Draft and Secondary Air Tubes

[ColdNorCal]

Have been looking at stoves and am curious to understand why two very similar stoves air tubes are very different. The two stoves are very close in terms of firebox size, EPA specs, BTU specs, minimum chimney height of 12 ft, 1 1/4" thick brick, made of steel... and 4 burn tubes. Two glaring differences are:

1 - One is an insert, the other freestanding.
2 - The freestanding 4 air tubes have a total of ~150 holes with all the holes being larger then the inserts ~100 holes.

With all things being very close to the same, I am curious which stove would be easier to draft. The freestanding with more holes that are also larger in size or the insert with fewer and smaller holes?

It would seem that more and larger holes would be less restrictive. However, it would take more air flow, cfm's, to feed the larger number and larger sized holes.

 

[begreen]

There are several factors. The longer the air has to travel and the more 90º turns in the path to the secondary tubes, the more resistance there will be that has to be overcome by draft. The size of the secondary manifold will also affect this.

 

[kennyp2339]

The chimney is the engine that drives the stove, a poor chimney will result in draft issues, couple that with poorly seasoned wood (wood that has a moisture content above 20%) and you will fight your setup the whole burning season.
Secondary tubes have little to do with draft of the entire stove. Most epa tube stoves have (2) different air intakes, the primary air intake, which can be controlled with a lever and then a smaller secondary intake which is designed much smaller but cannot be controlled.
Secondary combustion works primarily off of heat with in the stove, you load the stove, get the fire rolling and up to temps above 500 and the secondary combustion naturally takes off by separating the cooler, more dense heavier smoke vs the much hotter flue gasses in the upper baffle (above the tubes) the smoke then gets super heated from the fire in the box, mixed with room air in the secondary system and re-burnt out the tubes, the by-product is hot flue gasses which rise higher in the secondary baffle again and then out the chimney.
To establish good draft (almost 100% of todays stoves have "the good draft" your looking for) is to follow the manufacture specs on chimney requirement, that means on an insert limiting any bends, having a chimney that's at least 15ft in height, using an insulated liner (keeps flue gasses hotter) using the right size liner for the stove.
On free standing stoves that means coming out of the flue collar and going straight up with the chimney, if you have to do a chimney on the outside of the house then coming out the stove using double wall pipe, using to 45deg elbows vs 1 90deg elbow (reduces turbulence) having a minimum chimney height of 15ft. If building a chimney using class a double wall insulated pipe vs straight masonry (warmer flue gasses) if venting into a masonry chimney then installing a insulated liner (warmer flue gasses) If your masonry chimney is not high enough, there are chimney kits that allow you to install a anchor plate that has a class a chimney adapter, then you can extend the height of the chimney with class pipe.
The moral of all of this is that stoves designed to meet all the current epa clean air rules and tested an proven, there is no real issue with a buyer to find a certain stove for draft, (this isn't the 70's & 80's) the stoves of today are designed on cad programs and ran through many tests, the buyer should really focus on there chimney when deciding on a stove, without good venting no matter what you buy, it will never hit the design tolerances it was tested to.

 

[ColdNorCal]

The chimney is the engine that drives the stove, a poor chimney will result in draft issues, couple that with poorly seasoned wood (wood that has a moisture content above 20%) and you will fight your setup the whole burning season.
Secondary tubes have little to do with draft of the entire stove. Most epa tube stoves have (2) different air intakes, the primary air intake, which can be controlled with a lever and then a smaller secondary intake which is designed much smaller but cannot be controlled.
Secondary combustion works primarily off of heat with in the stove, you load the stove, get the fire rolling and up to temps above 500 and the secondary combustion naturally takes off by separating the cooler, more dense heavier smoke vs the much hotter flue gasses in the upper baffle (above the tubes) the smoke then gets super heated from the fire in the box, mixed with room air in the secondary system and re-burnt out the tubes, the by-product is hot flue gasses which rise higher in the secondary baffle again and then out the chimney.
To establish good draft (almost 100% of todays stoves have "the good draft" your looking for) is to follow the manufacture specs on chimney requirement, that means on an insert limiting any bends, having a chimney that's at least 15ft in height, using an insulated liner (keeps flue gasses hotter) using the right size liner for the stove.
On free standing stoves that means coming out of the flue collar and going straight up with the chimney, if you have to do a chimney on the outside of the house then coming out the stove using double wall pipe, using to 45deg elbows vs 1 90deg elbow (reduces turbulence) having a minimum chimney height of 15ft. If building a chimney using class a double wall insulated pipe vs straight masonry (warmer flue gasses) if venting into a masonry chimney then installing a insulated liner (warmer flue gasses) If your masonry chimney is not high enough, there are chimney kits that allow you to install a anchor plate that has a class a chimney adapter, then you can extend the height of the chimney with class pipe.
The moral of all of this is that stoves designed to meet all the current epa clean air rules and tested an proven, there is no real issue with a buyer to find a certain stove for draft, (this isn't the 70's & 80's) the stoves of today are designed on cad programs and ran through many tests, the buyer should really focus on there chimney when deciding on a stove, without good venting no matter what you buy, it will never hit the design tolerances it was tested to.

