressure for natural gas at a domestic appliance inlet should be 20 mbar when measured with the appliance operating at full load, slightly below the 21 mbar standing pressure.
Question 3: When commissioning an open-flued gas appliance, which test confirms that the flue is performing correctly and removing combustion products safely?
- A pressure test using a manometer
- A spillage test using a smoke match or smoke pellet (Correct answer)
- A flow rate test using an anemometer
- A temperature measurement using an infrared thermometer
Correct answer: A spillage test using a smoke match or smoke pellet
A spillage test using a smoke match or smoke pellet placed near the draught diverter confirms that combustion products are being safely drawn up and out of the flue rather than spilling into the room.
Question 4: What should an engineer do if a gas appliance cannot achieve the correct working pressure during commissioning?
- Commission the appliance and note the pressure deficit in the paperwork
- Adjust the appliance gas valve to compensate for the low pressure
- Do not commission the appliance and investigate the cause of the low pressure (Correct answer)
- Inform the customer and leave the appliance in service at reduced input
Correct answer: Do not commission the appliance and investigate the cause of the low pressure
An appliance that cannot achieve its design working pressure must not be commissioned, as incorrect gas pressure leads to incomplete combustion, poor efficiency, and potentially dangerous operating conditions.
Question 5: What is the primary reference document for setting the correct burner pressure on a gas appliance during commissioning?
- Gas Safety (Installation and Use) Regulations 1998
- The appliance manufacturer's installation instructions (Correct answer)
- BS 6400 Part 1 — Domestic gas meter installation
- IGE/UP/1 — Strength and tightness testing
Correct answer: The appliance manufacturer's installation instructions
The appliance manufacturer's installation instructions specify the exact burner pressure settings for that appliance, and these must always be followed during commissioning as they are specific to each appliance design.
Question 6: What must an engineer do immediately if a CO alarm activates during the commissioning of a gas appliance?
- Reset the alarm and continue commissioning at reduced input
- Ventilate the room and continue commissioning with windows open
- Shut off the appliance immediately and investigate the cause (Correct answer)
- Check the CO alarm's battery and continue if the alarm stops
Correct answer: Shut off the appliance immediately and investigate the cause
If a CO alarm activates during commissioning, the appliance must be immediately shut off and the cause fully investigated before any further work proceeds, as carbon monoxide is an odourless, potentially fatal gas.
Question 7: What percentage of CO2 would you typically expect in the flue gases of a correctly adjusted natural gas boiler?
- Less than 3%
- 4–6%
- 7–9% (Correct answer)
- 12–14%
Correct answer: 7–9%
A correctly adjusted natural gas boiler running with the appropriate amount of excess air should produce approximately 7–9% CO2 in the flue gases, indicating efficient combustion.
CCN1 Gas Appliance Installation and Commissioning 2
What is the acceptable CO/CO2 ratio in flue gas for a correctly adjusted natural gas boiler?