On most combi boilers, the pressure gauge should read between 1 and 1.5 bar when the heating's cold, usually shown as a green zone on the dial. It rises a little, to around 2 bar, once the heating's running and the water's expanded. Below around 0.5 bar means the system's lost water and needs topping up; well above 2.75 bar means it's overfilled.
This page covers what the reading means, how to check and top up pressure safely, and when dropping pressure is a sign of something worth getting looked at.
Is 0.6 bar of pressure on my boiler low?
Yes. 0.6 bar is below the normal 1 to 1.5 bar range for a cold system, so the boiler's lost some water and needs repressurising. Most boilers have a filling loop for topping up pressure yourself, usually a grey or silver lever or valve near the bottom of the boiler, with instructions in the manual.
If you've topped it up before and it's dropped to 0.6 bar again within weeks, that's not normal wear, it usually means there's a small leak somewhere in the system or a faulty pressure relief valve, and it's worth having an engineer check rather than topping up repeatedly.
How do I check the pressure on my Vaillant boiler?
Most Vaillant boilers show pressure on a digital display or an analogue gauge on the front of the unit, usually alongside the controls. Check it when the heating's been off for a couple of hours, since pressure reads higher while the system's hot. A normal cold reading is 1 to 1.5 bar; your model's manual will confirm the exact range for your boiler.
If you can't find the pressure reading or the display's showing a fault code instead, that's worth noting down before you call an engineer, since it narrows down the problem quickly.
Why do boilers get low pressure?
The most common causes are bleeding a radiator (which releases a little water each time), a small leak somewhere in the pipework or a radiator valve, or a faulty pressure relief valve letting water escape slowly. Topping up occasionally, once or twice a year, is normal. Needing to top up every few weeks is not.
A boiler that keeps losing pressure despite topping up usually has a leak that isn't visible, sometimes inside a wall or under flooring, and it's worth having an engineer trace it rather than continuing to top up.
My Worcester boiler pressure gauge isn't working. What should I do?
If the needle isn't moving at all, or the digital display isn't showing a reading, the gauge itself may have failed rather than the pressure being wrong. Don't assume the system's fine just because nothing looks alarming; a broken gauge means you've lost the ability to spot a real pressure problem early.
Worcester boilers show pressure in the green zone (1 to 1.5 bar cold) as normal, red as too low or too high. If the display's blank or stuck, it's worth getting a Gas Safe registered engineer to check whether it's the gauge or the sensor behind it.
Questions people ask
How often should you repressurise a boiler?
Once or twice a year is typical, usually after bleeding a radiator or over time as the system settles. If you're topping up more often than that, there's likely a leak somewhere in the system or a faulty pressure relief valve, and it's worth having it looked at rather than continuing to top up.
What is the operating pressure on a boiler?
Most combi boilers run at 1 to 1.5 bar when the system's cold, rising to around 2 bar when the heating's on and the water's expanded. The exact range can vary slightly by model, so check the markings on your own pressure gauge or the boiler's manual.
What should I do if my boiler pressure is too high?
If the gauge reads above roughly 2.75 bar when the system's cold, the boiler's been overfilled. Bleeding a radiator for a few seconds, using a radiator key, releases a little water and brings the pressure back down; stop once the gauge is back in the normal range. If pressure keeps climbing back up on its own, that points to a fault rather than overfilling, and needs an engineer.
Is 2.2 bar too high for a boiler?
It depends whether the system's hot or cold. While the heating's running, pressure naturally rises toward 2 bar, so 2.2 bar while it's on isn't unusual. If 2.2 bar is the reading once the system's cooled down, that's above the normal 1 to 1.5 bar cold range and worth bleeding down slightly.
What should I do if the pressure in my Vaillant boiler is too low?
Use the boiler's filling loop to top up the pressure to the normal range, usually 1 to 1.5 bar cold, following the instructions in the manual for your model. If the pressure drops again within a few weeks of topping up, that's a sign of a leak or a faulty part rather than normal wear, and it's worth getting a Gas Safe registered engineer to find the cause.
Sources
Last updated