ACS Gas Assessment Routes Explained — CCN1 & Domestic Appliances
Use this guide to distinguish core natural gas, appliance and meter routes, then build a revision plan around the assessment combination confirmed by your training provider, assessment centre or certification body.
ACS is an industry route for demonstrating Matters of Gas Safety competence. Assessment is delivered through approved centres under certification bodies. This Study Hub explains the launch competence areas — including CCN1, CPA1, CENWAT, CKR1, HTR1 and MET1 — how they combine, and a practical revision sequence for new entrants and returning engineers preparing for five-yearly reassessment. It does not reproduce a live assessment.
Everything you need to understand ACS gas revision
ACS can feel complicated at first because the route depends on the core, appliance and dwelling categories relevant to your work. This page is here to simplify that.
- How ACS competence routes are structured
- What CCN1 and domestic appliance codes mean
- How CENWAT, CKR1, HTR1, MET1 and CPA1 relate to CCN1
- Why assessment is not one universal MCQ paper
- How certification, Gas Safe registration and five-yearly reassessment differ
- A practical sequence for independent revision
If you want detailed answers on routes, certificates and practice tests, read our ACS FAQ.
How ACS gas assessment works
Candidates demonstrate knowledge and practical competence for a core area and the appliance and premises categories relevant to their work. Assessment is delivered at an approved centre. Certificates of competence are issued by the candidate's applicable UKAS-accredited certification body. Current, relevant competence evidence is then used when applying for or maintaining the appropriate Gas Safe Register work categories — certification and registration are separate processes.
Core Domestic Natural Gas
CCN1 is the domestic natural-gas core competence and the usual foundation for domestic appliance competencies.
Appliance and meter routes
Add CENWAT, CKR1, HTR1, DAH1 or MET1 according to the work categories you hold or plan to add.
Practical plus knowledge
Centres assess practical performance and knowledge. There is no single standardised public online MCQ paper across every certification body.
Certificate cycle & reassessment
Gas safety competence certificates are normally renewed every five years. Returning engineers should plan reassessment within the permitted early-renewal window under the applicable scheme rules. Gas Safe business registration is separate and annual.
A practical revision sequence
Good ACS revision respects scope boundaries. Keep core, dwelling and appliance competencies separated so one certificate is never mistaken for universal permission to work.
Confirm your exact assessment combination
Use your booking confirmation or speak to the assessment centre. Record the core, appliance and dwelling codes you are actually taking.
Start with current core principles
Revise legislation, emergency actions, combustion, ventilation, pipework, tightness testing, unsafe situations, controls, pressures, gas rates and chimneys before appliance detail.
Add one appliance route at a time
Keep CENWAT, CKR1, HTR1, DAH1 and MET1 in separate topic blocks to avoid cross-contamination.
Use questions diagnostically
After each set, record the principle, source and reason for every error instead of memorising answer positions. Finish with timed mixed practice only after topic accuracy is stable.
Choose the competence code on your assessment booking
Questions are filtered by the exact core, appliance and dwelling scope, with current criteria metadata displayed on every result.
Core natural gas
CCN1 covers core domestic natural-gas safety and is used with relevant appliance competencies.
Domestic appliances
CENWAT, CKR1, HTR1 and MET1 are the highest-demand appliance and meter routes; DAH1 is included where warm-air heaters are on your booking.
CPA1 combustion performance
CPA1 is searched heavily and sits within current CCN1 Issue 19 for many providers — confirm the exact code combination with your centre.
What the competence codes actually mean
Below is an overview of the launch ACS routes. Use these as editorial guidance alongside the live competence pages from the question bank.
Core Domestic Natural Gas — CCN1
Legislation and emergency actions
Revise GSIUR duties, emergency priorities, escape control and emergency-service escalation.
Combustion, CO and ventilation
Cover combustion performance, carbon monoxide risks, free area, grille types and multi-appliance air supply.
Pipework, tightness and purging
Focus on materials, jointing, installation volume, let-by, permissible movement and current IGEM/UP/1B procedures.
Unsafe situations and chimneys
Understand GIUSP classification principles, notices, flue-flow and spillage testing, and safe recommissioning.
Domestic appliances and meters
CENWAT
Domestic central-heating boilers and water heaters — location, flues, controls, commissioning and combustion analysis.
CKR1
Domestic cooking appliances — siting, clearances, stability, burners, flame supervision and commissioning.
HTR1
Gas fires and wall heaters — fireplaces, catchments, flue-flow and spillage tests, ventilation and unsafe conditions.
DAH1
Ducted-air heaters — system layout, combustion air, fan and limit controls, airflow and heat-exchanger safety.
MET1
Domestic natural-gas meters — location, installation, exchange, tightness testing, pressure regulation and restoration of supply.
Route cards at a glance
CCN1
Core Domestic Gas Safety — Natural Gas. Foundation for domestic appliance competencies.
CENWAT / CKR1 / HTR1
Boilers and water heaters, cookers, and fires or wall heaters — each requiring relevant core competence.
DAH1 / MET1
Warm-air systems and domestic natural-gas meters within the stated assessment scope.
CPA1
Combustion performance analysis — often included with current CCN1; confirm presentation with your assessment centre.
The live competence and quiz names on this site are pulled from the ACS question bank. Use those dynamic pages for detailed practice.
How to use each study tool
Different tools are useful at different stages of ACS gas revision.
Topic quizzes
Short, focused sets mapped to one competence area and one technical theme.
Practise by SubjectFlash cards
Rapid recall for definitions, controls, documents and appliance categories.
Study Flash CardsDiagnostic mocks
Timed multiple-choice practice with topic breakdowns — not a claim to duplicate a certification body's format.
Try a Mock AssessmentProgress tracking
Best for identifying which competence areas or topics still need more work before assessment day.
View Membership Plans
Your assessment centre remains the authority for your booking
Certification bodies may present assessment documentation and scheduling differently while assessing the same required competence.
Use this site to revise published learning areas, then confirm prerequisites, permitted reference materials, assessment duration, reassessment arrangements and certificate combinations directly with your centre.
A simple way to approach ACS revision
Popular ways to revise
Start with the code on your assessment booking
Choose a CCN1-only or mixed diagnostic set and use the result to prioritise your next revision session.