18 March 2026
Boiler losing pressure? Try this before you call anyone
Low boiler pressure is the most common heating fault, and often a two-minute DIY fix. Here's how to repressurise safely, and the signs it's time to call an engineer.
If your heating's stopped and the display shows a pressure warning (or the needle on the gauge has dropped into the red), don't panic, and don't reach for the phone yet. Low pressure is the most common boiler fault there is, and most of the time you can fix it yourself in two minutes.
What the gauge should say
Find the pressure gauge on the front or underneath of your boiler. When the heating is cold, it should read around 1 to 1.5 bar, most gauges mark the happy zone in green. Below about 0.5 bar, most boilers lock out and won't fire.
How to repressurise (the two-minute fix)
Every boiler has a filling loop, usually a silver braided hose with one or two small valves, underneath the boiler. Some newer boilers have a filling key or lever instead. Your boiler's manual (or a quick search of the model name) will show yours.
- Turn the heating off and let the boiler cool.
- Open the filling loop valve(s) slowly, you'll hear water flowing.
- Watch the gauge climb. Close the valves when it reaches about 1.2 bar.
- Restart the boiler. If it needed it, press the reset button.
Don't overfill, above 2.5–3 bar the boiler will dump water through the pressure relief pipe outside, and you'll be back where you started.
When low pressure means something's wrong
Topping up once or twice a year is normal. But if you're doing it every few weeks, the water is going somewhere:
- A small leak, check visible pipework and under radiator valves for damp, green staining on copper, or salt-like crust. - A weeping pressure relief valve, look at the copper pipe that exits through your outside wall near the boiler. If it drips, the valve isn't sealing. - A failing expansion vessel, the classic sign is pressure that climbs high when hot and collapses when cold, needing constant top-ups. - Bleeding radiators without topping up, every bleed lowers system pressure a little. Always check the gauge after bleeding.
The first is sometimes DIY-findable; the rest are engineer jobs, none of them are usually expensive, but all of them get worse if ignored, and repeated fresh water top-ups feed corrosion inside your system.
When to call us
If your boiler needs topping up more than every couple of months, drops pressure overnight, or won't hold pressure at all, it's time for a proper look. Call 07969 110679, no call-out charge, and we cover Eastleigh, Chandler's Ford, Winchester, Southampton and the surrounding SO postcodes, 24 hours for emergencies.
And if the pressure problem is one fault too many on an ageing boiler, you can see what a replacement would cost in 90 seconds, fitted price, no sales visit.
