How to Re-pressurise Your Boiler — Step-by-Step

If your boiler pressure has dropped below 1 bar, the boiler will refuse to fire as a safety measure. Repressurising via the filling loop takes about three minutes and is one of the few boiler tasks safe to do yourself. This guide walks through the process, explains why pressure drops in the first place, and tells you when the issue needs a plumber instead.

Why boilers need pressure

A sealed central heating system runs at a pressure of 1.0 to 1.5 bar when cold. The pressure rises to around 2.0 bar when the heating is hot (water expands as it heats). This pressure ensures water circulates correctly through the radiators and that the heat exchanger inside the boiler doesn't overheat.

If pressure drops below about 0.5 bar, the boiler shuts down to avoid damage. You'll see this as no heating and no hot water, with a fault code (commonly F22 or E22 on most brands) and a low reading on the pressure gauge.

What you'll need

• A torch or phone light (filling loops are often in dark cupboards)
• An old towel for any minor drips
• Your boiler's user manual if you have it — confirms the correct cold pressure for your specific model

You don't need any tools for most modern boilers. Older units may need a small adjustable spanner to operate the filling loop valves.

Step 1: Turn off the boiler and let it cool

Switch the boiler off at its electrical isolator and wait at least 30 minutes for the system to cool. Working on a hot boiler is dangerous and the pressure readings will be misleading — pressure changes significantly between hot and cold.

Step 2: Find the filling loop

The filling loop is a short braided silver hose with one or two valves. It connects the cold mains supply to the heating return pipe. Typical locations:

• Underneath the boiler, attached to the pipework
• In the airing cupboard near the cylinder (system boilers)
• In a built-in compartment under the boiler casing (some modern combis)
• As a removable kit that's normally stored separately (older systems)

If your boiler has no visible filling loop, check the manual — some Worcester and Vaillant models have built-in fillers operated via a key or lever on the underside.

Step 3: Check the pressure gauge

The pressure gauge is on the front of the boiler. It usually has a green zone marked between 1.0 and 1.5 bar (correct cold) and a red zone above 2.5 bar (too high). If the needle is below 1 bar, repressurising will fix it.

Step 4: Open the filling loop valves slowly

Open both valves on the filling loop. There are usually two valves — one at each end of the braided hose. Open them slowly, a quarter turn at a time. You should hear water flowing into the system.

For boilers with a single key or lever-operated filler: turn the key to the open position and hold it open.

Step 5: Watch the pressure gauge rise

The needle on the gauge will start to climb. Stop adding water when it reaches 1.2–1.5 bar. Don't exceed 2.0 bar — overpressure can trip the pressure relief valve and discharge water through the overflow pipe outside.

If you overshoot, you can release the excess by carefully bleeding any radiator until pressure drops back into range.

Step 6: Close both valves firmly

Close both filling loop valves and check the gauge holds steady at the new pressure. If the gauge is dropping again immediately, one of the valves isn't fully closed.

Step 7: Reset the boiler

Switch the boiler back on at the electrical isolator. If a fault code was showing, press the reset button (usually a flame icon or "R" on the control panel). The boiler should now fire normally.

Step 8: Monitor pressure over the next 24 hours

Check the gauge a few times the next day. The pressure will rise slightly when the heating is on (to around 1.5–2.0 bar) and settle back to 1.0–1.5 bar when cold. Both readings should be within range.

Why pressure drops

Small pressure drops are normal — air working out of radiators, slow seepage at joints, occasional bleed-down. Topping up once or twice a year is fine. But if pressure drops repeatedly (every few days or every couple of weeks), there's a leak somewhere. Common locations:

• Radiator valves — particularly TRVs and the union nuts at the bottom of each radiator
• Pipe joints in concealed pipework (under floors, behind walls)
• Pressure relief valve discharge pipe outside — check for water dripping from the white plastic pipe on the external wall near the boiler
• Expansion vessel internal failure — the vessel inside the boiler that absorbs thermal expansion
• Heat exchanger leak — internal to the boiler, requires professional diagnosis

If you're topping up more than once a month, stop and call a plumber to find the leak. Continually repressurising masks the problem and accelerates damage to internal components from the constant flow of fresh oxygenated water through the system.

When to call a plumber instead

• Pressure drops to zero within 24 hours of repressurising
• Visible water leak anywhere in the system
• The pressure relief valve outside is discharging water
• The gauge needle won't move despite opening the filling loop (filler is blocked or filling loop isn't connected to mains)
• Boiler still shows a fault code after repressurising and resetting
• You're unsure where the filling loop is

Our central heating engineers diagnose pressure-loss issues across all PE postcodes. Most leak-source diagnoses are completed in a single visit. See also our guide on why boiler pressure drops for more on persistent pressure issues.