Radiator Cold at the Top? It Needs Bleeding — Here's How
A radiator that is warm at the bottom but cold at the top has trapped air inside. Bleeding it takes five minutes and you do not need a plumber. Here is exactly how to do it.
By Nicholas King, Gas Safe Engineer July 2026 4 min read
If you have noticed your radiator is hot near the bottom but lukewarm or cold across the top, you have trapped air in the radiator. This is one of the most straightforward heating problems to fix yourself — all you need is a radiator key (available from any hardware store for a couple of pounds) and five minutes. Here is the complete guide.
Why Is the Top of My Radiator Cold?
Your central heating system circulates hot water through every radiator in the house. Air, being lighter than water, naturally rises to the highest points in the system — which means it collects at the top of radiators, creating a cold air pocket that prevents hot water from filling the whole radiator.
Air enters the system in several ways: whenever fresh water is added to top up the boiler pressure (all water contains dissolved gases that separate out in the system), through micro-leaks in valves or joints, or during any maintenance work where the system is opened.
The fix is to bleed the radiator — release the trapped air through a small valve at the top of the radiator until only water remains.
What You Will Need
A radiator bleed key — a small square key that fits the bleed valve (£1-£2 from any hardware store or DIY shop). Some modern radiators accept a flat-head screwdriver instead.
A cloth or small towel to catch drips
A bowl or tray is useful for older radiators that may release more water
How to Bleed a Radiator — Step by Step
Turn the heating on and let the radiators heat up fully. This builds pressure that helps push the air out.
Identify which radiators need bleeding by feeling for cold spots at the top. Write them down so you do not miss any.
Turn the heating off and wait 20-30 minutes. You want the radiators warm enough to have pressure, but cool enough to handle safely without scalding yourself. Never bleed a radiator when the heating is running — this can let air back in and disrupt the system.
Start with the radiator furthest from the boiler and work your way back. On a two-storey house, start upstairs, then do the ground floor.
Find the bleed valve — it is a small square-headed plug, usually at the top of one end of the radiator (often the side away from the thermostatic valve).
Hold a cloth underneath the bleed valve, then insert the radiator key and turn it anti-clockwise by about a quarter turn. You will hear a hissing sound — that is the air escaping.
Wait for the hissing to stop and for a steady drip or trickle of water to appear. This means all the air has been released and the radiator is full of water.
Close the valve by turning the key clockwise until it is snug. Do not overtighten.
Repeat for every radiator that needed bleeding.
Check your boiler pressure once you have finished. Releasing air and water lowers the system pressure slightly. The gauge should read between 1 and 1.5 bar. If it is below 1 bar, repressurise the boiler using the filling loop.
Turn the heating back on and check that the radiators are now heating evenly from top to bottom.
After Bleeding — What to Expect
Once bled correctly, your radiators should heat evenly top to bottom within 15-20 minutes of the heating coming on. You should also notice your home warming up more quickly, and potentially see a slight reduction in your gas bill as the system is working more efficiently.
The water that comes out of the bleed valve is usually slightly discoloured — a light brownish colour is normal. If it is very dark or black, that suggests sludge in the system, which is a different problem (see our guide on radiators cold at the bottom).
If You Need to Bleed Radiators Repeatedly
Bleeding once or twice a year is normal. But if you find yourself needing to bleed the same radiators every few weeks, something else is going on:
A slow leak somewhere in the system is letting air in. Check for damp patches under radiators or near valves.
Repeatedly repressurising the boiler introduces fresh water (and fresh air) into the system, causing air pockets to keep recurring. The root cause needs fixing.
A faulty automatic air vent on the boiler or system that is not doing its job.
Corroding pipework releasing gases as part of the corrosion process.
In these cases, call a Gas Safe engineer to diagnose the underlying problem. An annual boiler service is the best way to catch these issues before they become bigger problems.
Radiator Cold All Over — Different Problem
If your radiator is completely cold all over (not just at the top), the problem is not trapped air — it is likely a closed lockshield valve, a stuck thermostatic radiator valve (TRV), or a circulation problem. Read our guide to find out why your boiler might be cutting out, or call us if you cannot identify the cause.
HEATING PROBLEMS?
Still Having Trouble with Your Radiators?
If bleeding has not fixed the problem, there may be a deeper issue. We cover Wolverhampton, Walsall, Cannock and across the West Midlands. Gas Safe registered #568305.
Trapped air has collected at the top of the radiator, preventing hot water from filling it. The fix is to bleed the radiator — release the air through the bleed valve at the top end of the radiator using a radiator key.
Turn the heating on to build pressure, then turn it off and let it cool for 30 minutes. Insert a radiator key into the bleed valve, hold a cloth underneath, and turn anti-clockwise a quarter turn. Let the air hiss out until water drips steadily, then close the valve. Check your boiler pressure afterwards and repressurise if needed.
Some air ingress is normal — dissolved air in water separates out in the system. But if you are bleeding the same radiators every few weeks, there is likely a slow leak letting air in, or you are repeatedly adding fresh water to top up the pressure. An engineer needs to find and fix the root cause.
