When a boiler is installed on an existing central heating system, it is essential that the system be chemically flushed out to remove any build up of ‘sludge' (generally iron oxides). The system should then be refilled and treated with a corrosion inhibitor / protector. Failure to carry out this procedure (as specified in the Benchmark Central Heating Log Book) during installation may cause the new boiler to be contaminated by debris from the existing system. Because ‘modern' boilers are hold less water and are much smaller than their predecessors, they are more liable to be affected by debris in the system: heat exchangers, pumps or divertor valves are most at risk. The guarantee of the boiler can be deemed void if the debris from the old system damages your new boiler.