You should always bleed radiators when the heating system is off and the radiators are cool, not when they are hot. This is both safer and more effective at removing trapped air.

Quick Scoop

  • Bleed radiators with the boiler switched off.
  • Let the system cool completely so the radiators are cool to the touch.
  • Only then open the bleed valve to release air until water flows steadily.

Why cold is safer

  • When the heating is on, the water and air inside the radiator are very hot and can spray out under pressure, causing burns or scalds.
  • Working on a cool radiator means you can handle the valve and any drips of water without serious injury risk.

Why cold works better

  • If the pump is running and the system is hot, opening a bleed valve can draw new air into the system instead of clearing what is already trapped, making the problem worse.
  • Bleeding with the system off and settled lets air rise and sit at the top of the radiator, so it vents cleanly when you open the valve.

Simple step order

  1. Turn the heating on briefly and check which radiators have cold tops or gurgling noises. These are the ones that need bleeding.
  1. Switch the heating off and wait until all radiators are fully cool.
  1. Use a radiator key to open the bleed valve slowly until air hisses out, then close it as soon as water flows steadily.
  1. Check boiler pressure afterwards and top up if needed, then turn the heating back on to confirm even heat.

Extra notes

  • Never taste radiator water; it can contain corrosion inhibitors and other chemicals that are not safe.
  • If radiators stay cold even after proper bleeding, the system may have other issues such as sludge or circulation problems and might need professional attention.

Information gathered from public forums or data available on the internet and portrayed here.