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How Chimneys Work

Posts: 1290
Location: Okanogan Highlands, Washington
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Does your heat come with a chimney?
If you heat with wood, coal, straw, dung, or propane, the exhaust has to go somewhere.
Some folks may not realize just how critical a good chimney is to the whole operation.
Off-grid living (and that stove you keep for snow days and emergencies, for that matter) come with chimney installation, maintenance, troubleshooting, and operational safety basics to learn.

I've learned a lot about the history of fire prevention (often learned from specific catastrophic fires, and from "great new ideas" that led to a wave of fires and then a regulation to prevent future ones).  
Some of that's as a heating geek, and more recently I've seen the flip side as a volunteer fire fighter in a tiny rural community. Only a few hundred year-round residents live in our district, yet we've had 3 or 4 chimney fires in the past few years.  
1% per year seems like a really high number for a preventable problem like chimney fires.  And the more I look around, the more it seems like we all think we know how chimneys work, and we really mostly don't.

Chimney fires suck.

One of our worst fires, last winter, happened to a couple who had just cleaned their chimney.  
Possibly some of the creosote got dislodged and settled down at the bottom closer to the heat, possibly some part of the chimney got bumped and ended up with inadequate clearance to the wall or ceiling combustibles.  Possibly the through-wall cleanout access wasn't securely installed, and cleaning opened a gap.  It was impossible to tell after the fire.

Both occupants were on site when it happened, and were lucky in that they had time to move some belongings out of the farthest rooms of the house.   The room with the stove in it already had fire rolling across the ceiling, and the whole structure was a total loss.
He was trying to laugh it off with black humor, she was still shaking from hearing the cat screaming in the attic before it died.

We did what we could, with volunteers from the two closest fire districts, but it wasn't much.  We helped protect some nearby structures, gave them phone numbers to call for emergency housing help.
We had to finish the job with shovels and buckets of ice, as it was -10 F and the fire trucks were freezing faster than they could pump water.

That incident really stuck in my mind.  

Sometimes when I mention chimney fires, folks tell me, "oh yeah, we used to have a chimney fire 2 or 3 times a year, the chimney would be red-hot and my dad would be up on the roof yelling at us to close the stove, if you opened it it would go "boosh" and singe your eyebrows off."  


I twitch now when someone tells me "I built my own chimney even better - I used well casing instead of stovepipe" or some other DIY "improvement" that, for a whole host of reasons, is likely to cause a major fire hazard.

Visible smoke, and mid-winter chimney cleaning, both seem to me now to be early warning signs of a possible, catastrophic, chimney fire.  
Creosote in any form is an early warning sign of a preventable emergency.  

A past chimney fire is not just a warning sign or a close call - it's a preventable emergency that you survived.
Modern, manufactured chimney components are designed to take ONE chimney fire without failure, meaning you might not lose your house just because you had a chimney fire.
Repeated chimney fires can weaken the parts, loosen the joints, and degrade critical insulation and precise (scanty) safety clearances.  Partially-burned and dry wood near chimneys can also become more flammable, easier to catch on fire next time.
Surviving one or two chimney fire does not mean your chimney is 'safe' - it just means you're still alive.

Chimneys and chimney-powered stoves have only been around a few centuries.  By contrast, people have been using fire since there were people - thousands or millions of years.
A few centuries is not a lot of time for us, culturally or evolutionarily, to get used to a new technology and develop common sense awareness of what's safe, smart, or best practices.

Here are a few things I wish more people knew about chimneys.
I'd welcome corrections, additions, or stories from others with chimney experience - installers, sweeps, fire fighters, there are a lot of folks with more experience than me, who may have lessons they don't often bother to share any more.

1) Smoke = unburned fuel = creosote.  
We've been using smoke for generations to drive away mosquitos and insects, and to preserve food.  
A lot of people have a hard time recognizing it as dangerous.  Especially since it "smells so good".  
I know it does - I love smoked jerky and roasts, but we make them in a smoker away from the house.  The local butchers lose their business after their smokehouse caught fire, and burned their refrigerated truck and most of their freezer facility.
Smoke inhalation is also one of the reasons for the high rates of childhood mortality in some outlying regions that still use open-hearth fires indoors.  Smoke inhalation contributes to pneumonia in children and elders, and even the able-bodied can develop a nasty cough from the equivalent of a 400-pack-a-day smoking habit.  Smoke from wildfires and winter hearths contributes to health problems including asthma, respiratory disease, and even heart attacks.

