Radiator: central heating
The central-heating side of the boiler: the hot water it pumps round the radiators. When it is lit, heating mode is on; on Vaillant and Worcester Bosch displays it flashes while the burner is actually heating the radiators.
What the manual says to do
Press the radiator key (Worcester Bosch, Vaillant) or turn the radiator knob (Baxi, Ideal) to see and set the maximum flow temperature: the temperature of the water sent to the radiators, not of the room. Worcester Bosch ships it at about 65 °C, adjustable from 30 to 82 °C; Ideal’s knob runs from about 30 °C to 80 °C. The room temperature is set on the room thermostat and the radiator valves.
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
A lit radiator does not mean the radiators are hot right now: it only says heating mode is on, and the boiler still waits for the timer or room thermostat to ask for heat. It is not a room-temperature setting.
- What it looks like
- A radiator seen from the front: a row of upright panels or fins. It may be steady, flashing, or crossed through, and it sits on a key, a knob or the display.
- Names used
- Heating (Worcester Bosch key); Heating mode active (Vaillant); Heating mode (Baxi); Radiator symbol (Ideal); Heating circuit active (Viessmann)
Where makers differ
Vaillant: steady means heating mode with a heat demand, flashing means the burner is on in heating mode. Worcester Bosch Greenstar 2000: the radiator flashes in heating mode and a separate burner symbol appears when it fires. Ideal Logic Combi2 draws a cross through the radiator when central heating is off. Viessmann calls the same thing ‘heating circuit 1’ or ‘2’ active.
Other symbols with the same shapes
Sources
No public standard defines this boiler symbol, so its meaning is taken from the manuals of 5 different maker groups (Vaillant, Worcester Bosch, Baxi, Ideal Heating, Viessmann), each fetched and read on the date shown:
- Vaillant, ecoTEC plus VU, VUW, VUI ..6/5-5: Operating instructions ecoTEC plus, 0020245033_02 (26.05.2021) GB, IE (PDF) — Displayed symbols: “Heating mode active — Permanently on: Heating mode heat requirement; Flashing: Burner on in heating mode”. Checked 2026-09-27.
- Worcester Bosch, Greenstar 2000 combi (GR2301iW): Greenstar 2000 user instructions, 6 721 839 343 (2023/04) UK/IE (PDF) — Screen display: “Heating mode On” and “Heating mode Off”; “The maximum flow temperature can be adjusted between 30 °C and 82 °C”; supplied with the CH control set at approximately 65 °C; the symbol “flashes in the display in heating mode”. Checked 2026-09-27.
- Baxi, Baxi Compact range (quick guide 7849761): Quick start guide to your boiler, 7849761 Baxi Compact quick guide user instructions (PDF) — Display symbols listed beside the control panel: “Heating mode”, “Hot water request”, “Burner lit”; central heating temperature control knob. Checked 2026-09-27.
- Ideal Heating, Logic Combi2 C24, C30, C35: Logic Combi2 user guide, UIN 237664 A01 (10/2023) (PDF) — “Press the mode button until there is not a cross through the radiator symbol”; central heating radiator temperature (approx.) minimum 30 ºC, maximum 80 ºC. 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) — Operating programs: “Heating circuit 1 and/or heating circuit 2 is active. The rooms of the selected heating circuit are heated in accordance with the specified room temperature or flow temperature and the time program”. Checked 2026-09-27.
Written in our own words; quotations are the makers’ wording.