firescience

Fire Science: Burning Wood

a Skill-Building Sampler

Current Events:

Some things most people know about fire:

1) It's hot.

2) It burns [fuel, or my fingers].

3) You can put it out [smother it with dirt; starve it of fuel; interrupt it with air or water to chill its heart, or a shovel to scatter its bones]. This is the "fire triangle:" fuel, air, and heat. Take any one away, and no more fire.

4) Heat can start it -- if it gets too big/hot, you can't blow it out anymore....
(That's right... you just aren't cool enough.)

5) Flames tend to go up. Hot air rises. It's hot above a fire. (And hot all around it ...)

6) Fire is bright, and it can makes things glow. (Glowing radiation means it's over a thousand degrees, unless it's some fancy trick like LED's or glow-worms.)

7) Fire makes soot, smoke, and 'bad' gases (don't breathe it).

8) Wet fuel doesn't burn well*, and some fuels burn better than others: Wood fuels chart (*damp fuels smolder, green wood smokes, oils can burn on the surface of water, alcohol vapors and gasoline can burn even when mixed with water, and there are crazy fuels like metals that will burn underwater.)

Some things not everybody knows about fire:

1) Radiant heat bounces off walls. You can direct the heat from fire with a fireback. Put a wall behind it, and some of the heat bounces back across toward you. Put it in a box (like a big square fireplace) and the heat stays near the center and goes straight up if there's no lid. Great for cooking. You can fan out the walls and bounce heat out toward you, great for warming yourself. Rings of wildfire can do lens-effects and light distant trees like a candle.

2) Hot chimneys draw. A pipe, tube, chimney, or hollow log can function as a "hot stack." The tube gives the gases a place to go without interruptions from cold air coming in sideways. The hotter the tube gets, the faster the gas inside it rise, drawing more air across the fire and channeling exhaust gases up and away. A tube or chimney acts like a "smoke vacuum," and can very quickly turn a smouldering campfire into a rushing tower of flame.

3) Smoke is unburned fuel.
If you can burn your fuel completely, so there's no smoke,
a) you need a lot less fuel to get the job done, in most cases, and
b) you don't get so much turpentine, methanol, soot, carbon monoxide, vinegar, tars, and all those other nasty things that make people cough and rub their eyes when they get a face-full of smoke. (Or make inky smears that may have inspired the first ancient art.)

When fire burns completely, it turns carbohydrate fuels (like wood/grass/paper) and hydrocarbon fuels (like oil/wax/fats) into... carbon dioxide, and water. It's just a lot of carbon and hydrogen, getting completely oxidized.

(Side rant: Yes, carbon dioxide is building up in our atmosphere. It's a product of all our combustion technologies -- smelting ores to make metals, burning gas and coal for fuel, refining limestone into lime and cement, and powering our houses and computers with the electricity produced in far-off, coal-burning plants made from cement and metal. Wood heat uses "current" carbon dioxide, assuming more wood is grown to replace the chopped fuel. Fossil fueled heat uses 'ancient' carbon, that has not been part of the atmosphere since the time of the dinosaurs. So while a clean-burning fire produces a little carbon dioxide, a 'clean' electric heater in reality produces far more. Either is an improvement on lots of nasty smoke.

If it helps put things in perspective, methane is an even worse greenhouse gas. So stop eating beans. And water is the worst of all. But we need it to survive.

Maybe these substances aren't "bad" as such, maybe they're even potentially "good" elements in some ways, but it's dangerous to alter the amounts in a balanced system. Burning massive amounts of fossil fuels over a short period (a few hundred years) releases trapped carbon, destabilizing the climate. As an individual, you can't put it back the way it was. But if you can provide for your needs from your own 'waste' wood, without depleting your woodlot, you know that your plants are pulling that carbon back out of the air as fast as you put it in. You can even return the mineral ashes to the same forest, along with some rich compost-type waste, to speed things up. It's not "waste" that's the problem, it's wasted waste.)

