New certified wood stoves have significant technological developments that greatly reduce emissions and lower wood consumption.  Old stoves were tested and found to be somewhere in the 20-40% efficiency range.  New stoves have to be tested as part of EPA certification and all have been found to be 65-75% efficient.  There are varying methods of achieving these efficiencies and clean burning but most of the stoves we sell use non-catalytic technology to burn the gasses more completely using strategic placement of air and high temperatures creating "secondary burn".