As the homeowner you can deal with this yourself or call in a professional service like Woolwich Plumbers who will carry out the task and give a guarantee. A fault with a central heating system is not generally an easy thing to deal with, as it involves water under pressure and also it is more than likely hot. A heating system issue, usually means that you have to drain all the water out to find the problem as a flood is another problem you don’t need. If you are brave enough.Basic Guide:1). The first thing to do is Isolate the system by turning off at the boiler, and wait for the whole system to cool. Turn off the boiler so the pump will not run. Turn off the supply to your feed and expansion tank.2). Up in the loft, use a wooden batten with a cord tied in the middle, place it across the top of the tank and tie the cord around the arm for the float valve keeping it raised up (or closed) position. Press a flexible rubber hose on the drain of the tank and run the hose out to a main drain, taking care to always make sure the flow is at least on the level and never uphill as this will stop the flow. Open the valve and allow the tank to drain, you should have some handy cloths just in case. A Woolwich Plumber carries all the correct tools to carry out this task. Keep a check on the hose and connections for any leaks.3). When the system is empty, you can carry out the repair. Then you can close the system up again and remove the cord and batten and open up the main stopcock. You will need to bleed all the radiators in the circuit and don’t forget the pump because air can also get into it too