The 109 fault code appears on 3 brands — Ariston, ATAG and Chaffoteaux — and it does not mean the same thing on each. The same two or three characters can signal a pressure problem on one boiler and an ignition fault on another, so identify your brand first and use the table below to jump to the right page.
The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.
| Brand | What 109 means | Severity | DIY? |
|---|---|---|---|
A Ariston | Fault code 109 on Ariston boilers indicates that the system water pressure has climbed above 3 bar, triggering a safety lockout to protect the boiler and heating circuit. Full guide → | Lockout — engineer likely | No |
A ATAG | Error code 109 on an ATAG boiler is a pressure-related fault from the 100-series group. Full guide → | Engineer may be needed | Some checks |
C Chaffoteaux | Fault code 109 on a Chaffoteaux boiler indicates that the system pressure has climbed above the safe threshold — typically beyond 3 bar. Full guide → | Lockout — engineer likely | No |
Fault code 109 on Ariston boilers indicates that the system water pressure has climbed above 3 bar, triggering a safety lockout to protect the boiler and heating circuit. At this level, pressure is genuinely dangerous — the boiler will not operate until the underlying cause is identified and resolved. Do not attempt to reset the boiler or continue using it while this code is showing. Affected models include the Clas Net One, Clas One Combi, and Clas System One ranges.
Error code 109 on an ATAG boiler is a pressure-related fault from the 100-series group. There is a genuine ambiguity between sources about its precise direction: the most likely interpretation, supported by multiple technical sources, is that it signals the system pressure has exceeded the maximum permitted level (sometimes described as 'Pressure > Pmax'). However, at least one Gas Safe engineer source associates this code with low pressure — specifically pressure falling below 1 bar or insufficient pump pressure increase. The key thing to do first is check your boiler's pressure gauge directly. If the needle is above 2.5–3 bar, you likely have a high-pressure condition. If it reads below 1 bar, this is a low-pressure situation. Both can trigger pressure-series codes on ATAG boilers, and both need addressing promptly to avoid further damage.
Fault code 109 on a Chaffoteaux boiler indicates that the system pressure has climbed above the safe threshold — typically beyond 3 bar. As a protective measure, the boiler locks out automatically to guard internal components against damage caused by excessive pressure. This is essentially the mirror image of fault code 108 (low pressure). The code is consistent across the main Chaffoteaux ranges including the Pigma, Inoa, Talia Green, and Britony series, though it is always worth confirming the definition against your specific model's manual.
Causes, fixes & costs for Chaffoteaux 109 → · All Chaffoteaux codes
No. 109 means different things on different brands — see the table above. Always identify your boiler's brand (it is printed on the front casing) before acting on any advice about the code.
On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. The underlying repair usually needs a registered engineer. Never open the boiler casing yourself.
For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. If you smell gas at any point, leave the property and call the National Gas Emergency line on 0800 111 999.
General guidance only — not a substitute for professional advice, and not certified by a registered engineer. Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.
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