CCN1 Cheat Sheet 2026

The 30 highest-yield CCN1 facts, distilled from real exam questions. Print it, save it as a PDF, or study it here — free, no sign-up.

1. What is the maximum permissible pressure drop on a let-by test of a gas meter? → Zero — no drop is acceptable
2. What colour flame indicates correct combustion on a natural gas burner? → Predominantly blue with a clearly defined inner cone
3. How often must a landlord arrange a gas safety check on appliances in a rented property? → Every 12 months
4. What is the minimum internal diameter of a flue liner suitable for a standard domestic gas boiler? → 125 mm
5. What is the minimum vertical distance a room-sealed balanced flue terminal must be from ground level or a surface people can stand on? → 300 mm
6. What dangerous gas is produced as a result of incomplete combustion of natural gas? → Carbon monoxide (CO)
7. What does a pressure regulator (governor) on a gas meter do? → Reduces and maintains the outlet pressure at a constant 21 mbar
8. Under RIDDOR, who must a Gas Safe registered engineer report a dangerous gas fitting to? → The HSE (Health and Safety Executive) via the online RIDDOR reporting system
9. What is the primary reference document for setting the correct burner pressure on a gas appliance during commissioning? → The appliance manufacturer's installation instructions
10. At what concentration in air does natural gas become explosive? → Between 5% and 15% gas in air (the Lower Explosive Limit and Upper Explosive Limit)
11. During a tightness test, what is the stabilisation period required before commencing the 2-minute observation? → 1 minute
12. When carrying out a flue flow test (spillage test) on an open-flued appliance, where should the smoke match be held? → At the base of the draught diverter or canopy
13. Which document must a Gas Safe registered engineer carry at all times when working on gas installations? → A Gas Safe Register ID card
14. When commissioning a gas appliance, the engineer must check the burner pressure. What tool is used for this measurement? → A digital manometer or U-gauge connected to the burner pressure test point
15. What type of flue system draws combustion air from the room and discharges products of combustion to the outside? → Open flue (Type B)
16. When commissioning an open-flued gas appliance, which test confirms that the flue is performing correctly and removing combustion products safely? → A spillage test using a smoke match or smoke pellet
17. When installing gas pipework, what is the minimum distance it should be kept from electrical cables where they run parallel? → 25 mm
18. What are the three elements required for combustion to take place, commonly known as the 'fire triangle'? → Fuel, oxygen, and ignition (heat)
19. What is the primary product of complete combustion of natural gas? → Carbon dioxide (CO2) and water vapour (H2O)
20. Which organisation is the statutory body responsible for overseeing gas safety in the United Kingdom? → Gas Safe Register
21. Under the Gas Safety (Installation and Use) Regulations 1998, who may legally carry out gas work on a domestic installation? → Only a Gas Safe Registered engineer competent in the specific type of gas work
22. When a chimney serves an open-flued gas fire, what test must be carried out to confirm the chimney is safe to use? → A smoke test (flue integrity test) and spillage test
23. What is the approximate air-to-gas ratio required for the complete combustion of natural gas? → 10:1
24. Under RIDDOR, within what timeframe must a dangerous gas fitting be reported to the HSE? → As soon as is reasonably practicable
25. What is the typical maximum working pressure drop between standing and working pressure at a domestic gas meter? → 2 mbar
26. What is the correct standing (static) gas pressure for natural gas at a domestic meter outlet? → 21 mbar
27. Under the Gas Industry Unsafe Situations Procedure (GIUSP), what classification is given to a gas installation that poses an immediate danger to life? → Immediately Dangerous (ID)
28. Before commissioning a new gas appliance, what electrical checks must the engineer perform? → Check that the appliance is correctly earthed and that polarity is correct
29. When installing a vertical flue from a gas boiler through a flat roof, what is the minimum height the flue terminal must extend above the roof surface? → 600 mm
30. Which document must be completed and left with the customer after commissioning a new gas boiler? → Benchmark commissioning checklist