Check certificate dates and categories, then revise what has changed since your last ACS: tightness testing, unsafe situations, CO alarms and Part L. Build the rest around your own gaps.
ACS reassessment checks current competence. Start with the certificates you need to renew, then test your knowledge across their full scope. Everyday experience can leave less familiar topics untouched, so use your revision results to decide where to spend the time before assessment. Standards also move in five years, so start with what has changed since your last certificate.
Start with the admin
- Check which certificates are due and which appliance modules you actually need for the work you do.
- Confirm the assessment centre requirements: ID, existing certificates, evidence, manuals, analyser calibration and any pre-course material.
- Do not leave expired paperwork until the week before. Business registration and personal competence dates are separate things.
What has changed since your last ACS
Five years is long enough for the standards behind the assessment to move, and these are the places where experience gives the old answer. Revise them first.
- Tightness testing: IGEM/UP/1B Edition 4 takes effect on 1 October 2026, and Edition 3 is withdrawn on 30 September 2026. 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. Ask your centre which edition its assessment follows.
- Unsafe situations: the procedure is IGEM/G/11, Edition 2, amended in July 2025, which replaced Gas Safe Register’s Technical Bulletin 001. Check your habits against it: two unsafe categories, Immediately Dangerous and At Risk, with a DANGER DO NOT USE label for both; no Not to Current Standards category, so a shortfall you have judged safe is recorded as advice; and a set order for turning off an At Risk appliance: the isolation valve, then the fuse, then the appliance control, never the room thermostat or timer.
- Carbon monoxide alarms: since 1 October 2022, Approved Document J expects a CO alarm in the room when a new or replacement fixed gas appliance, other than one used only for cooking, is installed in a home in England. Before then its alarm guidance covered solid fuel appliances only.
- Boiler replacements: Approved Document L’s 2021 edition, in force since 15 June 2022, sets 92% ErP, a system designed to run at 55 °C where feasible, boiler interlock and upgraded controls. Its 2026 edition, from 24 March 2027, adds thermostatic control in each heated room when a boiler is replaced.
Refresh CCN1 first
CCN1 is the base layer. If that is rusty, the appliance modules become harder than they need to be. Give priority to tightness testing, emergency controls, ventilation, flueing, combustion basics, unsafe-situations procedure and warning documentation.
Drill the unsafe situations
- ID and AR: know the unsafe classifications and actions; record safe NCS shortfalls separately.
- Practise scenarios where the customer refuses permission to disconnect.
- Revise warning labels, written notices and reporting routes.
- Do not treat “I know it when I see it” as enough. The assessment wants clear classification and clear action.
Rehearse calculations and analyser judgement
Ventilation, gas rate, heat input and CO/CO2 ratio are easy marks when rehearsed and slow, expensive mistakes when rusty. Practise the method until you can explain the steps, not just get a number once.
Module-specific pass through
- CENWAT: boiler controls, interlock, flueing, commissioning and water-heater context.
- CPA1: sampling, stabilisation, analyser care, ratio interpretation and what to do with bad readings.
- CKR1 and HTR1: clearances, stability, flame supervision, spillage and user instruction.
- MET1: meter location, ECV access, labels, ventilation and tightness after work.
The final week
Do not cram random facts. Run mixed questions, revisit every wrong answer, and practise explaining why the wrong options are wrong. That is close to the real demand of reassessment: not memorising a phrase, but making a safe decision under time pressure.
Check coverage as well as accuracy. A high score on repeated emergency questions does not tell you how you would handle an unseen flueing or tightness scenario. Give each required topic a first pass, then return to weak areas. For each error, write down the missing observation, the decision it changes and the action that follows.
How the written questions are marked
Every written question has to be answered correctly in the end. A wrong answer gets 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.
Renewal dates: early reassessment and expired certificates
BPEC states that ACS certificates normally last five years. Reassessment within the final six months can preserve the remaining certificate time. Present the previous certificates so the centre can check the evidence and dates.
Under BPEC rules, an element expired by more than 12 months requires an initial assessment. That does not by itself mean repeating a complete new-entrant training programme. Ask the centre to confirm eligibility for each element before paying, especially if your core and appliance certificates expire on different dates.
Appliance certificates stop being valid if the core certificate they depend on expires, so keep the core in date even when an appliance certificate has longer to run.
Compare ACS reassessment costs on the same basis
A low headline price is hard to compare if one quote covers only the domestic core and another includes appliance assessments and refresher days. Send each centre the same list of categories and expiry dates, then ask for an itemised total.
BPEC does not require refresher training for applicants who meet the assessment prerequisites. Decide how much preparation you need from your knowledge gaps; confirm what the centre includes in its particular booking.
- Which core and appliance elements are included, and which are extra?
- Does the total include VAT, certification fees and any refresher training?
- How are resits charged, and what are the cancellation or date-change terms?
- Which days are training and which are assessment? What evidence, references and equipment must you bring?
- For tightness testing, which edition of IGEM/UP/1B will the assessment use? Edition 4 takes effect on 1 October 2026.
Quick answers
How often do you have to reassess your gas ACS?
Normally every five years. Check each certificate separately, because your core and appliance elements may have different expiry dates. Gas Safe business registration is a separate requirement.
What has changed since my last ACS?
Revise these first: tightness testing to IGEM/UP/1B Edition 4 from 1 October 2026; the unsafe situations procedure, now IGEM/G/11 as amended in July 2025; carbon monoxide alarms with new or replacement fixed gas appliances in England since 1 October 2022; and the Approved Document L rules for boiler replacements in force since 15 June 2022, which change again on 24 March 2027.
What is the pass mark for ACS reassessment?
Every written question has to be answered correctly in the end. Wrong answers get a second attempt; a result between 80% and 99% after that brings a third attempt or oral questions, and below 80% the section is failed. A failed section can be retaken within three months.
What happens if I let my ACS certificates expire?
Do not rely on an expired certificate as current evidence of competence. BPEC requires an initial assessment where an element expired more than 12 months ago. Confirm your route with the centre and your permitted work categories with Gas Safe Register before resuming that work.
Is ACS reassessment easier than the first assessment?
Prior experience helps, but reassessment still checks current competence. Use mixed practice across the categories you are renewing, including topics you rarely encounter at work, rather than assuming familiar daily tasks cover the assessment.
How long does ACS reassessment take?
The timetable depends on the categories being assessed and any refresher training booked alongside them. Ask the centre for assessment days and training days separately, then check its joining instructions rather than assuming one course duration applies to everyone.
What should I revise before ACS reassessment?
Use your own errors and unpractised topics to set priorities. Include unsafe-situations decisions, ventilation and gas-rate calculations, flueing, combustion interpretation and warning records so that strong recall in one area does not hide a gap elsewhere.