How to drain your central heating system

Drain your Central Heating

Here, we give you a useful guide on draining your central heating system for alteration work, or if you have a caravan, you can apply these principles to drain down your radiators in the winter. Also, if you have a combination boiler and the pressure is too high, you can drain some water out and reduce the pressure by doing the following.

What type of system have you got?


If you are unsure of your system type, please click here for an overview. It is either tank fed, or a sealed system fed via a manual filling tap assembly, such as a combination boiler.


  • Turn off your boilers electrical supply and central heating controls where applicable
  • For tank fed systems, tie your ball valve up, by placing a flat piece of wood across the small tank in the loft, and then using string or shoelace to firmly tie the arm to the wood. Alternatively, turn off your mains water stop cock and this will stop the tank from refilling.
  • Locate the drain point at the lowest part of the system, normally in the hall, kitchen, downstairs toilet, garage, or externally poking out of the wall


    Radiator Drain Valve

    Pipework Drain Valve










  • Attach a hose pipe to this drain point, secure with a jubilee clip where possible, and run to outside, into a foul drain ideally, as the water may be dirty. Avoid draining onto grass as the chemicals in the water may kill plants and grass.
  • You may wish to place a dustsheet or old towel under the drain valve as there may be a little water dripping from the drain valvewhen opened
  • Open the drain valve using a small spanner on the square end, and turn anti clockwise until you here water coming out
  • Open the radiator bleeds on your all radiators to let the water drain out
  • Wait for the water to finish draining

Refilling your system


So, you've done the necessary work, and now wish to refill your central heating system?


  • Close all radiator bleeds
  • Close the drain off point you've just used!
  • Add fernox inhibitor to system via tank, or via radiator bleed/cap
  • Where applicable, untie the ball valve or turn on the mains water stopcock
  • For a combination, or sealed system click here on how to refill
  • Bleed all radiators, starting from downstairs, working your way up. Please note, we need to bleed the radiators cold for now, and then check them when the system has been running, firing the boiler with no water in can damage the boiler.
  • If necessary, top up the system pressure again after bleeding the rads.


Things you may need



Jubilee Clips


Drain Down Hose


Waterproof Dustsheet








Inhibitor for Tanks



This can be poured straight into your small header tank in the loft, before filling the system




Inhibitor for Combis



This can be injected into your system via a radiator, or the filling loop, using a standard caulking gun, instructions are provided in the box.