How to Use a Boiler Filling Loop
Do not overpressurise the system. Stop adding water when the gauge reaches 1.5 bar. If you need to repressurise more than once a month, you have a leak or a failed expansion vessel — call a plumber.
The filling loop is the valve or flexible hose used to add water to a sealed central heating system when the pressure drops. Almost every combi and system boiler in the UK has one. Repressurising your boiler is one of the most important DIY skills for any homeowner — and it is completely safe if done carefully.
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Locate the filling loop — it is usually a short flexible silver braided hose or a built-in valve underneath or connected to the boiler. It has two valves, one at each end.
Check the boiler pressure gauge. If it reads below 1 bar when the heating is cold (and the system has cooled down), the system needs repressurising. Do not repressurise a hot system — always wait for it to cool.
Slowly open both valves on the filling loop (turn the handles a quarter turn, in line with the pipe direction). You will hear water flowing into the system.
Watch the pressure gauge closely — it will rise as water enters. Stop when the gauge reaches 1.5 bar. Close both valves firmly.
On some Vaillant, Worcester, and Ideal boilers, the filling loop is internal — there is a small lever or knob on the front of the boiler rather than a separate hose. The same principle applies — open it until pressure reaches 1.5 bar, then close it.
Press the boiler reset button if it went to lockout due to low pressure. The boiler should fire and run normally.
Check the pressure again after the system has heated up and cooled down — it should remain between 1 and 2 bar. If it drops again within a few days, there is a leak or a failed expansion vessel — call a plumber.
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