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Old 11-09-2011, 02:00 PM
JaysinSpaceman's Avatar
JaysinSpaceman JaysinSpaceman is offline
Master Fabricator
Join Date: Feb 2008
Location: Skull County, Ca
Posts: 1,637
Wood Stove w/ secondary combustion.

Well this post is mostly for CarterKraft but it might help out others too.

First, I have to say that I am not a wood stove designer and the stove that I built is not certified by anyone but me. If you choose to build a wood stove do the first test firings out side in a fire safe area and build in fail safes.

Now that I am done with that crap. I was asked by a family mamber if I could build a wood stove for a very specific opening in a rammed earth (think adobe) wall with a built in chimney. The hole in the wall was about 20" tall 18" wide and 22" deep, there wasn't any insert on the market that was even close to fitting. So I said sure. I have taken an interest in wood burning technology ever since my wife and I bought a new EPA approved secondary combustion wood stove to heat our house, so this was a chance to test what I learned.

A little background on these stoves. As wood burns it does so incompletely, the smoke you see is essentially the fire burning rich (just like when you see black smoke coming from an old car in need of a tune up) and the smoke is flammable and carrying a lot of heat energy. The Idea with a secondary combustion wood stove is that by adding fresh air above the burning wood you can cause the smoke to combust and both release more energy and reduce particulate pollution.

Here's a side view drawing of the air/smoke path in my stove.

Primary air comes in through the front door goes up the sides and washes down the door glass (to keep it clean) and feeds the fire at the front bottom. The smoke rises and travels forward, up and around the smoke shelf and across the secondary combustion tubes finally going back along the roof of the stove to the chimney. Secondary air travels from the front lower corners through 1" x 2" rectangular tube along the bottom corners and up the back corners and feed out through 4- 1" stainless steel tubes with a series of 3/16 holes drilled down their length. The long path for the secondary air allows it to preheat before it enters the hot smoke. I hope that all makes sense.

To have secondary combustion occur the smoke and fresh air needs to be really hot (somewhere between 700 and 1200 degrees F, the info on the net is pretty vague on this) so the fire box needs to be lined with refractory insulation. The fire box is everything below the smoke shelf, the smoke shelf should also be refractory insulation.

Here you can see the insulation and the secondary combustion tubes.

The door needs to have a rope seal all the way around it so that primary air can have positive control, we don't want an uncontrolled over fire.

Here you can see the seal. My primary air control is in the door so I also needed to seal this to the air inlet in the front of the fire box.

And here is a video of the first test fire. I had 9 feet of chimney pipe, you have to have a chimney to make the stove draft.

First Fire

In the video you can see there is no visible smoke coming out of the chimney. This is the great thing about secondary combustion, the smoke is burned before it goes up the chimney giving you more heat and reducing the particulates your stove puts into the air we breathe.

I am sure there will be questions and things I forgot. I didn't really get construction photos as I just forgot to take them along the way.

Thanx for looking,
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