ACS gas categories explained: what can you legally work on?

Nearly every page that ranks for “ACS categories” is a training provider selling a course, which makes it oddly hard to get a straight answer to a simple question: which qualification lets you do which work? This is the neutral map: how the ACS assessments you sit turn into the categories on the back of your Gas Safe ID card, and what each category legally lets you touch.

How do ACS assessments become ID card categories?

ACS, the Accredited Certification Scheme, is the competence assessment behind Gas Safe registration. The structure is a core plus appliance modules: the scheme's own framework makes the core a prerequisite for every appliance assessment, and says appliance assessments must be selected to cover the areas of work you actually undertake. Pass an assessment and the matching work category appears on your Gas Safe ID card, which is the public record of your legal scope; Gas Safe Register tells customers to check the back of the card before letting anyone near an appliance. No category, no work: an engineer qualified on boilers but not fires cannot legally work on a gas fire, however experienced.

What does CCN1 actually cover?

CCN1 is the core domestic gas safety assessment for natural gas. Its competencies are the fundamentals that apply on every job: gas safety legislation, emergency actions, combustion, ventilation, installation of pipework and fittings, tightness testing, meter regulators, burner pressures and gas rates, safety devices, unsafe situations, and chimney standards and testing. On the ID card, that core corresponds to the pipework category: core domestic gas safety knowledge including the installation and testing of gas pipework up to 35mm in a domestic setting. CCN1 on its own therefore covers pipework work but no appliances; every appliance type needs its own assessment on top.

Which assessment maps to which card category?

• CENWAT: central heating boilers and water heaters. The card's boiler category covers install, exchange, commission, disconnect, service, repair, safety checking and breakdown of domestic gas central heating and hot water boilers, including combis, circulators, storage and instantaneous water heaters.
• CKR1: domestic cookers, hobs and ovens. Range cookers with a chimney or flue sit under their own card category.
• HTR1: fires and space heaters, covering gas fires, wall heaters, convector heaters, stoves and flueless fires.
• MET1: domestic gas meters, covering install, commission, exchange and removal.
• CPA1: combustion performance analysis with an electronic analyser. The scheme lists it as a prerequisite for CENWAT, and it underpins commissioning and servicing checks across appliances.
• The longer tail: DAH1 for ducted warm air heaters, LAU1 for laundry appliances, LEI1 for leisure equipment, CMDDA1 for fumes investigation, plus the caravan, park home and boat sectors on the LPG side.

What happened to CEN1?

It was retired into CENWAT. Longer-serving engineers hold CEN1 (wet central heating) and WAT1 (water heaters) as separate certificates from the days when boilers and water heaters were assessed apart. The current ACS framework's renewal chart folds both into the combined CENWAT reassessment, which covers central heating boilers and water heaters in one module. If your paperwork still says CEN1, nothing is wrong: it simply renews as CENWAT, and new entrants sit CENWAT from the start.

Do your categories cover LPG or commercial work?

No, and this is where scope-checking earns its keep. Categories are fuel-specific and sector-specific. CCN1 is the natural gas domestic core; extending to LPG means a changeover assessment (CoNGLP1) taken for the sector you will work in, such as permanent dwellings, park homes, caravans or boats, after which the appliance categories you already hold carry across to the second fuel. The framework is blunt about the dependency: LPG assessments become invalid if the natural gas core expires. Commercial work runs on its own cores, such as COCN1 for commercial heating, with catering and laundry equivalents, and is a separate path with its own record types; our CP17 guide covers what the paperwork side looks like.

Where do G3 and Part L fit?

They don't, and that is the point worth knowing. Two tickets that training providers often bundle alongside ACS are not gas categories at all and never appear on your Gas Safe ID card. G3 covers unvented hot water cylinders under the Building Regulations, and Part L is the Building Regulations' energy efficiency side that heating work has to comply with. Both matter to a working installer; neither is ACS. Treat them as parallel qualifications with their own renewal arrangements, not as part of your gas scope.

How do you keep your categories current?

Every ACS certificate runs on the five-year cycle, and each category you hold carries its own expiry date, so an engineer who added CKR1 two years after CENWAT has two dates to watch. The renewal window and the consequences of a lapse are covered in our ACS renewal guide; the short version is that you can usually reassess up to six months early without losing time, and from the day a certificate expires you cannot legally work on that category. Manifold tracks your customers' renewal dates automatically so the CP12 work keeps flowing; your own certificates are the one set of dates the software cannot sit for you, so diary each expiry the day the certificate arrives, exactly as you would a landlord's.