CCN1 Pipework Installation & Tightness Testing — Questions and Answers
Question 1: What does a gas tightness test confirm?
- That the gas installation pipework contains no leaks — confirmed by applying a test pressure and observing for pressure drop (Correct answer)
- That the pipework is correctly sized for the appliance load
- That the correct gas pressure is reaching the appliances
- That all appliances are correctly commissioned
Correct answer: That the gas installation pipework contains no leaks — confirmed by applying a test pressure and observing for pressure drop
A gas tightness test applies a test pressure to the pipework and monitors for any pressure drop over a specified period. No pressure drop confirms the installation is gas-tight and free from leaks.
Question 2: What test pressure is used for a domestic gas installation tightness test?
- Twice the operating pressure (typically around 20 mbar for natural gas) or a minimum of 20 mbar whichever is greater (Correct answer)
- 10 mbar — the normal operating pressure
- 100 mbar to find micro-leaks
- No specific pressure is prescribed — the engineer decides
Correct answer: Twice the operating pressure (typically around 20 mbar for natural gas) or a minimum of 20 mbar whichever is greater
For domestic installations operating at low pressure (typically around 20 mbar), tightness testing is carried out at approximately twice the operating pressure — a minimum of 20 mbar. The specific pressure depends on the type of installation.
Question 3: What is the standard gas tightness test procedure for a domestic installation?
- Isolate appliances, apply test pressure, allow to stabilise, record pressure, wait specified time, record final pressure and compare (Correct answer)
- Simply check for smell of gas with all appliances running
- Apply pressure and use a match near joints to detect leaks
- Pressurise the system and listen for hissing sounds only
Correct answer: Isolate appliances, apply test pressure, allow to stabilise, record pressure, wait specified time, record final pressure and compare
The standard tightness test involves isolating all appliances, applying the correct test pressure, allowing stabilisation for the specified period, recording the starting pressure, waiting the required time, and recording the final pressure to identify any drop.
Question 4: What action must be taken if a tightness test reveals a pressure drop?
- Trace and locate the leak(s), repair, then re-test to confirm tightness before proceeding with any gas work or restoring supply (Correct answer)
- Report to the gas transporter and await instructions
- Increase the test pressure to identify the leak location
- Re-test after 24 hours as pressure drops can be due to temperature change
Correct answer: Trace and locate the leak(s), repair, then re-test to confirm tightness before proceeding with any gas work or restoring supply
Any pressure drop during a tightness test indicates a leak. The leak must be traced (using suitable leak detection fluid or an electronic gas detector), repaired, and the tightness test repeated to confirm the installation is gas-tight.
Question 5: What material must NOT be used for gas pipework installations in the UK?
- Lead — lead pipework is prohibited for gas installations due to its toxicity and poor joint integrity (Correct answer)
- Copper — it is not suitable for gas
- Steel with threaded joints — only press-fit is permitted
- Plastic — all plastic pipework is prohibited for gas
Correct answer: Lead — lead pipework is prohibited for gas installations due to its toxicity and poor joint integrity
Lead pipework is completely prohibited for gas installations in the UK. Materials approved for gas pipework include copper, carbon steel, and approved flexible hoses. Each must be joined using appropriate approved methods.
Question 6: What pipe sizing consideration is critical to ensure correct gas appliance operation?
- The pipe must be sized to deliver the required gas flow rate (volume) without excessive pressure drop — undersized pipes starve appliances of gas (Correct answer)
- Pipe sizing only affects the time taken to purge the system
- Oversized pipes always improve appliance performance
- Pipe size is only relevant for commercial installations
Correct answer: The pipe must be sized to deliver the required gas flow rate (volume) without excessive pressure drop — undersized pipes starve appliances of gas
Gas pipes must be sized to deliver adequate gas volume to all appliances simultaneously at the required pressure. Undersized pipes cause excessive pressure drop, starving appliances of gas and causing incomplete combustion.
CCN1 Pipework Installation & Tightness Testing
What does a gas tightness test confirm?