Hear a ticking or gurgling sound in your radiators, or do they stay cold at the top while warm at the bottom? Then there's air trapped in your heating system. The good news: in most cases you can bleed your boiler yourself, in around ten minutes.

Why bleeding your boiler matters

Air in the system stops hot water from flowing properly through your radiators. The result: cold spots, longer heating times and a higher energy bill. Your boiler has to work harder to deliver the same warmth. Bleeding regularly keeps your system efficient and prevents unnecessary wear.

When to bleed? Check your radiators at least once a year, ideally at the start of the heating season. If you notice cold spots or gurgling sounds, bleed them straight away.

What you'll need

  • A radiator bleed key (available at any DIY store) or a flat-head screwdriver for newer radiators
  • A cloth or small container to catch any water
  • A filling loop to top up the boiler afterwards, if needed

How to bleed your boiler: step by step

  1. Turn off the boiler. Switch it off or turn the thermostat right down and wait for the radiators to cool. This stops the pump from circulating air and prevents you from being scalded by hot water.
  2. Check the water pressure. Read the pressure on the display or gauge. It should sit between 1.5 and 2 bar. Note the value, as the pressure usually drops after bleeding.
  3. Start with the lowest radiator. Begin with the ground-floor radiator furthest from the boiler, then work upwards. Air rises, so bleed the highest radiators last.
  4. Open the bleed valve. Hold the cloth under the valve and slowly turn it a quarter turn open with the bleed key. You'll hear a hissing sound: that's the escaping air.
  5. Close when water appears. When a trickle of water comes out instead of air, the radiator is air-free. Turn the valve closed again.
  6. Top up and check. Look at the water pressure again. If it has dropped below 1 bar, top up using the filling loop to 1.5-2 bar. Turn the boiler back on and check that all radiators heat up evenly.

Common mistakes

The most common slip-up is forgetting to check the water pressure after bleeding. If the pressure drops too far, your boiler will go into fault mode (often a low-pressure error code). Also, never undo the bleed valve completely: a quarter turn is enough and stops the valve from shooting out.

Key takeaways

  • Bleed your radiators at least once a year, at the start of the heating season
  • Turn off the boiler and work from the lowest to the highest radiator
  • Always check the water pressure and top up to 1.5-2 bar

When to call an engineer

Do the radiators stay cold after bleeding, does the pressure keep dropping or does the boiler keep faulting? Then the problem runs deeper — for example in the pump, the expansion vessel or a leak in the system. Our engineers will fix it quickly. Read more about boiler service or see what we do for a boiler breakdown.

Written by the engineering team at ComfortCV — your boiler specialist in Amsterdam and the surrounding area.