Why Is My Boiler Losing Pressure? Causes and Fixes

What your boiler pressure should be

Most sealed system and combi boilers are designed to run at between 1 and 1.5 bar when the heating is cold. When the system heats up, the water expands and the gauge can rise to around 2 bar, which is normal. Below about 0.5 bar, most modern boilers lock out with a low pressure fault code, such as F22 on a Worcester Bosch, F1 on an Ideal Logic or E119 on a Vaillant.

A one-off drop over several months is nothing to panic about. Small amounts of air escape through automatic vents over time. What matters is the pattern: if you are topping up weekly, or the gauge falls overnight, water is leaving the system somewhere and it needs tracing.

The most common causes of pressure loss

In our experience, pressure loss almost always comes down to one of a handful of culprits. Some are simple to spot, others hide under floors or inside the boiler casing itself.

A leak does not need to be dramatic to drain your system. A weep the size of a pinhead on a radiator valve can lose enough water over a week to drop the gauge noticeably, and in older terraced houses around Burnley with original steel pipework, corrosion pinholes under suspended floors are a regular find.

• Leaking radiator valves or towel rail connections, often showing as green or white crusting on the fitting
• Pinhole leaks in pipework, sometimes only visible as a damp patch on a ceiling or a musty smell under floorboards
• A failed expansion vessel, where the boiler pressure spikes when heating then drops right back, often forcing water out of the pressure relief valve
• A passing pressure relief valve, which you can check by looking at the copper discharge pipe outside; if it drips, the valve is letting by
• A faulty filling loop that was left open or is passing, which causes rising rather than falling pressure but is often confused with the same fault
• Recently bled radiators, since removing air lowers system pressure and simply needs a top-up

How to repressurise your boiler safely

Topping up is a job most homeowners can do themselves, and it is worth learning because it saves an unnecessary callout. Find the filling loop, usually a braided silver hose or a built-in key beneath the boiler, and make sure the heating is switched off and cooled.

Open the valve or valves slowly and watch the gauge climb. Stop at 1 to 1.5 bar, close everything fully, and check the gauge again after the heating has run and cooled. If it holds for a week or more, you likely just had air in the system. If it drops again within a day or two, stop topping up and get it looked at, because constantly adding fresh water brings in oxygen and minerals that corrode radiators and sludge up the system, and in the harder water parts of Lancashire it also encourages scale inside the heat exchanger.

When it is time to call an engineer

Call a Gas Safe registered engineer if you are topping up more than once a fortnight, if the pressure relief pipe outside is dripping, if the gauge swings wildly between hot and cold, or if you can see water anywhere it should not be. Expansion vessel and pressure relief valve faults involve opening the boiler casing, which legally must be done by a Gas Safe engineer.

Costs depend on the fault. Recharging an expansion vessel is often a quick job done within a standard callout, while replacing a vessel or tracing a hidden pipework leak takes longer and costs more, particularly if floors need lifting. A straightforward valve or vessel repair typically lands somewhere in the low hundreds of pounds, but an honest engineer will diagnose first and quote before committing you to anything. If your boiler is over 12 to 15 years old and losing pressure alongside other faults, it is worth weighing repair costs against replacement rather than chasing repeated fixes.