4) Fire flows. Air is a fluid, so are smoke and water. And it's the gases released from hot wood that burn, just like the fumes from brandy or gasoline - the wood itself only chars.

You can use the same tools to channel hot air that you use to hold and move water. Just have to turn them upside down. And upside-down bucket is an oven or hot-air balloon. An upside-down pipe is still a pipe, or a duct. An upside-down drain is a chimney. And an upside-down siphon is.... well, it's something that most people haven't really thought about using on a fire. We call it a "j-tube," or an "l-tube," or a "thermosiphon," and use it to make fire burn sideways. Ancients used them to make long, hot furnaces for heating palaces, firing ceramics, and processing ores.

5) Remember convection, conduction, radiation?

Convection is hot air rising and wind blowing. Sunlight on your face is radiation. Conduction is why having a cuddle or hot shower is a lot warmer than watching one.

The chimney facilitates convection: it convects heat straight out of your house. We can use reflective masonry to bounce some of the radiation back at us (hence the dish-like diagonal walls of elegant Rumford fireplaces). But like cats, many of us like to sit on a warm surface as well. As long as your body is between the fireplace glow and the cold chair, your seat will only get as warm as your shadow.
Conduction is the process of transferring heat by touch -- and it's by far the most direct way to transfer heat. It's like radiation with zero distance -- heat transfer is limited only by the materials themselves. But we rarely use it for comfortable heat-- stoves are too hot, hearths too sooty, water-bottles and hot showers are too leaky and wet to use while reading a good book. Microvavable heat-packs are about the extent of it.

6) If we use everything we know about fire, we can get it to do amazing things, just by how we arrange its 'house'.

- A Rumford fireplace (diagram left) uses a narrow throat to keep the chimney from dropping cold air into the house when the fire is out (cool air flows down just as warm air flows up, with less desirable results in winter. The curved back helps scoop the smoke up the chimney when the fire is going, and slanted masonry sides reflect and re-radiate the heat back into the room.

A jug stove (photo, right) does almost the opposite - it uses dry earth to insulate and focus the heat directly beneath a chimney, which allows an African traveler to heat tea in a barren land with plenty of dry earth but little fuel.

-A "pocket rocket" concentrates the flames at the bottom of a can, where they can warm our toes and convection carries the heat all the way up to the rest of us. It's a good emergency heater for cold hoboes, or a mountain man's hunting cabin. (pictured, left)

- A masonry stove sends the exhaust from a single, batch-burned fire through a bunch of baffles on its way out, so it heats a massive masonry chimney and creates safe, warm heat for hours. Some larger examples such as the Russian stoves can combine cooking hearths, ovens, and even platform beds: a warm, raised surface owners can actually sleep on in the extreme cold of northern European winters. Others, such as German and Swedish tile stoves, offer a smaller but still efficient decorative heater for a central room.

-A rocket masonry stove combines these, plus extra features:

- a vertical (gravity-fed) feed tube, optional lid
- Multiple 90-degree turns, and masonry surfaces creating just the right amount of turbulence to get your smoke and air to mix, heat, and burn completely. (like an incinerator)

-A short, insulated, and therefore very hot chimney, creating powerful draw and a very hot spot at the top where the last gases re-burn in a torus of baffled energy.
- A barrel which radiates heat quickly like a woodstove, while directing exhaust gases downward to a horizontal heat-exchange mass.
- A horizontal system of heat-exchange pipes to capture the rest of the heat from our exhausted smoke, convenient for built-in seating or bed designs,
- and conventional vertical (or unconventional horizontal) exhaust outlets to release the combustion gases outside the building.

Of course, in trapping all the heat in metal and masonry, we've invented a fireplace that lets off almost no light at all. On a dark night, the reflected flickering might be just enough serve as a night-light for a small child .. if they found underground flames reassuring.

But no system is perfect.

Light, and other needs like summer cooking or portable heaters, call for other solutions: less mass, different designs, and wise use of nature's gifts (our two feet, candles, and daily sunlight, among them). This Rocket Stove is an example of specialized but resourceful design to create efficient, clean, and comfortable heat with minimally-processed materials.

