Refill and bleed a central heating system

Top up your heating system's pressure and clear trapped air from radiators safely, without needing a plumber.

RB-01Turn off the heating and let it coolSwitch off the heating at the programmer or thermostat and wait at least an hour for the system to cool. Bleeding or topping up a hot system risks scalding from pressurised hot water shooting out. While you wait, find your boiler's pressure gauge and note the reading — most domestic boilers run at 1.0 to 1.5 bar when cold, but check your boiler's manual for the exact figure.

RB-02Check each radiator for trapped airFeel each radiator with the heating still off. If a radiator is cold at the top but warm at the bottom, air is trapped at the top and it needs bleeding. Make a list of which radiators need doing before you start.

RB-03Bleed the radiators, one at a timeStart with the radiator furthest from the boiler, or work from the ground floor upwards if you're doing several. Hold a cloth under the bleed valve and turn it slowly with the bleed key until you hear hissing air, then close it firmly the moment water starts to dribble out steadily. Move to the next radiator on your list and repeat.

1 each · Radiator bleed key

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1 each · Old cloth or rag

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1 each · Small cup or container

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RB-04Check the boiler pressure gauge againBleeding air out of the system will drop the pressure. Look at the gauge — if it's now below 1.0 bar, or below the level your boiler's manual recommends when cold, you need to top it up using the filling loop.

RB-05Find your filling loopThis is usually a flexible braided silver hose fitted underneath or beside the boiler, with a lever or tap valve at each end. Some boilers have an internal filling key instead — check your manual if you can't see an external loop. Make sure both valves are fully closed before you touch anything else.

RB-06Top up the pressure through the filling loopOpen both valves slowly and watch the pressure gauge climb. Stop as soon as it reaches the correct level for your boiler when cold (usually 1.0 to 1.5 bar) — do not overfill. Close both valves firmly the moment you're done; leaving them open even slightly will let pressure keep rising and can trip the boiler.

RULES

1 each · Adjustable spanner

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KNOW THE RULES

Under the Water Supply (Water Fittings) Regulations 1999, a filling loop left permanently connected to the mains must have a proper backflow prevention device (a WRAS-approved double check valve) fitted. If your loop is a removable, temporary type, disconnect it after use rather than leaving it attached.

RB-07Disconnect a temporary filling loopIf your filling loop is the flexible braided type that isn't permanently plumbed in, unscrew it from both ends once the pressure is correct and both valves are closed. Have a cloth ready to catch any drips. Store the loop somewhere you'll find it next time.

1 each · Old cloth or rag

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RB-08Turn the heating back on and check for problemsSwitch the heating on and let it run for 15 to 20 minutes. Check each radiator you bled feels evenly warm all over, and check the filling loop connections and bleed valves for any drips. If the pressure gauge shows an error or the boiler has locked out, check your manual — most boilers need the reset button pressed after running low on pressure.

RB-09Tidy up and note the resultWipe up any water around the boiler or radiator valves and dry the floor to avoid slip risks. If the pressure keeps dropping over the next few days, that usually means a leak somewhere in the system rather than just trapped air — stop topping it up repeatedly and get it checked instead.

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