The modern American would generally be embarrassed by indoor smoke, but tolerates it outdoors.  
We also have the 'scarcity' mindset, coupled to our "rugged independence" lifestyle.  DIY rural buildings, smaller family sizes where one or two people struggle to maintain a household while working for a living, the push for more and cheaper households full of more and cheaper clutter - all this leads to more expensive heating bills (three couples heating three houses use a LOT more fuel than six people heating one house).  
The chore of filling the woodshed or paying the propane bill falls heavily on more and frailer shoulders.  A lot of DIY chimneys and woodstove installations going up in "tiny homes" and trailers, that may not pass muster in a larger, better-equipped household.  A lot of operators are trying out wood heat for the first time, or to save money, or to scrape by, who don't have time to sit by the fire and enjoy the learning curve. And a lot of families and older folks are trying to heat with wood, but don't have enough people in the house to divvy up evening and morning chores, and still get a full night's sleep.

Smoke is an early warning sign of unsafe wood heat, and an unacceptable pollutant in crowded modern cities and crowded neighborhoods.
If you don't have time to learn to operate your wood fire without smoke, consider sticking with automated heat as a back-up until you're ready for full-time homestead life.

2) Chimney Temperatures: Chimneys draw when warm.  Not just warmer than air, but also warmer than the dew point of any liquids in the smoke or exhaust.  (Clean-burning wood exhaust is slightly heavier than air, at the same temperature.)
- A chimney that puts out wood smoke must be over 350 F to be safe, i.e. in order not to condense creosote/tar inside the chimney.  If you are damping down a fire, or burning wet wood slowly, your chimney is probably also getting too cool for safety at the worst possible time.  Modern US woodstove regulations include a minimum chimney temperature - the chimney must be at least 350 F to safely operate.  Most woodstove chimneys operate between 400-600 F, and some may get hotter.  This is hotter than most kitchen ovens, and more than hot enough to start things on fire if it is anywhere close to combustibles.

- A chimney that puts out a cleaner exhaust (like propane, or coal, or a smokeless wood fire produced by burning dry, cured wood nice and hot) will not have creosote, but will still have water in the exhaust, and the chimney can stall if that water condenses while still inside the chimney. 200-300 F is a typical target temperature for masonry heater exhaust, to stay above the dew point of water for reliable draft.

- A chimney that operates close to room temperature, or below room temperature, is at risk for drafting backwards in certain wind and weather conditions.  All chimneys are likely to have start-up problems if they've been unused for a while.  These problems can be worse on warm days, or in warm climates, compared to a frequently-used chimney in a cold-climate home.
Because regulators understand that all chimneys need to be warm to operate safely (or at all), efficiency comparison numbers on different solid-fuel devices can be misleading.

- Starting Tip: Light a twist of newspaper under the chimney opening "to check for draft" before lighting a fire in any fireplace or woodstove.  Not only does this let you check for the correct damper position on unfamiliar chimneys, it also effectively primes the draft.

3) Good Chimneys, Poor Chimneys:
Size matters.
Chimney draw is proportional to the area of the chimney.  It's also related to the temperature, and the height.  
Clogged chimneys draw substantially less.  Short chimneys draw poorly, especially if they are shorter than surrounding buildings that may cause downdrafts and wind eddies.  Cold chimneys draw poorly, and may even "draw" backwards.  
If you like equations, here's the chimney effect (stack or 'flue' effect): https://www.engineeringtoolbox.com/stack-flue-effect-d_1424.html.  