Act II: The Building Codes and Legal Combustion of Solid Fuels

There were a great many horrific fires in historic cities - Rome, London, Chicago and San Francisco notoriously burned to the ground on several occasions, and these fires increased in frequency with the introduction of coal-fired home furnaces. Most pioneer towns had at least one close call.

Modern building codes require permits for installation of most combustion devices, to ensure their prior approval for occupant and building safety. Since 1970, approved devices also need to past tests specified by the EPA to diminish widespread air pollution. These codes are an interesting historic phenonenon: they combine centuries of experience-driven 'best practices,' with modern, mostly commercially-driven technical amendments. Unfortunately, many of the more recent and efficient European masonry heater techniques were not yet common knowledge in the US when the codes were first adopted, and only with much effort on the part of volunteer masons have the masonry heaters at last achieved a modicum of recognition. Testing of 'kits' still allows a much easier approval path than site-built heaters, at the cost of some flexibility for master masons to design heaters that may better meet a household's particular needs.

The cost of altering building codes is substantial, giving a tremendous advantage to industries which can spread the cost of testing and lobbying over thousands of retail units. Masonry does not retail well - much easier to ship a glorified metal barbecue grill, than a more efficient brick heater. So site-built technologies must be approved individually, at a significant disadvantage in cost and time delays. Categorical exemptions have already been made by the EPA for masonry heaters, but local building officials can be reluctant to accept such an exemption without a signed letter from... someone ... on EPA letterhead.

So where does that leave our traditional, earthy, masonry fireplaces?

- Rumford fireplaces are already in code - in many parts of the US they are approved under the same EPA categories as a certified insert or woodstove, based on their much lower contribution to local smog compared to cruder square fireplaces. In Portland, you can even build a Rumford fireplace with cob (monolithic adobe) under a local code variance that was just created through Portland's Alternative Technologies Advisory process. (Rumford diagram from www.HJMasonry.com, Maryland)

- Masonry heaters have been permitted in the US for at least 15 years, and exempted from EPA regulation by weight (each is site-built of tons of masonry, and can't effectively be transported to a testing lab. But they are known to be cleaner-burning than most commercial alternatives. (They have been a proven, high-end combustion technology for home heating in Europe for several centuries, and larger versions have been approved for some commercial buildings.)

So what about Rocket Mass Heaters?

Are they ready to 'come out' into mainstream built environments?

It is possible to build a Rocket Mass Heater completely according to masonry heater code. And, though more expensive than the junkyard variety, it would still be one of the least expensive masonry heaters and the easiest for an ordinary owner-builder to do with mostly their own labor. (The ASTM specifies that masonry heaters are complex and should be built, or supervised, by an experienced masonry heater builder. None of the builders currently listed on the masonry heater guild websites are conversant with rocket mass heaters. But the standard does not restrict the term 'experienced' to members of these guilds. Arguably, we would qualify as experienced builders under this standard.)

However, the process is complicated by the fact that few jurisdictions understand what codes and standards even apply to the European-style masonry heaters, let alone something with earthen masonry and a metal bell.

Here's the story of our attempt to build a Rocket Mass Heater, legally, under City of Portland building codes. (Journal entry at www.Journalscape.com/Ecca )

So, we have this ... thing ... in our living room. Pictures of The Thing

We built it out of recycled bricks, metal, sand, dirt, and a little bit of fancy stovepipe, tile, and plaster.

It burns wood. But it weighs over 6000 lbs, so officially it does not qualify as a woodstove. We gave it a factory-built chimney suitable for a woodstove, and some heat shielding and insulation from the walls like a woodstove, anyhow.

At that weight, and given that it's mostly made of masonry, it might be a masonry heater. (These are exempt from UL testing according to the testing companies and masonry heater guys. Oregon building code finding 93-47.pdf describes a type of European masonry stove, and it seems to apply pretty well to what we have here. Except for the shape.).

