I am looking to replace a rad in a room with a bigger rad as it is too small. I asked a plumber and he said I would have to flush the system after - hence higher price. I though it was just a case of closing off valves, pouring out contents, and sorting the fixings to the wall. Once the rad is replaced and valves etd open is there anything else that needs to be done? I know there's the valve that you open to increase the pressure in the system and I was assuming I might have to do this if, because of the increased rad size, the pressue at the boiler drops. Any tips?