What to check, in order
- Read the pressure with the boiler cold
Worcester's Greenstar 4000 manual gives 0.8 to 1.7 bar when the boiler is cold and hasn't been firing, and says to top up below 0.8 bar. The pressure menu shows green when it's OK.
- Top up with the filling link
Keyless link: pull the lever down and let go when the pressure menu reaches the green zone. Keyed link or external filling loop: open it slowly, watch the pressure, and close it again at the green zone.
- Overfilled? Bleed a radiator
If it reads more than 1.7 bar, bleed one radiator until the pressure is back between 1 and 1.7 bar.
- Reset if the code stays
Hold the up and down arrow keys until Reset shows on the display.
- If it keeps dropping, look for leaks
Check visible joints and radiator valves. Outside, look at the end of the pressure relief pipe: if it's dripping, the pressure relief valve may be faulty.
When to call a Gas Safe engineer
Worcester says topping up once or twice a year is normal. If you're doing it much more often, or the relief pipe is dripping, contact your installer or a Gas Safe registered engineer.
Parts people end up buying
Only if the checks above point to a part. Match it to your model number first: it's on a label on the data badge, usually under the boiler or behind the front cover.
- Radiator bleed keySearch on Amazon
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Checked against
- Worcester Bosch Greenstar 4000 user instructions (PDF)
- Worcester Bosch: how often should I top up the pressure?
- Worcester Bosch: the heating system is constantly losing pressure
Last reviewed September 2026. Manufacturers update their pages, and your manual is the final word.