How to Bleed Radiators: A Step-by-Step Guide for London Homes
Radiators that are cold at the top but warm at the bottom have trapped air. Bleeding them is a simple 5-minute task that can restore full heating efficiency.
Why Radiators Need Bleeding
Air can enter a central heating system through various routes — small leaks in joints, during refilling after a power flush, or through the system water itself. Air is lighter than water and rises to the top of radiators, forming a cold pocket that prevents hot water from filling the full radiator. Bleeding releases this trapped air.
What You Need
- A radiator bleed key (fits most UK radiators — available for around £1 from hardware stores)
- An old cloth or small container to catch water
Step-by-Step Guide
- Turn the heating on and let the system warm up fully. This makes it easier to identify which radiators have trapped air (they will feel colder at the top than the bottom).
- Turn the heating off and allow the system to cool for 30–60 minutes before bleeding. This reduces the risk of scalding from hot water.
- Start with the downstairs radiators and work your way upstairs. The radiator furthest from the boiler is often the worst affected.
- Insert the bleed key into the bleed valve (usually at the top right or left of the radiator, a small square valve in a round fitting).
- Turn anticlockwise slowly — about a quarter to half turn. You will hear a hissing sound as air escapes.
- Hold the cloth below the valve. When water starts to drip steadily (rather than spit), all the air has escaped.
- Close the valve clockwise — do not over-tighten.
- Check the boiler pressure gauge. Bleeding removes water as well as air and can drop the pressure. If it has fallen below 1.0 bar, repressurise via the filling loop.
After Bleeding
Turn the heating back on and check each bled radiator heats evenly from top to bottom. If problems persist — particularly if the same radiators need bleeding repeatedly — this points to an ongoing air ingress issue or a failing pump. Book an engineer to investigate.
Frequently asked questions
How often should I bleed my radiators?
Most homeowners bleed radiators once a year, typically at the start of autumn before the heating season. If a radiator becomes cold at the top outside this schedule, bleed it as needed.
What if no air comes out when I bleed the radiator?
If water flows immediately without any hissing, there is no trapped air in that radiator. Close the valve and move to the next one. If the radiator is still cold, the problem may be sludge blockage rather than air — this requires a different approach.
Can I bleed a radiator while the heating is on?
You can but it is not recommended — the water will be under higher pressure and very hot, increasing the risk of scalding. It is safer to let the system cool first.
Why does my boiler pressure drop after bleeding radiators?
Bleeding removes both air and a small amount of water from the system, reducing the total volume of fluid and therefore the pressure. Check the gauge after bleeding and repressurise if below 1.0 bar.