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.
Low boiler pressure is one of the most common heating issues homeowners face, and it's usually nothing to panic about. Your central heating system naturally loses pressure over time due to tiny leaks in pipework, radiator valves, or connections — even a slow drip you can't see will eventually show up on the gauge. If you've noticed your radiators aren't heating as well as they should, or the pressure needle has crept below 1 bar, this guide will walk you through topping it back up using your boiler's filling loop. It's a straightforward job that takes just a few minutes and requires no special tools. However, if you notice the pressure dropping again within a week or two, there's likely a leak somewhere in the system, and you should contact a qualified plumber to track it down before it causes damage.
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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.
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Frequently asked questions
How often should I need to repressurise my boiler?
Under normal circumstances, you shouldn't need to repressurise more than once a year. If you're topping up every few weeks, it indicates a leak in your pipework, radiators, or connections that needs professional attention. A Gas Safe engineer can help locate and fix the source of pressure loss.
What happens if boiler pressure gets too high?
If pressure exceeds 2.5 bar when the system is cold, or 3 bar when hot, the safety relief valve will automatically discharge water to bring it back down — you might see water pooling under the boiler. This is a safety feature, but persistent over-pressurisation suggests the expansion vessel may have failed and needs replacing.
Can I repressurise a boiler while it's still warm?
No — always allow at least 30 minutes for the boiler to cool completely before attempting to repressurise. Cold water entering a hot system can cause thermal shock and damage internal components, and you won't get an accurate pressure reading whilst the water is hot.
Why won't my filling loop valves open?
If the valves are stiff or won't turn, apply a little penetrating oil and wait 10 minutes before trying again. Never force them — if they're genuinely stuck, stop and call a plumber, as forcing them can break the valve handles or crack the connection points.
What's the difference between cold and hot pressure readings?
Cold pressure is what you read when the system has cooled (should be 1–1.5 bar). As the boiler heats the water, it expands and pressure rises naturally to around 1.5–2 bar — this is completely normal and nothing to worry about. The expansion vessel accommodates this safely.