Drain down a central heating system

Drain a central heating system safely to repair or replace a radiator, valve or pipework.

CH-01Turn off the boiler and let it coolSwitch the boiler off at its own controls, not just the room thermostat. Wait at least an hour for the radiators and pipework to cool down. Draining a hot system risks scalding and can crack a cold radiator if you flush cold mains water through it too fast.

CH-02Isolate the power and close the filling loopTurn off the electrical supply to the boiler at the fused spur or unplug it. If your boiler has a filling loop (most combi boilers do), make sure both valves on it are fully closed so mains water can't top the system back up while you're draining it.

CH-03Find the drain valveThis is usually a small valve near the boiler or at the lowest radiator on the ground floor, with a hose tail on it. It looks like a tap with a square or slotted spindle instead of a handle. If you can't find one, check the pipework under the lowest radiator on the system.

CH-04Connect a hose and run it outsidePush a garden hose onto the drain valve's hose tail and secure it with a jubilee clip so it can't blow off under pressure. Run the other end out through a door or window to an outside drain, gulley or soakaway — not into the garden if you can help it, as the water can be dirty and slightly corrosive from years of inhibitor and sludge.

1 each · Garden hose (10-15m)

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1 each · Jubilee clip

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CH-05Open the highest radiator bleed valves firstUse a radiator key to open the bleed valve on the highest radiators in the house a quarter turn. This lets air into the system as the water level drops, so it drains faster and more completely. Leave these open for now.

1 each · Radiator bleed key

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CH-06Open the drain valveTurn the drain valve spindle slowly with pliers or a spanner. Water and old sludge will start flowing out through the hose — it's often black or rusty-brown, which is normal. Check outside that the hose end is secure and actually draining where you expect.

1 each · Adjustable spanner or pliers

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CH-07Open the remaining bleed valves as the level dropsWork down through the house, opening each radiator's bleed valve in turn as the flow from the drain valve slows. This clears trapped water and air pockets from every radiator, not just the ones downstream of the drain point.

CH-08Confirm the system is emptyOnce no more water comes out and only air hisses from the bleed valves, the system is drained. If you're removing a specific radiator, double-check its own isolation valve and lockshield are both fully closed before disconnecting it, as there will still be a small amount of water trapped in it.

CH-09Close everything up and tidy awayClose the drain valve and all the bleed valves. Disconnect the hose, keeping the end up to avoid dripping sludge on the carpet, and mop up any spills straight away as the water can stain. Wrap a rag around the hose tail before you carry it through the house.

2 each · Old towels or rags

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1 each · Bucket

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CH-10Refill and re-pressurise before switching the boiler back onOnce repairs are done, close all drain and bleed valves, then refill via the filling loop or header tank to the correct pressure shown in the boiler manual, usually around 1-1.5 bar cold. Bleed each radiator again to clear air, add inhibitor if the system was fully drained, and only restore power to the boiler once it's fully refilled — running the pump dry can damage it.

1 bottle · Central heating inhibitor

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