Up and down arrows, + and −, or a knob: temperature up and down
Raise or lower the water temperature the boiler aims for: the flow temperature to the radiators, or the hot-water temperature, depending on which one is selected.
What the manual says to do
Pick the radiator or the tap first, then adjust. Worcester Bosch: press the radiator or tap key, then the up or down arrow; the setting saves itself after a few seconds (two on the Greenstar 4000, three on the 2000). Vaillant: adjustable values flash, and a change must be confirmed. Baxi and Ideal: turn the knob clockwise for hotter. Ideal says turning the heating knob lower lets the boiler run more efficiently, and higher may be needed in winter.
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
The numbers are water temperatures, not room temperatures: the room thermostat and the radiator valves decide how warm a room gets. The arrows do not clear a fault.
- What it looks like
- Two arrow keys, a minus and a plus, or a knob with a scale from min to max next to a radiator or a tap.
- Names used
- Up arrow / Down arrow (Worcester Bosch); − and + buttons (Vaillant); Temperature control knobs (Baxi, Ideal)
Where makers differ
Ranges differ: Worcester Bosch 30–82 °C flow, supplied at about 65 °C, with a scalding warning above that; Ideal about 30–80 °C for heating and 40–65 °C for hot water. Worcester Bosch advises noting the installer’s settings before changing them.
Other symbols with the same shapes
Sources
No public standard defines this boiler symbol, so its meaning is taken from the manuals of 4 different maker groups (Worcester Bosch, Vaillant, Baxi, Ideal Heating), each fetched and read on the date shown:
- Worcester Bosch, Greenstar 4000 combi (GR4700iW C): Greenstar 4000 user instructions, 6 720 891 170 (2020/09) UK/IE (PDF) — Overview of keys: “Up arrow — Menu navigation, Increase values; Down arrow — Menu navigation, Reduce values”; “The setting is saved automatically after two seconds.” Checked 2026-09-27.
- Worcester Bosch, Greenstar 2000 combi (GR2301iW): Greenstar 2000 user instructions, 6 721 839 343 (2023/04) UK/IE (PDF) — “Press arrow key … to set the required maximum flow temperature … Otherwise the setting will be saved automatically after 3 seconds”; flow temperature adjustable between 30 °C and 82 °C; advises making a note of the installer’s settings before adjusting. 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) — Operating concept: the buttons are for “Reducing or increasing the set value”; “Adjustable values are always displayed as flashing. You must always confirm a change to a value.” Checked 2026-09-27.
- Baxi, Baxi 800 Combi: Baxi 800 Combi user guide, 7728204-01 (PDF) — “Turn the control knob with the symbol over it clockwise to increase the DHW temperature. The temperature will be shown on the display screen.” Checked 2026-09-27.
- Ideal Heating, Logic Max Combi2 C24, C30, C35: Logic Max Combi2 user guide, UIN 228290 A06 (October 2022) (PDF) — Central heating radiator temperature (approx.) 30 ºC to 80 ºC; hot water 40 ºC to 65 ºC; “To operate your boiler efficiently (using less gas) turn the central heating knob … In winter periods it may be necessary to turn the knob clockwise towards a higher temperature”. Checked 2026-09-27.
Written in our own words; quotations are the makers’ wording.