What Pressure Should My Boiler Be?

Updated July 2026 · Emergency Plumber Chelsea

The pressure on the gauge is the pressure of the water inside your sealed heating circuit, not the pressure of the water coming out of your taps. A combi or system boiler pushes the same water round the pipes and radiators again and again, and that water needs to be held at a steady pressure so the pump can move it and the boiler's safety sensors are happy. That is why the gauge sits at the front of the boiler, usually on the control panel or just underneath it, and why the reading has nothing to do with how strong your shower is.

Reading the gauge is simple once you know what you are looking at. An analogue gauge is a small dial marked from 0 to 4 bar, often with a green band between roughly 1 and 2 bar and a red band or red marker above 3. Many modern boilers show the pressure on a digital display instead, either as a number such as 1.3 or as a bar graph, and some only show it when you press a button or scroll through the menu. If the dial has two needles, the red one is a fixed marker that an engineer can set as a reference point, and the black one is the live reading. Your boiler manual will show exactly where the figure appears on your model.

Cold and hot readings are different, and both are normal. Water expands as it heats, so when the heating comes on the needle climbs. A system that reads 1.2 bar first thing in the morning might sit at 1.6 to 1.8 bar with every radiator hot, then fall back to 1.2 bar overnight. A rise of about half a bar is what you would expect. A jump from 1 bar to nearly 3 bar every time the heating runs is not normal, and it usually means the expansion vessel inside the boiler has lost its air charge. Our guide to boiler pressure that is too high explains that fault and how to bring the pressure down safely.

Low pressure is the more common problem, and it tends to show up in autumn. If the gauge reads below 1 bar cold, many boilers will light but run poorly, and below about 0.5 bar most will lock out and show a fault code. Pressure often drifts down a little over the summer, and it always drops after you bleed radiators, because you let some water out with the air. Topping up once at the start of the heating season is normal maintenance. Topping up every week or two is not, and our guide on why a boiler keeps losing pressure walks through the usual leaks, from a weeping radiator valve to a dripping relief valve outside.

To top up a combi or system boiler, turn the heating off and let it cool first, so you are working from the cold reading. Find the filling loop, which is either a braided silver hose with a valve at each end under the boiler, or a built-in key, lever or blue tap on the underside of the boiler itself. Open the valves slowly and watch the gauge. You will hear water running in. Close them again when the needle reaches about 1.2 to 1.5 bar, and make sure both valves are fully shut afterwards, because a valve left slightly open lets mains water creep in and pushes the pressure too high. If your boiler needs a separate filling key that has gone missing, or the loop is not where the manual says, it is better to call than to force a valve.

Not every boiler has a pressure gauge, and that is not a fault. Older heating systems with a regular boiler, a hot water cylinder and two tanks in the loft are usually open-vented. The heating water is fed from a small feed and expansion tank in the loft, and the pressure comes from the height of that tank above the radiators. There is nothing to top up at the boiler. If you are not sure which type you have, our guide to mains pressure and gravity-fed plumbing explains how to tell them apart. In a tall Chelsea terrace with an open-vented system, cold radiators on the top floor are more often a sign of air or a tank problem than of low pressure.

Check the pressure at the moments when it is most likely to have moved. The first time you put the heating on after the summer is the obvious one, and our guide on when to turn the heating on covers the other first-start checks. Look again after you have bled any radiators, after a plumber has drained down part of the system to change a radiator or valve, and after a power cut if the boiler has shown a fault. A quick glance once a month through the winter is enough to spot a slow leak before it turns into a lockout on a cold night.

Bleeding radiators and pressure go together. Air in the system makes radiators cold at the top and can make the boiler noisy, and releasing it lowers the pressure. The right order is to bleed first, starting downstairs, then check the gauge and top up if it has dropped below about 1 bar. Our step-by-step guide on how to bleed a radiator covers this. If you find yourself bleeding the same radiator every few days, air is getting into the system, which is worth having looked at.

A pressure fault is often the reason a boiler shows an error code. Worcester, Baxi, Ideal and Vaillant boilers all have codes for low water pressure, and some also flag high pressure. Before pressing reset, check the gauge. If it is low, top it up first, because a reset will not work until the pressure is back in range. Our guide on how to reset a boiler explains the checks to make before you press the button, and when to stop trying. On a Glow-worm, low pressure shows as F22, and our guide to Glow-worm boiler pressure covers that make in detail.

Mansion block flats in Chelsea and South Kensington are a slightly different case. If your flat has its own combi or system boiler, the rules above apply exactly. If heating and hot water come from a communal plant room, there is usually no pressure gauge in the flat at all, and a pressure problem in the building is for the managing agent or porter to deal with. If you have a heat interface unit in a cupboard, it may have its own gauge, but the building's maintenance contractor normally looks after it.

Call an engineer when the pressure will not stay put. That means a gauge that drops below 1 bar within days of a top-up, a reading that climbs towards 3 bar, water around the boiler or dripping from the small copper pipe outside, or a boiler that keeps locking out even though the pressure looks right. Emergency Plumber Chelsea sends Gas Safe registered engineers for boiler repairs and central heating repairs across Chelsea, Knightsbridge, South Kensington, Sloane Square and Fulham. We find the leak or the failed part, explain the options, and agree the work before we start. There is no call-out fee on jobs over 3 hours. Call 020 3002 6667. If you are worried whether a pressure fault makes the boiler unsafe to use, our guide is my boiler dangerous? explains which warning signs matter.

Quick tips

• Read the gauge with the heating off and the radiators cool; that is the figure that counts.
• Aim for 1 to 1.5 bar cold; up to about 2 bar with the heating on is normal.
• Close both filling loop valves fully after every top-up, or the pressure will creep up.
• Bleed radiators first, then check the pressure and top up if it has fallen below 1 bar.
• If you top up more than once or twice a season, there is a leak to find.

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