What to do if your boiler pressure is too high

To ensure your boiler heats your home and provides hot water to your taps on demand, it must remain correctly pressurised at all times…never too low or too high. 

Whilst small changes in boiler pressure are to be expected, any larger fluctuations can have a serious impact on its ability to meet your household’s needs. What’s more, high boiler pressure can lead to leaks, excess wear on components and long-term damage to your central heating, so it’s important to deal with any pressurisation issues promptly.

Understanding why your boiler’s pressure is too high and what might happen as a result can be tricky, which is why we’ve put together this guide. Don’t worry if you’re not a professional plumber or Gas Safe registered engineer; we’ll walk you through the common causes of high boiler pressure in simple terms to help you know exactly how to get things fixed.

What is boiler pressure?

Regardless of whether it’s inside your central heating system or sitting in a pan on your stovetop, water expands as its temperature increases. As it expands, it takes up more space, increasing the pressure inside wherever it’s being held. This is why your boiler’s pressure will be higher when it’s fired up and running compared to when it’s switched off. 

Boiler pressure refers to the balance between the quantities of water and air inside the system. The expansion vessel controls this balance to keep it at the right level, and the pressure relief valve (PRV) steps in to release additional pressure from the system automatically, bringing it back down to a safe level.

What causes high boiler pressure?

There are several reasons why your boiler’s pressure might be too high. For ease, these can be split into two main categories.

Too much water in the system

The most common reason for high boiler pressure is too much water inside your central heating system. If you've recently repressurised your boiler due to low pressure and have added too much water by mistake, you'll need to remove some by bleeding your radiators to release excess pressure.

Faulty valve or boiler parts

Faulty boiler components can also lead to high pressure. These include the domestic heat exchanger, filling loop and expansion vessel.

If your plate or domestic heat exchanger is split, this can cause the boiler pressure to increase. If your boiler’s filling loop has been left slightly open or is “passing”, new water may be entering the system constantly, thereby increasing the pressure.

Your boiler’s expansion vessel may need recharging or could be faulty, also causing fluctuations in pressure, both high and low.

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Five signs that your boiler pressure is too high

1. A high pressure gauge reading

One of the most obvious signs is an unusually high reading on the pressure gauge. If the needle exceeds the recommended range, typically above 2.5 bar, it's a clear indication that the pressure is too high.

2. Leaking or dripping pressure relief valve

If you notice water dripping or leaking from the pressure relief valve, it's a strong sign of high boiler pressure. The valve is designed to release excess pressure, and when it's constantly leaking water, it signifies that the pressure is exceeding safe levels.

3. System pressure fluctuations

If you notice frequent fluctuations in the pressure gauge reading, with it constantly rising and falling, it suggests that the pressure is not stable and may be too high during peak periods.

4. Strange noises or vibrations

Excess pressure can cause the boiler and its components to operate under stress, resulting in unusual noises or vibrations. You may hear banging, gurgling, or whistling sounds and feel vibrations coming from the boiler or pipes.

5. Loss of heating or hot water

In severe cases, high pressure can lead to boiler shutdowns or lockouts. If you experience a sudden loss of heating or hot water, it could be due to the safety mechanisms triggering to protect the system from high pressure or temperature.

How to reduce boiler pressure

1. Bleed radiators to release any airlocks

If you've recently repressurised your boiler, you'll need to bleed your radiators using a radiator bleed key to twist the bleed valve anti-clockwise slowly.

You should first hear air escaping before water starts to leak from the valve. Let out a small amount of water from each radiator - checking the pressure after each one.

Continue to bleed your radiators until your pressure gauge is back in the safe zone.  Do this when the system is cool to avoid scalding and also get an accurate pressure reading.

2. Tighten or replace any faulty valves

Check the pressure relief valve pipe

This is a copper pipe that usually protrudes from the outside wall behind the boiler. If the pressure relief valve is working, this pipe should be dripping when the water pressure usually exceeds 3 Bar, if not the valve may be faulty. A Gas Safe Registered engineer can fix this issue for you in no time.

Check the filling loop

A fault with the filling loop valves can also cause issues when they don't close properly. The valve may not have been closed if you've had your boiler system repressurised recently due to low boiler pressure.

This one is fairly easy to fix and can be checked by looking at both filling loop valves and seeing whether they're turned off by turning them clockwise.

What is the optimum pressure for a boiler?

The majority of combi, system and standard boilers have pressure gauges displaying a pressure range between 0 and 4 bar.

Your boiler should have a pressure gauge reading of around 1 bar when the heating is off and 1.5 to 2 bar when the heating is on. You should always check your boiler pressure when the heating is off to get an accurate reading.

Is your boiler pressure still too high? Book a repair

We understand the frustration that comes with high boiler pressure, and we're here to help if the above steps haven't improved things. Ensure peace of mind and book a boiler repair with one of our Gas Safe registered engineers for just £89.

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Check out our other boiler maintenance guides and blogs for advice on how to keep your boiler running efficiently.