Fix a radiator that's cold at the bottom

Drain, flush and refill a radiator that stays cold at the bottom to clear a sludge blockage.

RAD-01Confirm the faultTurn the heating on and feel the radiator after 10 minutes. If the top is hot and the bottom is cold, this is sludge (rust and debris) sitting in the bottom of the radiator, not trapped air. If the top is cold and the bottom is hot, the fix is bleeding, not flushing.

RAD-02Gather tools and protect the floorLay old towels or a dust sheet under and around the radiator. Have a wide, shallow bucket or roasting tray ready to catch water, plus extra towels. Get an adjustable spanner, a radiator bleed key, and PTFE tape before you start.

2 each · Old towels or dust sheet

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1 each · Shallow bucket or roasting tray

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1 each · Radiator bleed key

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1 each · Adjustable spanner

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1 roll · PTFE tape

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RAD-03Turn off the heating and let the radiator coolSwitch the heating off at the thermostat or programmer. Wait at least an hour for the water inside the radiator to cool fully. Working on a hot radiator risks scalding when you open the valves.

RAD-04Isolate the radiator's valvesEvery radiator has two valves: a wheelhead valve on one end and a lockshield valve (with a plastic cap) on the other. Turn the wheelhead valve clockwise until it stops. Remove the lockshield cap, use the spanner or pliers to turn its spindle clockwise until it stops, and note how many turns it took so you can set it back the same way later.

RAD-05Drain the radiatorPlace the bucket under the lockshield valve end. Loosen the nut connecting the radiator to that valve with the spanner, keeping a towel wrapped around the joint. Let the water drain fully into the bucket, topping it up as needed, then retighten the nut once empty.

RAD-06Remove the radiator from its bracketsWith both valve connections undone, lift the radiator off its wall brackets and carry it outside or to a bath — it will still hold some dirty water, so keep it level. Two people make this much easier on a double radiator, as they're heavy and awkward alone.

RAD-07Flush the sludge outStand the radiator on its end outside or in a bath. Push a garden hose or tap hose into one connection and run water through until it comes out the other end completely clear, tipping and shaking the radiator to dislodge trapped debris. For heavy sludge, add a chemical radiator cleaner, leave it to work for the time stated on the bottle, then flush again with clean water.

1 each · Garden hose or tap hose adaptor

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1 bottle · Chemical radiator cleaner (e.g. Sentinel X400)

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RAD-08Refit the radiator and reconnect the valvesWrap new PTFE tape around both connection threads. Lift the radiator back onto its brackets and hand-tighten the nuts onto the wheelhead and lockshield valves, then nip them up with the spanner — don't overtighten, as this can crack the olive or valve tail. Turn the lockshield valve back to its original position, counting the same number of turns as before.

RAD-09Refill the system and bleed the radiatorOpen the wheelhead valve fully. If you have a sealed system, top up pressure at the filling loop to the level shown in your boiler manual (usually around 1 to 1.5 bar). Bleed the radiator with the bleed key, holding a cloth ready, and close the valve the moment water (not air) comes out steadily.

RAD-10Add inhibitor and check for leaksAdd a fresh dose of central heating inhibitor through the filling loop or a bled radiator, following the bottle's instructions for your system size — this protects against future sludge build-up. Turn the heating on and check every joint you touched for drips or damp patches.

1 bottle · Central heating inhibitor (e.g. Sentinel X100)

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RAD-11Tidy up and confirm the fixWipe down the radiator and floor, and wring out or wash any towels used. After 30 minutes of running heat, recheck the radiator top to bottom by hand to confirm it's evenly hot all the way across.

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