How to Refill Central Heating System

Central heating systems are one of the most relied upon features in UK homes. They keep our living spaces warm, provide hot water, and ensure year-round comfort. Like any system, however, they require care and attention to remain efficient and safe. One of the most common maintenance tasks is refilling the central heating system. This may be necessary after draining the system for repairs, following a leak, or if the boiler pressure has dropped too low. Knowing how to refill your heating system correctly can save you from costly callouts, but it must always be done carefully to avoid damaging the boiler or introducing air locks into the pipework.

Why central heating systems lose pressure

Most sealed systems work at a pressure of between one and two bar, which is usually displayed on a gauge on the front of the boiler. Over time, it is normal for pressure to drop slightly, particularly if radiators have been bled or if minor leaks are present in the system. A significant or sudden loss of pressure, however, is a sign of a fault that may need professional investigation. In these situations, refilling the system will restore pressure, but it is important to consider whether the underlying issue also needs addressing.

Safety before starting

Before attempting to refill a central heating system, it is important to ensure the boiler is switched off and has cooled down. Working on a pressurised system while it is hot can be dangerous and may damage components. You should also check your boiler’s manual for manufacturer guidance, as different models may have specific instructions. If you are unsure at any stage, or if your system repeatedly loses pressure after refilling, it is best to contact a Gas Safe registered engineer for advice.

Refilling sealed systems using a filling loop

Most modern combi boilers and sealed central heating systems are topped up using a filling loop. This is a flexible or fixed silver hose connected to the pipework beneath the boiler. The filling loop has two small taps or levers that control water flow into the system. To refill, both ends of the loop must be securely connected. You then slowly open the taps to allow cold mains water to flow into the system, watching the pressure gauge carefully. Once the gauge reaches the recommended pressure, normally around 1.5 bar, the taps should be closed firmly to prevent overfilling. After this, the filling loop should be disconnected if it is removable. Leaving a removable loop attached is not permitted under UK regulations as it can create a risk of contamination between mains water and the heating system.

Topping up open vented systems

Older properties may still have open vented systems with a small feed and expansion tank, usually located in the loft. These tanks automatically supply water to the heating system, so manual refilling is rarely required unless the tank itself runs dry. If you notice that your radiators are not heating properly or that the system is empty after draining, you will need to ensure the tank has a working float valve and a supply of clean water. If the tank is dirty or corroded, it should be cleaned and maintained to prevent debris entering the pipework.

Bleeding radiators after refilling

When you refill a heating system, air can become trapped in the radiators and pipework. This air prevents hot water from circulating properly and can cause gurgling sounds or cold spots on radiators. To resolve this, radiators should be bled after refilling. Using a radiator key, turn the bleed valve slowly until air escapes, then close it as soon as water begins to flow. After bleeding radiators, the pressure in the system may drop slightly, so you may need to top up again through the filling loop until the correct level is restored.

Using inhibitors when refilling

Whenever you drain and refill a heating system, it is essential to add an inhibitor. This chemical protects against corrosion, limescale build-up, and sludge formation inside radiators and pipework. Without inhibitor, systems are at higher risk of blockages and leaks, which reduce efficiency and shorten the life of the boiler. Inhibitors are usually added through a radiator or dosing point before the system is topped up to full pressure.

Avoiding overfilling

One of the most common mistakes when refilling a central heating system is allowing the pressure to rise too high. Overfilling can cause stress on the system and trigger the boiler’s safety valves, leading to water discharge outside the property. If this happens, the system may need to be drained slightly to return pressure to a safe level. It is best to open the filling loop taps slowly and keep your eye on the gauge at all times.

When professional help is needed

Although refilling a central heating system is a straightforward task in many cases, recurring pressure loss is not something that should be ignored. Continuous drops in pressure often point to leaks in hidden pipework, a failing expansion vessel, or problems with the pressure relief valve. In such cases, adding water repeatedly is not a long-term solution and could cause internal corrosion. If you find that your system needs topping up more than a few times a year, a heating engineer should be called to investigate.

Long-term care for your heating system

Keeping your heating system in good condition involves more than just topping up the pressure when needed. Regular servicing, flushing out sludge when required, adding inhibitor, and checking radiators for cold spots all help extend the life of the system. An annual boiler service, carried out by a qualified engineer, will also ensure safety features are working correctly and highlight any issues before they develop into costly repairs.

Conclusion

Learning how to refill a central heating system is a useful skill for homeowners and landlords, as it allows you to maintain the correct boiler pressure and keep the system running smoothly. By following the correct procedure and understanding when professional assistance is needed, you can ensure your heating system remains safe, efficient, and reliable throughout the year. While topping up occasionally is normal, keeping on top of system maintenance and arranging regular checks is the best way to avoid problems in the future.