Whilst there can be a multitude of reasons the most frequent is to flush the heating radiators. Before any work is attempted you need to turn off the water heating pump and central heating boiler. To successfully complete this chore you also need to check that no more water enters the central heating system. Almost all contemporary home heating systems are supercharged by very economical condensing boilers. Costly gas prices and the chances of more such increases suggest you must find an effective heating boiler for your household. If you have a central heating boiler maintenance plan such as British Gas Homecare you might want to speak to your provider to see what services they offer.
central heating systems are normally filled up through a expansion cylinder in the attic. The expansion tank uses a ball valve similar to that in a toilet system. Rising water levels in the tank mean that the valve is blocked as the ball floats higher on the water surface. However when some water is pulled off the cylinder, the ball drops, the valve opens and more water gets into the cylinder. This process will need to be stopped if the central heating system is to be fully flushed. One of the better ways to stop this is by putting a piece of wood across the water tank and splicing the value arm to it, which will close this valve.
Now take a hose pipe emptying into an external drain and connect this to the outlet valve on the bottom of your metal radiator. Water will now current out of the metal radiator through the hose pipe into the external drain. The hosepipe needs to be connected to the lowest point in your central heating system which will be one of the ground floor radiators. You now need to be diligent and check that all the water has discharged out and that no more water is getting in before any other work is executed on the gas heating system.
If you need to complete this process rapidly you can also vent the bleed valves on the metal radiators on the top floors. You can start work on the central heating system once all the water has flowed out.