Repressurise a combi boiler

Top up your combi boiler's pressure using the filling loop when the display shows low pressure or a fault code.

RB-01Check the pressure gaugeLook at the boiler's pressure gauge or digital display. Most combi boilers should sit between 1 and 1.5 bar when cold. If it's below 1 bar, or a low-pressure fault light is showing, you need to repressurise.

RB-02Find the filling loopThe filling loop is a braided flexible hose with a valve at each end, usually underneath or beside the boiler. Some are permanently fitted, others are a detachable hose kept nearby. If you can't find one, check the boiler manual, as some models fill differently.

1 each · Filling loop hose (if not already fitted)

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RB-03Prepare the areaTurn the boiler off using its power switch or the on/off button. Lay an old towel underneath the filling loop, as a few drips are normal. Make sure both valves on the loop are fully closed before you start.

1 each · Old towel or cloth

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RB-04Connect the loop if detachableIf your filling loop is a separate hose, screw one end onto the valve near the boiler and the other end onto the valve on the cold mains pipe. Push firmly and hand-tighten only, then check both connections are snug with no gaps.

RB-05Open the valves slowlyOpen one valve fully, then slowly open the second valve. You should hear water flowing into the system. Watch the pressure gauge closely as it rises, this only takes a few seconds.

RB-06Close the valves at the right pressureAs soon as the gauge reaches 1 to 1.5 bar, close both valves fully, starting with the one furthest from the boiler. Don't overfill past 2 bar, as this can trigger a pressure relief valve to open and dump water. If you overshoot, some systems have a small release button to bleed a little pressure off.

RB-07Disconnect the loop and check for leaksIf the loop is detachable, unscrew both ends carefully and catch any drips with the towel. Wipe the connection points and check nothing is dripping five minutes later. Store the hose somewhere handy for next time, as combi boilers often need topping up every few months.

RB-08Turn the boiler back onSwitch the boiler back on and check the display clears any fault codes. Some models need you to press a reset button. Give it a minute or two, then check the pressure gauge has settled at the right level.

RB-09Bleed a radiator if neededIf pressure drops again quickly or a radiator has cold spots, there may be trapped air. Bleed the coldest radiator with a radiator key until water runs out steadily, then check the boiler pressure again and top up if needed.

1 each · Radiator bleed key

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RB-10Know when to call a Gas Safe engineerIf the pressure keeps dropping within days, or won't hold at all, there's likely a leak somewhere in the system or a fault inside the boiler itself. Don't open the boiler casing yourself. Any work inside the boiler must be done by a Gas Safe registered engineer.

RULES

KNOW THE RULES

Only the filling loop and external controls are safe for a homeowner to use. Opening the boiler casing or working on its internal gas or flue components is illegal for anyone who isn't Gas Safe registered.

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