How to increase boiler pressure?

Question

My boiler pressure has dropped below 1 bar, which I don't think is good? What's the correct way to top it up safely?

Mark · Accepted Answer

Call an engineer as you might have an issue with your boiler

Asif · Answer

Boiler will have filling loop. This should be done when the system is cold

Michael · Answer

depending on the boiler. If you look up the model of the boiler and make you can find the instructions this will guide you to the fill loop on your system.

Ryan · Answer

Depending on boiler type you will have 2 tabs underneath at the right side of the boiler turning them both horizontally you will hear a water filling noise keep an eye on pressure and top it to 1.5 bar remember to move both tabs back to there normal position

Alex · Answer

With out knowing your actual model can't tell you exactly where you filling loop is as some have a intregal and some have one below with a silver flexible hose between try youtube for your model then you want to.pressure it to 1 to 1.5 bar while cold

Curtis · Answer

You’re right to question it—below 1 bar is getting too low, and most boilers will start to complain or shut down around that point. Topping it up is straightforward, but it’s important to do it carefully.
🔧 What you’re aiming for
Cold system (heating off): ~1.2 – 1.5 bar
Don’t go much above 1.5 bar when cold
🛠️ How to top it up safely (filling loop method)
Most UK systems use a filling loop (a silver braided hose under the boiler, or sometimes a built-in key system).
Step-by-step:
Turn the boiler off and let it cool
(you want a cold reading, not hot pressure)
Locate the filling loop
Usually a flexible metal hose with two small valves (or a key + valve setup)
Open the valves slowly
You’ll usually turn them 90° (quarter turn)
You should hear water flowing in
Watch the pressure gauge closely
As it rises, aim for ~1.2–1.5 bar
Close both valves firmly
Don’t leave them open
Turn the boiler back on
⚠️ Important things to avoid
❌ Don’t overfill (above ~2 bar cold)
❌ Don’t leave the filling loop open
❌ Don’t do it while the system is hot (gives a false reading)

Alamgir · Answer

Depend on installation. Check under the boiler if there is flexi pipe two black knobs open them and hear water moving.

Sajad · Answer

You can use the filling loop on the boiler cold pipe to the return pipe.

Riccardo · Answer

Depends on the boiler. Most will have a small filling loop under the boiler or separate filling loop on the cold to return pipework which will be 2 small levers that require to be in turned on to fill and then off again once it’s up to pressure.
Some worcester boilers will have a keyless filling link which is a little blue lever under the boiler which you just pull down and release to top up.

Susan · Answer

You’re right — if your boiler pressure has dropped below 1 bar, it will need topping up as most systems operate best between 1.0 and 1.5 bar when cold.
The usual way to top it up is via the filling loop (a silver braided hose or built-in valve underneath the boiler). You would:
Slowly open the filling valves
Watch the pressure gauge
Stop once it reaches around 1.2–1.5 bar
Close the valves fully
It’s important not to overfill, as too much pressure can cause the system to discharge water.

That said, if your pressure has dropped, it’s worth checking why — it could be something simple like recently bleeding radiators, or it could indicate a small leak or an issue with the expansion vessel.

How to increase boiler pressure?

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