Pressure too high

1 Read it cold first

Water expands when heated, so a higher reading with the heating on is normal, and Vaillant's website gives 3 bar as an upper limit while running¹. Before doing anything, let the system cool, and compare with boiler pressure.

2 Three likely reasons

Vaillant's website gives three: too much water added during a recent top-up, an external filling device left not fully closed or passing water, or a fault in the boiler¹. The first is easy to spot if you have just filled it. The second shows as a steady climb, covered under filling loop.

3 What the manuals say to do

The advice is not the same across makers, and Vaillant's own documents differ with age.

Vaillant's website is broader than most: under 2.5 bar at room temperature needs no action, though bleeding will lower it, and over 2.5 bar it advises reducing the pressure before running the boiler, by bleeding or with help¹. That differs from Worcester's stricter 1.7 bar on the 4000, which is why the manual for your own boiler comes first.

4 If your manual allows bleeding

Turn the boiler off first and let the radiators cool. Worcester's 4000 says not to vent with the heating on, and not to let pressure fall below its normal setting³. Watch the gauge while it comes down, and top up if you overshoot. The how-to is under bleeding a radiator.

5 When to call someone

Call as well if the reading climbs again after bleeding, or if water runs from the pressure relief valve pipe outside. Vaillant's leak page says to turn off the heating and water supply in that case and call an engineer, naming a faulty vessel or a passing filling loop as possible sources¹⁰. The expansion vessel entry covers that part.