Boiler pressure too high? How to lower it safely

What counts as too high

The gauge on the front or underside of a combi or system boiler shows the pressure in the sealed heating circuit, not the pressure at your taps. Manufacturers generally want it between about 1 and 1.5 bar when the system is cold. As the water heats up it expands, so a rise of around half a bar while the radiators are hot is normal and nothing to worry about.

The reading you care about is the cold one. Look at the gauge first thing in the morning before the heating has come on, or an hour or two after it has gone off. Many gauges have a green band or a red marker for the upper limit. Your boiler manual gives the exact figures for your model, and it is worth checking because a few boilers are set up differently.

How to lower boiler pressure by bleeding a radiator

Bleeding a radiator is the simplest way for a householder to let a little water out of the sealed system. It needs a radiator key, an old towel and something to catch drips. Do it with the heating off and the system cool, because hot water under pressure can scald and a hot system gives a misleading reading.

Go slowly. Each radiator only needs to release a small amount of water to drop the pressure noticeably, and it is easy to overshoot and end up below 1 bar, which will stop many boilers firing. If you do overshoot, the boiler will need topping up through the filling loop, following the manual.

• Turn the heating off at the programmer or thermostat and wait for the radiators to cool.
• Pick a radiator, ideally downstairs, and hold a cloth under the bleed valve at the top corner.
• Turn the key a quarter turn anticlockwise. You will hear air hiss, then water will start to come out.
• Watch the boiler gauge, or have someone else watch it, and close the valve as soon as it reads about 1 to 1.5 bar.
• Tighten the bleed screw firmly but gently and wipe the valve dry so you can spot any later drip.
• Turn the heating back on and check the reading again once the system has cooled the next day.

Do not open the boiler casing, loosen fittings on the boiler or try to adjust the expansion vessel. Anything inside the case is work for a Gas Safe registered engineer. If you smell gas, turn off the supply at the meter if it is safe to do so, open windows and call the National Gas Emergency Service on 0800 111 999.

Why the pressure goes up in the first place

A sealed heating system does not gain water on its own. If the cold reading keeps creeping up, water is getting in from somewhere or the system is losing its ability to absorb expansion. The pattern tells an engineer a lot about which one it is.

The most common cause is simple: the filling loop was left open, or not closed fully, after the system was last topped up. The filling loop is the braided hose or built-in key under the boiler that connects mains cold water to the heating circuit. Both valves should be firmly shut once the gauge reaches the right figure. If they are shut and the pressure still climbs slowly, the valve itself may be letting water through. If you mainly notice the climb overnight or while the heating is off, our article on boiler pressure rising when heating off explains what to record for the engineer.

• Filling loop left open or a filling valve that no longer seals properly.
• An expansion vessel that has lost its air charge, so normal heating expansion pushes the gauge into the red every time the heating runs.
• On a combi, an internal leak in the plate heat exchanger letting mains water cross into the heating side, often noticed as pressure rising after hot taps have been used.
• A faulty gauge that reads high when the pressure is actually fine, which an engineer can confirm with a separate test gauge.

When high pressure only happens with the heating on

If the gauge is fine when cold but shoots up towards 3 bar every time the heating runs, the expansion vessel is the usual suspect. The vessel contains a cushion of air behind a rubber diaphragm that absorbs the extra volume as the water warms. When that air charge leaks away, there is nowhere for the expanding water to go and the pressure spikes.

Bleeding radiators will bring the cold reading down, but the next heat-up will simply spike it again, and you may end up with a system that is too low when cold and too high when hot. Recharging or replacing the vessel is the fix. Some boilers have the vessel inside the case, which makes it Gas Safe work; others use a separate vessel nearby. Our expansion vessel replacement page explains what that repair involves.

Water dripping from a pipe outside

Most sealed boilers have a pressure relief valve that opens at about 3 bar to protect the system. Its discharge pipe usually runs through the wall to the outside, often ending near a drain. If you notice that pipe dripping or leaving a wet patch on the wall below, the valve has been lifting.

Once a relief valve has opened a few times it may not reseat properly, and it can keep weeping even after the pressure is back to normal. That then shows up as the opposite problem: a system that slowly loses pressure. If you have seen the outside pipe dripping, mention it when you book, because the valve may need replacing as well as the original cause being fixed.

Is high boiler pressure dangerous?

High pressure on its own is rarely an immediate danger in a modern sealed system, because the relief valve is there to release it. The risk is more practical: repeated high pressure strains joints, radiator valves and the relief valve itself, and many boilers will show a fault code or stop firing when the pressure is outside their working range.

It is still worth dealing with the same day if the gauge is sitting at or near 3 bar, if water is discharging outside, or if the boiler has locked out with a fault code in cold weather. If there are elderly people, young children or anyone unwell in the property and the heating has stopped, tell us when you call so the visit can be prioritised.

Getting it looked at

If you have lowered the pressure once and it has stayed put, you probably left the filling loop slightly open and the job is done. If it has risen again, book an engineer before you spend weeks bleeding radiators. Pressure that keeps coming back is a symptom, and the cause is usually a filling valve, an expansion vessel or, on combis, a heat exchanger, each of which needs testing rather than guessing.

Loyal Plumbers has 10 vans working 24 hours a day across London and everywhere inside the M25, and gas work is carried out by Gas Safe registered engineers. When you call or send an enquiry, tell us the boiler make and model, the cold and hot gauge readings, whether the outside pipe has been dripping and when the system was last topped up. That lets the engineer bring the right parts on the first visit. If your pressure is doing the opposite and falling, see our boiler losing pressure guide instead.