I have got a one pipe system in an old terraced house I recently bought, I've had a plumber there, take out the old primatic cylinder and replaced the cylinder with an indirect cylinder and new header tank. The back boiler is working perfect I've been told. We've replaced the pump and drained, cleaned and flushed the system. We've bled the radiators but the system isn't working, the boiler only fires up when you are bleeding the system, what's wrong?, it´s got the plumber baffled. Thank you very much.