How to repressurise a boiler safely, step by step

Your boiler has shown a low pressure warning, or the needle on the front gauge has dropped below the marked range, and you want to top it up yourself. On many sealed heating systems that is a reasonable household job, as long as you go slowly and follow your own boiler manual. This guide is the practical procedure. If you want to understand why pressure falls in the first place, our post on a boiler losing pressure covers the causes.

Before you touch anything: is this your kind of system?

Repressurising only applies to a sealed system, which is the type fitted with most combi boilers and many system boilers. You can usually tell because there is a pressure gauge on or under the boiler and no feed and expansion tank in the loft. If your home has an open vented system with tanks in the loft, there is nothing to top up in this way.

Next, look for the way your boiler is filled. There are two common arrangements:

• An external filling loop. A short braided or silver hose, often under the boiler, linking the cold mains pipe to the heating pipework, with a small valve or tap at one or both ends.
• A built-in filling key or lever. Some boilers have a filling device underneath, operated by a removable key or a lever, described in the user manual.

If you cannot find either, or your manual does not describe one, do not improvise. Do not try to borrow parts, fit a hose yourself or open the casing to look for one. Call an engineer instead.

The procedure

Keep the manual open beside you, because the exact layout varies by make and model. The general sequence is:

1. Switch the heating off and let the boiler cool. Pressure reads higher when the system is hot, so a cold reading is the honest one.
2. Find the gauge and the correct range. Most gauges have a shaded band or markers showing where the needle should sit when cold. Your manual will confirm the right range for your boiler. Use the markings on your own gauge rather than a figure from somewhere else.
3. Locate the filling loop or key. On an external loop, check both ends are connected firmly. On a built-in device, fit the key or find the lever as the manual shows.
4. Open the valve slowly while watching the gauge. Open a little at a time. The needle can climb quickly, so keep your eyes on it the whole time.
5. Close the valve fully once the needle reaches the marked range. Close both valves on an external loop. Make sure they are firmly shut, not just nearly shut, otherwise water can keep creeping in.
6. Check for drips. Look at the loop, its connections and the pipework below the boiler. A dry tissue held under each joint will show a slow weep you might miss by eye.
7. Restart or reset the boiler as the manual says. Some boilers clear the warning on their own; others need a reset. Our guide on resetting a boiler safely explains how to do that without repeated attempts.

Once the heating has run for a while, glance at the gauge again. It is normal for the needle to move up a little when the system is hot. Compare it with the hot and cold guidance in your manual.

If you overfill it

Overshooting the range is an easy mistake. Do not open any valve on the boiler itself to let water out, and never touch the pressure relief valve. Bleeding a radiator can lower the pressure slightly, but if the gauge is well above the marked range, or a pipe outside is dripping, read our post on boiler pressure that is too high and call us if you are unsure.

Stop and call an engineer if you see any of these

• Water leaking from the boiler, the filling loop or any pipe while you are filling. Our guide on a boiler leaking water helps you work out where it is coming from.
• Pressure falling again soon after you topped it up.
• Pressure rising too far, or continuing to rise after the valves are closed.
• A boiler type or filling arrangement you do not recognise, or no filling device described in the manual.
• Any smell of gas. Leave the property, do not use switches, and call the National Gas Emergency line on 0800 111 999 first. Our gas leak page explains what happens once the property is safe.

What repeated topping up is telling you

A sealed system should hold its pressure. Needing to refill it now and again over a long period can happen, but topping up every week or every few days means water is escaping somewhere or a component is not doing its job. Common places include a weeping radiator valve, a joint under the floor, the pressure relief valve or the expansion vessel inside the boiler.

Finding the cause matters, because every refill brings in fresh water and oxygen, which over time encourages corrosion and sludge inside radiators and pipes. A slow leak can also damage ceilings and floors before anyone notices. If you can see water at a radiator, our guide to a leaking radiator valve shows how to tell a gland nut from a pipe joint.

Where nothing is visible, our central heating leak detection service is the sensible next step, and faults inside the boiler are covered by boiler repair. For wider questions about how your heating is set up, a heating engineer visit lets us look at the whole system rather than one symptom.

Keeping a simple record

If the pressure keeps dropping, a short note on your phone helps a great deal. Write down the date, the reading on the gauge when cold, and when you topped it up. Add anything you noticed, such as a damp patch, a radiator that went cold or a dripping pipe outside. When an engineer arrives, that record narrows down where to look and saves time on site.

Need a hand?

If you would rather not top up the boiler yourself, or it will not hold its pressure, call our Sevenoaks team on 01732 444555 or send a request through our contact page. We are open Monday to Friday 7am to 10pm and Saturday to Sunday 9am to 8pm. Our minimum charge is £90 + VAT, including the first hour on site, and larger jobs get a fixed, no-obligation price.