A radiator that won't heat up is one of the most common heating complaints - and one of the most satisfying to fix, because the cause is usually simple. The trick is to read where the cold is. A radiator that's cold at the top has a completely different problem from one that's cold at the bottom, and knowing which you've got tells you whether it's a five-minute job or a call to an engineer.
Cold at the top = trapped air, bleed it. Cold at the bottom = sludge, needs a flush. Cold all over while others are warm = a stuck valve.
Read the Cold Patch First
Before you touch anything, turn the heating on, wait ten minutes, then feel the radiator carefully with the back of your hand. Where the heat stops is the single most useful clue you have.
- Cold at the top, warm at the bottom
- Air is trapped inside, sitting above the water and blocking the flow. Bleeding the radiator almost always fixes it.
- Warm at the top, cold at the bottom
- Sludge - rust and debris - has settled along the base. No amount of bleeding will shift it; the system needs flushing.
- Cold across the whole radiator
- Either both valves are shut, or one is stuck. Other radiators heating fine points to this one's valve, not the boiler.
Why Radiators Stop Heating Evenly
Central heating is a sealed loop of water pushed round by a pump. Over time, two things creep in: air, which gets drawn in through tiny gaps or released as gas when the system is topped up, and sludge, a black iron-oxide mud that forms as the inside of steel radiators slowly corrodes. Air rises to the highest points and sits at the top of radiators; sludge is heavy and sinks to the bottom. That's why the location of the cold patch is such a reliable guide to the cause.
The Five-Minute Fix: Bleed It
If the cold is at the top, bleeding releases the trapped air so hot water can fill the whole panel again. You'll need a radiator bleed key and a cloth.
Let the system cool so you don't get scalded and so any air settles at the top.
Fit the key to the small square valve at the top corner and turn it a quarter-turn anticlockwise. Hold the cloth underneath.
You'll hear air hissing out. The moment a steady trickle of water appears, the air is gone - close the valve.
Bleeding lets water out, so the boiler gauge may drop. Top it back up to 1-1.5 bar using the filling loop, then restart the heating.
Want the full walk-through with photos of what to look for? Read our step-by-step guide on how to bleed a radiator.
Cold at the Bottom Means Sludge
If the radiator is warm at the top but cold along the bottom, bleeding won't help - the water simply can't flow through the silt. This is a job for a power-flush or a chemical clean, where an engineer pushes cleaner through the system at pressure to break the sludge up and pump it out.
- Several radiators cold at the bottom - the whole system is silting up
- Murky black water when you bleed - a classic sludge sign
- A magnetic filter fitted to the boiler will catch debris and slow it recurring
- Left long enough, sludge wrecks pumps and blocks the boiler's heat exchanger
One Room Always Colder? Balance the System
If a radiator works but never gets as hot as the others - typically the one furthest from the boiler - the system is unbalanced. Water takes the path of least resistance, so the nearest radiators hog the heat. Balancing means slightly closing the lockshield valves on the greedy radiators so more flow reaches the far ones. It's fiddly but free, and a heating engineer can do it in under an hour.
- Radiators nearest the boiler scalding, far ones lukewarm
- Upstairs hot, downstairs cold (or vice versa)
- The far radiator bleeds clear and isn't cold at the bottom - so it's flow, not air or sludge
When It's the Valve, Not the Radiator
A radiator that's stone cold all over, while the rest of the house is toasty, usually has a valve problem. Check the obvious first: thermostatic valves can be knocked shut, and the pin inside can seize after a summer of sitting still.
- Make sure the thermostatic valve is turned up, not at the frost setting
- Check the lockshield valve at the other end is open
- A seized TRV pin can sometimes be freed by gently tapping the valve and working the pin up and down
- If both valves are open and it's still cold, the valve likely needs replacing
When to Call a Heating Engineer
- Bleeding and checking the valves hasn't worked
- Cold at the bottom across several radiators - book a flush
- No radiators warming at all - that's the boiler, not the radiators
- You spot a leak from a valve or the radiator itself
If nothing here brings the heat back, the problem is likely the boiler or pump rather than the radiator - run the boiler not working checklist next, or see typical repair prices in our boiler repair cost guide. Stuck with no heat on a cold night? Here's how to get an emergency heating engineer on the way on the way within the hour.