Radiator cold at the top but hot at the bottom: what to do?

If your radiator is cold at the top and hot at the bottom, it is usually a sign of trapped air or a buildup of sludge inside the radiator. This is a common issue that can often be fixed with a simple bleed, but sometimes it requires professional help.

What is happening?

When air gets trapped in the radiator, it rises to the top, preventing hot water from filling the whole unit. Alternatively, sludge (a mix of rust and debris) can accumulate at the bottom, blocking heat. Both cause uneven heating: cold at the top, warm at the bottom.

What to do first

1. Turn off the heating and allow the radiator to cool completely to avoid burns.
2. Bleed the radiator: Use a radiator key to slowly open the bleed valve at the top corner. You should hear a hiss as air escapes. Once water starts to dribble out, close the valve tightly.
3. Check boiler pressure: After bleeding, the system pressure may drop. If it is below the recommended level (usually 1-1.5 bar when cold), top it up using the filling loop, following the boiler manual.
4. Turn the heating back on and see if the radiator heats evenly. If not, repeat bleeding after it cools.

What NOT to do

• Do not bleed a hot radiator – scalding water can cause serious burns.
• Do not over-tighten the bleed valve – it can break or strip the thread.
• Do not ignore persistent cold spots – they may indicate a bigger issue like sludge buildup or a failing pump.
• Do not attempt to remove the radiator yourself unless you are competent; it can cause leaks or damage.

When to call a professional

If bleeding does not solve the problem, or if the radiator remains cold at the top after several attempts, call a qualified heating engineer. They can flush the system, check for sludge, and balance the radiators. Also seek professional help if you notice leaks, unusual noises, or if the boiler pressure keeps dropping.

What to monitor

After bleeding, monitor the radiator over a few heating cycles. Check for consistent heat distribution and watch for any water leaks around the valve. Keep an eye on the boiler pressure gauge; if it falls frequently, there may be a leak or air ingress.

How to prevent it

Regular maintenance helps: bleed radiators annually before winter, consider adding a magnetic filter to trap sludge, and have the system serviced by a professional every few years. This keeps your heating efficient and prevents cold spots.

If you are unsure or uncomfortable performing these steps, contact a qualified heating engineer. Safety first: never work on a hot radiator or a pressurized system without proper knowledge.