How to repressurise a boiler safely (filling loop guide)

If your combi boiler has suddenly lost pressure and the heating has stopped, it can be worrying. The good news is that in many cases you can safely repressurise the system yourself using the filling loop, as long as you follow your manual and take your time.

Understanding boiler pressure and safe ranges

Combi boilers are sealed systems, which means they rely on the correct water pressure to move hot water around your radiators. Too little pressure and the boiler may lock out; too much and safety devices will activate to prevent damage.

Every manufacturer gives a recommended pressure range in the user manual. As a general guide, many modern combi boilers sit around 1.0 to 1.5 bar when cold, and may rise to around 2.0 bar when hot. The pressure gauge is usually on the front of the boiler, sometimes just underneath.

If you are not sure of the correct pressure for your model, always follow the boiler manual first. If you do not have the manual, you can usually download it from the manufacturer’s website using the boiler model number.

Common reasons boiler pressure drops

A small drop in pressure from time to time is normal, especially on older systems. The problem is when the pressure keeps falling and you find yourself topping up every week or two.

Typical causes of low boiler pressure include:

• Bleeding radiators and letting out air, which also releases a little water from the system

• Minor system leaks from radiator valves, pipe joints or towel rails

• Faulty expansion vessel causing pressure swings between hot and cold

• Leaking pressure relief valve (PRV), sometimes after a previous overpressure event

If you ever see obvious water leaks, signs of corrosion, or damp patches around pipes, valves or under the boiler, stop and contact a Gas Safe engineer rather than continuing to repressurise.

Safety first before you use the filling loop

Before touching any valves, make sure the boiler is cool enough to work on safely. If the heating has been on, switch the boiler off and let it cool for at least 30 minutes so you are not dealing with very hot water.

Important safety note: This is a general guide only. Always follow the exact instructions in your boiler’s user manual. If anything looks different to the diagrams in the manual, or you feel unsure at any stage, stop and contact a Gas Safe registered engineer.

Step-by-step: repressurising with an external filling loop

An external filling loop is a short, flexible silver hose, usually under the boiler, with a valve at each end. Sometimes the hose is kept separately and only attached when needed.

Follow this generic process, but only after checking your own boiler instructions:

1. Locate and check the filling loop
Find the silver braided hose and its two isolation valves, usually shaped like small levers or flat-head screw heads. Make sure both valves are currently in the closed position, usually at 90 degrees to the pipe.

2. Attach the hose if it is not already fitted
If the hose is separate, connect it firmly to both valve connections and tighten the fittings by hand. Do not overtighten with tools, as this can damage the threads or washers.

3. Note the current pressure
Look at the boiler’s pressure gauge and note the reading. Many boilers will not operate below about 0.5 bar, and some may show a flashing low-pressure warning.

4. Open the valves slowly
Turn the first valve gently to the open position, usually inline with the pipe. Then slowly open the second valve while watching the pressure gauge. You should hear water flowing in.

5. Stop at the recommended pressure
Allow the pressure to rise gradually until it reaches the level specified in your manual, often around 1.2 to 1.5 bar when cold. Close both valves firmly but gently as soon as you reach this point.

6. Remove the hose if required
Some manufacturers require the filling loop hose to be removed after use. If so, place a towel underneath, undo the connections, and store the hose safely.

Step-by-step: repressurising with an internal filling loop

Many modern combi boilers now have an internal filling loop built into the casing. Instead of a flexible hose, you may see blue or black plastic taps, or a single key-operated valve.

Always check the manual for your specific boiler, as internal filling loop designs vary a lot. In general, the process is similar:

1. Identify the filling controls
Remove the lower cover if needed and look for clearly labelled filling valves or a filling key slot. The manual should show exactly where these are.

2. Ensure all valves are closed
Before starting, double-check that the filling valves or key-operated levers are in the closed position. This prevents accidental overfilling when you turn the boiler back on.

3. Start filling slowly
Turn the appropriate tap or lever to begin filling, while watching the pressure gauge on the boiler display. Never open the valve fully in one go; small movements give you better control.

4. Close at the target pressure
Once the pressure reaches the recommended cold setting, return the taps or keys fully to the closed position. Make sure they are firmly shut to avoid any slow, unnoticed filling.

Checks to make after repressurising

Once you have finished topping up, switch the boiler back on and set the heating to run. Let the system warm up and keep an eye on the pressure gauge as the radiators heat.

The pressure will usually rise a little as the water heats, but it should stay within the green or normal range on the gauge. Check again after the system has cooled to confirm it has settled back close to your target cold pressure.

Troubleshooting common filling loop problems

If the pressure will not rise

If you open the filling loop valves and the pressure gauge does not move, or you cannot hear water flowing, there may be an issue with the valves or your mains water supply. First, make sure both valves are actually open and not stuck.

If the valves are open and nothing happens, do not force anything. Close the valves and call a Gas Safe engineer to investigate, as internal components or the filling loop itself may be faulty.

If the pressure rises too quickly

If the gauge shoots up rapidly as soon as you open the valves, close them immediately. This usually means the valves are opened too far or the mains pressure is very strong.

Allow the system to settle, then try again, opening the valves only a tiny amount. Never let the boiler pressure climb into the red zone, and if you accidentally exceed the recommended pressure, contact an engineer for advice.

If the pressure keeps dropping after topping up

Needing to repressurise your boiler more than a few times a year usually signals an underlying fault. Common culprits include hidden leaks in pipework, a failed expansion vessel, or a pressure relief valve that is no longer sealing properly.

The pressure relief valve (PRV) discharges water outside through a copper pipe, often pointing towards a wall. If you see water dripping or marks below this pipe, the PRV may be releasing pressure and should be checked by a Gas Safe engineer.

Risks of repeatedly topping up boiler pressure

It is tempting to keep using the filling loop whenever the pressure drops, but frequent topping up is not a long-term solution. Each time you add fresh water, you introduce more oxygen and minerals into the system.

Over time this can lead to increased corrosion, sludge build-up, and limescale, which may damage key components such as the heat exchanger, pump and radiators. A persistent pressure loss is a sign that the system needs proper diagnosis and repair, not just more water.

When to call a professional for boiler pressure issues

If you are at all unsure about using the filling loop, cannot find the correct valves, or the pressure behaves unpredictably, it is safer to get professional help. A qualified engineer can check for leaks, test the expansion vessel and PRV, and ensure your boiler is running safely.

• You need to top up more than a few times a year

• The pressure keeps dropping back within hours or days

• Water is dripping from the PRV pipe or from any part of the system

• The boiler shows error codes related to pressure or lockouts

If your boiler pressure will not stay where it should, arrange a thorough boiler repair to find and fix the root cause rather than relying on the filling loop. It is also wise to keep up with regular boiler servicing to catch small pressure issues before they turn into major faults.

For friendly, expert help with persistent boiler pressure problems, contact JLN Plumbing, Heating, Electrical & Renewables on 02476677667. A Gas Safe registered engineer can safely diagnose the issue and get your heating system running reliably again.