Bleeding Radiators: A Definitive Guide

Why Do Radiators Need Bleeding?

When air enters a central heating system — through a leak, during refilling, or naturally over time — it rises to the top of radiators and prevents hot water from filling the full radiator. The result is a radiator that's warm at the bottom but cold at the top, reducing efficiency and comfort. Bleeding releases the trapped air and allows hot water to fill the space.

When Should You Bleed Radiators?

Bleed radiators when:

What You Need

Step-by-Step Instructions

Step 1: Turn on the central heating and allow all radiators to fully warm up — this makes it easier to identify which ones need bleeding.

Step 2: Feel each radiator carefully. Identify those with cold patches at the top.

Step 3: Turn off the central heating and allow the system to cool for 10–15 minutes. This makes the process safer.

Step 4: Starting with the radiator furthest from the boiler (usually upstairs), locate the bleed valve — a small square fitting at the top end of the radiator.

Step 5: Hold your cloth under the valve and insert the bleed key. Turn anticlockwise slowly — just a quarter to half turn. You'll hear a hiss as air escapes.

Step 6: When water begins to dribble out, close the valve by turning clockwise. Do not overtighten.

Step 7: Repeat for all radiators that need attention.

Step 8: Check your boiler pressure gauge. If it has dropped below 1 bar, repressurise using the filling loop.

For help with persistent cold radiators or pressure problems, call Halo Heating on 01483 676108.