ACS Gas Qualifications Explained — Units, Costs & How to Pass
ACS (Accredited Certification Scheme) qualifications are the practical assessments that make you legally eligible to work on gas appliances in the UK. Without the relevant ACS units, you cannot be Gas Safe registered. This guide explains every domestic ACS unit, what the assessment involves, and how to prepare.
What are ACS qualifications?
ACS is a competency-based certification scheme developed by the gas industry and approved by the Health and Safety Executive (HSE). It tests engineers on their ability to safely work on specific types of gas appliances — not just their theoretical knowledge, but their practical skills in a real assessment environment.
ACS certificates are valid for 5 years. At renewal, you sit a shorter reassessment to confirm your competencies are up to date. If your certificate lapses, you must sit the full assessment again before you can renew Gas Safe registration.
Core domestic ACS units
The CCN1 (Core Domestic Natural Gas) unit is mandatory for any engineer who wants to work on domestic gas appliances. It cannot be skipped. On top of this, you add the appliance-specific units that cover the work you will carry out.
- →CCN1 — Core Domestic Natural Gas: gas safety, pipework, tightness testing, purging, ventilation. Mandatory for all domestic gas work.
- →CPA1 — Central Heating Boiler: commissioning, servicing, and fault-finding on gas boilers. The most common additional unit.
- →HTR1 — Gas Fires: installation, commissioning, and servicing of open-flued, balanced-flue, and flueless gas fires.
- →WAT1 — Water Heaters and Boilers: instantaneous and storage water heaters.
- →CENWAT — Combined Central Heating Boilers and Water Heaters: covers both CPA1 and WAT1 in one unit.
- →CKR1 — Cooker and Hob Installation: cookers, hobs, and range-style appliances.
- →MET1 — Meters: meter installation and exchange — required if you work for a gas network or energy company.
- →LPG — Liquefied Petroleum Gas units: for engineers working on LPG appliances (caravans, off-grid properties).
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