Boiler filling loop: how to top up pressure safely

Low boiler pressure can sometimes be corrected by carefully using the filling loop, but topping up is not always the right solution and should be approached with caution. Understanding what the filling loop looks like, whether it’s an internal or external valve, helps you identify the correct part to use without confusion. Knowing exactly how far to raise the pressure—typically between 1 and 1.5 bar, but always check your boiler manual—ensures you don’t overfill the system.

Recognising when to stop filling, such as when the pressure gauge reaches the recommended range, is crucial to avoid leaks, damage to boiler components, or triggering the pressure relief valve. Taking these steps seriously reduces the risk of costly repairs and keeps your heating system running safely and efficiently.

What a boiler filling loop looks like

A filling loop is a short section of flexible braided hose that connects your central heating system to the cold mains water supply. It is usually located underneath the boiler or nearby pipework, although its exact position varies between models.

Some boilers have an external filling loop with one or two small valves at either end of the braided hose. Other modern boilers have an internal filling link, key or integrated filling device, which may be operated using a removable key or a small control beneath the boiler. Always check your boiler manual before touching anything, as designs differ.

Before starting, make sure you are dealing with the heating system pressure gauge, not a gas control or electrical component. If the filling arrangement is unclear, ask a Gas Safe registered engineer to identify it for you.

Typical pressure ranges

Many sealed heating systems sit at around 1 to 1.5 bar when cold, often shown within a green section on the gauge. The correct range can vary, so use the figure in your manufacturer’s instructions as the main guide rather than relying on a generic number.

• Check the pressure with the heating turned off and the system cool.

• Use the boiler manual to confirm the recommended cold pressure.

• Stop before the gauge reaches the upper limit or red zone.

• Never force a valve that feels stuck or difficult to turn.

How to top up boiler pressure safely

Turn off the heating and allow the boiler and radiators to cool. This gives you a more reliable pressure reading and reduces the chance of burns from hot components or water.

Locate the filling loop or internal filling device. If you have an external braided hose, check that it is properly connected at both ends and that there are no visible signs of damage. Place a cloth underneath in case a small amount of water escapes.

Open the valve or valves slowly. You may hear mains water entering the system, while the pressure gauge begins to rise. Keep watching the gauge continuously and close the valve as soon as the pressure reaches the manufacturer’s recommended cold level.

With some filling loops, both valves need to be opened to allow water through. Close them securely afterwards, but do not overtighten them. If the hose is designed to be removed, only remove it when the manual instructs you to do so, and make sure any caps are replaced.

Check around the filling loop and underneath the boiler for drips. Turn the heating back on and monitor the gauge as the system warms up. A small increase is normal, but a significant rise needs professional attention.

When not to top up the system

Topping up is intended as an occasional adjustment, not a way to manage an ongoing fault. Stop immediately and arrange a repair if the pressure shoots up as soon as the heating starts, reaches the red zone or falls again shortly after topping up.

• The pressure rises sharply or approaches the maximum marked on the gauge.

• The pressure relief valve discharge pipe outside is dripping or releasing water.

• You can see water around the boiler, radiators, valves or pipework.

• You need to refill the system repeatedly.

Do not remove the boiler casing, interfere with gas components or keep resetting the boiler to hide a pressure problem. A discharge pipe dripping outside can indicate that the pressure relief valve has opened, while repeated pressure changes may point to a leak, expansion vessel issue or another internal fault.

Repeated top-ups can damage the system

Every time you add fresh mains water, you also introduce dissolved oxygen and minerals into the heating system. Over time, this can encourage corrosion, sludge and scale, reducing efficiency and potentially affecting pumps, valves and the heat exchanger.

Central heating inhibitor helps protect system water, but repeated draining or topping up can dilute it. If your boiler pressure keeps dropping, an engineer should investigate the cause and test the system water where appropriate, rather than simply adding more water.

For background on the common causes behind falling pressure, read our guide to why boiler pressure drops. If the boiler is due a routine check, arranging a boiler service can also help identify developing faults and assess system condition.