And it represents more material that must be pre-heated so as not to chill the smoke to below 1200°F. Getting back to our example, if we mix our 1400°F smoke with an equal amount of room temperature secondary air, the resultant temperature of the mixture will be less than 800°F, far less than the necessary 1200°F. No reaction. Poor efficiency. A lot of creosote. A lot of pollution. Bad. You can probably see that you are going to need a source of secondary combustion air at about 1000°F or higher under these conditions. A pre-heater will be necessary to boost the room air to 1000°F.
Unfortunately, there are some conditions of low fire (severely held back) where the smoke itself is under 1200°F within inches of the logs. In that case secondary combustion is almost out of the question. It is ironic that in the situation of a severely suffocated fire that most needs the effect of secondary combustion, it is most difficult to obtain. When the fire is burning relatively freely (and therefore cleanly), that is when it is easiest to initiate secondary combustion.
Again let's get back to the example at hand. We need to pre-heat air to 1000°F. It will be necessary to use a heat exchanger to do this. Some current products have a 6" long tube to pre-heat the air as it passes through. We'll see that this isn't even close to enough exchanger surface. Assuming that the stove consumes 35 CFM of primary air for the fire, we will also need 35 CFM of secondary air as described above. To heat 35 CFM from 70°F to 1000°F will take about 35 x (1000-70) x 0.24 / 28 * 60 or approximately 17000 Btu/hr. The 0.24 is the air's specific heat; the 1/28 is the air's specific volume at the mean temperature; 60 is the number of minutes in an hour.
When we are talking about a unit that is only going to develop 20,000 or 30,000 Btu/hr, you can see that we are going to have to use more than half of the capability for pre-heating. The secondary combustion might add 25% to the output (maybe 7000 Btu/hr) but you use 17000 to do it. A losing proposition. Except for the safety considerations, it would be foolish to consider.
Conventional heat exchange analysis (see other sheets in the 300 series) will give the necessary areas of heat exchange for this pre-heater. We will avoid the math here. A two stage boost heater is most logical and effective here, where the first stage heats the air to (600°F in our example). The necessary area in a 700°F part of the stove for this exchanger is 1.6 sq. ft. The air then passes to the second exchanger to be heated further (to 1000°F) in a hotter part of the firebox right over the flame tips. The necessary area of this exchanger is 1.5 sq. ft., assuming the smoke temp is 1300°F in that part of the firebox.
If the supply tube is 2" in diameter, the first exchanger must be nearly 9 feet long (wrapped around inside the firebox) and then the second will also be about 9 feet long. The secondary combustion air supply would have to pass through a total of 18 feet of specially located heat exchanger to ensure good secondary combustion. There would not be much room left in the firebox in most stoves for any exchangers for USEFUL heat. And remember, even then there are conditions when secondary combustion still won't occur. Is there any wonder why currently available products with a stub tube pre-heater don't work?