How to Top Up Boiler Pressure — Step-by-Step UK Guide (Combi & System)

Topping up boiler pressure takes about 5 minutes. You find the filling loop (the silver braided hose under the boiler), open both valves slowly, watch the pressure gauge climb to 1.0 to 1.5 bar cold, then close both valves and reset the boiler if needed. That’s it.

This guide walks through it step-by-step for combi and system boilers, flags the four common mistakes that catch people out, and tells you when you should stop and call a plumber instead. If you’re in Leicester or the surrounding LE postcodes and want a Gas Safe engineer to do it, call 0116 488 6858 — though honestly, if it’s just a top-up, doing it yourself is fine.

Before you start — what you need to know

• Target pressure: 1.0 to 1.5 bar cold. Not hot. Read the gauge first thing in the morning before the heating has fired.
• Most modern combi and system boilers have an internal or external filling loop. Older models (pre-2010) more often have a keyed filling key instead — same idea, different mechanism.
• Always work with the boiler off. Switch the heating off at the boiler’s main switch or the on/off button before topping up.
• The gauge will rise quickly once both valves are open — water flows in fast. Don’t walk away.

Step-by-step: combi boiler with a filling loop

Step 1 — Switch the boiler off

Press the on/off button on the front of the boiler, or flip the fused spur on the wall nearby. The display goes blank or shows a standby light. This is so the boiler can’t try to fire up while you’re filling.

Step 2 — Find the filling loop

Look underneath the boiler. The filling loop is a short braided silver flexible hose, about 30cm long, with a small valve at each end (usually a black or chrome lever or a slot for a flathead screwdriver). On most modern boilers it’s permanently fitted in place. On some older ones it’s stored separately and you screw it back on with two threaded couplings before use.

If you can’t see one, check inside the boiler casing under the front panel — some Worcester Bosch and Vaillant models hide it behind a flap. The user manual will show its location (and is usually on the manufacturer’s website if you’ve lost the paper copy).

Step 3 — Open both valves slowly

Watch the pressure gauge as you do this. Open one valve a quarter turn — you’ll hear water start to flow. Then open the second valve a quarter turn. The needle should start climbing.

Don’t crank the valves fully open — quarter to half turn is enough. Slow flow gives you time to react if anything’s wrong.

Step 4 — Stop at 1.2-1.5 bar

When the gauge reaches the green zone (typically marked 1.0-1.5 bar), close both valves firmly — back to their original position, perpendicular to the hose for lever-type valves. Listen — the water flow should stop completely.

Aim for around 1.2-1.3 bar cold. If you overshoot to 1.8 or higher, see “if you overfilled” below.

Step 5 — Switch the boiler back on and reset

Switch the boiler back on. If it had locked out with a low-pressure error (F22, E119, EA, depending on brand), press and hold the reset button for a few seconds. The error should clear and the boiler should fire normally on the next heating or hot water demand.

Step-by-step: system boiler with a filling loop

The process is identical to a combi — the filling loop is just usually located on the airing-cupboard pipework near the cylinder rather than under the boiler itself. Same routine: switch off, open both valves, watch the gauge (the gauge is on the boiler itself), close both valves at 1.2-1.5 bar, switch back on.

If you have a vented (open-vent) heating system with a header tank in the loft, you don’t have a filling loop at all — the tank tops the system up automatically. Pressure issues on vented systems usually mean a faulty ball-valve in the tank, not a top-up problem.

If you overfilled (above 2.5 bar cold)

Common mistake — easy to fix. Bleed a single radiator using a bleed key (the small square brass nipple at the top of the radiator). Hold a cup or cloth under it, turn the key a quarter turn anti-clockwise. You’ll hear water hiss out — let it run for 10-15 seconds, watching the gauge until pressure drops to 1.2-1.5 bar. Close the valve.

If you’re at 4+ bar, the boiler’s pressure-relief valve will release automatically — you’ll hear or see water dripping outside via the small overflow pipe on the wall. Bleed two radiators if it’s that high.

The 4 mistakes people make

• Topping up while the heating is hot. Hot pressure is naturally 0.5-0.7 bar higher than cold. If you top to 1.5 hot, you’ll be at 0.8 cold the next morning — under-pressure again. Always top up first thing, before the heating fires.
• Walking away with the valves open. Water flows in fast — pressure can hit 4 bar in 30 seconds. Stay there, watch the gauge.
• Leaving one valve cracked open. The filling loop is a one-way connection from mains water to your heating circuit. If a valve doesn’t close fully, fresh oxygenated mains water keeps trickling in over weeks — silently corroding your radiators and heat exchanger. Close both valves firmly. Listen for the flow to stop.
• Topping up over and over. If you’re topping up more than once a month, there’s a leak or a failing component. See our guide on what causes a boiler to lose pressure — chasing top-ups is masking the real problem.

When to call a plumber instead

• You can’t find the filling loop and the manual doesn’t help.
• The valves on the loop are seized and won’t turn.
• You’ve topped up but the pressure drops back within hours.
• The boiler shows an error code that doesn’t clear after a top-up and reset.
• You see water dripping from the boiler casing or on the floor underneath.

Frequently asked questions

What pressure should my boiler be at?

1.0 to 1.5 bar cold (heating off, first thing in the morning). Hot, with the heating running, 1.8-2.2 bar is normal. Below 0.8 bar cold the boiler will usually lock out.

Where is the filling loop on a Worcester Bosch combi?

On most Worcester Greenstar models it’s underneath the boiler — a short silver braided hose with a black lever valve at each end. On some compact models the loop is internal, behind the front panel. Check your manual for the exact location for your model number.

Can I damage the boiler by overfilling?

Slightly overfilling (1.8-2.5 bar cold) is harmless — the boiler will run a little stressed but the pressure-relief valve will release any dangerous spike. Severely overfilling (above 3.5 bar) can stress joints and the heat exchanger. Always close both valves firmly and bleed a radiator if you overshoot.

My pressure gauge is in the red — what now?

If it’s in the upper red zone (above 2.5 bar cold), bleed a radiator to drop it back to 1.2-1.5 bar. If it’s in the lower red zone (below 1 bar), top up via the filling loop. If it keeps drifting back into the red after you correct it, that’s a fault — call a plumber.

How often should I have to top up?

Once or twice a year is normal. Pressure naturally drifts down a touch over time, especially after a radiator is bled. More than once a month means there’s an active leak or a failing component — see our boiler losing pressure guide.

Need a plumber to take a look?

If something’s not right and you’d rather a Gas Safe engineer handle it, call 0116 488 6858. We cover Leicester and the surrounding LE postcodes — Hinckley, Loughborough, Oadby, Wigston and 33 surrounding towns. Free quote, no upfront deposit, no pressure.