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Building my own woodstove: any advice?
Q: I don't want to hurt your feelings, but I just don't like any of the woodstoves you offer, or any of the
woodstoves I've seen anywhere else. They're all too small, and they all have such dinky little viewing windows.
I'm pretty handy with a welder, and want to build my own woodstove to heat my 1200 sq.ft. home (very well
insulated). The stove I have in mind will need to meet the EPA emissions standards without a catalytic converter,
and have a firebox like the woodstove my Dad used to have, big enough to hold 30" pieces of wood, or even
bigger (I cut my own wood, and don't want to have to whack up small pieces). I also want to have a larger glass
door than the ones I see on manufactured woodstoves. I've been all over your website, and you seem to know
your stuff. I feel like if I just understood the theories I could weld something up and make modifications to it until
I get it to work right. Can you tell me the basics I'll need to know to design this thing? Also, if you have time, any
tips you care to share that might be helpful. It's getting cold outside!
Let's start with a basic overview of how non-catalytic woodstoves clean up the smoke emissions. The
concept is pretty simple: in order to meet EPA standards, you'll need to create a second fire chamber inside the
woodstove's firebox to reburn the exhaust from the wood fire. The secondary burn chamber in an EPA approved
stove is located at the top of the firebox, and is designed in such a way that the exhaust from the fire must pass
through it on the way out the chimney. In operation, the wood exhaust ignites inside the chamber, creating a
1200+ degree flame (much hotter than the wood fire below) which burns up approximately 90% of the smoke
particles as they pass through it. In order to fire off your secondary burn chamber, three things must be present at
the same time and in the right amounts: fuel, heat and oxygen. The fuel part is easy: it is the smoke from the
wood fire. Likewise the heat: ceramic blanket insulation is used above the chamber to trap the heat from the fire
below and ensure lightoff temperatures. The oxygen part gets a little tricky: since the primary fire has consumed
most of the oxygen available in the firebox, preheated air must be introduced into the reburn chamber so
secondary combustion can occur. Your design challenge will be to figure out a way to preheat the secondary
combustion air to the right temperature and cause it to be drawn into your secondary burn chamber in measured
quantities that automatically adjust with the draft control that supplies air to the primary fire, keeping in mind that
the secondary flame must keep burning and cleaning up the exhaust even when the primary draft control is at its
lowest setting.
Tip: Scores of woodstove manufacturers have gone out of business in recent years as a result of being unable to
engineer a stove that would burn clean enough to meet EPA emissions standards. The sad fact is, unless you are
an exceptionally talented thermal design engineer with a background in wood combustion and lots of time and
materials for prototype testing, your secondary burn chamber probably isn't going to work.
Next, let's look at woodstove viewing window basics. Early attempts at adding viewing windows to woodstoves
were miserable failures, as the windows turned black almost immediately from contact with the wood smoke. Since
it is kind of silly to go to the expense of incorporating a viewing window if you can't see through it, woodstove
manufacturers soon learned to incorporate an "airwash" in the design of the stove, taking a portion of the
incoming combustion air, preheating it, and directing it across the inside surface of the window to keep the smoke
away. Airwash designs are a little tricky, as they require building the necessary preheating chambers into the
design of the stove and then providing a means to direct the airflow across the inside window surface without
adversely affecting burning efficiency.
Tip: Manufacturers know that a big view of the fire will appeal to woodstove buyers, and make every effort to
provide the largest viewing window that can be kept clean by the airwash system. As you modify and retest your
stove with various airwash delivery systems and various sizes of windows, you'll find that there's a limit to how big
the window can be and still be kept clean by the airwash system. You could save yourself a lot of futile effort by
resigning yourself to a viewing window that is about the same size as the ones you see on manufactured stoves.
Finally, let's tackle firebox sizing. Woodstove manufacturers know that people who buck up their own fuelwood
want fireboxes that can hold larger pieces of wood, because cutting wood is hard work, and larger pieces mean
fewer cuts. So why, in recent years, have these manufacturers been making their fireboxes SMALLER, instead of
larger? In a word, heat. Too much heat. Remember that 1200+ degree burn in the secondary burn chamber? Your
Dad's stove didn't have one of those. If it had, and was as big as you describe, he wouldn't have been able to
stand the heat long enough to go into the house, let alone get close enough to the stove to add one of his 30"
chunks of wood to the fire.
Tip: Our highest output woodstove, the Pacific Summit, has a firebox measuring just 20" x 18" x 13" (2.7 cubic
feet), yet generates 97,000 BTU's per hour (enough to heat a 3,000 sq.ft. house). If you do manage to design and
build a reburn chamber that works, and want to heat your 1200 sq.ft. house without blasting your family out into
the yard, you'll want to plan a firebox that will burn your 30" logs afer they've been cut in half. This won't cause
you any extra labor at cutting time, because if your reburn chamber works, you're only going to burn about half
the wood your Dad did to heat the same area.
Here's something you didn't ask about, but need to know: Federal and state laws prohibit installation of any
woodburning heater that isn't UL listed and EPA approved. Obtaining this approval is not practical for a single
stove: even if you were able to build a stove that would be safe enough and clean-burning enough to pass, you'd
be out about ten grand for the safety and emissions testing. If you were to go ahead and install your home-built
woodstove without listing labels from the test labs, you most likely wouldn't be able to get a permit from your
Code Authority to install it. In the event of a fire claim, the insurance adjuster might notice the lack of listing
labels or ask to see your installation permit, at which point your coverage would probably hit the fan.
