How to repressurise a boiler using the filling loop
⚠️ Do not repressurise a hot system — allow the boiler to cool for 30 minutes first. Cold pressure should be 1–1.5 bar. Hot pressure will rise to 1.5–2 bar; this is normal.
Not sure if this matches your problem?
Use our interactive tool — answer a few questions and get a personalised diagnosis.
Most likely cause & what to check
The filling loop is a braided flexible hose (typically 300–500mm long) connecting a valve on the cold mains supply to a valve on the central heating return pipe. On most boilers it is found underneath the boiler.
Ensure the boiler is switched off and cool. Look at the pressure gauge — if below 1 bar, you need to add pressure.
Locate both valves on the filling loop — they typically have flat-head screw slots or lever handles. Open both valves (turn screws so the slot aligns with the pipe, or levers to inline position).
Watch the pressure gauge rise slowly. Stop when the gauge reads 1.5 bar. Close both filling loop valves.
Switch the boiler on and allow it to heat up. The gauge may rise to 1.8–2 bar as the system heats — this is normal. It must not exceed 3 bar.
Check the filling loop valves are both fully closed — a partially open filling loop will continuously add pressure and will cause the pressure relief valve to discharge.
Prefer to have it done for you?
Find me an engineer →🛠 Tools & materials you may need
Get a vetted local engineer to handle it — no hassle, Gas Safe registered.
Was this guide helpful?