Boiler Pressure Too High? Causes and How to Fix It

Most boilers run at a pressure of around 1 to 1.5 bar. If the needle on your pressure gauge has climbed into the red zone — usually above 2.75 bar — your boiler pressure is too high. It’s a common problem, it’s rarely dangerous thanks to built-in safety valves, and in many cases you can bring it back down yourself. Here’s what causes high boiler pressure and how to fix it.

What is normal boiler pressure?

A healthy combi boiler sits at roughly 1 to 1.5 bar when the heating is off and cold. When the heating is running, it’s normal for the pressure to rise to around 2 bar — that’s nothing to worry about. It’s only a concern when the pressure stays high, or climbs towards 3 bar and into the red section of the gauge.

What causes high boiler pressure?

Overfilling during a top-up: the most common cause. If too much water is added through the filling loop, or the filling loop valves are left slightly open, pressure builds up.
A faulty expansion vessel: this part absorbs the expansion of hot water. If it loses its charge, the pressure has nowhere to go and rises sharply when the heating is on.
A faulty pressure relief valve: the safety valve that releases excess pressure can stick or fail over time.

Is high boiler pressure dangerous?

In almost all cases, no. Every modern boiler has a pressure relief valve designed to release excess pressure safely — you may notice water leaking from your boiler or dripping from a small pipe outside your home, which is the valve doing its job. That said, high pressure shouldn’t be ignored, as it puts strain on the system and points to a fault that will keep coming back until it’s fixed.

How to reduce boiler pressure yourself

Bleeding a radiator is one of the safest ways to bring the pressure down, and it can also help if you have a radiator that won’t heat up. Work through these steps:

Check the filling loop is fully closed. This is the flexible braided hose beneath the boiler. If either valve is even slightly open, close it fully — this alone often solves the problem.
Bleed a radiator. Releasing a little water from a radiator lowers the pressure in the system. Our guide on how to bleed a radiator walks you through it. Keep an eye on the gauge as you go and stop once it returns to around 1.5 bar.
Re-check the gauge. With the heating cold, the needle should now sit between 1 and 1.5 bar.

When to call a Gas Safe engineer

If the pressure keeps rising after you’ve closed the filling loop and bled a radiator, the cause is likely a faulty expansion vessel or pressure relief valve. These are sealed parts of the boiler and should only be repaired or replaced by a Gas Safe registered engineer — never attempt to work on internal gas components yourself. You should also call an engineer if water leaks continuously from the outside overflow pipe, or if the boiler locks out and won’t fire.

Our Gas Safe registered team offers fast boiler repairs across West London and can diagnose and fix a pressure fault, usually in a single visit.

Frequently asked questions

What pressure should my boiler be at?

Around 1 to 1.5 bar when the heating is cold. It’s normal for it to rise to about 2 bar while the heating is running.

Can I use my boiler if the pressure is too high?

It will usually keep working, and the pressure relief valve protects it, but you should bring the pressure down and have any underlying fault checked to avoid repeated problems.

Why does my boiler pressure keep rising?

If it climbs again after being reset, the most likely causes are a filling loop that isn’t fully closed or a faulty expansion vessel. The expansion vessel needs a Gas Safe engineer to re-pressurise or replace.

Need help with your heating in West London?

If your radiators or boiler still aren’t behaving after trying the steps above, our Gas Safe registered engineers can help. We provide fast, reliable plumbing and heating services across West London, from radiator and boiler repairs to full system checks.

Tel: 020 3561 4415