Radiator cold at the top or bottom? What it means
Where a radiator is cold tells you a lot. Cold at the top usually means trapped air, and you can bleed it yourself. Cold at the bottom usually means sludge, which needs a power flush. If every radiator is cold, the boiler, the pump or a valve may have failed.
Updated 02/11/2026 · Express Contractors
Some cold radiators are easy to sort out at home. Others point to a fault in the boiler or the heating system. If the checks below don't help, you'll need a boiler repair from a Gas Safe engineer. This guide is part of our common boiler problems series.
Where is the radiator cold?
Where a radiator is cold points to the cause. Cold at the top usually means trapped air, and cold at the bottom usually means sludge. One cold radiator often has a closed valve. If every radiator is cold, the boiler, the pump or a valve may have a fault.
To check, turn the heating on and let it run for 15 minutes. Then feel each radiator with a light touch. It can be very hot.
| Where it's cold | Likely cause | What to do |
|---|---|---|
| Cold at the top, warm at the bottom | Trapped air | Bleed the radiator, then check the pressure |
| Cold at the bottom, warm at the top | Sludge | The system needs a power flush or a clean |
| One radiator cold all over | A closed or stuck valve | Check the TRV and the lockshield valve |
| Cold upstairs, warm downstairs | Air, low pressure or a stuck zone valve | Bleed the radiators and top up the pressure only if the gauge is low. A stuck zone valve needs an engineer |
| All radiators cold | A fault with the boiler, pump or a valve | Check the thermostat, timer and pressure, then call an engineer |
| Near ones hot, far ones lukewarm | The system needs balancing | Adjust the lockshield valves |
Each of these is explained below.
Why is my radiator cold at the top?
A radiator that is cold at the top has air trapped inside it. Air rises above the hot water and stops it filling the radiator. You can usually fix this yourself by bleeding the radiator, which lets the air out.
Air gets into heating systems over time. Hot water can't fill the space it takes up, so the top stays cold.
How do I bleed a radiator?
To bleed a radiator, turn the heating off and let it cool. Then open the bleed valve at the top corner with a radiator key. Close the valve again as soon as water comes out. You can buy a radiator key at a hardware shop. Some newer radiators use a flat screwdriver instead.
- Turn the heating off and let the radiators cool. The water inside can scald you.
- Find the bleed valve. It's a small square or slotted screw at one top corner of the radiator.
- Hold an old cloth under it to catch any drips.
- Turn the key slowly anticlockwise, about a quarter turn. You'll hear air hissing out.
- Close it when water comes out in a steady trickle. Don't over-tighten it.
- Do the same on any other radiator that's cold at the top.
- Check the boiler pressure gauge afterwards.
Bleeding lets a little water out, so the boiler pressure drops. On a combi or system boiler, the gauge should read about 1 to 1.5 bar with the heating off. If it's lower, see our guide to a boiler losing pressure to top it up. Older systems with a small tank in the loft refill by themselves.
If the water from the bleed valve looks black, that's a sign of sludge as well. If you need to bleed the same radiator every few weeks, air may be getting in. Or rust may be making gas inside the system. Both need looking at.
Safety first
Smell gas? Call the National Gas Emergency line on 0800 111 999 straight away. Open doors and windows. Don't use light switches or anything that could spark. Turn the gas off at the meter if you can do it safely, then go outside.
Headaches, dizziness, sickness or a lazy yellow flame? These can be signs of carbon monoxide (CO), a poison gas you can't see or smell. Turn the boiler off, open windows, get everyone outside and call 0800 111 999. See a doctor if anyone feels unwell. Our carbon monoxide advice has more.
Never open the boiler case. Only a Gas Safe registered engineer should take the cover off a gas boiler. The checks in our guides are all ones you can do from the outside.
Why is my radiator cold at the bottom?
A radiator that is cold at the bottom usually has sludge in it. Rust from inside the system forms a heavy black sludge that sinks and blocks the hot water. Bleeding won't fix it. The system needs a power flush or a system clean.
Most radiators are made of steel. Over the years, the inside slowly rusts. The rust mixes with the water and turns into sludge. It's heavy, so it settles at the bottom of the radiator.
Other signs of sludge include:
- black or dirty water when you bleed a radiator
- radiators that take a long time to warm up
- a boiler that bangs or gurgles
- several radiators cold at the bottom, not only one
A power flush pumps water and a cleaner through the radiators and pipes at speed. It washes the sludge out. A gentler system clean uses a cleaner left in the water for a while before it's drained. We do power flushing. See our plumber page for more.
After a clean, an inhibitor is usually added to the water. This is a liquid that slows down rust and sludge coming back. A magnetic filter on the pipe near the boiler also helps. It catches bits of rust before they settle.
Why is only one radiator cold?
If only one radiator is cold, check its two valves first. A TRV turned down, a stuck TRV pin or a closed lockshield valve are the usual causes. If the valves are fine, the radiator may be full of air or sludge.
A radiator has a valve at each bottom corner. Check them in this order.
- Check the TRV dial. A TRV (thermostatic radiator valve) is the dial with numbers on it. Make sure it's turned up, not set to 0 or a frost symbol.