Our Thing is spread out around the room, about 18 inches high, and has a factory-built chimney instead of a masonry chimney. (A masonry heater is usually a giant stone or brick chimney, which puts the smoke from a fireplace or woodstove through some baffles to slow it down and collect more heat from it. Masonry Heaters Association, http://mha-net.org)

Does it need a reinforced foundation, if there's no possible way it could fall on anybody or bring the house down with it?

It can boil water. But it's not our sole cooking device; we use it occasionally for making tea/soup, for fun or in power outages. (A cookstove that is your only way to cook is exempt from EPA even if it smokes like a demon.)

It can heat the house pretty well - it was able to bring the thermostat to 61 degrees, in 13-degree weather. We have a furnace in the house, but we don't use it when The Thing is working.

And it's really comfy to sit on. (Part of it is a heat-exchanger, where the flue gas runs through a cob bench, so it makes a full-body heating pad or heated sofa/bed.)

The flue gas doesn't smell like woodsmoke, or like much of anything. It's basically steam and CO2. On warm days, it makes no visible smoke. Just clear flue gas. On cool days, it puts out clear steam that immediately condenses into white clouds, and then dissolves again as it drifts away.

We've had friends over with small children, and lit it for them, and they felt comfortable and safe.

We just had two inspectors from Portland's building code team, and one from DEQ, out to have a look at it. They didn't give it a permit, but they didn't deny it, either.

It doesn't fit into any existing box.

Here's the long version of the story:

Ernie was part of the research team for the 2005 edition of the book, Rocket Mass Heaters: Superefficient Woodstoves You Can Build (and cuddle up to!). When he got hurt, we did a workshop as a fund-raiser and built one outdoors at Tryon Life Community Farmwww.tryonfarm.org). It went over well, and one of the participants had us do another workshop the following year at his house - to heat an outdoor bench and small outbuilding.

There's a lot of interest from folks in being able to build one indoors, in a "normal" house. (We have friends and mentors on Oregon's south coast, who have installed these in small cob houses where they are the only cooking and heating device, and they work great. We also know of a few people in various places who have installed them, up-to-code or otherwise, as owner-builders in conventional and/or log houses, and they work well there, too.)

We've helped people build a couple of them outdoors for a "warming fire," and they work pretty good - but outdoors is not necessarily where most people want to stay warm. The "Pocket Rocket" was developed for people who have no indoors to go to, and need a warming fire - but it's kinda bulky for most people. Ernie knows a guy who has one on his boat.

We can help someone learn how to build a clean fire, and how to clean out their Rumford fireplace so it works better(or refit an "ox-cooker" fireplace to a more Rumford-like configuration for better heat efficiency). And we can show people how to build a Thing as a fancy outdoor cooking or heating device. www.ErnieAndErica.InfoWe think these Things are an improvement over what's out there for home solid-fuel heating, in a lot of ways.

For example, they put out way less visible smoke that the currently available UL-certified woodstoves, even when the latter are run by competent fire-makers with properly cured wood. (With inexperienced people or wet wood, there's no comparison.)

They use about 1/5 to 1/10 the wood for heating that a conventional woodstove would use in the same place. This means a normal, suburban family could get the heat they need without quitting their day job to split wood.

They provide an extraordinarily comfortable, direct body-heating option, which means that you can have a variety of people in the house and maintain a temperature gradient that keeps everybody happy.

So we'd like to be able to tell people, "Get a permit, and you can put one of these in your home; here's how you do it to code so you don't invalidate your insurance."

But nobody's done that yet, at least in Oregon. So we don't know how it would work.

So we decided to try it ourselves.

The place we're currently renting is part of the Dana property in the Sylvan hills. It's an attached cottage, known as the Annex or "Little House," next door to "the Big House."

The family has been thinking about developing this two-family residential property into a sort of ecological retreat or B&B, to preserve its rural character in an increasingly suburban area. (Recent infill has contributed to stormwater and other problems, and the owners are not excited about the idea of seeing their childhood home replaced by 5 or 6 big-footprint McMansions.)

