ACS gas exam practice questions
There is a strong natural-gas smell near an understairs cupboard. Someone reaches for its light switch to look for the meter. Which response is appropriate?
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What is the ACS gas exam?
The ACS gas exam is the assessment that proves you are competent to do gas work, and the evidence Gas Safe registration is built on. It is not one paper: you sit CCN1, the core domestic gas safety assessment, then a module for each type of appliance you will work on, and each module combines knowledge questions with practical tasks watched by an assessor. The questions above come from our CCN1 bank, the same items used in the full revision product.
ACS at a glance
- Core assessment
- CCN1, core domestic gas safety
- Appliance modules
- CENWAT, CKR1, HTR1, MET1 and others
- Assessed by
- UKAS-accredited certification bodies, at approved centres
- How it is assessed
- Practical tasks, written questions, oral questions where needed
- Pass standard
- Every question answered correctly, with retries
- Certificates last
- Five years, then reassessment
The pass standard and the five-year cycle come from the ACS scheme document, so they are the same at every centre. The number of questions and the timetable are set by each certification body, and your centre’s joining instructions are the authority for your date.
CCN1 and the appliance modules
Everyone starts with CCN1. The appliance modules then match the work you intend to do, and your Gas Safe registration can only cover the categories your certificates cover. A typical domestic heating engineer holds CCN1 and CENWAT and adds the others as the work demands.
| Module | What it covers | Usually taken by |
|---|---|---|
| CCN1 | Core domestic gas safety: the law, emergencies, tightness testing and purging, pressures, ventilation, flues, combustion and unsafe situations | Everyone, first |
| CENWAT | Central heating boilers and water heaters | Most heating engineers |
| CKR1 | Domestic cookers | Engineers who fit or service cookers |
| HTR1 | Gas fires and space heaters | Engineers who work on fires and wall heaters |
| MET1 | Domestic gas meters | Engineers who fit or exchange meters |
| CPA1 | Combustion performance analysis with a flue gas analyser | Included in CCN1 since April 2012 |
| CMDDA1 | Carbon monoxide and carbon dioxide atmosphere and appliance testing | Engineers who investigate CO alarms and fume reports |
What CCN1 tests
CCN1 is the widest module because every appliance module assumes it. Its knowledge questions and practical tasks come from the same areas:
- Gas safety law: who must do what under the Gas Safety (Installation and Use) Regulations, and the limits of your own competence.
- Gas escapes and emergencies: first actions, the emergency control valve and making an installation safe.
- Tightness testing and purging to IGEM/UP/1B, and deciding what a gauge reading means.
- Pressures and gas rate: standing, working and burner pressure, and working out heat input at the meter.
- Ventilation and flues: air supply for each appliance type, flue types, and the flue flow and spillage tests.
- Combustion and carbon monoxide: complete and incomplete combustion, combustion analysis with a flue gas analyser, and what the signs of CO mean for your next step.
- Unsafe situations: classifying an installation as Immediately Dangerous or At Risk, and the warning, labelling and reporting each needs.
What assessment day looks like
Each module has three parts: a practical assessment, a written assessment and, where the assessor needs more evidence, oral questions. The written questions are multiple choice or short answer, and CCN1’s are split into separate papers by subject, chimney standards being one. They put you in a situation and ask for the decision: how an installation should be classified, whether a test result can be accepted, what an appliance needs for its air supply. Most turn on one condition in the wording, such as whether an appliance is room-sealed or open-flued.
In the practical tasks an assessor watches you work on rigs and appliances: a tightness test, a working pressure check, a flue flow or spillage test, making a situation safe. Expect to be asked why you did each step. Knowing the procedure in order, and the reason for each part of it, is what gets you through that questioning.
LCL Awards, one of the certification bodies, puts a first assessment of CCN1 with CENWAT, CKR1 and HTR1 at about four days, and the same set as a reassessment at two to two and a half days. Bring photo ID, and for a reassessment your previous certificates; the centre confirms the rest.
How to revise for it
Work in short sessions against real questions rather than reading notes end to end. Answer, read the explanation whether or not you were right, and note the standard or regulation the answer rests on.
Where a question turns on a figure, such as a permitted pressure loss, a ventilation free area or a flue termination distance, learn which document and which condition it comes from rather than the number alone. The figure changes with the appliance, the gas and the edition of the standard.
