CCN1 Pipework and Fittings 2 — Questions and Answers
Question 1: What is the purpose of purging a gas installation after completing pipework installation or modification?
- To test the tightness of the installation
- To remove air from the pipework and replace it with gas, preventing a potentially explosive air-gas mixture (Correct answer)
- To check the burner pressure of connected appliances
- To measure the gas flow rate
Correct answer: To remove air from the pipework and replace it with gas, preventing a potentially explosive air-gas mixture
Purging removes the air from newly installed or modified gas pipework and replaces it with gas. This is essential because a mixture of air and gas within the pipe could form an explosive mixture. Purging must be carried out safely in accordance with IGE/UP/1B, ensuring proper ventilation and that no ignition sources are present.
Question 2: What is the minimum pipe size typically required for a gas meter connection to a domestic boiler with a rated input of 30 kW?
- 15 mm
- 22 mm (Correct answer)
- 28 mm
- 35 mm
Correct answer: 22 mm
A 22 mm pipe is typically the minimum size for supplying a 30 kW domestic boiler from the meter, depending on the pipe run length and the number of fittings. Pipe sizing must be calculated using the appropriate formula or sizing tables to ensure the pressure drop does not exceed 1 mbar from the meter to the appliance.
Question 3: What is the maximum permissible pressure drop between the gas meter outlet and any appliance in a domestic installation?
- 0.5 mbar
- 1 mbar (Correct answer)
- 2 mbar
- 4 mbar
Correct answer: 1 mbar
The maximum permissible pressure drop between the gas meter outlet and any gas appliance in a domestic installation is 1 mbar. This ensures that each appliance receives adequate gas pressure for safe and efficient operation. If the pressure drop exceeds 1 mbar, the pipework may need to be upsized.
Question 4: When sleaving gas pipework through a wall, what must the sleeve extend to on each side?
- Flush with the wall surface on both sides (Correct answer)
- At least 25 mm beyond the wall surface on each side
- At least 50 mm beyond the wall surface on the external side only
- The sleeve should be sealed at both ends
Correct answer: Flush with the wall surface on both sides
When passing gas pipe through a wall, the sleeve must extend flush with the plaster or wall surface on each side. The gap between the pipe and the sleeve should be sealed at both ends with a suitable fire-resistant sealant to prevent gas migration through the wall cavity. The sleeve protects the pipe from corrosion where it passes through masonry.
Question 5: During a tightness test, what is the stabilisation period required before commencing the 2-minute observation?
- No stabilisation is required
- 30 seconds
- 1 minute (Correct answer)
- 3 minutes
Correct answer: 1 minute
Before commencing the 2-minute tightness test observation period, a stabilisation period of 1 minute must be allowed. This permits the gas temperature and pressure to equalise throughout the installation after pressurisation. Without this stabilisation period, temperature-related pressure changes could give a false reading.
Question 6: What action must be taken if a tightness test reveals a pressure drop exceeding the permitted limit?
- Report the installation as safe but recommend repair
- Turn off the gas supply, investigate and locate the leak using leak detection fluid, and repair before re-testing (Correct answer)
- Increase the gas pressure to compensate for the loss
- Leave a warning label and advise the customer to call their supplier
Correct answer: Turn off the gas supply, investigate and locate the leak using leak detection fluid, and repair before re-testing
If the tightness test fails (pressure drops below the permitted limit), the gas supply must be turned off at the ECV, the leak must be found using approved leak detection fluid (LDF), and the fault must be repaired. A further tightness test must then be carried out to confirm the installation is gas-tight before the supply is restored.
CCN1 Pipework and Fittings 2
What is the purpose of purging a gas installation after completing pipework installation or modification?