Old wood stoves generally had a steel plate in the upper area of the firebox to slow the smoke down before it exited out the chimney. So a good portion of the heat went out the chimney; this is why in old homes you would sometimes see black smoke pipe snaking throughout different rooms. The homeowner was trying to extract the heat from the pipes before it was lost through the chimney.
The new high-efficiency wood stoves burn the wood and smoke to extract the heat more completely. This is achieved by one of two designs, catalytic or secondary burn. The most common type of wood stove you will see is the secondary burn stove although catalytic stoves are becoming more popular again.
The secondary burn stoves have a baffle with stainless steel air tubes above the firebox. Air is preheated and forced out through these air tubes & the heated air ignites the smoke and gases coming from the firewood. You may see flames swirling above the wood and this is the smoke burning.
Catalytic stoves use a catalyst to help burn the smoke by routing the exhaust gases through the combustor. The catalyst is a coated ceramic honeycomb which lowers the ignition temperature of the combustion gases. This allows these stoves to burn cleanly at low heat output settings resulting in longer burn times.
These new stoves can save you up to 30% in your firewood consumption and reduce smoke and emissions from your chimney by 90%. Good for your pocketbook & good for the environment!