How do I repressurise a boiler?

A sealed heating system runs at a set pressure. Bleeding radiators, small leaks and normal seepage let it fall, and below about 0.5 bar most boilers shut down as a protection measure. Topping up is a routine homeowner job; needing to top up often is not.

Safety

What you'll need

• The boiler manual
• Filling loop, if not permanently fitted (optional)
• Torch (optional)

Step by step

1. Read the pressure gauge with the heating off and cold.Cold pressure should sit between 1 and 1.5 bar on most systems. It rises when hot, which is normal and expected.
2. Turn the boiler off before doing anything.Repressurising a running boiler can cause a fault lockout, and hot system water under pressure is worth avoiding.
3. Find the filling loop.Usually a braided silver hose beneath the boiler with a valve at each end, or an internal key or lever on newer models. The manual identifies it.
4. Open both valves slowly.You should hear water flowing. Slowly is the operative word — opening fully causes a rapid overshoot.
5. Watch the gauge and close the valves at 1.5 bar.Close them firmly. It moves faster than expected, so do not look away.
6. If you overshoot, bleed a little water out of a radiator.Bleed briefly with a cloth and a container until the gauge falls back into range. Above 2.5 bar the pressure relief valve will discharge.
7. Disconnect the filling loop if it is a removable type.Leaving it connected can allow mains water to creep into the heating system, which is why many manufacturers require removal.
8. Restart the boiler and check for a fault code.Most clear automatically once pressure is restored. If not, the manual lists the reset procedure.

Tips

• Photograph the gauge each month. A slow steady fall shows a leak; a sudden drop after bleeding radiators is normal and expected.
• Check the pressure relief discharge pipe outside — if it drips, the system has been over-pressurised or the valve has failed.
• If pressure falls repeatedly with no visible leak, the expansion vessel has usually lost its charge. That is an engineer job and is straightforward for them.

Common mistakes

• Opening the filling valves fully — Pressure rises very quickly and overshooting means bleeding water back out again, or triggering the relief valve.
• Topping up repeatedly and ignoring why — The water is going somewhere. A slow leak under a floor can do serious damage before it is visible.
• Leaving a removable filling loop connected — It risks mains water entering the heating system and is contrary to most manufacturers' instructions and water regulations.

Questions people ask