Radiators Not Heating Up Properly — Causes and Fixes

Radiators that are cold at the top, cold at the bottom, or completely cold all have different causes. This guide helps you pinpoint and fix the problem.

Diagnosing Radiator Problems

The location of the cold area on your radiator tells you a lot about the underlying cause:

Cold at the Top — Air Trapped in the Radiator

This is the most common radiator problem and the easiest to fix: bleed the radiator. See our full guide on how to bleed a radiator. After bleeding, check the boiler pressure and top up if it's dropped below 1 bar.

Cold at the Bottom — Sludge Build-Up

Black or brown magnetic sludge settles at the bottom of radiators, blocking the circulation of hot water. Bleeding won't fix this — the radiator needs to be removed and flushed, or the whole system may need a power flush. The water that drains when you bleed the radiator will look dark brown or black if sludge is present.

Completely Cold Radiator

If an individual radiator is completely cold while others heat normally, check:

TRV (thermostatic radiator valve): Try turning it up. If the head is stuck, remove it and check that the pin underneath moves freely — sometimes it seizes in the closed position.
Lockshield valve: The valve on the return side may be fully closed. Open it a quarter turn and see if the radiator heats.
Air lock: Bleed the radiator even if you can't hear air — sometimes the bleed point needs to be opened for water to flow properly.

All Radiators Cold

If all radiators are cold, the problem is likely with the boiler, circulation pump, or zone valve rather than individual radiators. See our guide on central heating not working for a full diagnostic checklist.