How to Bleed a Radiator
Cold at the top, warm at the bottom? Trapped air is the problem. Bleeding a radiator takes five minutes and costs nothing.
Difficulty
Easy
Time
5-10 mins
Cost
Free
Safety First
The water inside radiators can be very hot. Turn the heating off and let the radiators cool down for at least 15 minutes before bleeding. The water that comes out is dirty and can stain carpets and clothing. Have a cloth and container ready to catch drips. Radiator water contains inhibitor chemicals so wash your hands afterwards.
If your radiator is hot at the bottom but cold at the top, trapped air is preventing hot water from circulating to the top of the radiator. Bleeding lets the air out so the hot water can fill the entire radiator. This is one of the simplest DIY jobs there is and something every homeowner should know how to do. You should bleed your radiators at the start of every heating season and whenever you notice cold spots. It takes about five minutes per radiator and improves your heating efficiency, which means lower energy bills.
What You Will Need
Tools
- - Radiator bleed key (from any hardware shop, under 1 pound)
- - Old towel or cloth
- - Small container to catch water
Materials
- - No materials needed (this job is free)
Step-by-Step Instructions
Turn off the heating
Switch off your central heating and wait at least 15 minutes for the radiators to cool. This reduces the pressure in the system and means the water will not be scalding when it comes out.
Find the bleed valve
The bleed valve is a small square fitting at the top corner of the radiator, usually on the side opposite to the thermostatic valve. It has a small square peg in the centre.
Prepare for drips
Hold your cloth or towel underneath the bleed valve and have a small container ready. The water that comes out is usually dark and dirty. Protect the carpet or flooring beneath.
Open the valve slowly
Insert the radiator key into the square peg and turn it anticlockwise very slowly, about a quarter turn. You will hear a hissing sound as the trapped air escapes. Keep the key in position and do not open it too far.
Close when water appears
As soon as the hissing stops and water starts to dribble out steadily, turn the key clockwise to close the valve. Do not overtighten. You only need a quarter turn back. Wipe up any water that has escaped.
Check the boiler pressure
Go to your boiler and check the pressure gauge. It should read between 1 and 1.5 bar when the system is cold. If the pressure has dropped below 1 bar after bleeding, you need to top up the system using the filling loop. This is the braided hose underneath the boiler with a valve at each end. Open both valves slowly until the gauge reads 1.2-1.5 bar, then close them both.
Which Radiators First
If you are bleeding multiple radiators, start with the ones on the ground floor and work upwards. Air rises, so the upper floor radiators usually have the most trapped air.
Ongoing Cold Spots
If you have to bleed a radiator frequently, there may be a bigger issue. A failing pump, a stuck valve, or sludge in the system can all cause persistent air pockets. A professional powerflush (around 300-500) cleans the entire system and often solves recurring problems.
Common Mistakes to Avoid
- - Opening the valve too far (water gushes out and is hard to close quickly)
- - Forgetting to check boiler pressure after bleeding (low pressure means the boiler may not fire up)
- - Not having a cloth ready (radiator water stains carpets and walls)
- - Bleeding with the heating on (hot water under pressure is dangerous)
- - Overtightening the bleed valve (can damage the valve and cause leaks)
When to Call a Professional
Call a plumber if the radiator stays cold even after bleeding, if the bleed valve is damaged or leaking, if the pressure keeps dropping after topping up (indicating a leak in the system), or if the radiator water is very dark and sludgy which suggests the system needs a powerflush.