I have got a Worcester Bosch combi boiler heating system with a MagnaClean filter. The heated towel rail in the bathroom is white and rusting, so I plan to replace it with a chrome rail to reduce the likelihood of future rusting. Am I right in thinking that when I disconnect the existing towel rail that a lot of the water in the system will drain out even with the valve which controls the supply to the boiler turned off? One of the radiator valves is faulty, it won't turn off, so I need to fit a pair of new valves on the new radiator, hence the question about water draining out.