How to Repressurise a Combi Boiler
Do not overpressurise — the gauge should read 1–1.5 bar when cold. Stop filling as soon as it reaches 1.5 bar.
Low boiler pressure is one of the most common issues homeowners encounter with combi boilers, and the good news is that in most cases it's straightforward to fix yourself in under ten minutes. The problem typically develops when air enters the system, a small leak gradually reduces water volume, or after you've had radiators bled or work done on your heating pipework. It's particularly common in older properties or those with slightly worn filling loop valves. This guide walks through the simple process of topping up your system safely, but it's worth noting that if pressure keeps dropping week after week, there's likely a leak somewhere that needs professional investigation—and that's when you should call a qualified heating engineer rather than keep refilling.
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Most likely cause & what to check
Check the pressure gauge on the front of your boiler — it should read between 1 and 1.5 bar when the system is cold. Below 1 bar, the boiler may shut down or perform poorly.
Turn off your boiler and wait for the system to cool completely (at least 30 minutes). Filling a hot system can give a false reading.
Locate the filling loop — a short braided hose or built-in valve connecting the cold mains to the heating system. On most combi boilers it's located beneath the boiler, sometimes inside a cupboard panel.
Open both valves on the filling loop (usually quarter-turn or screw-type). You will hear water entering the system.
Watch the pressure gauge carefully. Stop filling when it reaches 1.5 bar — close both valves firmly.
Turn the boiler back on. Reset if needed. The boiler should fire up normally. Check for any leaks around the filling loop or radiator valves.
If the pressure drops repeatedly (more than once every few weeks), there is likely a leak somewhere in the system — in a radiator valve, pipework, or the boiler itself. Have this investigated by a heating engineer.
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Frequently asked questions
Why does my boiler keep losing pressure?
If your boiler pressure drops frequently, there's almost certainly a leak in your system—either in a radiator valve, the pipework, or occasionally the boiler itself. A single drop over several months is normal and nothing to worry about, but repeated drops mean you need a heating engineer to locate and fix the leak properly.
Is it safe to repressurise a hot boiler?
No—always allow at least 30 minutes for the system to cool completely before refilling. A hot system gives a false pressure reading and filling it while warm can damage components and give you an inaccurate sense of whether the pressure is correct.
What happens if I overfill the boiler?
Overfilling above 1.5 bar can damage the expansion vessel and create excessive pressure in the system, potentially triggering the pressure relief valve or causing leaks. This is why it's essential to stop filling the moment the gauge reaches 1.5 bar and close both valves firmly.
My boiler still won't fire up after repressurise—what should I do?
Try a manual reset using the button on the boiler (usually red or black), then give it 10 minutes to try again. If it still won't fire up after reset, there may be another fault—either call the boiler's helpline or contact a Gas Safe engineer to diagnose the issue safely.
Where exactly is the filling loop on my boiler?
It's typically a short braided hose or pair of built-in valves located beneath the boiler, sometimes hidden inside a cupboard panel or under the unit itself. Check your boiler manual or look for a hose with two small valves close together—that's your filling loop.
Can I repressurise my boiler if it's an older model?
Yes, the process is the same, though older boilers may have a slightly different filling loop setup—always consult your manual first. If you're unsure about locating or operating the filling loop, it's worth having a heating engineer show you how to do it safely.