The mechanics of Top Down, Why it works so well.

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bigealta

Minister of Fire
May 22, 2010
1,814
Utah & NJ
So with pushing Top Down set ups about as far as possible, i think i've figured out much of why it works so well in woodstoves,
Especially modern epa stoves with Secondary Combustion.

Today i did probably one of the most Aggressive set ups possible in my jotul f400.
2 East / West rows of Full Size splits that topped out about 2 inches from the baffles.
The bottom splits rest on only 3 sleepers 1" to 2" thick, and the
2nd row sits on 1" thick and smaller sliver and stick sleepers resting on top of the bottom splits.

With barely enough room to get much kindling on top of the 2nd row, the small V gap between the front row and back row held 4 pine cones and small stick kindling.
A very few pieces of small sized sticks were jammed in between the top row and the baffles.
I lit the pine cones in 3 places with an oak sliver.
It caught and burned well with the door closed immediately after lighting.

With such a small amount of starting kindling, i knew i would have to add at least 2 more pieces of stick kindling. Which i did after 1 or 2 minutes.

Closed the door immediately again.
Secondary flames start appearing Very quickly, In just 3 minutes or so and built from there.

Full Secondary flames in 5-6 minutes with flue temp reading 325F in about 6 minutes.
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So why does it work so well?

The biggest reason i think is you've created a Flexible size fire box.
The small top fire is essentially starting in up to an 80-90% smaller firebox. With the splits underneath taking up the majority of the space below and a "platform" to start on, you are practically "Raising" the floor of your box. The more you raise it with splits the smaller the "Starting" size of your box.

This "Smaller" box is now also Much closer to the baffle and secondary tubes. The starter flames immediately touch the tubes heating them, and the baffle, instantly. As the starter fire is small with limited fuel the smoke it can produce is very limited. Since the flames do not have to run thru larger splits (as it does with a bottom up) the flames are not cooled on a long path to the baffle. And the warming of those splits are not producing extra smoke from the weak limited heat.

As the Top down burns down, the firebox size grows as the splits below start to burn. The smoke from those splits now travels up thru the starter fire flames and into the already fully burning secondary tubes where it gets the chance to be reburned. As the fire progresses, the top air gap increases allowing more air velocity under the baffle and thru the splits below. The increased air flow, as the firebox size "grows", pumps the vigorous burning to higher levels.

This process continues as the fire works it's way down to the base of the firebox.

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Follow up showing a clearer view of how high a top down start can be set up.

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Top down fires with full loads close to the baffle definitely take off faster than half loads for the reasons you described. I typically don't bother with the sleepers. Sleepers shave off some time for sure and I may use them if in a rush but I'm usually not. My stove isn't super deep being only 1.8 cu ft so I don't have a ton of appropriate sized sleeper pieces. I just make a U shape with splits loading EW and leave 1-2 out of the middle to fill with kindling. It eventually collapses on itself as it takes off and leads to a nice quick starting fire. I have good draft and can often close the door right away as well.

I've actually never tried a bottom up fire in my wood stove. No plans to. I remember doing it that way with my grandfather in the fireplace growing up. It worked but was always so smokey. Top down for life. I even do it at my mom's in her open fireplace. Works great.
 
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My buddy moved and gave me a bunch of shorts mostly 8”, 9”, & 10”. I Re split them for sleepers. In my stove they really help get the secondaries going much faster. Got ‘em stacked up and ready to go.

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Caw already said it, but I’d leave the sleepers out. Even the bottom ones. It would buy you the room for the extra kindling you needed to add in after the fire was going.

Are you calling the middle row of kindling sleepers also? Maybe leave that out and…
When I had the Englander, I would put large splits on the bottom, E/W, with a little smaller, N/S, and a little smaller E/W, each reducing by about 50% in size. Then I’d place the kindling on top, from 1” kindling to 3/16” scraps and a piece of fat wood I harvested.
It always lit off well.
 
In my smaller stove I have now, this is offend quite sufficient. Some pine kindling on three dry oak splits.

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Caw already said it, but I’d leave the sleepers out. Even the bottom ones. It would buy you the room for the extra kindling you needed to add in after the fire was going.

Are you calling the middle row of kindling sleepers also? Maybe leave that out and…
When I had the Englander, I would put large splits on the bottom, E/W, with a little smaller, N/S, and a little smaller E/W, each reducing by about 50% in size. Then I’d place the kindling on top, from 1” kindling to 3/16” scraps and a piece of fat wood I harvested.
It always lit off well.
I don't need any extra kindling, only needed that 1 time when i maxed out split sizes and only had like 1" space under the baffle. If i have about 2" or 3" of space that's all i need in my stove for good starts. Just light it and close the door right away and come back in 2 to 3 hours to reload on the coal bed.

Yeah you were doing what i call a Combo load in the englander. In that case i would not use sleepers/spacers as the air flow is much better. But my bottom row would be N/S then E/W (I have a youtube showing this).

In my stove with E/W loads, the airflow is blocked too much, especially when loading large splits. I've tried it many, many times both ways. For my set ups with large E/W splits, starts are much better with sleepers than without them.

I could leave the middle spacers out, but again there is a noticeable improvement in cold starts with them in my stove. I live in a pretty tightly packed neighborhood and want to minimize my smoke output, while still getting a long uninterrupted burn time.
 
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Ok. Yep. Doing all E/W, the intermediate sleepers probably help a lot.
I was a lot looser in the Englander like you said.
 
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I think that the key is the kindling is not wasted trying get a thick block of wood burning, and the firewood is not wasted bringing secondary tubes up to temperature. The kindling quickly heats the secondary tubes to "working" temperature, and at the same time, heats and chars the wood below enough to generate the smoke that the secondary tubes need.
Additionally, because the firewood hasn't been blazing, it's far easier to step down the control of a top down fire.

It's fast,efficient, and completely counterintuitive.

I often use sleepers on the bottom, because I load E-W, and without sleepers, the primary air is blocked by the front log.

TE
 
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I find 2 pieces of fatoowd work well enough as bottom sleepers if you don't have wood short enough for the EW loaders out there who want to use them.
 
Depending on how clean the stove is, you could just take a trough down the middle for the bottom sleepers.
 
I have a E/W loader 1.6 box and use kiln dried construction scraps as sleepers. Huge mesh bag for $6 last me 2 months. Two medium splits on them and two smaller diagonal splits across them. Sometime a cutoff chunk on each end. One pinecone and sticks from my property for top down ignition. Sometimes I add one piece of fatwood. Works well.