Short answer: To bleed a radiator, turn the heating off and let it cool, hold a cloth under the bleed valve at the top corner, and turn it a quarter turn with a radiator key until air hisses out. Close it when water appears, then check the boiler pressure and top up if needed. Cold at the bottom means sludge, not air.
Bleeding radiators is one of the few heating jobs most people can safely do themselves. It is also one of the most misunderstood. Air shows up as a cold top. Cold at the bottom is a different problem.
Is it air?
| Symptom | Likely cause |
|---|---|
| Cold at the top, warm at the bottom | Trapped air |
| Cold at the bottom, warm at the top | Sludge |
| Cold all over, others warm | Valve, balancing or blockage |
| Upstairs cold, downstairs warm | Pump, air or balancing |
| Gurgling noises | Air in the system |
If the top of a radiator is cold, bleeding should help.
What you need
- A radiator key, unless your valves take a flat screwdriver
- A cloth or old towel
- A small container
How to bleed a radiator
- Turn the heating off and let the radiators cool. Hot water can scald, and a running pump can draw air in.
- Find the bleed valve. It is at one of the top corners, a small square or slotted fitting.
- Hold the cloth under it.
- Turn the key a quarter to half turn anticlockwise. You will hear air hissing.
- Close it as soon as water appears. Do not unscrew it fully.
- Repeat on other radiators that need it. Start downstairs and furthest from the boiler.
- Check the boiler pressure. Bleeding lets out water, so pressure drops.
- Top up if needed using the filling loop, following your boiler manual. Close the loop taps afterwards.
- Turn the heating back on and check the radiators warm evenly.
Open vented systems
If you have a regular boiler with tanks in the loft, there is no pressure gauge to top up. The loft tank refills the system automatically. See our post on regular boilers and cylinders.
If you need to bleed often
Air should not keep getting in. If you are bleeding the same radiator every few weeks, something is wrong:
- A small leak letting air in as water escapes
- An open vented system pumping over, where water circulates through the loft tank
- Gas forming inside the system from corrosion, often a sign of rust
Corrosion gas is a warning sign. It means steel radiators are rusting inside, which also makes sludge.
When it is sludge
Radiators that are warm at the top and cold at the bottom have sludge sitting in them. It is black iron oxide, called magnetite, that forms when steel radiators rust inside. You may notice:
- Dark or black water from the bleed valve
- Radiators slow to heat
- A noisier boiler
- Repeated pump problems
Bleeding will not fix sludge. It needs a filter clean, a chemical clean or a power flush, depending on how bad it is. Our magnetic filter clean and system health check page explains how an engineer decides, and our post on power flush or magnetic filter compares the two.
Older Putney houses
In Putney, many Victorian and Edwardian houses still have tall old radiators that have been on the system for decades, alongside newer ones added for loft conversions. Old steel and cast iron radiators can hold a lot of sludge, and mixed systems are often unbalanced. Our Putney area page covers the housing there.
Balancing
If all radiators are free of air and sludge but some still heat slowly, the system may need balancing. That means adjusting the lockshield valves, the valves with plastic caps on the other end of each radiator, so water flows evenly. An engineer can do this, or talk you through it.
Safety
- Always let radiators cool first.
- Keep the key turned only far enough to release air.
- If water keeps flowing and the valve will not close, hold the cloth on it and call for help.
- Never open anything on the boiler itself. That is gas work for a Gas Safe registered engineer.
Which radiators to bleed first
On a two storey house, start with the downstairs radiator furthest from the boiler, then work back towards the boiler, then do upstairs the same way. Air tends to collect in the highest radiators and towel rails, so upstairs often needs it most. In a flat on one floor, start with the radiator furthest from the boiler. Check the pressure gauge after every two or three radiators, not just at the end.
When to book
Book a check if:
- You bleed often and air keeps coming back
- Water from the bleed valve is black
- Radiators are cold at the bottom
- Pressure keeps dropping after you top up
At a service the engineer can clean the magnetic filter, check the water and inhibitor, and advise.
Gas work in the UK must be done by a Gas Safe registered engineer. Check the ID card and the Gas Safe Register.
Quick questions
How often should I bleed radiators?
Only when needed, when a radiator is cold at the top. If you need to bleed often, air is getting in and the cause should be found.
Why did my boiler stop after bleeding radiators?
Bleeding lets out a little water, so pressure drops. Top up using the filling loop as your manual shows, then reset.
Which radiator should I bleed first?
Start downstairs, furthest from the boiler, then work towards it and upstairs.
Last updated 23 October 2026. Written for homes in London. Rules can change, so check the linked sources for the latest position.