What pressure should my boiler be? Reading the gauge properly

Normal boiler pressure at a glance

The figures below are typical for a sealed heating system, which is what nearly every combi boiler and most system boilers are connected to. Treat them as a guide and check them against the manual, because a few boilers and some larger systems are set up to run slightly differently.

What the gauge is actually measuring

The pressure gauge on a boiler shows the pressure of the water inside the closed loop of pipes and radiators that makes up your central heating. It has nothing to do with the pressure at your taps or shower. A boiler can read a perfect 1.2 bar while the kitchen tap trickles, and a strong mains supply does not mean the heating is correctly pressurised.

The system needs some pressure so that water reaches the highest radiator, the pump runs without drawing in air, and the boiler's own sensors are satisfied it is safe to fire. Too little and the boiler shuts itself down. Too much and the safety valve lets water out through a pipe, usually a small copper pipe that runs to the outside wall.

Older houses with a small tank in the loft that tops up the heating are different. These are open vented systems, and many of them have no pressure gauge on the boiler at all. If yours has a feed and expansion tank in the loft and no gauge, the figures on this page do not apply. Our guide to open vented or sealed heating systems explains how to tell which you have.

How to read the gauge

Most boilers show pressure in one of two ways. Older and many current models have a round dial with a needle, often behind a flap on the front or underneath the casing. Newer boilers often show pressure as a number on the digital display, sometimes only after pressing a button marked with a gauge symbol or scrolling to an information menu.

On a dial, the numbers usually run from 0 to 4 bar. Many dials have a green band for the normal zone and a red mark near 3 bar. Some have a second, adjustable red needle that an engineer sets to show where the pressure should be after filling. If your needle is sitting on or just above that red marker when the system is cold, the pressure is where it was set.

Read it cold. Early in the morning before the heating comes on, or a couple of hours after it goes off, is best. Taking a reading while the heating has just been running gives a higher figure that can look like a problem when it is only normal expansion.

• Find the gauge or the pressure reading in the boiler menu.
• Check whether the heating has been off for at least an hour.
• Note the reading and the time, and compare with the ranges in the table.
• If it is outside the normal range, check again at the same time the next day before doing anything else, unless the boiler has already locked out.

Why boiler pressure goes up and down during the day

Water expands when it is heated. As the system warms up, the same amount of water tries to take up more room, and the pressure rises. A rise of about half a bar between cold and hot is normal. A sealed system has an expansion vessel, a metal tank with a cushion of air on one side of a rubber diaphragm, that absorbs this expansion and stops the pressure from shooting up.

So a boiler that reads 1.2 bar at 6am and 1.8 bar at 7pm with the heating on is working as intended. What matters is whether the cold reading stays roughly the same from one day to the next.

If the pressure climbs by a bar or more as the heating warms up and then drops right back, the expansion vessel has often lost its air charge. That puts extra strain on the relief valve and is worth having checked, even if the boiler is still working.

If the pressure is too low

A cold reading below 1 bar is low for most boilers. If it has only just dropped, for example after you bled several radiators, topping it up through the filling loop is normally all that is needed. The filling loop is a pair of small valves, sometimes joined by a braided hose and sometimes built into the underside of the boiler, that lets mains water into the heating. Follow the manual for your model, open the valves slowly, and close them as soon as the gauge reaches about 1 to 1.5 bar.

If you have to top up more than once, the water is going somewhere. Common causes are a small leak at a radiator valve or under a floor, a relief valve that is letting by, or a failed expansion vessel. Our guide to a boiler losing pressure goes through how to narrow it down, including checking the outside discharge pipe and timing the loss.

Do not remove the boiler casing or adjust anything inside it. Topping up through an external filling loop is a normal user task. Work inside the boiler, on the gas valve or on a sealed expansion vessel inside the case, must be done by a Gas Safe registered engineer.

If the pressure is too high

A cold reading of 2.5 bar or more is too high. The usual householder fix is to let a little water out by bleeding a radiator with the heating off, watching the gauge as you go. Our guide on how to lower boiler pressure explains the method step by step.

The more useful question is why it went up. The two usual answers are a filling loop valve that has been left slightly open or is letting by, and an expansion vessel that has lost its charge. If the pressure keeps creeping up even with the heating off, our guide to boiler pressure rising when the heating is off explains what to look for. If the copper pipe outside is dripping, the boiler pressure relief valve has been opening, and after it has lifted a few times it may not seal properly again.

Where to set the pressure after a repair or top-up

After refilling, aim for the cold figure in your boiler manual, which for most homes is about 1 to 1.5 bar. In a house where the highest radiators are well above the boiler, such as a boiler in a basement feeding a third-floor bedroom, an engineer may set the cold pressure a little higher so the top of the system is properly filled. If your gauge has an adjustable red needle, that is often where the engineer has marked it.

Do not overfill to be safe. A system filled to 2 bar cold can reach 3 bar when hot, which is exactly when the relief valve opens. That wastes water and inhibitor, and a relief valve that has opened several times is more likely to start dripping for good.

When to call an engineer

Pressure on its own is rarely an emergency. The ones that need attention quickly are pressure dropping to zero overnight, water appearing from a ceiling, wall or the boiler itself, and a relief pipe that is running rather than dripping. In those cases turn the boiler off and, if there is a visible leak, isolate the water. A single top-up that holds for weeks is not a fault.

Loyal Plumbers repairs boilers and sealed heating systems across London and everywhere inside the M25, 24 hours a day, with 10 vans on the road and Gas Safe registered engineers for any work on the boiler. We carry £5m public liability cover, and our call-out charge is £60. Call 020 4652 8585 or send an enquiry with the cold reading, how fast it changes and whether anything is dripping outside.