air in central heating system?

When you bleed the radiators, is the water that eventually comes out of them black? If so, it suggests that there is a certain amount of sludge build up in your system. This sludge (called magnetite) actually produces its own gas - largely hydrogen - (its an old plumbers trick to actually set it alight - impresses the punter). It could be time for a system flush if this is the case.

John
April 2009