Why do different woods need different air settings?

[d.sebens]

Assuming moisture content are the same, why does different woods require different low burn air settings? Seems like soft woods and light hardwoods need turned down a lot more on the air but dense wood like hedge seems like it requires more air or it goes out.

I get softwoods can burn hot and fast due to the extra surface area of the wood that can interact with oxygen but why would a dense wood need more air? My guess is possibly the heat it is taking to warm the log maybe?

 

[bholler]

Assuming moisture content are the same, why does different woods require different low burn air settings? Seems like soft woods and light hardwoods need turned down a lot more on the air but dense wood like hedge seems like it requires more air or it goes out.

I get softwoods can burn hot and fast due to the extra surface area of the wood that can interact with oxygen but why would a dense wood need more air? My guess is possibly the heat it is taking to warm the log maybe?

Different density

 

[d.sebens]

So while the right amount of air for clean combustion for a specific weight of wood doesn’t change, adding denser wood makes a fuel rich situation with the same air setting due to the increased amount of fuel being consumed? Is that what you mean?

In car terms the most efficient stoich ratio is 14.7:1. Is this comparable to saying denser wood is like pressing the accelerator more, thus requiring more air to keep up with the fuel? Essentially keeping the same ratio of fuel and air in the fire box?

 

[fire_man]

I thought all hardwood needed about the same air until I started burning Locust. The firebox needs to be heated to a much higher temperature for the Cat to light off with Locust and it was very old (4+ year) seasoned wood.

Once it lights off, I can throttle it down to the low setting I normally burn.

 

[stoveliker]

Indeed, the chemistry outcome does not change (much) - perfect combustion from cellulose and other components will be the same no matter what the density is. But the pathway to that end result may be different; the intermediary steps and gases produced may be different - meaning that the air supply may need to be different. E.g. in some cases you may need more primary air to create the gases that secondary combustion (tubes, or cat) would further oxidize.

There are a lot of parameters dictating the optimal air setting; temperature wanted in the home, outside temperature (draft), wood dryness, wood density, wood packing (tetris) in the stove, etc. It's going to be hard to tease out the influence of one of these parameters as some of the others are not easily controlled by us. Burning a lot will result in you knowing what works best (even if not always being able to reason exactly how or why due to this large parameter space).

 

[EbS-P]

My explanation is the higher density take more heat to reach off gassing temps and the off gassing is slower . Generating less secondary combustion. Stoves with unregulated secondary air softwoods get to off gassing temps faster and then “burning” down require as much primary as the jet generated by the increased secondary combustion creates self sustaining fire with non red for primary air.

 

[fire_man]

My explanation is the higher density take more heat to reach off gassing temps and the off gassing is slower . Generating less secondary combustion. Stoves with unregulated secondary air softwoods get to off gassing temps faster and then “burning” down require as much primary as the jet generated by the increased secondary combustion creates self sustaining fire with non red for primary air.

That makes sense but I didn't think Locust was that much denser than Red Oak and Locust needed a much higher firebox temp to get cat lightoff. It really surprises me how much longer I have to heat up the load before bypassing, or I end up with a smoldering unlit cat.

The Locust and Oak were both stacked and covered for 4+ years in the same pile.

 

[sneefy]

So while the right amount of air for clean combustion for a specific weight of wood doesn’t change, adding denser wood makes a fuel rich situation with the same air setting due to the increased amount of fuel being consumed? Is that what you mean?

In car terms the most efficient stoich ratio is 14.7:1. Is this comparable to saying denser wood is like pressing the accelerator more, thus requiring more air to keep up with the fuel? Essentially keeping the same ratio of fuel and air in the fire box?

Sounds like you already know the answer and are overthinking it. Denser (more) fuel, be it wood or gas, needs more oxygen.