A radiator that is warm at the bottom but cold across the top usually contains trapped air. In a properly operating wet central-heating system, hot wet underfloor heating and heat pumps should circulate through most of the radiator’s internal channels. Air is lighter than water, so any trapped pocket rises and collects near the top, preventing hot water from filling that part of the unit.
Bleeding the radiator releases this air and is often a straightforward homeowner task. Before starting, examine the radiator pattern carefully. A cold top and warm bottom points toward air, while a cold bottom and warmer top more often suggests sludge, restricted circulation, or sediment. A completely cold radiator may have a closed valve, stuck thermostatic radiator valve pin, balancing issue, airlock, or wider system fault.
How to bleed the radiator
Gather a radiator key, an old cloth, and a small container. Run the heating briefly so you can identify affected radiators, then switch the system off and allow it to settle. Bleeding while the circulation pump is running can draw more air into the system or release hot, pressurized water.
The bleed valve is usually located at one of the upper corners. Protect the wall and floor with the cloth, position the container underneath, and insert the key. Turn it slowly counterclockwise, normally no more than a quarter or half turn. You should hear a hiss as air escapes. Do not remove the bleed screw. When the hissing stops and water begins to emerge in a steady trickle, close the valve clockwise. It should be snug but not overtightened.
If several radiators need attention, it is often sensible to begin downstairs and then move upward, although you should follow any guidance supplied for your particular heating system. After bleeding, check each radiator for drips and wipe the valve dry so that any continuing leak is easy to spot.
Bleeding can lower the pressure in a sealed system. Check the boiler gauge when the system is cool. The correct pressure varies, so use the appliance manual or the marked operating range on the gauge. Many domestic systems operate at roughly 1 to 1.5 bar when cold, but that is not universal. If pressure is low, follow the manufacturer’s instructions for repressurizing through the filling loop. Stop if you are unfamiliar with the controls or if the pressure rises abnormally.
If bleeding does not solve the problem
A radiator that remains cold at the top after bleeding may have another circulation problem. Possible causes include:
- A blockage inside the radiator
- Incorrectly adjusted lockshield valves
- A weak or failing circulation pump
- Low system pressure
- A stuck thermostatic valve
- Poor system balancing
- Pipework restrictions
Repeated air accumulation is also significant. If the same radiator needs bleeding every few weeks, air may be entering through a small leak, or gas may be forming because of internal corrosion. Frequent pressure loss, visible corrosion, or water around valves should be inspected by a heating engineer.
Bleeding treats trapped air; it does not clean sludge from the system. Cold areas at the bottom, dirty water from the bleed valve, and noisy circulation may indicate contamination requiring chemical cleaning, filtration improvements, or professional flushing. A magnetic system filter can help collect circulating iron oxide, but it should form part of a properly assessed maintenance plan.
Diagnose the heat pattern first, use the simple fix only when appropriate, and seek professional help if the symptom quickly returns. Persistent circulation problems can reduce comfort, increase running time, and place unnecessary demand on the boiler, so treating the underlying cause is better than repeatedly addressing the same cold spot.