Why is my radiator hot at one end and cold at the other?

Water enters at one end and leaves at the other, giving up heat as it goes. If enough water is flowing it does that quickly and the panel is even. If the flow is slow, the water gives up all its heat near the inlet and arrives at the far end already cool.

What you'll need

• A radiator bleed key
• A small spanner or an adjustable for the lockshield
• A cloth

Step by step

1. Feel the whole panel and be precise.A gradient from hot to warm along its length is normal. Hot for a quarter and stone cold for the rest is not.
2. Check which end the valves are on.On a standard radiator, water enters at one bottom corner and leaves at the other. Cold at the far end from the inlet means it is not reaching there.
3. Confirm both valves are fully open.Open the TRV fully and check its pin is free. Take the lockshield cap off and open it a couple of turns, counting them.
4. Compare with the other radiators.If the ones nearest the boiler are scorching and this one is barely warm, the system is unbalanced and this radiator is losing out.
5. Balance rather than opening everything.Close the lockshields on the hottest radiators nearest the boiler by a turn or two. That pushes flow along the circuit to the ones at the end.
6. Work methodically if you balance properly.Open every lockshield fully, then close them all and reopen each by a set amount, working from the boiler outwards and allowing the system to settle between changes.
7. Check for a bottom-opposite-end arrangement.Some radiators are piped in at one bottom corner and out at the other. On very long radiators this arrangement genuinely struggles to heat the far end at low flow.
8. Consider sludge in a long radiator.A cold far end that is also cold along the bottom is sludge lying in the base, not a flow problem. That needs flushing.
9. Check the pipework has not been altered.A radiator that was fine and is now cold at one end often follows work elsewhere — a partly closed valve, a new radiator added to the circuit, or debris in the valve.
10. Accept a modest gradient.Every radiator is cooler at the outlet end. If the far end is warm rather than hot, it is working correctly.

Tips

• A radiator should show a temperature drop of roughly 10 to 20 degrees between inlet and outlet. Warm at the far end is right; cold is not.
• Count every turn you make on a lockshield. Balancing depends on relative settings, and untracked changes are hard to undo.
• Balance with the system fully up to temperature, not from cold.

Common mistakes

• Bleeding a radiator that is cold sideways — Air sits along the top, not at one end. Bleeding a sideways-cold radiator drops the system pressure and changes nothing.
• Opening every lockshield fully to be safe — With all lockshields open, water takes the shortest easiest path back to the boiler, so the near radiators run hot and the far ones stay cold. Balancing is deliberately restrictive.

If it doesn't work

Questions people ask