How to check and top up boiler pressure

What boiler pressure means

The gauge or display shows the water pressure in the central-heating circuit. It is different from the pressure of water coming from a tap. On many sealed domestic systems the pressure when cold is around 1 to 1.5 bar, but this is not a universal target: the correct range depends on the boiler and installation. Use the figure in the boiler user manual or on the installer’s label. If you cannot find the correct range, ask your installer before topping it up.

Check the reading when the system is cool and note it. A low reading may follow maintenance, but pressure that keeps dropping can indicate a leak or another fault. Topping up is not a repair for the cause.

Safe steps before topping up

1. Identify the boiler. Note the make, model and whether the pressure display is digital or analogue.
2. Read its user guide. Find the correct filling arrangement for that exact model. It may use a filling loop with one or two valves, a removable key, or an integrated filling link.
3. Check for a visible leak. If you see water around the boiler, radiators or pipework, or water is discharging outside, do not top up; contact an engineer.
4. Follow the model instructions. Some manufacturers advise switching the boiler off and allowing it to cool first. Use only the identified user filling control, open it slowly and watch the pressure gauge continuously.
5. Stop at the manual’s cold pressure target. Close every filling valve fully, or remove and secure the key as instructed. Never leave a filling link open.

Do not force a stiff control or operate a valve just because it is near the boiler. The visible pipework may include other valves with a different purpose.

Instructions from the boiler makers

The following official guides show why the correct method depends on the appliance:

• Ideal Heating user guides — find the guide for Logic MAX, Vogue MAX or your exact model.
• Worcester Bosch: what to do if your boiler loses pressure.
• Vaillant: how to repressurise a boiler — methods include model-dependent filling loops or keys.
• Baxi: how to repressurise a boiler, with separate guides for some EasyFill and digital-gauge models.

When to stop and call an engineer

Close the filling control if you can do so safely and arrange a visit if the pressure rises above the model’s target, does not rise while filling, or drops again soon afterwards. A repeated need to top up can point to a leak or a component fault. Do not keep adding water to hide the symptom, and do not try to repair the boiler or sealed heating circuit yourself.

If a radiator has just been bled, the pressure may need checking afterwards. Follow the boiler manual; if you are not confident using the filling arrangement, ask a Gas Safe registered engineer. For an existing fault, note the display code and any visible water so you can describe it clearly when booking.