how do i get air out of my central heating system all my radiators constantly have air in them and my pressure keeps dropping have tried bleeding them in the usual way but doesnt seem to be working i have a vokera combi boiler linea rangegeorge

george, April 2009

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