If your boiler was fitted pre 2005 its likely you have a standard efficiency non - condensing boiler, which means the flue gas temperature of your boiler could be 200 degrees centigrade and all that heat is being wasted. A modern condensing boiler will often have a flue gas temperature as low as 40 degrees centigrade meaning all that extra heat is finding its way into your radiators rather than disappearing off in to the world.