It’s quite common to notice that the top of a radiator feels cold while the bottom is warm. In most cases, this isn’t a serious fault. It’s usually caused by trapped air inside the radiator, which prevents hot water from filling the entire panel.
While it can reduce heating efficiency, it’s often easy to fix.
The Most Common Reasons This Happens
1. Trapped Air Inside the Radiator
The most common reason a radiator is cold at the top is air buildup. Air rises above the hot water inside the radiator, creating a cold section at the highest point.
If your radiator also makes light ticking or bubbling sounds, air may be the cause. (See Why Is My Radiator Ticking?)
2. Recently Refilling the Heating System
After topping up boiler pressure or carrying out maintenance, small air pockets can form in the system. These pockets often collect at the top of radiators.
If you’ve recently adjusted pressure, you may want to check Why Is My Boiler Pressure Low?
3. Infrequent Heating Use
If a radiator hasn’t been used for a while, small amounts of air can accumulate naturally. This is more common in spare rooms or areas where the heating is rarely turned on.
4. Circulation Imbalance
In some systems, poor circulation can allow air to gather at high points. This may happen in older systems or if radiators aren’t balanced properly.
If the entire radiator feels cool rather than just the top, see Why Is My Radiator Not Getting Hot?
Is This Normal?
Yes — a cold top and warm bottom is one of the most common radiator issues.
It’s normal if:
• The rest of the radiator is warm
• The heating system works normally otherwise
• The issue developed gradually
It may not be normal if:
• The radiator is completely cold
• Multiple radiators are affected
• Boiler pressure keeps dropping
When Should You Worry?
You should investigate further if:
• Bleeding the radiator doesn’t fix the issue
• Pressure drops repeatedly after bleeding
• You hear loud banging instead of light ticking
• Several radiators stay cold
Persistent issues may indicate a larger circulation or pressure problem.
What You Can Do
If the top of your radiator is cold, you can:
• Turn off the heating
• Use a radiator key to bleed the trapped air
• Check boiler pressure afterward
• Repressurise the system if needed
Bleeding usually restores full heating quickly.
In most situations, a radiator that’s cold at the top simply needs bleeding. Once the trapped air is released, it should heat evenly again. If the problem returns frequently, a professional can inspect the system.