BoilerFaultCodesLow boiler pressure: how to repressurise your boiler
Low boiler pressure is one of the most common boiler problems — and one of the few you can safely fix yourself. If your pressure gauge reads below 1 bar, or your boiler has shut down showing a pressure fault code, this guide will walk you through topping it up.
What's the correct boiler pressure?
Cold system: 1.0–1.5 bar · Hot system (running): 1.5–2.0 bar. Anything below 0.8 bar when cold is too low.
How to repressurise your boiler
Check the pressure gauge
Find the pressure gauge on your boiler — usually a circular dial or digital display on the front. If it reads below 1 bar with the heating off, it needs topping up.
Turn off and cool down
Switch the boiler off at the controls and wait at least 30 minutes for it to cool. Never repressurise a hot system.
Find the filling loop
Look underneath the boiler for the filling loop — a short, flexible silver or braided hose connecting two pipes. Some boilers have a built-in filling key or lever instead.
Open the filling valves slowly
Open both valves on the filling loop (usually a quarter-turn so the slot aligns with the pipe). You'll hear water entering the system. Open them slowly — the pressure rises quickly.
Watch and stop at 1.2–1.5 bar
Keep your eye on the pressure gauge. Close both valves as soon as the needle reaches 1.2–1.5 bar. Overfilling causes a different problem (pressure too high).
Restart and monitor
Turn the boiler back on. It may need a reset — hold the reset button for 3–5 seconds. Once running, check the pressure again when the system is hot: it should read 1.5–2 bar. If it drops again within a few days, there's a leak somewhere.
Fault codes for low pressure
When to call an engineer
- › Pressure drops again within days of topping up (indicates a leak)
- › You can't find the filling loop or filling key
- › The boiler still shows a fault code after topping up and resetting
- › Pressure is dropping very rapidly
- › You can see water leaking from the boiler or pipework
Pressure keeps dropping?
A Gas Safe engineer can find and fix the leak.