Why a radiator is cold at the bottom

If a radiator is warm at the top but cold at the bottom, the most common cause is a build-up of sludge or debris that's settled at the base of the radiator over time, restricting water flow.

Why this happens

Over years of use, central heating systems can accumulate rust, limescale and other debris. This settles at the lowest point of each radiator, which is why the bottom is affected while heat still reaches the top.

What can be done

1. Bleeding the radiator will remove trapped air, but this is a different problem — trapped air usually makes the top of the radiator cold, not the bottom.
2. A power flush can clear sludge and debris from the whole system, restoring proper flow and improving efficiency across all your radiators.
3. In some cases, an individual radiator may need removing and flushing out separately if it's particularly badly affected.

When to get it looked at

If more than one radiator is affected, or your heating system feels generally less efficient than it used to, it's worth getting a proper assessment rather than living with reduced performance.