CCN1 is the core everyone holds first; the appliance modules then match your work — CENWAT (boilers and water heaters), CPA1 (combustion analysis), CKR1 (cookers), HTR1 (fires), MET1 (meters). Most domestic heating engineers start with CCN1 plus CENWAT.
Domestic gas ACS looks like alphabet soup until you group the modules by job type. CCN1 is the core domestic gas-safety base. The appliance modules then prove competence for the work you want on your card: boilers and water heaters, cookers, fires and space heaters, metering, and combustion analysis.
Start with CCN1
CCN1 is the domestic core. It is the base layer for gas safety: legal scope, emergency controls, tightness testing, pipework, ventilation, flueing, combustion basics and unsafe-situations judgement. If CCN1 is weak, every appliance module feels harder.
CENWAT
CENWAT covers domestic central heating boilers and water heaters. The revision focus is not only boiler parts; it is safe installation context, commissioning, controls, flues, ventilation where relevant, water-heater operation and what the engineer must check before leaving the appliance in use.
CPA1
CPA1 is combustion performance analysis. Revise it as a decision process: sample at the correct point, let readings stabilise, understand CO and CO2, calculate the ratio, compare against the limit used in training, and decide what action follows if combustion is poor.
CKR1, HTR1 and MET1
- CKR1: domestic cookers, including stability, clearances, flame supervision and user instruction.
- HTR1: gas fires and other space heaters, including flueing, ventilation, spillage checks and appliance condition.
- MET1: domestic gas meters, including location, ECV access, labels, pressure checks and leaving the installation safe.
Which modules do you actually need?
You do not take every module — you take CCN1 plus the appliance modules for the work you actually want to do. For a typical domestic heating engineer that usually means CCN1 and CENWAT (boilers and water heaters), with CKR1, HTR1, MET1 or CPA1 added as the work demands. There is no point holding a cooker ticket you never use, and no benefit to skipping one you need: match your modules to the appliances you will legally work on. If you are unsure, start with CCN1 and CENWAT and add the rest as your role grows.
Where LPG and the water add-ons fit
Beyond the core domestic set, some engineers add CONGLP1 for LPG changeover work, while unvented hot water (G3) sits alongside gas as a separate plumbing competence rather than an ACS gas module. These are additions taken when your work needs them, not a substitute for CCN1 — which remains the base everything else depends on.
A sensible revision order
- First pass: CCN1 legal duties, emergency actions, tightness testing and unsafe situations.
- Second pass: gas rate, ventilation, flueing and combustion calculations.
- Third pass: your appliance modules, starting with the work you do most often.
- Final pass: mixed mock questions so you practise switching topics under time pressure.
Do not confuse assessment with a course
ACS is an assessment of competence for defined gas work. Training providers can help you prepare, but the important question is whether you have enough supervised experience and evidence for the assessment route you are taking. Check the current requirements with your chosen centre before booking.
Quick answers
Is CCN1 enough to work on boilers?
CCN1 is the domestic core. Boiler and water-heater work normally sits with the relevant appliance competence such as CENWAT, so check the exact scope on your certificates and Gas Safe registration.
Is CPA1 the same as using a flue-gas analyser?
CPA1 is broader than pressing buttons on an analyser. It includes correct sampling, interpreting readings and taking the right safety action when combustion is outside limits.
Which ACS module should I revise first?
Revise CCN1 first because it supports the rest. Then work through the appliance modules you are taking, with extra time on calculations and unsafe-situations scenarios.
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