To sum up, the most likely product of your efforts will be an illegal, wood-gobbling, creosote manufacturing smoke
dragon with a viewing window you can't see through. And no insurance coverage in the event that this monster
burns your house down.
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Q: I am mailing in response to your letter to someone wanting to build his own woodstove. Tuff luck that someone
doesn't sell a design for this fellow. My whole problem with EPA certified woodstoves is that once the unit is sold,
the manufacturer has no control over how it is used anyhow and I have seen burn-clean stoves belching forth
smoke all the live long day. I mean, it sure seems to be an awfully delicate balance to maintain and I can't
understand how this is achieved from the board room table. I used to own a house with a fireplace insert and run it
steady without a visible trace of smoke and heat the house with it solely (-15to-30C). I think the claims of EPA
ratings should be made after the public sector is factored in as I'm suspicous the stoves are tested in equally
delicate balance. Anyhow, I would also like to build my own fireplace insert for our new house.
Thomas McFarland
Hi Thomas,
Thanks for the input! Sorry to say, we can't supply plans for your build-it-yourself wood insert project.
Manufacturers guard their EPA certified designs jealously: even though we've established a close relationship
with several manufacturers over the years, we've never been allowed to see a diagram or drawing you could build
a stove from.
I'm curious about your assertion that you've seen EPA approved woodstoves belching forth smoke all the live
long day. In the early days of clean-burning technology, we were involved in a test of the low-emissions designs of
the day, and no matter how hard we tried, we couldn't get any of them to produce any visible smoke. This wasn't
because the primary fire in an EPA approved wood stove doesn't produce just as many particulates as any fire, it
was because EPA approved stoves incorporate a secondary burn chamber where these particles are automatically
reburned, resulting in no visible emissions.
Because of the way they work, the only way EPA approved wood stoves could belch smoke out the chimney all
day long as you describe would be if the secondary burn was extinguished somehow. You imply that some sort of
"delicate balance" must be maintained by the operator to stop that from happening, but I can only think of three
ways it could possibly happen.
One way to extinguish the secondary burn would be to smolder the fire (starve it for air). This can't be what
caused the smoke you saw, unless the clean air stove in question was one of the very earliest models: today's
stoves are designed so that the secondary burn gets sufficient air for combustion no matter how low the draft
control is adjusted. In fact, EPA emissions testing is performed with the draft conrol set as far from a "delicate
balance" as you can get: the stove must pass emissions testing with the draft control set to the worst-case
scenario, closed down all the way.
The second way to extinguish the secondary burn would be to deliberately bypass the secondary burn system,
which is not possible with most models. Even if the stove in question was one of the few that have a bypass, it is
highly unlikely that's what produced the smoke you saw: since it is neither efficient nor economical to burn with
the secondary burn system bypassed, it is hard to believe that anybody trying to heat their house with a
woodstove would make that mistake very often.
Which leaves us with the only likely cause of the smoke you observed: the operator was burning wet or green
fuelwood. Burning green or wet wood will cause excessive smoke to flow out a chimney venting any woodstove, no
matter how high-tech: you can read about why in our Sweep's Library by clicking here. Green or wet fuelwood
should never be burned in any stove, EPA approved or otherwise, not only because of the extreme emissions, but
also because of other resulting headaches like excessive creosote formation, incomplete combustion, lousy heat
output and blackened viewing windows.
The bottom line is, the manufacturers of today's EPA approved non-catalytic woodstoves have really done an
excellent job building stoves that burn clean automatically, without any need for any special tweaking or
adjustment by the operator. All you have to do is avoid burning green or wet wood, which has always been a no-no
in any woodstove anyway.
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Q: if you arent looking for pretty use a 50 gallon drum and you can make a baffle box out of 1/8" steel and
alternate the steel so that the spaces are at opposite endsand leave about 2" of gap, this also bumps up the heat
output. or use an old catalytic converter from a truck. the EPA guys arent gonna go looking for you if there is
some smoke if its yer only heat source. ive got worse. a metal and cinder cube with a chimney, door and vents with
no glass in my house and i dont get bugged about it.
Kelly Waliser
rednek- - - -@hotmail.com
Hi Kelly,
I'm afraid you might be missing the point here. Your barrel stove idea is nothing new: similar designs have been
around since..... well, since barrels. I've seen several variations of your double-baffle idea over the years, and
while incorporating baffles in the design might improve the burn of barrel stoves a bit, I hope you realize that
you're never going to build a stove that even approaches the efficiency of an EPA approved design (I'm going to
assume that your idea of incorporating a catalytic converter out of a truck is not something you've actually tried,
because there's no way that would ever work).
The point is, EPA approved designs are better than homemade stoves in several respects:
1) They heat the same area while burning about half the fuel (saves half the cutting, splitting, hauling, not to
mention usage of the wood resource).
2) Their stay-clear viewing windows allow you to ensure your fire is burning properly from across the room
(prevents smoldering, assures utmost burning efficiency).
3) They cut the formation of creosote in the chimney by about 90% (less frequent need for sweeping, less chance
of having a chimney fire).
4) If you ever do have a fire, their UL safety listing ensures that your insurance company will pay your claim.
5) They are MUCH kinder to the environment, as they exhaust only 1/20th of the particulate emissions into the
airshed as non-EPA approved stoves. Since our generation has finally come to the realization that protecting the
environment is everybody's business, deliberately burning a homemade smoke dragon like you describe could be
looked at like tossing your empty PBR cans out of your truck window.
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