Pressure gauge or bar symbol: system pressure too low
The water pressure in a sealed central-heating system, in bar. The boiler needs it within a band its maker sets; too low, and it warns you and then stops firing to protect itself.
The pressure each manual asks for
The numbers differ by model and by maker, and your installer may set a different figure for your system; these are the ranges the manuals print.
| Maker, boiler | Range | What else the manual says |
|---|---|---|
| Vaillant ecoTEC plus VU, VUW, VUI ..6/5-5 | 1.0–2.0 bar | “System pressure: 0.1 … 0.2 MPa (1.0 … 2.0 bar)”; fill below 0.8 bar; switches off below 0.5 bar (F.22) Source, checked 2026-09-27. |
| Worcester Bosch Greenstar 2000 combi (GR2301iW) | 1–1.5 bar | cold (static pressure); 1–2 bar while firing; re-charge below 0.8 bar; display warns below 1.1 bar Source, checked 2026-09-27. |
| Baxi Baxi 800 Combi | 1.0–2.5 bar | “Is the Central Heating System Pressure between 1.0 and 2.5 bar?”; “If the reading falls below 1.0 bar repressurise the system” Source, checked 2026-09-27. |
| Ideal Logic Combi2 C24, C30, C35 | 1–2 bar | “original installation pressure of 1-2 bar”; top up to 1 to 1.5 bar; “will not operate if the pressure has reduced to less than 0.3 bar” Source, checked 2026-09-27. |
What the manual says to do
Read the gauge or display with the boiler cold. If it is below the maker’s minimum, the manuals let you top it up yourself through the filling loop, slowly, to the figure they give (Ideal: “Wait for the pressure gauge to reach 1 to 1.5 bar”; Worcester Bosch: fill to between 1 and 1.5 bar), then close the valves. If you have to top up again and again, the manuals say to call your installer or engineer: it points to a leak.
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 reading that rises as the boiler heats up is not a fault in itself: Worcester Bosch gives 1–1.5 bar cold and 1–2 bar while firing. It is not the mains water pressure at your taps, and it is not a reason to open the boiler case; only the filling loop outside the case is the householder’s part.
- What it looks like
- A dial gauge marked in bar (often with a green band), a digital reading in bar, or a symbol of a drop with bars or a bar graph that flashes when the pressure is out of range.
- Names used
- System pressure / pressure indicator (Worcester Bosch); Filling pressure (Vaillant); Water pressure too low (Baxi); System water pressure (Ideal); System pressure (Viessmann)
Where makers differ
Ideal’s boiler will not operate below 0.3 bar. Vaillant says fill below 0.8 bar and switches off below 0.5 bar with fault F.22. Worcester Bosch says re-charge below 0.8 bar, and the Greenstar 2000 display warns below 1.1 bar. Baxi and Viessmann say top up below 1.0 bar.
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) — Pressure symbol: “Permanently on: Filling pressure in the permitted range; Flashing: Filling pressure outside the permitted range”; system pressure 1.0 … 2.0 bar; below 0.80 bar fill the installation; below 0.5 bar the product switches off and shows F.22. Checked 2026-09-27.
- Worcester Bosch, Greenstar 2000 combi (GR2301iW): Greenstar 2000 user instructions, 6 721 839 343 (2023/04) UK/IE (PDF) — “Re-charge the system if the pressure indicator reads less than 0.8 bar”; static pressure 1–1.5 bar, operating pressure 1–2 bar; low-pressure warning “Displayed when system pressure is lower than 1.1 bar”; constantly re-pressurising “is an indicator of potential leaks”. Checked 2026-09-27.
- Baxi, Baxi 800 Combi: Baxi 800 Combi user guide, 7728204-01 (PDF) — Start-up checks: “Is the Central Heating System Pressure between 1.0 and 2.5 bar?” and “If the reading falls below 1.0 bar repressurise the system as described in Maintenance”; display symbol “Water pressure too low”. Checked 2026-09-27.
- Ideal Heating, Logic Combi2 C24, C30, C35: Logic Combi2 user guide, UIN 237664 A01 (10/2023) (PDF) — “If the pressure is seen to fall below the original installation pressure of 1-2 bar over a period of time and continue to fall then a water leak may be indicated”; “THE BOILER WILL NOT OPERATE IF THE PRESSURE HAS REDUCED TO LESS THAN 0.3 BAR”; top up until the gauge reads 1 to 1.5 bar. 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) — Switching on: “If the pressure shown is below 1.0 bar: Please top up with water or notify your heating contractor.” Checked 2026-09-27.
Written in our own words; quotations are the makers’ wording.