Yes, but does the draft "pull" the air from the secondary tubes? And would more and larger holes make it easier or more difficult to draft the stove compared to the same stove with fewer and smaller holes?

btw - I recently installed a stove. Just have been looking at it and am curious how it works. This stove requires 12 ft min chimney, measured from top of stove, and allows 2 90's. I installed 1 90 adjusted to ~75 with the flex liner making a nice round bend to make the vertical run with a total of 15 1/2 ft chimney height. Insulated liner installed inside existing chimney in middle of house.

 

[kennyp2339]

Yes, but does the draft "pull" the air from the secondary tubes? And would more and larger holes make it easier or more difficult to draft the stove compared to the same stove with fewer and smaller holes?

btw - I recently installed a stove. Just have been looking at it and am curious how it works. This stove requires 12 ft min chimney, measured from top of stove, and allows 2 90's. I installed 1 90 adjusted to ~75 with the flex liner making a nice round bend to make the vertical run with a total of 15 1/2 ft chimney height. Insulated liner installed inside existing chimney in middle of house.

The draft doesn't change, due to the secondary air tubes, the secondary system function is dependent on heat from the main fire, now there have been noted issues with uncontrollable stoves due to strong draft.
The mechanics is different, some members have encountered a compound issue, they had a strong draft, good dry wood an experienced high internal fire box temps as a result, even with the primary air turned as low as it could go, the secondary reburn system then caused even higher temps because the initial fire was burning higher, thus they had stove top temps in the high 700's low 800's which could be considered an over fire, the solution for some members to slow things down some was to limit or block some of the combustion air going into the secondary manifold with foil or a heavy duty magnet, many had good results, others only had decent results once the firebox got up to temp.

 

[ColdNorCal]

The draft doesn't change, due to the secondary air tubes, the secondary system function is dependent on heat from the main fire, now there have been noted issues with uncontrollable stoves due to strong draft.
The mechanics is different, some members have encountered a compound issue, they had a strong draft, good dry wood an experienced high internal fire box temps as a result, even with the primary air turned as low as it could go, the secondary reburn system then caused even higher temps because the initial fire was burning higher, thus they had stove top temps in the high 700's low 800's which could be considered an over fire, the solution for some members to slow things down some was to limit or block some of the combustion air going into the secondary manifold with foil or a heavy duty magnet, many had good results, others only had decent results once the firebox got up to temp.

I wonder if the opposite is also possible. Rather then blocking the intake of the air tubes due to high temps and secondary burn, force a small amount of air into the tubes. In other words, if some do not have a good secondary burn would it be possible to push a small amount of air into the air tubes manifold, creating a good secondary burn. I read about "lazy" burns from some and this made me think about the possibility of forcing air into the tubes, like a turbo charger

Update: Are you saying that the heat from the fire heats up the air tubes which then draws colder air from the manifold?

 

[kennyp2339]

Are you saying that the heat from the fire heats up the air tubes which then draws colder air from the manifold?

not exactly, the heat from the fire causes a circulating motion that allows the hot gases to go up the chimney and the cooler flue gases that have many unburnt particles to sink going into the secondary reburn system, the air entering the secondary manifold is preheated also by the initial fire, essentially the air entering the secondary system is being used for its oxygen to mix with the unburnt particles to re-fire them off again creating a cleaner burn, not all stoves that have secondary reburn capabilities have tubes, some stoves use a down draft technology, others have a rigid stainless steal baffle with holes incorporated in them, its just that tubes have been around a long times and most prevalent in popular stoves.

 

[begreen]

I wonder if the opposite is also possible. Rather then blocking the intake of the air tubes due to high temps and secondary burn, force a small amount of air into the tubes. In other words, if some do not have a good secondary burn would it be possible to push a small amount of air into the air tubes manifold, creating a good secondary burn. I read about "lazy" burns from some and this made me think about the possibility of forcing air into the tubes, like a turbo charger

Update: Are you saying that the heat from the fire heats up the air tubes which then draws colder air from the manifold?

If one is burning well seasoned wood properly, the last thing you want to do is turbo-charge the fire at the strong outgassing stage. That can lead to overfiring or possibly a lot of secondary combustion occurring out of the stove and in the stove pipe or liner. The result is more heat up the chimney and less in the house. Lazy flames are a good sign that the gases are staying resident in the stove and are being more completely combusted. The term is relative to both the stove, the wood and how the air supply is managed. I strongly recommend not tampering with the stove's design or configuration unless absolutely necessary.

Here's a short video of our stove with a full load of wood. Note the billowing flame coming down from top to bottom. This is caused by secondary air mixing and turbulating the fire.

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This is a different stove design with tubes. Note how it is burning much more at the top where secondary air is mixing with the wood gases.

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