Radiator Cold at the Top or Bottom? What It Means and How to Fix It
A radiator that’s cold at the top and warm at the bottom almost always has trapped air. You can usually sort it yourself by bleeding the radiator.
A radiator that’s warm at the top and cold at the bottom usually has sludge, a build-up of rust and debris settling inside the panel. Bleeding won’t fix that. It needs the system cleaning.
If some radiators are hot and others stay cold all over, the problem is usually balancing, a stuck valve or the pump, rather than the radiator itself.
Quick guide: what the cold spot tells you
| Symptom | Most likely cause | Can you fix it yourself? |
|---|---|---|
| Cold at the top, warm at the bottom | Trapped air | Usually. Bleed the radiator |
| Warm at the top, cold at the bottom | Sludge in the radiator | No. It needs a flush or clean |
| Cold in the middle or in patches | Sludge, often in older radiators | No |
| Whole radiator cold, others fine | Stuck valve or TRV pin, or the radiator needs balancing | Partly. Check the valves |
| Upstairs cold, downstairs fine (or the other way round) | Balancing, the pump, or air in the system | Sometimes |
| All radiators cold, hot water fine | Boiler, pump, programmer, thermostat or diverter valve | Check the controls, then call an engineer |
Cold at the top: bleeding the radiator
Air gets into heating systems over time and collects at the highest point of each radiator. That stops hot water filling the panel.
To bleed a radiator:
- Turn the heating off and let the radiators cool. Hot water can spit out.
- Put an old cloth under the bleed valve. That’s the small square valve at one top corner.
- Using a radiator key, turn the valve anticlockwise about a quarter turn. You’ll hear hissing as the air escapes.
- Close the valve as soon as water starts to come out steadily.
- Check the boiler pressure. On a combi or system boiler, bleeding lets water out of the system, so you may need to top the pressure back up through the filling loop.
If one radiator needs bleeding every few weeks, air is getting in from somewhere. It could be a small leak, or corrosion producing gas inside the system. Get an engineer to look at it.
If the pressure has dropped after bleeding, here’s how to go about topping your boiler pressure back up and what to check if it keeps falling.
Cold at the bottom: sludge
Steel radiators and pipework slowly corrode from the inside. The rust turns into a dark sludge (magnetite) that settles at the bottom of radiators, where the water flow is weakest. Signs include:
- cold patches along the bottom or in the middle of the radiator
- dark, dirty water when you bleed it
- radiators that take longer and longer to warm up
- a noisy boiler or pump
- repeat breakdowns on the boiler, because sludge can also clog heat exchangers and pumps
Depending on how bad it is, the fix will be one of these:
- A chemical flush, where a cleaning chemical is circulated through the system, drained and refilled with inhibitor. This is often enough for mild sludge.
- Taking off and flushing individual radiators that are badly blocked.
- A powerflush, where a machine pumps water at high flow around the whole system. It’s usually kept for heavily sludged systems.
After cleaning, the system should be refilled with corrosion inhibitor, and ideally fitted with a magnetic filter to catch new debris.
Some radiators hot, some cold
If whole radiators are staying cold while others get hot, work through these checks:
- Is the thermostatic radiator valve (TRV) turned up? TRV pins sometimes stick down, especially after summer. With the TRV head removed, the small pin underneath should move up and down freely. If it’s stuck, gently freeing it sometimes works. If you’re not comfortable doing this, leave it for an engineer.
- Is the lockshield valve open? That’s the valve at the other end of the radiator, usually under a plastic cap. It may have been closed during decorating.
- Does the system need balancing? Radiators nearest the boiler often take most of the flow. Balancing means adjusting the lockshield valves so heat is shared evenly. It’s fiddly but not dangerous, and an engineer can do it quickly with the right thermometers.
If all the radiators are cold but you still have hot water, the problem is more likely with the boiler, the pump or a diverter valve (on a combi). Check the programmer and room thermostat settings first, then call an engineer.
When to call an engineer
- Bleeding doesn’t fix a cold top, or the radiator needs bleeding again and again.
- Radiators are cold at the bottom or in patches.
- Water from the bleed valve is black or sludgy.
- You keep losing boiler pressure.
- Several radiators, or whole floors, stay cold after you’ve checked the valves.
- The boiler is noisy, or keeps locking out.
Any work on the boiler itself needs a Gas Safe registered engineer. Radiator valves, balancing and system cleaning are heating work, and a qualified heating engineer should do them.
FAQs
How often should I bleed my radiators?
Only when they need it. That usually means a cold top, or gurgling noises. Many people check once at the start of the heating season. If you need to do it more often than that, it’s worth finding out why.
Is a powerflush always needed for sludge?
No. A chemical flush, or cleaning individual radiators, is often enough for mild sludge. A powerflush is usually kept for heavily contaminated systems or before fitting a new boiler into a dirty system. Ask the engineer to explain why they’re recommending it.
Why is my new radiator cold at the bottom?
With a new radiator, it’s more likely to be a valve setting or a balancing problem than sludge. Check that both valves are fully open.