— blog
If your boiler pressure keeps dropping, it can be worrying, especially when radiators stay cold or the boiler keeps switching off. The good news is that low pressure is usually fixable, and there are some safe checks you can do before calling an engineer.
Most modern combi and system boilers are designed to run at around 1 to 1.5 bar of pressure when the system is cold. You can see this on the pressure gauge on the front of your boiler or nearby pipework.
When the heating is on and the water heats up, the pressure often rises slightly, usually staying under 2 bar. If your gauge regularly drops below 1 bar, or down to zero, the boiler may lock out or stop working to protect itself.
Common signs of low boiler pressure include:
Radiators not heating fully or staying cold at the top
Boiler cutting out and needing a reset
Gurgling or trickling sounds in pipes and radiators
Flashing low pressure or fault codes on the boiler display
Before you touch anything, make sure your boiler manual is to hand. It will show where your pressure gauge, filling loop and isolation valves are, and explain any boiler-specific steps.
Look at the gauge on the boiler. If the needle is in the green area or between about 1 and 1.5 bar when cold, the pressure is usually fine. If it is below 1 bar, or in the red low section, your system may need re-pressurising.
Note how quickly it is dropping. If you top it up and it falls back within hours, that usually points to a fault that needs a Gas Safe engineer.
Walk around your home and check each radiator, valve and visible pipe. Pay close attention to radiator valves, pipe joints and any towel rails in bathrooms, as these are common leak points.
Run your hand carefully along pipework (if safe and cool) and look for drips, green or white staining, or rust marks. Also check for damp patches on ceilings, skirting boards or flooring that might indicate a hidden leak.
Inspect the pipes underneath the boiler and the safety discharge pipe that usually goes outside. If you can see water staining or dripping from any part of the boiler casing or the pipework, turn the boiler off and do not remove any covers.
Any water appearing from the boiler itself, any signs of scorching should be treated as urgent. In those cases, do not try to fix it yourself: contact a Gas Safe engineer straight away.
If you cannot see any obvious leaks and the pressure is just below 1 bar, you may be able to re-pressurise using the filling loop. Always follow your boiler manual and only do this if you feel confident.
Step 1: Switch the boiler off and let it cool for at least 15 to 30 minutes.
Step 2: Locate the filling loop, usually a silver flexible hose with two small taps or levers.
Step 3: Open both taps slowly so you can hear water entering the system, watching the pressure gauge rise.
Step 4: Close both taps once the pressure reaches around 1 to 1.5 bar.
Step 5: Turn the boiler back on and reset it if required.
Safety note: Never leave the filling loop open, never exceed the recommended pressure in your manual and never remove the boiler casing yourself. If in doubt, stop and call a professional.
After topping up, monitor the pressure over the next 24 to 48 hours. If it stays steady, it may just have been a one-off drop, for example after bleeding radiators. If it drops again, there is likely an underlying issue.
Even a tiny drip on a radiator valve or a pinhole in a pipe can slowly let water out and cause pressure loss. Sometimes these leaks only show as slight staining or are hidden under floors.
Plumbers can use leak detection techniques to find and fix these problems. If you are in an older home or have had work done recently, small leaks are a common culprit.
The pressure relief valve (PRV) is a safety device that lets out water if the pressure gets too high. If it has lifted once, it can sometimes stick slightly open, allowing water to escape over time.
You might see a copper pipe outside, often bent towards the ground, that drips when the boiler is running. If this is wet or stained regularly, the PRV or another component may need replacing.
The expansion vessel takes up the extra volume when water heats and expands. If its internal air charge is lost or the vessel fails, pressure can swing too high when hot and drop too low when cold.
Signs of this include the pressure gauge rising a lot when the heating is on, then falling back to almost zero when off. Recharging or replacing the expansion vessel is a job for a Gas Safe engineer.
If you have recently let air out of radiators, some pressure loss is normal, because you are effectively letting water out of the system. The gauge may drop slightly afterwards.
In that case, a single careful top-up is usually all that is needed. If you keep needing to bleed radiators and re-pressurise, it suggests an ongoing issue that should be checked.
If you are re-pressurising your boiler every few days or weeks, it is a sign that the system is losing water somewhere. Constantly adding fresh water brings more oxygen into the system, which can lead to corrosion, sludge and damage over time.
It is always better to find and fix the underlying cause rather than live with the problem. A professional inspection can often pinpoint the issue before it turns into a breakdown, especially in busy heating periods in areas such as Willenhall.
You should arrange a visit from a Gas Safe registered engineer if any of the following apply:
Pressure drops from normal to low in a matter of hours
You need to reset the boiler frequently to get hot water or heating
You can see water leaking from the boiler casing or nearby pipework
The discharge pipe outside is regularly dripping
It is also sensible to book an annual boiler service to check the expansion vessel, PRV and overall system health. Regular servicing can reduce the chance of pressure issues and unexpected breakdowns.
If your boiler pressure keeps dropping and you are unsure what to do next, it is safer to get expert help rather than keep topping it up. A qualified engineer can test for leaks, check safety devices and make sure your system is running efficiently.
For friendly, professional support with boiler repair or routine boiler servicing, contact Heatfire Ltd on 01902239130. We can diagnose pressure problems, repair faults and help keep your home warm and safe.
