Boiler Pressure Too High? Causes and How to Lower It

Boiler pressure is too high if the gauge reads above roughly 2 to 2.5 bar when the heating is cold, or gets close to 3 bar when it is running. You can lower it yourself by turning the heating off, letting the system cool and bleeding a radiator until the gauge drops back to about 1 to 1.5 bar. If the pressure keeps climbing again afterwards, something is letting water into the system or the system can no longer absorb expansion. The usual culprits are a filling loop left open or letting by, a failed expansion vessel, or on a combi, an internal leak in the plate heat exchanger.

What Normal Boiler Pressure Looks Like

Most sealed heating systems are designed to run at about 1 to 1.5 bar when cold. As the water heats up it expands, so it is normal for the gauge to rise by around half a bar while the heating is running and fall back when it cools. Check your manual for the figure your manufacturer recommends, but as a rule of thumb:

• 1 to 1.5 bar cold: normal.
• Up to about 2 bar hot: normal expansion.
• Above 2.5 bar cold, or near 3 bar hot: too high. Bring it down and find out why.

At 3 bar the pressure relief valve is designed to open and dump water through a pipe to the outside. That protects the system, but it is not something that should happen routinely.

How to Lower Boiler Pressure Safely

The simplest way to release pressure is through a radiator bleed valve:

1. Turn the heating off and wait until the radiators are cold. Bleeding a hot radiator risks scalding, and you will get a false reading while the water is still expanded.
2. Check the filling loop is fully closed. On most combis this is one or two taps on a braided hose or a lever under the boiler. If it is slightly open, the pressure will just climb back up.
3. Put a towel and small bowl under a radiator bleed valve, preferably one downstairs.
4. Open the valve slowly with a radiator key, about a quarter turn. Air will hiss out first, then water.
5. Watch the boiler gauge, or have someone else watch it. Close the valve when it reads about 1 to 1.5 bar.

Do not try to release pressure by opening the pressure relief valve by hand. It may not reseat properly afterwards and can start dripping. If you need to top pressure back up after bleeding too much, our guide to repressurising a boiler explains how.

Why Boiler Pressure Keeps Rising

If you bring the pressure down and it creeps back up over the following days, the cause is almost always one of these:

1. Filling loop letting by

The filling loop connects your heating system to the mains water supply. When it is closed, no water should pass. If one of its valves is worn, mains water slowly leaks through into the heating system and pushes the pressure up. This is especially likely if the pressure rises even when the heating is off. The fix is a new filling loop valve, and on some systems disconnecting the loop after use as the regulations intend.

2. Failed expansion vessel

The expansion vessel is a tank with an air cushion on one side of a rubber diaphragm. It absorbs the extra volume when the water heats up. If it loses its air charge, or the diaphragm splits, there is nowhere for that expansion to go, so the pressure shoots up whenever the heating comes on and drops back when it cools. That big swing is the giveaway. Over time the relief valve will start opening, which then causes pressure loss. Our guide to how to tell if an expansion vessel has failed covers the symptoms.

3. Plate heat exchanger leak on a combi

Inside a combi, the plate heat exchanger has heating water on one side and mains water on the other. If an internal seal fails, higher-pressure mains water can leak through into the heating circuit. It looks just like a filling loop that is letting by, so an engineer will rule the filling loop out first.

4. Over-filling

Sometimes the answer is simply that too much water went in the last time the system was topped up. If the pressure comes down after bleeding and stays down, that was all it was.

Is High Boiler Pressure Dangerous?

Modern boilers are designed to protect themselves. The pressure relief valve opens at 3 bar, and many boilers will lock out with a fault code before that. So high pressure is unlikely to cause a sudden failure, but it does wear things out. Repeated relief valve discharges damage the valve seat, leading to a dripping pressure relief valve. Radiator valves and joints are put under strain, and a boiler cycling between too high and too low pressure will eventually lock out.

If you see the pressure gauge in the red zone, water pouring from the discharge pipe outside, or water coming from the boiler itself, turn the boiler off and call an engineer.

When to Call an Engineer

If bleeding a radiator fixes the pressure and it stays put, you do not need anyone. If it climbs back within a few days, the filling loop, expansion vessel or plate heat exchanger needs testing. Recharging an expansion vessel and replacing internal boiler parts is work for a Gas Safe registered engineer.

Our engineers diagnose pressure faults across London and can usually tell a letting-by filling loop from a failed expansion vessel within the first visit. You can book a diagnostic visit or read about our boiler pressure repair service. If your problem is the opposite and the pressure keeps dropping, start with our boiler losing pressure guide.