Chimney-sweep or flue-gas test mode
A test mode for the engineer: the boiler runs at a fixed maximum or minimum output so its flue gases can be measured.
What the manual says to do
Leave it to the engineer. Worcester Bosch: “Only to be operated by a qualified heating engineer”. Vaillant: “For competent persons only”. Viessmann: it “must only be activated by your flue gas inspector during the annual inspection”, and it ends by itself after 30 minutes.
Only your boiler’s own manual says exactly what a symbol means on your model. The manuals let the householder reset a fault once, top up the pressure through the filling loop and change the settings; anything else on a gas boiler is a job for a qualified heating engineer (Gas Safe registered in the UK), as the manuals themselves say.
What it does not mean
It is not a cleaning function and it does not mean the chimney or flue needs sweeping. It does not save gas or heat the house faster.
- What it looks like
- A symbol for a chimney or flue, or a named key, lit on the display while the test runs.
- Names used
- Chimney sweep mode (Worcester Bosch, Vaillant); Maximum output operation, for chimney sweeps only (Vaillant); Emissions test mode (Viessmann)
Where makers differ
Worcester Bosch calls it chimney sweep mode, at maximum or minimum output; Vaillant ‘maximum output operation (for chimney sweeps only)’ and, in its 2023 manual, chimney sweep mode to “carry out combustion analysis”; Viessmann ‘emissions test mode’.
Other symbols with the same shapes
Sources
No public standard defines this boiler symbol, so its meaning is taken from the manuals of 3 different maker groups (Worcester Bosch, Vaillant, Viessmann), each fetched and read on the date shown:
- Worcester Bosch, Greenstar 2000 combi (GR2301iW): Greenstar 2000 user instructions, 6 721 839 343 (2023/04) UK/IE (PDF) — Screen display: “Chimney sweep mode — Displays during service when the appliance is set to maximum or minimum output. Only to be operated by a qualified heating engineer.” Checked 2026-09-27.
- Vaillant, ecoTEC plus VU, VUW, VUI ..6/5-5: Operating instructions ecoTEC plus, 0020245033_02 (26.05.2021) GB, IE (PDF) — Operator panel symbol: “Maximum output operation (for chimney sweeps only)”. Checked 2026-09-27.
- Vaillant, ecoTEC plus VU../VUW../VUI.. (current range): Operating instructions ecoTEC plus, 0020308119_03 (28.11.2023) (PDF) — “Call up chimney sweep mode (carry out combustion analysis). For competent persons only.” Checked 2026-09-27.
- Viessmann, Vitodens 100-W/111-W/111-F/141-F (black/white screen): Operating instructions for the system user, 6167584 GB (12/2020) (PDF) — “Emissions test mode for testing flue gas must only be activated by your flue gas inspector during the annual inspection … Emissions test mode ends automatically after 30 minutes.” Checked 2026-09-27.
Written in our own words; quotations are the makers’ wording.