In short
- If the radiator is cold at the top and warm at the bottom, air has got into it: bleed it.
- If it is cold at the bottom and warm at the top, sludge and deposits have built up in the radiator: it needs radiator cleaning (flushing).
- If none of the radiators heat up but you still get hot water, the cause may be a combi boiler setting, the room thermostat or a part inside the boiler.
Cold at the top, warm at the bottom
There is air in the radiator. The air collects at the top and stops the hot water from reaching it.
- Switch off the boiler and wait for the radiators to cool down a little.
- Hold a cloth or a container under the bleed valve at the top corner of the radiator.
- Open the valve slowly with a screwdriver or a radiator key. Air comes out first, with a hissing sound.
- When the air stops and water starts to come out, close the valve.
- Check the boiler pressure; if it has dropped below 1 bar, add water.
You get better results if you start bleeding with the radiator on the top floor or the one furthest from the boiler.
Cold at the bottom, warm at the top
Over the years, sludge, rust and deposits build up in the system and settle at the bottom of the radiator. Bleeding does not solve this. In radiator cleaning, a machine is connected to the system, the build-up is flushed out with the water, and then the system is refilled with clean water. About radiator cleaning
One radiator does not heat up at all
- The radiator valve may be closed; open it fully.
- If there is a thermostatic radiator valve, turn the head to the highest setting. On valves that have been closed for a long time, the pin inside can get stuck; in that case, a technician should look at it.
- It is also possible that the radiator is completely full of air or sludge.
Rooms far from the boiler do not heat up
If the radiators near the boiler get very hot while the ones further away stay lukewarm, the system may be out of balance, the pump may have weakened or the system may be dirty. Turning down the valves on the nearer radiators a little can restore the balance; if it does not, the system needs to be checked.
None of the radiators heat up
- The boiler may have been left in summer mode (hot water only); switch on heating mode.
- If you have a room thermostat, check that it is on, that its battery has not run out and that it is set correctly.
- There may be an error code or a low-pressure warning on the boiler display.
- If you get hot water but the radiators do not heat up, the three-way (diverter) valve or the pump inside the boiler may be faulty. In this case, contact a technician.
Dereli Teknik · 25 September 2026