How to Bleed a Radiator: How and When to Do It Correctly
A radiator that stays cold at the top while it is hot at the bottom, or that makes gurgling and clicking sounds, almost certainly contains air. This trapped air stops the hot water circulating across the whole surface of the radiator, which reduces your comfort and increases your consumption. Bleeding a radiator is a simple operation, no plumber needed, which takes a few minutes per radiator. Here is how to do it properly.
When to bleed a radiator
Bleeding is needed as soon as you notice one of these signs: the top of the radiator is cold while the bottom is hot, you hear gurgling or hissing sounds, or the heating takes a long time to come up to temperature. It is generally recommended to bleed all the radiators at the start of winter, then mid-season if necessary. If your system was bled last year without a problem, a simple annual check is enough.
The equipment you need
- A bleed key (supplied with the radiator or 2 to 4 € at a DIY store)
- A cloth or sponge to protect the floor
- A small container (cup, bowl or cut bottle) to catch the water
- A flat screwdriver as backup if the bleed valve is too small
- A bucket in case of a very old radiator or one full of air
Preparing the bleed safely
Before starting, turn off the heating (boiler or heat pump) and wait for the water to cool down: bleeding a scalding radiator is dangerous and the water can splash you. In a communal heating system, the operation is also usually done with the boiler running, but at least with the circuits cooled down. Get your cloth and container ready, then locate the bleed valve: it is at the top of the radiator, on one side, often in the shape of a small square or cross-head nut.
Bleeding a radiator step by step
- Place the container under the bleed valve and wrap the cloth around it to absorb the splashes
- Insert the bleed key into the slot and turn it gently anticlockwise (a quarter turn is often enough)
- A hissing sound is heard: that is the air escaping, let it out until a steady trickle of water flows
- Close the valve as soon as the water flows regularly, without over-tightening so as not to damage the thread
- Wipe up any drops and repeat the operation on the other radiators
- Turn the heating back on and check the boiler pressure (it must stay between 1 and 1.5 bar)
The mistakes to avoid
- Bleeding when the heating is at full power: risk of burns and poor pressure adjustment
- Opening the valve a full turn: the valve can unscrew completely and water sprays everywhere
- Forgetting to check the boiler pressure after bleeding: too low a pressure blocks the whole circuit
- Bleeding continuously on a sealed system without ever putting water back into the installation
After bleeding: restarting
Once all the bleeds are finished, turn the heating back on and listen: the gurgling should disappear and the heat should spread evenly from the bottom to the top of the radiator. If the boiler pressure has dropped below the minimum mark, reset it using the pressure gauge by adding water via the filling loop, then bleed again for a few seconds to expel the air introduced.
Manual bleed or automatic air vent
On recent radiators, an automatic air vent can replace the bleed valve: it lets the air escape on its own, without intervention. Handy on cast-iron radiators, it nevertheless remains more fragile and can seize up with limescale. On a standard system, an annual manual bleed remains the most reliable method, especially if your water is hard, because it also lets you check the condition of the valves and gaskets of your radiators.
If, after a bleed, the air comes back very quickly or the water that comes out is murky and full of sludge, your circuit no doubt contains heating sludge. In that case, a specific cleaning product for heating circuits or a power flush by a professional is needed before winter, to avoid corrosion of the radiators and recurring air pockets.
By bleeding your radiators every year before winter, you optimise the efficiency of your system, you limit your energy consumption and you avoid circulation breakdowns. It is a simple, quick and free maintenance gesture that also protects your boiler from the damage of air and limescale.