So Emily Dana was in favor of installing an efficient wood-burner in the rental house. She paid for the permit application and the parts required for code, as a "capitol investment," and got her hands dirty mixing cob, laying masonry, and cobbing-in the heat-exchange ducting.

We got the permit application in September, and we had the stove mostly built by the end of October. We had to wait a while between layers, because of damp (the little house is pretty damp to begin with).

Our final work-party to finish the exterior plaster was the weekend after Chinese New Year. The 180-day deadline for our first inspection was still a month or two out, and we thought about calling around to get advice ... but we decided that we'd rather hear it from the officials in charge, than try to please everybody and then hear it from the officials anyway.

So we called the Bureau of Development Services, and scheduled an inspection.

We were all excited about it, and tidied up the living room, and rescheduled our tax appointment when the inspector needed to come earlier in the day.

Inspector John came out, and said, "Wow." Or maybe, "Whoa." He looked in both ends, we lit it off for him, he asked to see the plans, we pulled out the Rocket Stove Book and our photo show, and hunted around for the original design drawings.

He said, "This is very cool - but I can't inspect it. I have no idea what to inspect it for. Let me put you in touch with my supervisor."

So we called his supervisor, Joe. The first thing he asked when he called back is whether it was a "cooking rocket," or a "rocket mass heater." Good researching!
(Cooking rockets were developed for third-world families as an improvement over a 3-brick indoor hearth; they have an L-shaped burn chamber, and are not designed to heat a home.bioenergylists.org or aprovecho.net)

We had an interesting phone conversation about whether we had a woodstove, a fireplace, or a cookstove. We said, "None of the above." He told us if it heats the house or cooks food it's an appliance (not the term I would have used for a fireplace or chimney, but apparently that's the building code language). And would need EPA approval and UL ratings. We pointed him toward Finding 93-47, which describes "masonry heaters" as an alternative category of solid-fueled device, hand-built, and not considered a UL-testable woodstove on account of the weight.

(We had already asked about this by calling OMNI-Test Labs. They certify woodstoves and other fuel-burning products. They said it sounds like we have a masonry stove, which is exempt; and also, the UL tests are run at three wood-consumption burn rates, all of which are faster than our Thing can swallow wood.)

After doing some research, Joe called back and set an appointment for the following Thursday morning, to come see it and bring a DEQ inspector along. We invited him to bring anybody else who wanted to see it, too.

Once again, we tidied up and prepared. I got all our design drawings and notes collected into a file, and drafted a new, larger drawing on official-looking architectural vellum.

Our inspectors arrived on time (Joe from BSA, Robert from DEQ, and later John stopped back by as well). We showed them what we have, and they asked to see it work, and we discussed options.

They could see the value in a "poor man's masonry stove," and Robert from DEQ seemed relieved to find himself among people who knew how to make fire without smoke.

But they couldn't pass it, based on the existing code categories.

One issue was the permit we'd gotten: it was for a woodstove installation, which means UL-approved or antique woodstoves. For a masonry heater we should have submitted the plans first for a mechanical permit, instead. (The office didn't know this, and they were between issues of the code at the time, so neither did we.)
Joe decided to keep our original permit open, meaning we'd have another 180 days to get it inspected again. We could submit the plans for a mechanical permit, and if they needed changes (like a reinforced slab footing suitable for a massive chimney) we could tear out the existing work and re-do it. Um....

Another option would be to submit the plans and information to "OMOA" (another division of BDS), and see if we could get it passed as an "approved alternative." (To my mind, this might be necessary anyway, because of the earthen masonry.)

Or it could go through testing at OMNI-Labs and get approved as a heating device, we could put a sticker on it, and they could pass it as an appliance, like a woodstove but different.

If we didn't have an electric range, it could be exempted under DEQ as a cookstove. But I kind of like having a kitchen. (My thoughts: Maybe there's an exception for emergency cooking / heating, like the woodstove Grandma used to have in her basement?)

