The water pressure inside a boiler system should usually be somewhere between 1 and 1.3 bar (but check your manual to find out its optimum range). If it’s too high or low outside this range, your system won’t work efficiently, as that’s the range at which all the components are designed to operate.

Is high boiler pressure dangerous?

High pressure is unlikely to be dangerous, even if it’s a lot higher, as your system should have a pressure relief valve that will trigger. However, you should not rely on this to prevent overpressurisation. First, it’ll be inefficient, and second, it could be dangerous if the relief valve is faulty or jammed.

Signs that the pressure is too high

How do you know if your boiler pressure is too high? Here’s how to find out.

Check the gauge

The first port of call should be your pressure gauge. This will be on the boiler itself, and will take one of two forms: an analogue (pointer) display or a digital display.

An analogue display will look like a clock face or speedometer, with a needle pointing to the pressure. It will have numbers around the edge, but it should also have some kind of indication of where the ideal range is, for example a shaded area or two lines marking the maximum and minimum pressures.

A digital gauge will be an LCD, LED or screen type, and might be multi-function, i.e. it shows other information such as timers, temperatures or whether the boiler is serving central heating or hot water. Sometimes you have to press a button to scroll through different measurements until you reach the pressure indicator. The screen might also show an error message if the pressure has tripped it out. Check your boiler’s manual for details.

Is there any leakage?

In the unlikely event that the pressure gauge is faulty, another tell-tale sign is leakage from your central heating system. It normally starts at the weakest link, so will show itself at joints, often where pipework joins radiators. If you see puddles, it’s worth checking your pressure. But you’ll also need to have the leak fixed by a plumber.

How to reduce the boiler pressure

Once you’ve diagnosed high pressure in your boiler system, the next step is to reduce it. There are several ways to do this. We’ve ordered them by ease, but the techniques are not all available to every system.

Open the filter valve

Many (but not all) central heating systems have a filter, which water continually cycles through to remove impurities. They also double up as drainage valves, and are perfect for relieving pressure in the system. You’ll normally find the filter near the boiler, as it should be located between the last radiator and the boiler to ensure your boiler is best protected. It’ll be either conical or cylindrical in shape, with a valve at the bottom. Put a bucket under the filter, open the valve and keep an eye on the gauge. Close the valve when it reaches 1 bar.

Use a drain-off valve

If you don’t have a filter, you might have a drain-off valve (also known as a drain cock). This is located in the pipework for the central heating system. It could be anywhere, and there could be several, but if there’s just one it will probably be downstairs. It will be a Y-shaped or T-shaped component branching from the pipework, with a valve that might require a spanner to open. Place a bucket or tub underneath it and open it up, checking the pressure on the boiler. Close the valve when it reaches 1 bar.

Bleed a radiator

As well as releasing air trapped in the system, bleeding can also be carried out to lower the water pressure. We’ve written an article on bleeding radiators here, but in summary, locate the bleed valve on a radiator, place a bucket or tub underneath or next to the valve, and open the valve gently. Water will squirt out, so you should also protect any wallpaper and carpets nearby. Bleeding a radiator will get the job done, but it’s very slow, so one of the methods above is preferable if available.

Undo a radiator nut

This is a more specialist job, and should only be attempted if none of the above are available, and you’re a confident DIYer. Place a container underneath a radiator nut that connects the pipework to the radiator, and gently undo the nut with a spanner. You’ll get a much faster flow than with bleeding, so be prepared for splashing and get ready to tighten up the nut quickly in case you need to.

Preventing high pressure from coming back

Once you’ve got your pressure back to normal range, you should investigate why it has happened in the first place. Take a look at the suggestions below.

Check the filling loop

The filling loop is where water enters the system from the mains, and consists of two valves. When both valves are open, the system lets more water in, and the pressure rises. A prime suspect for high pressure is these valves being left open accidentally. They turn by 90 degrees, and if the handle is aligned with the pipe, it means it’s open. Make sure they are both closed – your system circulates its water, and rarely needs topping up.

Sometimes there’s a detachable length of hose that connects to the two valves in the filling loop, rather than permanent pipework. If it’s the hose type of loop and the hose is not connected, you can rule out the valves being open as you’d see water pouring out!

Overfilling

It’s possible that someone has responded to low pressure in the past (for example after bleeding the radiators) by adding more water via the filling loop. While that’s the right thing to do, people sometimes overfill it by accident.

Faulty expansion vessel

The expansion vessel helps keep the pressure consistent and prevents sudden changes. If this is faulty or damaged, the pressure might be raised. Do not attempt to inspect or fix this yourself – it’s a professional job.

Get a service

If you haven’t had your annual service in the past 12 months, book one now, as it might reveal an issue with your system. Similarly, if you keep getting high pressure and don’t know why, it’s time to call out a Gas Safe Registered engineer to have a look at your boiler.


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