Downdraft stove coal orientation optimization

[BurningIsLove]

The following is specific to Dutchwest/VC non-catalytic "everburn" stoves, but it may be of use for any downdraft stove that has a similar design. I still encounter the ever annoying 'everburn stall' from time to time, where the firebox has a thick hot coal bed, coaled dry splits, etc., but it still stalls out rapidly (read the various threads on this for details). Often times, simply shuffling the splits around and tweeking the coal bed orientation has been enough to spontaneously go from repeated stalls to optimum operation, but sometimes w/o success.

The following is a result of a lot of trial & error and what I've learned. Although the principal of downdraft operation is to pull secondary and primary air through the coal bed to heat it prior to entering the fountain area above the refractory shoe, the movement of that primary air & gas seems to be more crucial. Too much coals and not the correct orientation can lead to a lot of stalls as the airflow isnt enough to sustain a secondary combustion.

One method to help is as follows. When the splits are largely coaled and you are ready to reload fresh splits, before you load the new splits take some time to create a channel in the coals leading from front-back running from the front of the stove to the shoe opening. (N/S or did I remember backwards?). It should look like the outline of a boat in water. I've tried to create an image below, but Im an engineer, not an artist. The idea is to create a clear channel that sucks air & hot gases from the front/top of the firebox down the front of the stove to the bottom, then n/s along the coal bed (but not through the coals themselves) to the shoe opening. This seems to assist in the drafting that is so critical in this style of stove's operation. As the bottom of the splits will be the most coaled, and the coal bed itself is darn hot, the gases will be heated greatly by following this path vs. moving over the top and down the back, which is traversing the coldest part of the firebox

The picture is viewed from the front of the firebox looking into the stove. Often times the freshly loaded splits may move the coal bed cutting off the channel. When this happens, or my channel is not large enough, I use a poker to SLOWLY push back towards the refractory shoe and widen the channel. If you encounter resistance, STOP and dont try it....the risk of punching through a blockage and ramming the shoe w/ the poker is not worth it. Move slowly, back and forth (E-W) if need be, but again, its not worth the risk if you cant. And please, dont pull burning splits out of the stove to reorganize, that's just plain silly. And dont try this w/o stove/welding gloves, you all know how hot it gets opening the front loading door w/ a big coal bed.

Thoughts/discussions welcome

 

[Diabel]

During the day I load my stove with whatever I have (lots of irregular splits) & I get different results every time, sometimes the placing is ok & I get a good burn, other times I have to reposition these "irregular" splits (no big deal). As for the overnight I pick & choose the perfect splits

 

[BurningIsLove]

Not at all surprising......the flow of air around splits when the stove is being 'finicky' is also important. At times like right now, where the stove is running great (almost too hot w/ the air completely choked off)...I could throw anything in there I wanted and it would burn well w/ a nice secondary rumble, clean emissions, etc. At times like these, I throw on the less seasoned splits one at a time every few hours and instantly close the bypass after loading. The splits get heated & dried from the other fuel. I dont care about the 'wasted heat' to dry the less seasoned splits as the stove is throwing out way more heat that I need. And at 1200 degrees flu gas, Im less concerned that any water vapor is going to condense causing creosote in the stack.

I do something similar as you during the day if its not that cold and Im not burning 24x7, I load in the 'less desirable' splits, and burn hot, quick fires w/ the bypass open. during the evenings, I burn the 'good stuff'.

 

[Diabel]

I do something similar as you during the day if its not that cold and Im not burning 24x7, I load in the 'less desirable' splits, and burn hot, quick fires w/ the bypass open. during the evenings, I burn the 'good stuff'.

During the day I burn a lot of branch wood...hence the uneven pieces, but I always close the bypass once ready for the everburn, otherwise I'd be wasting fuel..

 

[ICY99]

Very interesting! Thanks for the input.

 

[bmwbj]

With my Intrepid, I always place my coal bed directly under the rear "hood" entrance
to the cat. and load my fresh splits in the front ---- I never had a stall condition when
I do it this way ---- any other method will cause a stall. I guess the hot coals preheat the
smoke before entering the cat.

 

[schortie]

I have the least amount of stalling when the splits are just above the shoe opening - I place them e/w on top of small n/s splits on either side of the shoe opening, which makes a "well" front to back. I suppose this is my version of what Burning is Love is discussing. But as the burn progresses, the secondary stalls when there are only coals in the back and the rest of the wood is towards the front of the stove - at least the rumble is gone. The stove keeps a consistent temp, but I do notice emissions from the chimney. Has anyone else experienced this? Any recommendations?

 

[Rowski]

Thanks for the explanation and graphic!!

I will give it a try tonight.

 

[BurningIsLove]

I have the least amount of stalling when the splits are just above the shoe opening - I place them e/w on top of small n/s splits on either side of the shoe opening, which makes a "well" front to back. I suppose this is my version of what Burning is Love is discussing. But as the burn progresses, the secondary stalls when there are only coals in the back and the rest of the wood is towards the front of the stove - at least the rumble is gone. The stove keeps a consistent temp, but I do notice emissions from the chimney. Has anyone else experienced this? Any recommendations?

Yes, I sometimes do the same thing, and have encountered the same situation.....you have a nice burn going, and after some time as the fuel is partially consumed, there is an audible settling of the splits, and the rumble INSTANTLY stops and the secondary combustion stops. It also works in reverse too, where the stove is silent, there is a settling, and the rumble instantly starts up.

I generally dont have much luck w/ piling the coals up too much in the back towards the shoe, as draft & circulation are so critical to downdraft stoves like this. I will put a few small coals into the throat/fountain opening, but not much as I dont want to negatively impact draft. The channeling of the gases through a well seems to heat up the waste products enough to achieve secondary, but certainly nothing is guaranteed.

 

[Diabel]

This is a bit off topic but what flue (using magnetic thermo.) temperatures are you running through out the burn cycle?

 

[BurningIsLove]

This has been discussed a lot in other threads, but in summation......

Typically my stove top thermometer runs around 450-550. But that is not always an accurate indication that its ready for secondary combustion on its own.

 

[Diabel]

I am not sure if it has been discussed that much. What I am trying to establish is that these everburns need a very high flue temp in order to work right. Mine needs to run at around 450-500* using outside magnetic thermo. which translates to around 900-1000* interior That is way higher than other technologies. Are we wasting that much fuel?

 

[BurningIsLove]

I'll post in some relevant threads so you can read 50+ pages of discussion/debate on the topic, hahahahaa!

Not specific to everburn (VC's trademark name for non-catalytic downdraft stoves), the rule of thumb is that they do need to be run hotter than a catalytic stove due to the lack of a catalyst for secondary combustion. On average, a catalytic needs 550 degrees (interior, not the stove top temp) in order to reburn the gases & smoke. A non-cat requires that temperature to be about double at about 1100 when the gases & smoke are pulled through the refractory material.

The above is one reason why a coal bed is so essential to proper use of a non-catalytic. By pulling the waste gases & smoke through a hot coal bed, they are brought up to the necessary (higher) temps required.

So its not that non-cats are wasteful or require more fuel per se, but they do have a higher operating temperature in order to burn cleanly. Once they are going, the non-cats reburning process becomes self sustaining and produces a lot of extra heat, so its not wasted btus.