How to Drain and Refill a Central Heating System

Updated July 2026 · Emergency Plumber Chelsea

You need to drain the system in a handful of situations, and it is worth being sure yours is one of them before you start. The common reasons are removing or replacing a radiator, fixing a leak on pipework that cannot be isolated, adding corrosion inhibitor after the old dose has been diluted by top-ups, or preparing for a power flush. If you only want to move or bleed a single radiator you often do not need a full drain-down, because most radiators can be isolated at their valves and drained on their own. A full drain empties the whole system, so keep it for jobs that genuinely require it.

Step one is to turn everything off and let it cool. Switch the boiler off at its controls and, for a combi, turn off the electrical supply to it as well. Set the programmer or thermostat so nothing can fire up mid-job. Hot water under pressure can scald, so give the system time to cool before you open anything, ideally an hour or more after the heating has been running. While it cools, gather your kit: a garden hose that fits the drain-off valve, a radiator bleed key, an adjustable spanner, old towels and a bowl for drips.

Step two is to stop water entering the system so it cannot refill as fast as you drain it. On a sealed or pressurised system, common in modern Chelsea flats and combi installs, close the filling loop and note the cold pressure on the gauge before you start. On an older open-vented system with a tank in the loft, you stop the feed by tying up the ball valve in the small feed-and-expansion tank or turning off its dedicated supply, so the tank cannot keep letting fresh water in as the level drops.

Step three is to attach the hose and open the drain-off. Find the lowest drain-off valve on the system, usually a small brass valve near the bottom of a downstairs radiator, on the lowest point of the pipework, or in a basement or lower-ground room in the taller Chelsea townhouses. Push a hose firmly onto its spigot, secure it with a jubilee clip if you can, and run the other end to an outside drain, gully or a suitably low point where the water can flow away safely. In a mansion flat with no easy external drain, running the hose into a bath or a large container you can empty is an alternative, but be ready for a lot of dirty, sometimes black, water.

Step four is to open the valve and let gravity do the work. Open the drain-off valve with your spanner and water will start to flow. To speed the draining and let air in behind the water, go to the highest radiators in the house and open their bleed screws with the key, then work downwards opening bleed screws floor by floor. The system empties from the top down as air replaces water. In a Chelsea property over three or four storeys this can take a while, so let it run until flow from the hose stops completely. Keep towels handy at each bleed point because a little water always weeps out.

With the system empty you can do the actual work, whether that is lifting off a radiator, repairing a joint, or simply preparing to add inhibitor and refill. If you are removing a radiator, close both its valves first, note the position of the lockshield valve so you can reset the balance later, and have towels and a bowl ready because a radiator holds water even after the system drains. This is also the natural point to consider a power flush if the water ran out very dark or full of sludge, which points to years of corrosion that a simple refill will not cure. A power flush uses a pump to circulate cleaning chemicals at high flow and clear the sludge that a gravity drain leaves behind, and it is a job for a professional with the right equipment rather than something to attempt during a DIY drain-down.

Step five is to close everything back up ready for refilling. Close all the radiator bleed screws you opened, then close the drain-off valve firmly and remove the hose. Double check that any radiator valves you touched are back to their original settings. It is easy to leave one bleed screw open and create a mess when you refill, so walk the whole property and confirm every point is shut before water goes back in.

Step six is to refill and re-pressurise. On a sealed system, open the filling loop slowly and watch the pressure gauge, bringing it up to the normal cold figure of around 1 to 1.5 bar, then close the loop. On an open-vented system, release the ball valve or turn the feed back on and let the loft tank refill the system naturally. Refill gently rather than in a rush, because filling too fast traps more air and makes bleeding harder. As the system fills you will hear it settling and may see the pressure move as air finds its way to the top.

Step seven, and the step most often skipped, is adding inhibitor. Central heating inhibitor is a chemical that protects the system from the inside by slowing the corrosion that produces sludge, cold spots and pinhole leaks, so it should go back in whenever you have drained and refilled. It is usually added through a radiator (via the bleed point on some designs) or dosed into the feed tank on an open system, and the correct amount depends on the number of radiators. As a rough guide one standard bottle treats a typical domestic system of up to around ten radiators, but always follow the product instructions for your own layout. If you are unsure how to dose it correctly, this is a good moment to have a professional finish the job, because running without inhibitor shortens the life of the whole system and undoes the point of draining it in the first place.

Step eight is to bleed all the air out. Refilling always introduces air, which collects at the top of radiators and leaves them cold at the top while warm at the bottom. With the system full, turn the heating on and let it warm up, then go round every radiator with the bleed key, opening each screw until the hissing air stops and water appears, then closing it. Start with the radiators on the lowest floor and work up, because air rises to the highest points. Once bled, check the pressure again on a sealed system and top up through the filling loop if it has dropped below the normal cold reading.

Finally, check your work. Look over every joint you disturbed, the drain-off valve and each bleed point for weeping over the next day or two, and feel each radiator once the system is hot to confirm it heats evenly across its whole surface. A radiator that stays cold at the top needs another bleed, while one cold at the bottom points to sludge rather than air. If pressure keeps dropping, a radiator will not heat through after repeated bleeding, or you find a leak you cannot stop, stop and call for help. Emergency Plumber Chelsea covers SW3, SW10 and nearby postcodes and can drain, refill, inhibit and, where needed, power flush a heating system correctly. Call 020 3002 6667.

Quick tips

• Only do a full drain-down when you must (radiator removal, an unisolatable leak, adding inhibitor or a power flush). A single radiator can usually be isolated at its valves instead.
• Photograph the lockshield valve position before removing a radiator so you can reset the balance when you refill.
• Refill slowly to avoid trapping air, and always add the correct dose of inhibitor for the number of radiators before you finish.
• Bleed radiators from the lowest floor upwards after refilling, then recheck sealed-system pressure and top up to around 1 to 1.5 bar cold.
• If the drained water runs black or full of sludge, a refill alone will not fix it. Consider a power flush to clear the corrosion.

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