Guide · Boiler
How to top up your boiler pressure
Topping up the boiler means putting water back into the radiator circuit when the pressure drops. With the boiler cold, open the filling tap slowly (almost always under the boiler) and close it when the gauge reaches the value in your manual, for many models between 1 and 1.5 bar. If you have to do it often, water is escaping somewhere and you need an engineer.
10 minutes Difficulty: EasyUpdated 25 October 2026
Under the boiler there are also the gas pipe and the mains water pipes, with taps that can look alike. Touch only the filling tap shown in the manual and, if in doubt, stop. Do not open the boiler casing. If you smell gas, leave the boiler alone: no switches and no flames, windows open, gas shut off at the meter if you can do it safely, then go outside and call the gas emergency service (free number on your bill) or 112 from outside.
What you need
- The boiler instruction manual
- A torch
- A dry cloth
Step by step
Tap a step to mark it as done.
How many bar to stop at: what the manuals say
The right value is the one in the manual for your model, and it varies more than you might think. For the ecoTEC plus, Vaillant gives 1 to 2 bar with the system cold and asks you to top up below 0.8 bar. Below 0.5 bar the boiler stops with code F.22. For the Luna Duo-tec E, Baxi gives 1 to 1.5 bar. For the Clas One, Ariston gives 0.6 to 1.5 bar.
On the Ciao Green, Beretta asks you to bring the pressure back to between 1 and 1.5 bar. Immergas gives 1 to 1.2 bar, Ferroli about 1 bar with a top-up advised below 0.8, Bosch about 1.2 bar. In homes on several floors a higher value may be needed, as Vaillant also writes. In that case the engineer who knows the system decides.
Where the tap is and what changes between brands
On Vaillant ecoTEC plus boilers with integrated hot water, the filling tap is on the lower side of the boiler. On heating-only models, the filling connection to the water supply is explained by the installer, and the manual itself advises you to have them show you where it is. On the Baxi Luna Duo-tec E, the filling tap is marked with a letter in the manual drawing and low pressure shows as code E118. The same manual has message 18, automatic filling in progress: if you see it, wait for the boiler to finish by itself.
On Ariston Clas One boilers the tap is at the bottom of the boiler. The manual says to open it, hold the MODE button for 5 seconds until PURGE appears, close the tap when you are between 0.6 and 1.5 bar, then press MODE again. On Beretta Ciao Green boilers, with code A04 and pressure below 0.3 bar, you turn the selector to off, fill to 1-1.5 bar and go back to summer or winter: a bleeding cycle of about 2 minutes starts.
On Immergas Victrix Tera boilers the filling tap is under the boiler, and low pressure shows as code 10. Recent Ferroli boilers have a filling knob to turn anticlockwise, which on some models must first be pulled out. The code is F37, and after filling the display shows Fh for a bleeding cycle of 300 seconds. Some newer Ferroli models, such as the Bluehelix Maxima, fill with a button on the display or fully automatically, depending on how the engineer has set them.
Why the pressure drops
When you bleed the radiators a little water comes out too and the pressure falls. A top-up after bleeding is normal, and so are one or two top-ups a year.
If it drops every week or every month instead, water is escaping somewhere: a fitting, a radiator valve, or the boiler's safety valve discharging into the pipe below. A useful clue: if the needle rises a lot when hot and collapses when cold, the expansion vessel (the tank that absorbs the expansion of hot water) has lost its air charge. Fixing it means opening the boiler, so it is a job for the engineer.
When to stop and call a technician
- You need to top up more than once a month, or the pressure falls again a few days after filling
- Water comes out of the discharge pipe under the boiler or you see drops on the fittings
- With the boiler hot, the needle gets close to 3 bar
- The pressure rises by itself even with the tap closed, a sign that the filling tap is not sealing
- You cannot find the filling tap or you are not sure which one it is: stop instead of turning valves at random
Frequently asked questions
What is the right boiler pressure?
With the system cold, for many boilers, between 1 and 1.5 bar. Some models allow different values, for example 0.6 to 1.5 bar on Ariston Clas One and 1 to 2 bar on Vaillant ecoTEC plus. The number in your boiler's manual is the one that counts.
Does the boiler fill itself?
Some recent models do. Certain Ferroli Bluehelix Maxima fill automatically or with a button on the display, and the Baxi Luna Duo-tec E shows code 18 for automatic filling in progress. On most boilers filling stays manual, from the tap under the boiler.
I added too much water. What do I do?
If you are only slightly above the manual's value, usually nothing happens. If the needle is very high, with the system cold you can bring it down by opening a radiator bleed valve for a few seconds, with a cloth underneath. If it gets close to 3 bar, call an engineer.
Boiler locked out with low pressure: what do I do?
First fill to the right value, then press reset once. If the lockout comes back with the correct pressure, the cause is something else and it needs checking.
How often is it normal to top up the boiler?
Once or twice a year, for example after bleeding the radiators. More often than once a month means there is a leak or a problem with the expansion vessel.
Who do I call if the pressure keeps dropping?
A qualified heating engineer, who will look for the leak and check the expansion vessel and safety valve. In Campania you can call us: even on a video call we can narrow down the causes by looking at the pipes and the gauge.
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