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Boiler pressure

Checked against its sources: Updated

Boiler pressure is the water pressure in a sealed heating circuit, and the right reading depends on the model, because manuals differ on when to top up and what to fill to.1 Vaillant's advice page gives 1-1.5 bar with the heating off as a general guide, and says to check the manual.2

Covers
Sealed systems on gas, LPG or oil, UK. An open-vented system does not rely on this gauge in the same way.
You
Read the gauge or display, compare it with your own manual, top up if that manual allows.
A professional
Repeated loss or rise, leaks, the expansion vessel, the relief valve, anything inside the casing.

1 Figures from the manuals

Use the line for your own boiler. If it is not listed, read your manual and do not borrow another maker's figure. These are from user documents, read on the issue shown; a later issue can differ.

Top-up and fill figures, by named user manual
Boiler and issueTop up belowFill to, or normal range
Worcester 8000 Life, 2021/0541 bar; p6 also says 0.8 bar or less1-1.5 bar; out of operation at 0.2 bar or less
Worcester i ErP, 2015/0351 bar1-1.5 bar
Worcester 8000+ regular, 2024/0631 barSettle at 1-1.5 bar
Worcester 4000, 2020/0910.8 barGreen zone; static 0.8-1.7, firing 1-2. Its p16 check says 1-1.5 cold
Vaillant ecoTEC plus, 28.11.202360.8 barAt least 1, at most 2.5, 1.5 a good value; add 0.1 bar per metre of height
Vaillant ecoTEC pro, 03.08.201770.8 bar1-2 bar cold; switches off below 0.5
Baxi Platinum, 1/1880.5 bar1-2 bar; not above 2.5 running
Main Eco Compact Combi90.7 bar on p18; its p15 flowchart says 1.01-1.5 bar
Baxi 600 Heat, 1/19, and Potterton Titanium Heat, 2/2010110.7 bar1-2 bar; 600 Heat p10 recommends 1.5
Glow-worm Energy7, 02.03.202012Outside 0.5-2.7Not specified further
Glow-worm Ultimate 3, 15.07.201913Outside 1.0-1.51-1.5 bar
Glow-worm Ultracom 2 cxi, 03/10140.5 bar; a picture on that page says 0.40.8-1.2 bar
Viessmann Vitodens 050-W, 05/2026151.0 barNot stated
Ideal Logic+, May 2014 issue on a merchant's site16Will not run below 0.31-1.5 bar; installed at 1-2

Vaillant's 2023 example is worth a look for tall houses: a system 8 metres from cellar to roof gets a target of 1.8 bar6. Its 2017 manual says only that a multi-storey house may need more, and to ask a competent person7.

2 Why the needle moves

Water expands when heated, so a rise while the heating runs is expected, and the reading falls back as it cools2. Read the gauge cold for a true comparison. Worcester's 4000 manual separates a static figure from an operating one for this reason1.

3 When the reading is wrong

Low reading: see repressurising a boiler, and if it keeps happening, pressure keeps dropping. High reading: see pressure too high. Many boilers show a code as well, and the meaning varies by maker; start at reading a fault code.

4 If there is no pressure to read

On an open-vented system the header tank keeps the pressure, which Worcester states in its 8000+ regular manual3. Its Heatslave oil guide says the gauge is not used on such systems, and to ask the installer if it rises above 1 bar17. To check which you have, see regular boiler.

Sources

  1. Worcester Greenstar 4000 manual 6720891170, pp7-9, 16, 2020/09, read 3 November 2026.
  2. What pressure should my boiler be, Vaillant read 3 November 2026. Web page, not a manual.
  3. Worcester 8000+ regular manual 6721890764, pp6-7, 2024/06, read 3 November 2026.
  4. Greenstar 8000 Life manual 6720883872, pp6-7, 2021/05, read 3 November 2026.
  5. Greenstar i ErP instructions 6720806946, pp18-21, 2015/03, read 3 November 2026.
  6. Vaillant ecoTEC plus instructions 0020308119_03, p14, read 3 November 2026.
  7. Vaillant ecoTEC pro instructions 0020244996_01, p9, read 3 November 2026.
  8. Platinum Combi 24-40 guide from Baxi, 7697821-01, pp3, 6, read 3 November 2026.
  9. Main Eco Compact Combi guide 7716571-04, pp15, 18, read 3 November 2026.
  10. Baxi 600 Heat guide 7726861-01, pp10, 19, 21, read 3 November 2026.
  11. Potterton Titanium Heat guide 7668906-06, sealed systems section, read 3 November 2026.
  12. Glow-worm Energy7 instructions 0020300878_00, p10, read 3 November 2026.
  13. Glow-worm Ultimate 3 instructions 0020239565_01, p9, read 3 November 2026.
  14. Glow-worm Ultracom 2 cxi manual 0020085232_01, PDF p15, read 3 November 2026.
  15. Viessmann Vitodens 050-W instructions 6173895, p27, read 3 November 2026.
  16. Ideal Logic+ Combi guide UIN 204979 A07, p11, an older issue on a merchant's site, read 3 November 2026.
  17. Greenstar Heatslave oil guide 8716106253, p8, 03.2010, read 3 November 2026.