So that's where we stand at the moment. Joe, John, and Robert are going to keep talking to their office mates, and showing pictures. We gave them Ernie's card with our website, and hopefully they can find the pictures.

We'll give Karl in OMOA a call in a little while once Joe has had a chance to mention this to him, and see what the process is for "approved alternative."

It may be a lengthy process, but hopefully we can get a prescriptive, or ideally descriptive, option worked out. That would allow a homeowner to build this "poor man's masonry stove," and burn less fuel with less smoke than any other option we know.

The Thing's good points:

Clean
Efficient: low-fuel, lots of heat
Local biofuel alternative energy
Low-cost
Low-maintenence (yearly inspection/cleanout, daily ash cleanout and one or two 2-hour firings per day during cold season)
Safety: Lower temperatures on exposed surfaces than most heaters, fewer toxic components, fire is mostly recessed away from curious hands.

Points of concern:

Foundations and reinforcement: Seems like overkill to pour a concrete slab with enough rebar to support a 10- or 20-foot chimney, when this stove is basically a slab with a center of mass under a foot from the ground. But existing slab-on-grade may not be enough.
Unusual materials: Earthen masonry, recycled metal barrel, and ducting/stovepipe installation specs:
These are substantially different from approved Masonry Stove construction methods. (Masonry chimney, high-heat ceramic flues, rebar and concrete footing, hearth or vertical burn chamber door, etc.)
We think they're better in many ways (better thermal contact between flues and mass, solid monolithic masonry with integrated tensile reinforcement instead of linear rebar, lower environmental impact and materials costs). And there's some evidence that concrete and rebar are not compatible with earthen masonry in certain applications. But it's not up to us entirely: somebody official needs to agree with us about that.

Other observations from the inspectors:
Robert wondered if a child could "fall into" or against the barrel and be burned.
The barrel is a substantially lower temperature than the woodstove we had in my home growing up, or the modern ones I've seen in other homes and meeting spaces since 2005. And we've built a fairly substantial cob structure around the stove core. It's ergonomically not that easy to get your body in position to fall into the barrel unless you actually climb onto the sloping cob mass. The kids we've had in the house seemed to get the picture quite easily, and sat happily on the bench or ran around in the other parts of the house. But it could happen; kids do run around and climb on things, and it might one day suffer the same fate as many a family sofa.

Proposed solution: John described a screen they'd built from square metal bars, three rails, that served as a "kid fence" without interrupting the radiant heat from their stove. I've been looking for decorative options anyway, and a wrought-iron lattice or sleek modern rail could add something nice to the stove's appearance. (And, Emily observes, it would be somewhere nice to dry mittens, socks, and warm towels for luxuriant showers.)

The option that Tryon Farm used for kid-safety (they host school groups, and have toddlers in residence) was
a) Ernie designed the stove so the barrel wouldn't overheat easily, and
b) they cobbed a sculptural motif around the main part of the barrel in addition, making it difficult to accidentally touch the metal but still possible to cook on it.

While thinking about safety, Joe liked the downdraft setup. It's not hot above the wood feed, and it avoids a lot of the problems associated with a "hearth" where fire can fall out onto the floor.
From our friends' experience, you'd have to do something really silly like perch a 4-foot long crooked stick of firewood in the stove (more than twice the design length), and then leave it unattended, leaning precariously at odd angles, in order to get it to fall out.

One of my favorite aspects is that we can actually burn a single log (of 4" to 5" diameter) and it self-feeds and self-regulates the air. Smokeless, just like the other fuel options.

Another speculative option that one of our inspectors suggested in a humorous mood ... would the fact that it's made of local dirt, as old as dinosaurs, and in a traditional manner, possibly qualify it as antique?

Stay tuned for updates on the Rocket Mass Heater permitting process: http://www.ErnieAndErica.info/rocketmassheaterpermitting

Fire Science: Burning Wood

Current Events:

Other Skill-Building Topics:

Some things most people know about fire:

Some things not everybody knows about fire: