ACS Cheat Sheet 2026

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

1. What are the three classifications used by Gas Safe engineers when they identify an unsafe gas situation? → Immediately Dangerous (ID), At Risk (AR), and Not to Current Standards (NCS)
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? → Incomplete combustion due to insufficient primary air
3. What is the principal role of a Flame Supervision Device (FSD) on a gas hob burner? → To automatically stop the flow of gas to the burner if the flame goes out.
4. If a let-by test reveals that the Emergency Control Valve (ECV) is faulty, what is the correct action? → Notify the gas transporter, as the ECV is their responsibility to repair
5. Under BS 6891, what is the maximum allowable pressure drop across a domestic gas installation during a tightness test? → 1 mbar over a defined test period after the permitted stabilisation time
6. What are the main products of the complete combustion of natural gas (methane)? → Carbon Dioxide (CO2) and Water Vapour (H2O)
7. Which of the following actions should be AVOIDED in a property where a gas leak is suspected? → Operating a light switch to improve visibility.
8. What is the product of complete combustion of natural gas (methane)? → Carbon dioxide and water vapour
9. Under GSIUR 1998, which work is exempt from the requirement to be carried out by a Gas Safe registered engineer? → Reading a gas meter — no work on the fitting itself
10. Under the Gas Safety (Installation and Use) Regulations 1998, what is the fundamental requirement regarding the air supply for a combustion appliance? → There must be an adequate supply for combustion and efficient working of the flue.
11. What is the primary function of an Over-Pressure Shut-Off (OPSO) device fitted within a domestic gas meter regulator assembly? → To shut off the gas supply if the downstream pressure rises to an unsafe level.
12. Which UK Gas Industry standard primarily covers tightness testing and purging of domestic natural gas installations? → IGEM/UP/1B
13. What must be confirmed before attempting to light an appliance after purging? → That gas is flowing freely and consistently at the appliance burner
14. Which British Standard provides guidance on the installation of domestic gas appliances? → BS 6798 for boilers and BS 5440 for flues and ventilation (with BS 6891 for pipework)
15. At what pressure should a tightness test be conducted on a domestic natural gas installation? → Working pressure (approximately 21 mbar)
16. If a gas supply has been interrupted and then restored, in what order should appliances be purged and re-lit? → One appliance at a time, starting with the one nearest the meter
17. Which of the following would indicate a failed tightness test? → The U-gauge shows a pressure drop during the test period
18. Which type of pipe is commonly used for internal domestic gas installation in the UK? → Copper or corrugated stainless steel (CSST) pipe with compression or press-fit fittings
19. Which of the following describes a key responsibility of a Gas Safe registered engineer when commissioning a new SMETS2 smart gas meter installation? → Demonstrating the functionality of the In-Home Display (IHD) to the customer.
20. What is the primary purpose of purging a gas installation? → To remove air and replace it with fuel gas
21. What is the permitted working pressure for a domestic low-pressure gas installation in Great Britain? → 21 mbar
22. How often must gas engineers renew their ACS competency certificates? → Every 5 years
23. What position should all appliance isolation valves be in during a tightness test? → Closed, isolating all appliances from the pipework
24. Which instrument is typically used to perform a tightness test on a domestic gas installation? → U-gauge manometer
25. When purging through an appliance burner rather than a dedicated purge point, what must the gas engineer ensure first? → The appliance burner tap is open and the appliance is ready for ignition
26. What does 'tightness testing' of a gas installation verify? → That there are no gas leaks in the installation pipework before it is put into service
27. During a let-by test, what does a pressure rise on the downstream side of the closed ECV indicate? → The ECV is allowing gas to pass through when in the closed position
28. Which of the following is a key requirement for labelling an Emergency Control Valve (ECV) according to UK gas industry standards? → The label must clearly indicate its purpose and the method of operation.
29. Under the Gas Industry Unsafe Situations Procedure (GIUSP), what classification applies to an unlocated gas escape discovered during a tightness test? → Immediately Dangerous (ID)
30. 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? → CO / CO2 ratio