If your radiator is cold at the top but warm at the bottom, it almost certainly needs bleeding. Here's exactly how to do it safely, and what to do if the problem comes back.
Why Radiators Need Bleeding
Air gets into central heating systems over time — through the water supply, through micro-leaks or when the system is drained and refilled. Air rises to the top of the radiator and stops hot water filling that space, leaving the top of the radiator cold while the bottom remains warm. The boiler has to work harder to heat the room and your energy bills go up. Bleeding releases the trapped air and allows the radiator to heat evenly again.
What You Will Need
You need a radiator bleed key — a small square-ended tool available from any hardware shop for under £1 — and a cloth or small container to catch any water that drips out. Some modern radiators have a slot-head bleed valve that can be turned with a flat-head screwdriver instead.
Step 1 — Identify Which Radiators Need Bleeding
Turn your heating on and let the system reach full temperature. Run your hand along each radiator from top to bottom. Any radiator that feels cold at the top but warm at the bottom has trapped air and needs bleeding. Make a note of which radiators need attention before turning the heating off and allowing the system to cool for at least 30 minutes — never bleed a pressurised hot system.
Step 2 — Bleed Each Radiator
Hold your cloth beneath the bleed valve at the top corner of the radiator. Insert the bleed key and turn it slowly anti-clockwise by a quarter to half a turn. You will hear a hissing sound as air escapes. When the hissing stops and a steady trickle of water appears, close the valve immediately by turning clockwise. Do not over-tighten. Start with the radiators furthest from the boiler and work towards it.
Step 3 — Check the Boiler Pressure
After bleeding, check the pressure gauge on your boiler. Bleeding releases trapped air and the system loses a small amount of water, which can cause pressure to drop. Most combi boilers should read between 1 and 1.5 bar when cold. If it has dropped below 1 bar, top up via the filling loop — a flexible hose usually beneath the boiler — until pressure reaches 1.2 bar, then close both valves. Check your boiler manual if you are unsure.
If the Problem Keeps Coming Back
If you find yourself bleeding the same radiators repeatedly, air is entering the system faster than normal. Common causes include a faulty automatic air vent, a small leak drawing air in, or internal corrosion producing hydrogen gas. A plumber can diagnose the root cause and may recommend a power flush and inhibitor treatment to clean and protect the system. Leaving recurring air problems untreated accelerates corrosion and can shorten the life of your boiler.