Eco, comfort or pre-heat: hot water kept ready
Chooses whether a combi boiler keeps its hot-water heat exchanger warm so hot water reaches the tap sooner (comfort or pre-heat on), or heats water only once you open a tap (eco, pre-heat off), using less gas.
What the manual says to do
Switch it with the key your manual names. Worcester Bosch: press eco until ‘eco’ shows (eco on, comfort off); press again to return to comfort. Vaillant: switch comfort mode on or off. Ideal: pre-heat Off, On or Timed; ‘Timed’ learns your hot-water use (over 7 days on the Logic Max Combi2, 24 hours on the Logic Combi2). Viessmann: the DHW ECO function, on combi boilers only.
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
Eco here does not make the radiators run cooler or change the heating at all: it only decides whether hot water is kept ready. Pre-heat is not a hot-water cylinder; the boiler just fires briefly now and then to keep its own heat exchanger warm.
- What it looks like
- The word ‘eco’ on a key or the display, a ‘Preheat’ key, or a tap symbol with a mark for comfort. The drawing here is a tap with a small clock: water kept warm ahead of time.
- Names used
- eco key, comfort mode (Worcester Bosch); Comfort mode (Vaillant); Preheat On / Off / Timed (Ideal); DHW ECO function (Viessmann)
Where makers differ
Worcester Bosch says the boiler “will use more gas in comfort (or pre-heat) mode”, and that in eco mode briefly opening and closing a hot tap heats the water once. Ideal ships its boilers with pre-heat off. Vaillant’s comfort symbol flashes when the burner is on for it.
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, Ideal Heating, 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) — “In comfort mode, the appliance is continually maintained at the set temperature … However, the appliance will use more gas in comfort (or pre-heat) mode. In eco mode, heating-up to the set temperature begins as soon as the hot water tap has been opened. To set eco mode: press eco until eco appears in the display.” 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) — “Comfort mode immediately supplies hot water at the required temperature, without you having to wait for the water to heat up”; symbol “Permanently on: Comfort mode active; Flashing: Comfort mode active, burner on”. Checked 2026-09-27.
- Ideal Heating, Logic Combi2 C24, C30, C35: Logic Combi2 user guide, UIN 237664 A01 (10/2023) (PDF) — “The boiler is factory set with the preheat turned off to give standard hot water delivery and reduce gas usage … timed intelligent preheat function which learns the usage patten for domestic hot water over 24 hours.” Checked 2026-09-27.
- Ideal Heating, Logic Max Combi2 C24, C30, C35: Logic Max Combi2 user guide, UIN 228290 A06 (October 2022) (PDF) — Preheat can be turned to ‘On’ or ‘Timed’; “If ‘Timed’ is selected then the intelligent preheat function is active which learns the usage patten for domestic hot water over 7 days.” 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) — “Setting the Eco function for DHW heating: This function is only possible in conjunction with an instantaneous water heater (combi boiler)”; symbol “DHW ECO function”. Checked 2026-09-27.
Written in our own words; quotations are the makers’ wording.