Track the topics you get wrong and return to them a few days later. Spacing the repeats is what moves a topic from recognised to known.
Tightness testing changes on 1 October 2026
IGEM/UP/1B Edition 4, the domestic tightness testing and purging standard, becomes effective on 1 October 2026, and Edition 3 is withdrawn on 30 September 2026. Under Edition 4 the pipework itself must be proved tight, any permitted loss for existing appliances depends on the fuel and the installation volume rather than the meter, and the purge volume is worked out from the installation volume.
If you sit CCN1 or reassess around the changeover, ask your centre which edition its assessment follows. Our tightness testing notes and questions follow Edition 4.
Initial assessment, reassessment and renewal
A new entrant reaches the initial assessment through a managed learning programme approved by IGEM: structured training plus supervised gas work recorded in a portfolio. Engineers already on the Gas Safe Register, and holders of certain qualifications, have their own routes.
Certificates run for five years. You can reassess up to six months before expiry and keep the time that was left; a certificate that expired less than 12 months before the reassessment can still be reassessed, and after that it means the initial assessment again. Appliance certificates stop being valid if the core they depend on expires.
Reassessment covers the same scope as the original modules and assumes current working knowledge, so standards that have changed since your last certificate, such as the tightness testing edition above, deserve most of your revision time.
Gas Safe registration depends on current certificates for the work you carry out. The assessment centre, not this site, confirms the arrangements that apply on your date.
Set up a free CCN1 mock test
Use the free mock-test setup for a timed run through CCN1 questions, then read the reasoning behind each answer. It practises the knowledge side of the assessment; the practical tasks and the assessor’s questions can only be proved at a centre.
The free test offers 5, 10 or 25 questions from this same set, with an optional practice timer. In Test mode, answers and explanations appear after you finish. Repeated tests reuse this bank, so a higher score can reflect familiarity.
Set up a free CCN1 gas safety testCommon questions
What is the pass mark for the ACS gas exam?
Every written question has to be answered correctly in the end. Wrong answers get a second attempt; if that brings you to between 80% and 99%, you get a third attempt or oral questions, and below 80% the section is failed. A failed section can be retaken within three months.
How many questions are in the ACS gas exam?
There is no single national figure. Each certification body writes its own papers for CCN1 and the appliance modules, and your centre confirms the format for your sitting. CCN1 has the most, split into separate papers by subject, because it covers the core the appliance modules build on.
Is the ACS gas exam open book?
Partly. The scheme lets centres allow your own reference material, such as the standards and manufacturer instructions, for open-book questions, but an assessment can also include closed-book questions. The joining instructions say what you may bring.
What is CCN1?
CCN1 is Core Domestic Gas Safety, the core ACS assessment for domestic natural gas work. Every appliance module depends on it, and since April 2012 it has included combustion performance analysis, which used to be the separate CPA1 assessment.
How long does ACS certification last?
Five years from certification. Reassess up to six months early and the remaining time carries over; a certificate that expired less than 12 months before can still be reassessed, but after that you sit the initial assessment again.
Who awards ACS certificates?
Certification bodies accredited by UKAS, including BPEC Certification, Blue Flame Certification, LCL Awards and NICEIC Certification, through approved assessment centres. The scheme itself is owned by the standard setting body that IGEM manages. Centres set their own fees.
Do I need ACS to join the Gas Safe Register?
You need current evidence of competence for each category of gas work you register for. For most domestic engineers that evidence is ACS certificates, starting with CCN1; some qualifications aligned to ACS are also accepted. Gas Safe Register publishes the current requirements.
Is CCN1 enough to work on boilers?
No. CCN1 is the core; boiler and water-heater work normally needs CENWAT as well. Check the exact scope on your certificates and your Gas Safe registration before taking on the work.
Can I practise ACS gas questions for free?
Yes. The questions on this page are free and need no account. They come from the same maintained bank as the full product, so the difficulty and the explanations are identical.
Revision to support these questions
Keep going
Passing ACS is a sequence, not one exam: core first, then a module per appliance category you want on your registration. The full CCN1 bank, the appliance modules and timed mocks sit behind one PlumbRevise account.