Diagnose why a radiator stays cold, bleed trapped air, check valves and boiler pressure, then test it's fixed.
RAD-01Work out what's actually wrongFeel the radiator all over. Cold at the top, warm at the bottom usually means trapped air. Warm at the top, cold at the bottom usually means sludge inside. Stone cold all over often means a closed valve or low system pressure. Check whether other radiators in the house are affected too.
RAD-02Get ready and stay safeTurn the heating off at the thermostat or programmer and wait 15-20 minutes so the pipework and valves aren't scalding hot. Lay an old towel under both ends of the radiator and keep a small bowl handy, because any water that comes out will be hot and can mark flooring or scald skin. Find your stopcock, usually under the kitchen sink, and check it turns, so you can act fast if anything leaks.
1 each · Old towel or rag RAD-03Check the TRV isn't stuckThe TRV is the valve with numbers on it, usually at one end of the radiator. Turn it up and down through its full range a few times. If the pin inside feels stuck and doesn't click, grip it gently with pliers and pull it out, or fit a new TRV head, which pushes on without tools.
1 each · Replacement TRV head RAD-04Check the lockshield valve is openThe other end of the radiator has a plain valve under a plastic cap, with no numbers. Pop the cap off and check it's not fully closed. Use an adjustable spanner to turn it anti-clockwise a little if it's shut, but don't open it further than it was, as it's set to balance heat across the whole system.
1 each · Adjustable spanner RAD-05Bleed trapped air from the topWith the heating off, hold the bowl and towel under the bleed valve at the top corner of the radiator. Push the radiator key onto the square fitting and turn slowly anti-clockwise until you hear hissing, then a trickle of water. Close it firmly the moment water flows steadily. Repeat on every radiator that feels cold at the top while you're at it.
1 each · Radiator bleed key RAD-06Check the boiler pressureBleeding radiators lowers system pressure, and low pressure on its own stops radiators heating properly. Check the gauge on or near the boiler; it should read around 1-1.5 bar. If it's low, top it up slowly using the external filling loop, following your boiler's instructions exactly.
RULESKNOW THE RULES
Only a Gas Safe registered engineer can work on the gas parts of a boiler. Topping up pressure through the external filling loop is normally fine for homeowners to do themselves, but never open the boiler casing or touch its internal gas or electrical parts.
RAD-07Deal with sludge if the bottom stays coldIf a radiator is warm at the top and cold at the bottom, bleeding won't help. This is usually sludge or rust built up inside, and needs a power flush or a chemical flush plus inhibitor added to the system. Treat this as a job for a plumbing and heating engineer if it affects more than one radiator.
1 bottle · Central heating inhibitor RAD-08Test the fix and tidy upTurn the heating back on and give it 15-20 minutes to check the radiator heats evenly all over, top to bottom. Wipe up any spilled water straight away, as it can mark wood or laminate flooring. Tip any drained water down an outside drain rather than onto plants, since it can contain rust and old chemicals.
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