Primary vs Secondary vs Tertiary Burns - What Gives???

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Boomstick Bruce

New Member
Nov 14, 2021
5
Montreal
I'm new to wood stoves. Just finished building my house and went with a Hearthstone Heritage IV as primary means of heating during the winter. Been working it for about a month now and I'm loving it. I've been reading up a lot on these forums and I see a lot about primary, secondary and tertiary burns.

From what I understand :
-Primary burn : The wood actually burning
-Secondary burn : The gases that eventually burn up in the firebox
-Tertiary burn : The remaining gases/fumes that are burned in the catalyst.

I read that some stoves have primary air and secondary air controls. Mine only has one air control. At times I've seen secondary burns happen (quite cool I might add) but it seems to happen somewhat randomly. Because of the stone design, I don't think I can ever see the tertiary burn cuz I can't see the catalysts.

So I was basically wondering. What gives with secondary burning. Is it a good sign ? Bad sign ? I think the metals tubes at the top of my stove are secondary air tubes, but give how I don't have a control, wasn't sure what they are for.

So yeah, just overall curious about how to manage secondary combustion and avoid or optimize it.

Cheers all!
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
26,867
central pa
I'm new to wood stoves. Just finished building my house and went with a Hearthstone Heritage IV as primary means of heating during the winter. Been working it for about a month now and I'm loving it. I've been reading up a lot on these forums and I see a lot about primary, secondary and tertiary burns.

From what I understand :
-Primary burn : The wood actually burning
-Secondary burn : The gases that eventually burn up in the firebox
-Tertiary burn : The remaining gases/fumes that are burned in the catalyst.

I read that some stoves have primary air and secondary air controls. Mine only has one air control. At times I've seen secondary burns happen (quite cool I might add) but it seems to happen somewhat randomly. Because of the stone design, I don't think I can ever see the tertiary burn cuz I can't see the catalysts.

So I was basically wondering. What gives with secondary burning. Is it a good sign ? Bad sign ? I think the metals tubes at the top of my stove are secondary air tubes, but give how I don't have a control, wasn't sure what they are for.

So yeah, just overall curious about how to manage secondary combustion and avoid or optimize it.

Cheers all!
Most stoves only have one air control. You really can't change the secondary burn at all other than with firebox temps.
 

EbS-P

Minister of Fire
Jan 19, 2019
1,713
SE North Carolina
Evidence by way its use in many/most epa stoves that secondary combustion is good. It happens at high firebox temps. 1100-1200 degrees sticks in my mind.

Many single control stoves have a fixed secondary air metering. (My F400 does) and you control the burn rate by changing the amount of primary air. Once I get a coal bed I can almost always close my primary air all the way and all I see is secondary combustion. Wood is hot off gassing no oxygen down low but plenty up high from the baffle. And it ignites.

The less primary air the more secondary combustion I see but I need some minimum amount of heat to sustain this.

I light a top down fire and can see good secondary combustion very early in my burn.

My question about secondary combustion is how clean and efficient is it? When I run full closed and rolling secondaries am I burning as clean as I can, or should I be opening up the air more for a cleaner burn? (I’ve never had much creosote and mostly fluffy fly ash).

Seems to me some stoves, to meet 2020 emissions, increases the secondary air available to the fire box. Hotter fire=cleaner fire I guess.
 

neverstop

Member
Oct 11, 2020
104
new hampshire
Most stoves only have one air control. You really can't change the secondary burn at all other than with firebox temps.
Not trying to thread-jack but I'll give my experience:
The indirect control on the secondary combustion can be slightly concerning. As I'm shutting down the primary after cat engagement it seems like the more I shut down the air the more intense the secondary flames become and the higher the stove top temperatures get. At the lowest primary air setting the secondaries can still be raging and the STT continues to climb. Basically a perpetual cycle of higher firebox temps result in more secondaries, results in higher firebox temps. Although my previous stove didn't have a glass door so for all I know it was an absolute raging inferno in there, and just being able to see the flames has me at a high pucker factor.

Is the only way to mitigate this to keep STT lower prior to reducing primary?
 

EbS-P

Minister of Fire
Jan 19, 2019
1,713
SE North Carolina
Not trying to thread-jack but I'll give my experience:
The indirect control on the secondary combustion can be slightly concerning. As I'm shutting down the primary after cat engagement it seems like the more I shut down the air the more intense the secondary flames become and the higher the stove top temperatures get. At the lowest primary air setting the secondaries can still be raging and the STT continues to climb. Basically a perpetual cycle of higher firebox temps result in more secondaries, results in higher firebox temps. Although my previous stove didn't have a glass door so for all I know it was an absolute raging inferno in there, and just being able to see the flames has me at a high pucker factor.

Is the only way to mitigate this to keep STT lower prior to reducing primary?
Bigger splits tighter packing of the firebox can slow down the out gassing. And yes closing air down sooner. This is why many use flue gas temps and not stove top temps as they are a better gauge of the rate of combustion.
 
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Highbeam

Minister of Fire
Dec 28, 2006
19,529
Mt. Rainier Foothills, WA
Not trying to thread-jack but I'll give my experience:
The indirect control on the secondary combustion can be slightly concerning. As I'm shutting down the primary after cat engagement it seems like the more I shut down the air the more intense the secondary flames become and the higher the stove top temperatures get. At the lowest primary air setting the secondaries can still be raging and the STT continues to climb. Basically a perpetual cycle of higher firebox temps result in more secondaries, results in higher firebox temps. Although my previous stove didn't have a glass door so for all I know it was an absolute raging inferno in there, and just being able to see the flames has me at a high pucker factor.

Is the only way to mitigate this to keep STT lower prior to reducing primary?

Since the primary air is all you have (partial) control over, the secondary air just supplies more oxygen as you reduce the primary air. It's like a ratio lever. When the chimney is pulling at a particular vacuum (or flow rate) and you reduce the opening for the primary air, the suction pulls even more air through the secondary opening to try and satisfy the chimney vacuum.

The hotter a fire burns, the stronger the chimney vacuum which sucks even more oxygen in to heat up the fire more, and on and on. I don't like this lack of control but it makes it so that the emissions are relatively low all the time.

On a stove prone to runaway, some are worse than others, it is very nice to have a key damper to regain control. I ran a Lopi noncat that would run away on us. It wouldn't get to the point of glowing on the outside but we were unable to reel it in to a reasonable temperature. Not cool!