How to Bleed Your Radiators

Radiators are the workhorses of older heating systems, but they are notorious for trapping air bubbles that stop hot water from circulating properly. If your radiator is cold at the top but warm at the bottom, or if you hear a persistent gurgling sound coming from the pipes, it is time to perform a bleed. This simple maintenance task pushes that trapped air out, allowing the system to fill completely with heated water. Done well, this project is a five-minute job that noticeably improves the comfort of a room and lowers your energy bills by forcing your boiler to work less. You are essentially resetting the circulation efficiency of the unit. The process is straightforward, but it requires a careful hand to ensure you do not release too much water or burn yourself on pressurized steam.

  1. Kill the Heat First. Switch off your central heating system completely and wait for the radiators to cool down. If the pump is running, it will continue to circulate air bubbles, making it impossible to bleed the system effectively.
  2. Spot the Square Peg. Find the bleed valve at the top corner of the radiator. It is usually a small square metal peg inside a circular housing, located on the opposite side of the thermostatic valve.
  3. Protect Your Walls Now. Place a rag or an old towel directly under the bleed valve to catch any water that escapes. Hold a small container against the wall just beneath the valve to catch larger spills.
  4. Release the Trapped Air. Insert the radiator key onto the square peg and turn it slowly counter-clockwise. You should hear a distinct hissing sound as the trapped air escapes from the unit.
  5. Seal It Tight. As soon as the hissing stops and a steady stream of water begins to sputter out, tighten the valve clockwise immediately. Do not overtighten, as this can strip the threads or damage the seal.
  6. Restore System Pressure. Go to your boiler and check the pressure gauge. Bleeding radiators can lower the system pressure, so you may need to top it up using the filling loop until it hits the green zone.