Domestic bliss ....................
Dear Ben ,
1 ) first generalisation . In well though out systems of fire installation air is ducted in from outside to ensure that room heat is not taken to drive the fire .
2 ) The burning of fuels is a mixed bag , if you for example burn wood , the material that may condensate out on the chimney lining , be it creosote , or humidity turning to water , trapping ash & filling , reducing / blocking the chimney .
I must admit , it ' s nice to see this work , but may be , leaving the chimney un ducted , even insulated , but have a top of chimney heat exchanger , be it air to water , or air to air & pull the heat off at one point , that could be easily pulled out to clean .
Regards Bob Woodliff