2026-07-15

Why Is My Radiator Cold at the Bottom But Hot at the Top?

If your radiator is warm at the top but stays stubbornly cold along the bottom, don't reach for the bleed key just yet - this is a different problem to trapped air.

What's really going on

Cold at the bottom, warm at the top usually means sludge - a build-up of rust and mineral deposits - has settled at the bottom of the radiator, blocking water flow through that section. Bleeding won't fix this, because there's no air pocket to release.

Signs it's sludge, not air

• ✓ The radiator has been on the system for many years without a flush
• ✓ Bleeding the radiator produces little or no air, or dirty water
• ✓ The problem is worse on radiators furthest from the boiler
• ✓ Your heating system takes longer than it used to warm the house

The proper fix

A power flush forces high-velocity water (sometimes with a cleaning chemical) through the whole system to break up and clear sludge from radiators and pipework. For a single bad radiator, it can sometimes be removed and flushed individually, but if several radiators are affected a full system power flush usually gives better, longer-lasting results.

Left unaddressed, sludge build-up makes your boiler work harder, pushes up energy bills, and can eventually cause pump or boiler component failures. Our heating team can assess whether a flush would sort your system out.