Part of the guide Troubleshooting a boiler fault
Radiators ticking or gurgling? Cold at the top, warm at the bottom? That is air in your system. Bleeding takes ten minutes per floor and makes your heating 10 to 20% more efficient straight away. Here is how.
When to bleed?
- With start of the heating season (September/October) — after a summer standing idle there is always some air in the system.
- Na topping up the boiler — vers leidingwater bevat opgeloste lucht die vrijkomt.
- When a radiator stays cold at the top while the rest heats up.
- With gurgling or ticking in the pipes.
Step-by-step: bleeding radiators
- Switch off the central heating pump. Turn the thermostat down to 5°C or switch it off. Wait 15 minutes so the air rises to the top.
- Start with the lowest radiator closest to the boiler. Put a small bowl or cloth under the bleed valve.
- Turn the radiator key a quarter turn open. You will hear hissing. As soon as water comes out instead of air, close it immediately.
- Work from low to high and from close to the boiler to further away. Finish with the highest radiator in the loft.
Top up the pressure after bleeding
Bleeding removes water, so the pressure drops. Check the boiler's pressure gauge — below 1 bar? Then you need to top up (see Topping up a boiler). Boven 1,5 bar? Prima, laat staan.
What if no air comes out?
Three causes:
- Blocked valve — carefully push a needle into the hole. Caution: water may suddenly spurt out.
- Closed radiator valve — check that the valve is open.
- No air present — then the problem is something else: pump, thermostatic head or a blockage.
Need a radiator key? See · or book a service with an installer.
Low-maintenance boilers
- Remeha Tzerra Ace-Matic 35c CW5 — automatic air vent built in, bleeding usually no longer needed
- Intergas Kombi Kompakt HRE 36/30 CW5 — proven low-maintenance Dutch design
Full range: · .
Also in this guide
- Topping up your boiler: step-by-step plan, correct pressure and tips per brand
- Does an Intergas boiler have a three-way valve? No, and this is what it has instead
- Original or alternative boiler part: where it is safe, where it is not, and what it means for your warranty
- Automatic air vent on a boiler or underfloor heating: how it works, when it is faulty, cap open or closed, and which one fits
- Back to the overview: Troubleshooting boiler faults