A boiler losing pressure usually means water is escaping from the sealed system: a recently bled radiator, a slow leak, a faulty pressure relief valve or a failing expansion vessel. British Gas says cold pressure should read 1 to 1.5 bar. If it drops again within a day or two, call a Gas Safe engineer.
If your boiler’s pressure gauge is sitting below 1 bar, it is losing water somewhere in the sealed heating system, and it is worth knowing whether that is a one-off (bleeding a radiator, for example) or a sign of something that needs an engineer. According to British Gas’s boiler pressure guide, most boilers should read between 1 and 1.5 bar when cold, rising by roughly three-quarters of a bar once the heating has been running and the water has expanded.
The usual suspects
- You recently bled a radiator. Releasing trapped air also releases a small amount of water from the sealed system, which drops the pressure. This is completely normal, and the fix is simply to repressurise afterwards.
- A slow leak somewhere in the system. Check around radiators, visible pipework, and the boiler itself for damp patches, staining, or drips. A leak does not have to be dramatic to steadily drain pressure over days or weeks, and it is worth checking thoroughly in an older terrace, where pipework tucked under floorboards can be harder to see than in a newer build.
- A faulty pressure relief valve (PRV). This valve is designed to release water if the system pressure gets too high, but if it is faulty it can leak even at normal pressure, often visible as a small pipe outside your house dripping water.
- A failing expansion vessel. This component absorbs the pressure increase as water heats up. If its internal diaphragm fails, pressure can behave erratically, sometimes too high, sometimes dropping unexpectedly.
How to repressurise it yourself
Most modern combi and system boilers have a filling loop, either built in or as a separate braided hose, that lets you top up the pressure without calling anyone out:
- Check your boiler’s specific instructions first. The exact steps vary between manufacturers, and the process is usually printed in the manual or on a label near the pressure gauge.
- Turn the boiler off before you start, and make sure it has cooled down.
- Locate the filling loop and slowly open the valve (or valves, on some models) to let mains water into the system.
- Watch the gauge closely and stop once it reaches the manufacturer’s recommended pressure, typically 1 to 1.5 bar when cold. Overfilling is a common mistake and can trigger the boiler’s safety cut-out.
- Close the valve fully, then turn the boiler back on.
When a top-up is not the answer
A single repressurise after bleeding a radiator, that then holds steady, is nothing to worry about. It is a different story if:
- The pressure drops again within a day or two of you topping it up, suggesting an active leak or fault rather than a one-off.
- You can see or find evidence of water anywhere in the system that was not there before.
- The boiler is displaying an error code related to pressure or water level, which varies by brand but is usually clearly flagged on the display.
- You are topping it up repeatedly, which is masking a problem rather than fixing it, and can eventually cause the boiler to lock out entirely as a safety measure.
At that point, it needs a Gas Safe registered engineer to find and fix the actual source, whether that is a leak, a faulty valve, or a failing expansion vessel, rather than an endless cycle of repressurising.
Getting it looked at properly
A boiler engineer diagnosing a pressure problem should check the whole system, not just refill it and leave, since a leak that is not visible from the outside can still be steadily losing water into a wall or under a floor. If your boiler is due a service anyway, this is exactly the kind of fault an annual service is designed to catch before it becomes a bigger problem. We check Gas Safe registration and independent reviews when we build our Top 5 lists, so a page for towns such as North Coventry is a quick way to find an engineer who is already vetted.
Frequently asked questions
What is the correct pressure for a boiler?
According to British Gas, most boilers should read between 1 and 1.5 bar on the pressure gauge when the heating is off and the system is cold. It is normal for this to rise slightly, by around three-quarters of a bar, once the system is hot and the water has expanded.
Is it safe to repressurise my boiler myself?
Yes, for most modern boilers with a built-in filling loop, topping up the pressure is a straightforward job you can do yourself by following your specific boiler's instructions, usually in the manual or on a label near the pressure gauge. If you are not confident, or your boiler does not have an accessible filling loop, ask a Gas Safe registered engineer to show you once so you know for next time.
Why does my boiler pressure keep dropping after I top it up?
If pressure drops again within a day or two of repressurising, that is not normal and points to an underlying fault, most commonly a leak somewhere in the system, a faulty pressure relief valve, or air in the system after bleeding radiators. A boiler that needs regular manual repressurising should be looked at by a Gas Safe registered engineer rather than topped up indefinitely.
Can low boiler pressure damage my boiler?
Yes. If the pressure drops too far, most boilers will lock out and stop working entirely as a safety measure, since there is not enough water in the system to circulate heat safely. Running a boiler for an extended period at very low pressure can also cause it to overheat internally and shorten its lifespan, so it is worth addressing promptly rather than living with a low reading.
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About the author
Stu Clark
Stu spent over 15 years in the trades, working as a plumber before running his own plumbing and bathroom renovation company with a team of eight. He founded Top5Trades to give homeowners the kind of shortlist and straight advice you would get from someone who has actually done the work. More about us →