Draining a Vented Central Heating System

Sometimes it is necessary to drain the water from a vented central heating system; this may be, to fit a new boiler, valve, pump or radiator, or just to carry out some maintenance on the system.

It should be possible to drain down a system without too much trouble or mess.

  1. Turn off the boiler - don't just set the thermostat low, actually switch off the power to the control circuit and turn off the gas. Or, in the case of a solid fuel boiler, put the fire out.
  2. Allow the water in the system to cool down.
  3. Turn off the water supply to the central heating feed and expansion tank, either turn it off at the stopcock in the supply pipework or hold up the ball cock arm to shut off the supply (put a piece of wood across the top of the tank above the ball cock arm and secure the arm under the wood using a piece of string).
  4. Locate the drain cock for the system, this will usually be found in the return pipe near to the boiler, or under the floor at the lowest point in the system.
  5. Connect a hose pipe to the drain cock and secure it with a hose clip. Run the hose pipe to an outside drain gully.
  6. Go to every radiator and make sure that each valve is fully open - if you want to be able to reset the valves, count and make a note of how many turns each valve is opened.
  7. Open the drain cock using a drain cock key or an adjustable spanner - don't use pliers as this will round the corners.
  8. Allow the water to drain away - when the flow slows, open the bleed valves in the radiators; start with the highest radiator and work your way down. If water comes out of any bleed valve as it is opened, tighten it back up and wait for the water level in the system to drop before opening it again. Leave any radiators where they are fed from above.
  9. As the water flows out of the system check to see what colour it is:
    • If coloured a dirty grey, this is normal.
    • If coloured a dirty black, it's a sign of some sludge in the system - see refill page for remedial action.
    • If coloured rusty, it's a sign of some corrosion in the system - see refill page for remedial action.
  10. When no more water comes from the hose, the system should be empty unless there are any inverted loops, such as radiators fed from above (often found over solid floors). In these cases, there will be additional drain cocks are the low point of each inverted loop. If these need to be drained as well:
    1. Shut the main drain cock.
    2. Move the drain hose to each of the other drain cocks in turn.
    3. Repeat the draining procedure from each additional drain cock.
  11. When all the water has been released from each drain cock, the system should be empty, however there may be some water held in the system by air locks, so keep the drain cocks closed unless there is a drain hose attached.

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