I need to change my old back boiler and I've been told I can but I need to qualify in order to exempt me from having to fit a condensing boiler, which leads me to the 2 questions I have. 1) is there such a thing as a condensing back boiler and if so who makes it, and 2) what conditions do I have to meet in order to allow me to replace my old conventional back boiler with a new conventional one