Classicat,
These microturbine units don't work by recovering energy that the fuel burning appliance wastes. As you note, modern fuel burning appliances are _very_ efficient, in terms of thermal energy released by the fuel versus thermal energy delivered to whatever you want to heat up.
Instead you should think of these microturbine systems in which electricity is generated relatively inefficiently, but then the waste heat goes to heating up something that you were going to heat up anyway.
With a modern gas water heater, you might put 98% of the thermal energy of the fuel into the water. With one of these turbines, then if you are lucky 30% of the thermal energy of the fuel is delivered as electricity, and maybe 55% or 60% is delivered as heat to your water, and 10% or 15% goes up your exhaust stack.
However the system is viable because electricity is a much more valuable form of energy than heat.
-Jon
