1. If the Radiator Is Cold at the Top, You Almost Always Need to Bleed It
Air naturally rises in a heating system, so the top of the radiator is the first place it collects. When the top is filled with air instead of hot water, the bottom warms because water is still flowing through the lower channels, but the top stays icy cold.
A simple bleed usually fixes this, and you’ll instantly hear that hiss of air escaping when you open the valve. When water starts coming out in a steady flow, that’s when you close it. Give the radiator five minutes and check again — most of the time, the top starts heating properly.
If you’ve bled it before and the problem keeps coming back every few days or weeks, that’s a sign of something else we’ll get into further down.
2. Your Boiler Pressure Might Have Dropped After Bleeding
Bleeding a radiator releases air but also lowers system pressure. If you’ve recently bled this or another radiator, there’s a good chance your boiler pressure is now sitting too low to push hot water through the whole system.
Check the gauge on your boiler. If it’s under 1 bar, top it up to around 1.2–1.5. Once the pressure is restored, the radiator should fill properly and the top will start warming.
3. The Lockshield Valve Might Be Restricting Flow
If your radiator gets warm at the bottom but never fully heats at the top, even after bleeding, the lockshield valve might be too tight. This valve controls how much water flows through the radiator. If it’s barely open, you’ll get weak circulation — just enough to warm the bottom, not enough to push heat upwards.
Remove the cap and turn it slightly open (just a quarter turn). Then let the heating run for 10 minutes. If the top starts warming, that was the issue.
I go deeper into balancing radiators here:
How to Balance Radiators Properly.
4. Your Pump Might Not Be Pushing Enough Water Through
A weak circulation pump doesn’t always cause full cold radiators — sometimes it just causes “half working” ones. The radiator will get warm at the bottom because that’s where water enters, but the top won’t heat because the flow isn’t strong enough to move hot water through the upper section.
If you’ve noticed other radiators also taking longer to heat up, or you hear gurgling noises in the pipes, this might be the cause. You don’t always need a new pump — sometimes the speed setting needs adjusting.
5. You Might Have a Small Amount of Sludge at the Bottom
Even though sludge usually causes the opposite problem (cold bottom, hot top), in a partially blocked radiator it can slow the flow so much that the water doesn’t circulate fully through the radiator channels.
Signs of this include:
- The radiator heats unevenly, even after bleeding
- The bottom feels hotter than it should
- The radiator cools down much faster than others
If you suspect this, your radiator may need a flush — but you can often improve things massively just by balancing the system.
If your problem is actually the reverse (hot top / cold bottom), here’s the guide for that:
Why Your Radiator Is Hot at the Top but Cold at the Bottom
6. Air Might Be Re-Entering the System Regularly
If your radiator keeps getting cold at the top even after bleeding, it’s a sign that air is being drawn into the system somewhere. This can happen with older boilers, minor leaks, or when the system hasn’t been pressurised properly.
It doesn’t mean anything dangerous — but it does mean your system isn’t sealed perfectly. A heating engineer can find micro leaks, but before going down that route, make sure the pressure is stable over 48 hours.
7. Putting It All Together
If your radiator is warm at the bottom and cold at the top, follow this order:
- Bleed the radiator properly
- Check boiler pressure and top up if needed
- Open the lockshield slightly
- Check TRV pin isn’t stuck
- Look for sluggish flow that points to pump issues
Most of the time, the fix is one of the first two. It’s rarely anything complicated.
8. Want to Warm the Whole House More Efficiently?
A single radiator not heating properly affects the entire heating system — the boiler has to work harder, other rooms heat unevenly, and the whole home feels colder than it should.
If you want the bigger picture on how to warm your home without wasting money, here’s the full guide:
