A radiator that is hot at the bottom and cold at the top, gurgling in the pipes, a room that no longer heats as it used to: there is air in your heating circuit. The fix is simple, quick and free: bleed the radiators.
When should you bleed the radiators?
- At the start of the heating season, every autumn.
- When a radiator stays cold at the top.
- When you hear water or gurgling noises.
- After topping up the system with water.
What you need
A radiator key (a few euros in a DIY store) or a flat screwdriver depending on the type of bleed valve, a container and a cloth.
Bleeding a radiator in 6 steps
- Turn off the heating and wait for the radiators to cool: the pump must be stopped so that the air rises.
- Fully open the radiator's thermostatic valve.
- Hold the container under the bleed valve, the small screw at the top of the radiator, on the side opposite the valve.
- Turn the screw a quarter or half turn: the air escapes with a hiss.
- As soon as a steady trickle of water comes out, close it without forcing.
- Move on to the next radiator.
In which order?
Start with the radiator closest to the boiler on the lowest floor, then work further away and up floor by floor. Air always collects at the highest points: finish on the top floor.
Do not forget the pressure
Releasing air lowers the system pressure. After bleeding, check the boiler's pressure gauge and top up to about 1.5 bar if necessary: everything is explained in our article on low boiler pressure.
What if the radiator stays cold?
- Cold at the bottom, hot at the top: sludge has settled in the radiator. The system needs flushing.
- Completely cold: the thermostatic valve may be stuck (common after the summer).
- All radiators cold: the problem is the boiler or the pump — see our article on boiler breakdowns.
- Air that keeps coming back: there is an air inlet or a faulty expansion vessel that needs checking.
Frequently asked questions
Should the heating be on or off when bleeding radiators?
Off, with the radiators cooled down: with the pump stopped, the air rises to the top of the radiators and escapes more easily.
Which radiator should I bleed first?
The one closest to the boiler, on the lowest floor, finishing with the highest.
Why is my radiator cold at the bottom?
A radiator that is cold at the bottom and hot at the top is usually full of sludge. It needs flushing; bleeding is not enough.