A central heating system with a boiler will need to be flushed out eventually. When you have hard water in you home, you will end up with lime scaling in the heat exchangers which will cause the boiler to make a knocking sound. You may have corrosion or just a simple flush needed to keep up the preventative maintenance on your system.
Step 1 - Drain the System
First turn off all power to the system. Taking your bucket, go to each radiator that has a drain valve or cock and open to drain the water out of the system. If the water is just gray, that is normal and you just need to flush the system. If it is dirty black, you have some sludge build up and if it is rusty there is corrosion. Make sure you drain each of the radiators.
Step 2 - Flushing
Now you are going to close all the drain valves on the system. Using your water hose put water in through the feed tank in the loft. If you are going to have to descale your system, then you need to introduce the descaler at this time according to the instructions. After you have filled and added the descaler it is recommended you start the system and run the boiler on high with all the radiators open for at least a week. Then turn it all off and drain and flush the system with water until it runs clear. This will get rid of any scale and sludge you have in the system.
If you only have gray water that drained you will not need to add the descaler. Just fill and flush the system until the water is running clear through it.
Step 3 - Refilling the System
Now you are ready to refill the system and run it again. go around to all the radiators and make sure all the vents and bleed valves are shut off. Fill the water in the feed system and let it run down into the boiler. If you are going to add a scale inhibitor you should add it now per the instructions. As it runs down, there will be air pockets that come into the system as well. Going around to each radiator, open each valve to let the air out and then close them again. Let more water come down and do the same thing, making sure to close the bleeder valve after each air bleed off. Once you have bled it all off and refilled the system, restore power to the system and start the boiler and let it run. You will have to bleed off the valves again in a day, week and then a months time to get all the air out of the system. The scale inhibitor should help your system run clean.