- Check the TRV pin. If the dial is up and the radiator stays cold, the pin under it may be stuck. Turn the dial to the highest number and unscrew the head. You'll see a small metal pin. Press it gently. It should spring back up. If it's stuck down, a few gentle taps may free it. Don't force it or use pliers hard, as the pin can break. Screw the head back on afterwards.
- Check the lockshield valve. This is the valve at the other end, usually under a plastic cap. It's set when the system is balanced, and then left alone. If someone has turned it fully clockwise, it's closed, and no water gets in. Pull off the cap and turn it anticlockwise a little at a time.
We fix stuck valves and fit new TRVs.
Why are my radiators cold upstairs but warm downstairs?
Cold radiators upstairs are often caused by trapped air, because air collects in the highest radiators.Bleed the upstairs radiators first, then check the boiler pressure. Low pressure can also stop water reaching the top floor.
Some homes have two heating zones, one for upstairs and one for downstairs. Each one has a zone valve. This is a small valve with a motor that opens to let hot water through. If one sticks shut, half the house stays cold. A stuck zone valve needs an engineer.
Why are all my radiators cold when the boiler is on?
If every radiator is cold, check the thermostat, the timer and the pressure gauge first. If those are fine, the pump, a zone valve or the boiler itself may have a fault. That needs a Gas Safe engineer to check.
Start with these simple checks.
- The thermostat: turn it up a few degrees above the room temperature. Change its batteries if it has them.
- The timer: check the heating is set to come on, and the clock is right.
- The pressure: check the gauge reads about 1 to 1.5 bar.
- A fault code: write down any code or flashing light, with the boiler model.
- A frozen pipe: on a freezing day, see our frozen condensate pipe guide.
If the hot taps work but the radiators stay cold, see hot water but no heating. If all of these look fine, the pump may have failed. The pump pushes hot water round your radiators. A zone valve stuck shut, or a fault in the boiler, can do the same.
What does balancing radiators mean?
Balancing radiators means setting the lockshield valve on each one, so every room gets its fair share of heat.It helps when radiators near the boiler get hot but the far ones stay lukewarm.
Hot water takes the easiest path. So the radiators closest to the boiler often get hot first and take most of the heat. To balance them, the closest radiators are turned down a little. The furthest ones are left more open. You can do it yourself, but it takes time and some trial and error. An engineer can do it for you.
When to call a Gas Safe engineer
Call a Gas Safe engineer if all your radiators stay cold, or the checks above haven't helped. Cold bottoms on several radiators, or bleeding every few weeks, point to a fault that needs a proper check.
- all your radiators stay cold with the boiler on
- the checks above haven't helped
- several radiators are cold at the bottom
- you have to bleed radiators every few weeks
- the boiler shows a fault code or keeps cutting out
The engineer first runs a thorough check of the boiler and the heating system. That finds the fault and the parts needed. We repair and replace heating pumps, zone valves and TRVs. See our boiler repair page for the faults we fix.
A yearly boiler service can spot worn parts early, before the heating stops on a cold night.
Other boiler guides
- Boiler losing pressure: what the gauge should read and how to top it up.
- Heating works but no hot water: checks to try, and the reverse problem.
- Frozen condensate pipe: how to thaw it safely.
- Leaking boiler: what to do first, and where the water is coming from.
- All common boiler problems.
Radiators still cold? Book a boiler repair and we'll find the cause.
Common questions
Why is my radiator cold at the top and hot at the bottom?
Air is trapped at the top of the radiator. Air rises, so it sits above the hot water and stops it filling the radiator. Bleeding the radiator lets the air out. Then check the boiler pressure, as it drops when air comes out. How to top up boiler pressure.
Why is my radiator cold at the bottom and hot at the top?
This usually means sludge. Rust from inside the radiators and pipes forms a black sludge that sinks to the bottom. Bleeding won't help. The system needs a power flush or a system clean to wash the sludge out. See our power flushing service.
Why is only one radiator cold?
Check its valves first. The dial on a thermostatic radiator valve may be turned down, or the pin under it may be stuck. The valve at the other end may have been closed. If the valves are fine, the radiator may have air or sludge in it.
Why are my radiators warm downstairs but cold upstairs?
Upstairs radiators sit higher, so air often collects in them. Low boiler pressure can also stop water reaching them. On some homes, a zone valve controls upstairs and downstairs separately, and it may be stuck. If bleeding and topping up don't help, call an engineer.
Why are all my radiators cold when the boiler is on?
Check the thermostat, the timer and the pressure gauge first. If they are fine, the pump may have failed, or a zone valve may be stuck shut. The boiler itself may also have a fault. These need an engineer to check. Book a boiler repair.
How often should I bleed my radiators?
Only when they need it, when a radiator is cold at the top. Once or twice a year is normal for some homes. If you have to bleed the same radiator every few weeks, air is getting in or rust is making gas inside. That needs looking at.
Radiators still cold?
Call us to book a boiler repair. A Gas Safe registered engineer will check the boiler and the heating system, find the fault and fix it.