Servicing a Vented Central Heating System


If, when the system was drained, the colour of the water indicated signs of sludge or corrosion in the system, the system should be flushed until the water runs clear.

The indication for sludge and corrosion are if the water was coloured:

  • Dirty grey, this is normal.
  • Dirty black, it's a sign of some sludge in the system.
  • Rusty, it's a sign of some corrosion in the system.

Sludge build up can be quite severe, especially after a number of years.

In severe cases, it would be easier, quicker and more effective to remove each radiator and flush it through using a hose pipe connected to the mains water supply. This can be messy but it will remove any large sludge build up much more effectively.

Once all the radiators have been flushed through in this manner, flushing the whole systems a couple of times should be enough.

If a new system or a new radiator has been installed, it is also worthwhile flushing through once to clear any solder flux residue or swarf from the pipework.

The sequence for refilling for flushing through is as below except that no inhibitor is added until the final refill after the water runs clear.


In hard water areas, lime scale builds up on the inside of the boiler's heat exchanger, in the pipework and radiators - this reduces the efficiency of the whole system and can cause the boiler to make a banging sound.

To de-scale the system use a mild de-scaler, harsher de-scalers can lead to added corrosion and leaks in systems which are old or already badly corroded.

  • Follow the manufacturers instruction when introducing de-scaler into a central heading system; usually it is fed into the feed and expansion tank in a similar manner as described below for inhibitor.
  • After the de-scaler has been added, run the central heating system continuously for at least a week with the boiler set to a high temperature and with all the radiators turned on.
  • Finally drain and flush the system three or four times.
  • Some de-scalers need to be naturalised before the inhibitor can be added with the final refill, so check the manufacturers instructions.


  1. When you come to fill a vented central heating system, go round to all the radiators and drain cocks and make sure that all the bleed valves and drain cocks are closed.
  2. Reconnect the water supply to the feed and expansion tank in the loft, either open the stopcock feeding them or release the ball cock arm if this was held up.
  3. As the tank fills, the water will flow down the system but will trap air in the radiators and any inverted loops.
  4. When the feed and expansion tank has stopped filling, add a corrosion inhibitor to the tank in accordance with the manufacturers instructions.
  5. Starting with the lowest radiator, open the bleed valve to release the trapped air, when water starts to come out of the valve, close it and move onto the next radiator working your way around the lowest floor level then working upwards.
  6. Check the pump and above the boiler for additional bleed valves, and if they are there, bleed these as well.
  7. As the radiators are bleed, additional water will fill the system and the inhibitor will be drawn down into the pipework. Once the system has been refilled, turn on and run the boiler and pump for at least 3 hours to pass the inhibitor around the system and mix it into the water.
  8. As the system settles down after refilling, it will be necessary to re-bleed the radiators as air trapped in the system will collect in them - it is suggested that the radiators are re-bled after one day, one week, one month and then every month until it is found that additional bleeding is unnecessary.

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