Inspired by a helpful YouTube guide. This walk-through is based on the popular "How To Top Up Your Boiler Pressure - Repressurise" video from the Heatable channel. Heatable is a UK boiler installation company and their guide covers the filling loop process clearly, including the important bit about not going over 1.5 bar.
1. Check the pressure gauge
Most combi boilers have a pressure gauge on the front panel, either a dial type or a digital display. The normal operating pressure when the system is cold should be between 1 and 1.5 bar. Below 0.8 bar or so, the boiler will typically lock out and refuse to fire.
Check the gauge first to confirm low pressure is actually the problem. If the gauge reads 1.5 bar or above, the issue is something else and re-pressurising will not fix it.
2. Let the system cool down
If the heating has been on recently, let the system cool before you add water. Adding cold water to a hot system gives you a misleading pressure reading, and the pressure will rise again as the system reheats.
Twenty to thirty minutes is usually enough. The boiler can still be warm to the touch — just not hot.
3. Locate the filling loop
The filling loop is usually a short flexible silver braided hose with a small valve or key at one or both ends. On most combi boilers it is underneath the boiler itself, connecting to the cold water inlet.
Some boilers have a built-in filling loop behind the front panel. Others use an external loop that hangs loose or clips into a bracket. Check your boiler manual if you cannot find it — the diagram inside the front panel often shows exactly where it is.
4. Open the filling loop valves slowly
Open the valve or valves on the filling loop slowly. You will hear water entering the system. Watch the pressure gauge carefully as you do this.
The pressure will rise as water enters. Stop when the gauge reads between 1 and 1.2 bar. Do not rush it — the gauge can lag slightly behind the actual pressure, so open the valves in short bursts and wait a moment to let it settle.
5. Close the valves and check for leaks
Once you have reached 1 to 1.2 bar, close the filling loop valves firmly. Check that no water is dripping from the hose connections or from around the valves before you put the boiler panel back on.
Mind you, a filling loop that drips from the connection points when you remove it needs to be inspected properly — even a slow drip over time can cause damage behind the boiler casing.
6. Reset and restart the boiler
Most boilers will reset automatically once the pressure is restored, but if the boiler is still showing a fault code, press and hold the reset button for a few seconds. It should fire up and run through its ignition sequence.
Run the heating for thirty minutes and then check the pressure gauge again. It should hold steady between 1 and 1.5 bar once the system is up to temperature.
7. If the pressure keeps dropping
A boiler that loses pressure regularly is losing water somewhere — either through a leak in the pipework or radiators, a faulty pressure relief valve, or a weeping filling loop valve. Re-pressurising fixes the symptom, not the cause. If you are topping up more than once or twice a year, it is worth getting a heating engineer to find out why.
When to call a handyman
Call Richard for general property checks and minor maintenance around the home. For boiler faults, gas work, and recurring pressure loss, a Gas Safe registered engineer is the right call.
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