Downdraft Stove Operation
Burning the newer Downdraft (base burning) - Type Wood Stoves
Definition: A wood stove which draws the smoke and exhaust DOWN through the burning load and embers in an attempt to extract as much heat as possible from the gases in the wood. Also sometimes called downdraft, cross draft and base burning.
Downdraft wood stoves have been around for hundreds of years, and even Ben Franklins orginal design tried to bring the smoke back down through the embers in order to burn more efficienctly. In fact, patent #86,074 from 1869, shows an extremely well developed base burning model with most all of the features in place.
But most of this technology was forgotten as wood burning stoves were junked in favor of coal and then oil/gas heating.
The mid -1970's saw a resurgence of wood burning, and with the new stove companies sought out designs which would provide high efficiency and reduced creosote and smoke (pollution) production. A company called Riteway came to market with a number of utilitarian models which used downdraft design. These were well received in the marketplace, but became a victim of cheap oil in the 1980's.
Pictured above are two designs separated by approx. 100 years!
In 1988, Vermont Castings reintroduced a downdraft stove called the Resolute Acclaim. This stove was created in order to meet the new EPA clean burning standards which required that any newly manufactured stove meet certain emission guidelines.
Cutaway drawing of Resolute Acclaim combustion system shown above - note - this drawing is from US Patent Office files, with some added color for explanation.
With the renewed interest in wood burning following the turn of the millenium, a number of new models with similar combustion systems have recently entered the market. These include, but are not limited to:
1. Vermont Castings Non-catalytic Encore and Defiant
2. Lopi Leyden
3. Avalon Arbor
4. Harman Oakwood
5. Certain Dutchwest Non-Catalytic cast iron models (Model 2479, etc.)
Although Hearth.com has some articles on starting and tending a wood fire, these do not specifically address the proper starting and burning of downdraft-type unit. Here are links to the aforementioned articles:
Starting a Wood Fire - http://www.hearth.com/econtent/index.php/articles/start_a_wood_fire
Tending a Wood Fire - http://www.hearth.com/econtent/index.php/articles/tending_a_wood_fire
Ok, now onward to the operation of a downdraft stove.
Short summary - as you can see in the drawing of the Resolute Acclaim, these stove depend on having a large bed of hot red embers in order to function. This often required burning though a good part (or all) of the first load of wood in order to establish this "critical mass". Typical operation in such a stove is:
1. Burn first load mostly with bypass and air inlets open to warm chimney, stove and establish bed of embers.
2. Add wood, close bypass and adjust output once you are familiar with your stove and chimney combination
3. When the load has burned down to red coals, open the bypass, add wood, and then close the bypass and adjust air
Here are suggestions from our forum members who are using these stoves
Here is the procedure that I follow when my stove (Dutchwest 2479 ) is up to temp and needs to be reloaded:
1) Open bypass
2) poke residual logs to collapse into coals (if necessary). Needs a good 2+ of coals, so plan accordingly
3) load fresh splits (see note below on orientation)
4) open air inlet 100% for about 10-15 minutes or until fire is very active (about 475 on the flu connector magnetic thermometer)
5) Damper down to about 1/3 air and let burn for another 10 minutes. This reduces wasteful burning that is just rocketing right up the chimney, but is necessary to pre-heat the new splits on the top
6) Open air inlet to 100% again for about 2 minutes to get an active fire again
7) Close bypass/engage downdraft.
8) If rumble persists, temps are good, smoke-free at the top of the stack, I Damper down to about 3/4 then 1/2 then 1/4.
If the stove stalls, it means that the coals weren't oriented right, there werent enough of them, or the fresh splits were ready for that stage yet. All the above assumes dry, seasoned hardwood. Also, I have a thick masonry chimney which has to be properly heated before it drafts well enough to use the downdraft combustion sytem. This takes about 2 hours in my set up.
Also, when the drafting is good and outdoor temps are low, lately Ive been experimenting with the following to reduce thermonuclear incidents. A freshly loaded stove holds about 6 medium sized splits on top of the coal bed. I have been putting two less-seasoned splits on the top row. That way they bake for a while and dry out before the splits below them reduce to coals.
Orienting the splits.......
When I add new splits, I create a small pocket in front of/ above the throat entrance to the reburning chamber. Split-size willing, I do this often by resting a split on the flat, top section of the block that houses the throat. When that split and the ones around it eventually reduce to coals, it falls into place on its own. For some reason, creating that pocket seems to help. Maybe it allows the air to not have to travel across cooler splits that have not completely caught yet, therefore making it more hot before it enters the throat.
In the morning
In the morning there are still coals remaining, but not the 2-3 required to jump right back into the downdraft process. Also the stove has dropped to about 200 degrees (surface), which isnt hot enough. But the stack is still warm and the draft is moderately strong, so after loading with fresh splits it is only a few minutes before the fire is roaring again. However, it does take a while to rebuild the thick bed of coals necessary.
So in the morning, the procedure is similar but different and goes something like:
1) open bypass and reload about 1/2 way with small diameter splits (2-3") and open up air inlet all the way
2) make big pot of coffee
3) once the splits are about halfway to coals, jumble them w/ the poker to break off some coals and build up the coal bed
4) completely load the firebox as normal, finish the pot of coffee, and revert to the previous posted steps and enjoy.
Every fire is different based on the fuel & weather conditions, but its a procedure that seems to work often enough. So the morning procedure takes about an hour before the combustion system is ready to go again, which is less than half the time it takes starting a cold stove/stack.