- If your chimney tends to blow back when starting the fire, or on bad weather days, it needs improvement.  Consider doing one or all of these things:  
a) Increase the chimney height to 3 to 5 feet above the roof, and 2 feet above anything within 10 feet (nearby roofs, farm structures, etc).  
b) Insulate the chimney where it passes through unheated spaces like the attic, and where it is exposed outside the building.  Make sure the insulation is non-combustible, and matches the chimney manufacturer's recommended installation.  
c) If your chimney has a LOT of exposed height outside the building (like it went out the wall and then up), you will probably need insulated chimney PLUS a "chase," basically a little insulated house for your chimney.  A chimney chase is a lot of work, and it still may not work as reliably as a chimney that was built indoors, near the roof ridge, to begin with.  

- If you are considering what to do for the size, height, and installation location for a new chimney, think seriously about the following factors:

a) A bigger chimney will draw WAY better.  If you're choosing between stoves with a 5", 6", or 8" chimney, the 8" chimney will be about twice as powerful as the 6" one.  The 6" one will be half-again stronger than the 5" one, and twice as powerful as a 4" chimney.  4" chimneys suck, and probably should not be used except for very, very short runs (like maybe a cookstove in a gypsy wagon).  There is such a thing as too-powerful draft, and too much heat for one room.  
The #1 factor is the design size of your stove.  Most stoves are designed for a specific chimney size, which you can see by the size of the connection collar on older stoves.  Get the right size stove or heater for your needs, and the right size chimney for your stove.
Location, location, location.

b) Treat the chimney right, and it will treat you right.  Many people want to stick the chimney up the outside of the house - to get it out of the way, or in the mistaken belief that it's safer "out there."  An outside chimney still needs to stay warm to draft properly, so you'll probably lose a lot more heat with a slap-on chimney, and you may even end up having to build it its own separate house or "chase" to get it to work adequately.  I prefer to put the chimney as central in the building as possible, where it can stay warm when you're not using it, and where any heat it loses is still usefully lost into the building you're trying to heat.
c) There will be fewer leak problems HIGHER on the roof.  An installation down low, near the eaves, has to deal with all the ice, snow, and rain being shed by the entire roof.  A thru-wall installation has to deal with the rain and ice running down the chimney itself, and keep these leaks out of the wall.  If you must go through the wall, try to exit as high as possible, on a gable side (the pointy side, where there is no drip line to dump roof water/ice onto the chimney).
d) There will be fewer structural problems installing a through-roof chimney than an add-on.  Unless your walls are masonry and your roof is thatch, it will probably be far easier to take the chimney straight up between roof rafters, and it will work better.  Attaching a chimney sideways to a wall, with adequate clearance and weight support, all the way up to the height where it gets reliable draft without building-pressure problems, is more of a job than you might think.

4) Air is not Heat
Chimneys work because they draw all the smoke, and a certain amount of the air, out of your home.  
Any functional chimney will cost you a fair amount of warm air.  Maybe not as much as a smoke-hole in the rafters, but still quite a bit.
Some people resent this loss of warm air, and try to damp down the chimney to trap more warm air in the house for comfort.
If your home is lightweight wood and fluff, and does not have heavy masonry or adobe inside to hold the heat, it is easy to think that losing warm air means losing all the heat out of your house.  
However, warm air doesn't actually hold that much heat - air is much better at removing heat, or preventing heat transfer, than at storing it.  

Consider improving the building's "heatability" - slow down heat losses, improve heat storage.
Hopefully you've already sealed drafts near the floor, and insulated or double-lined any cold windows to prevent the cold-air "waterfall" effect (adiabatic flow).
Next (or first, if you're starting fresh), pay attention to insulation, detailing, and airtight sealing of upstairs ceilings, windows, stairwells, attic hatches.  
Invest in good insulation, especially for ceilings and exterior walls.  (Don't insulate the roof itself unless it's designed for it; insulation inside a vapor barrier can trap condensation on the cold outer surface, causing mold and rot problems.)
For fresh air, consider heat-exchanging ventilation (builditsolar.com has designs you can build with pop cans, if you can't afford a mechanical system).
And as far as possible, within the load limits and rental/lease agreements, consider improving good heat storage within the building in terms of thermal mass or strategically-placed heat reservoirs (hot water bottles, insulated warm bricks).

In a few cases, letting the fire burn out and shutting off the chimney before bed (with no remaining unburned fuel in the stove) may actually preserve more heat than trying to maintain an overnight fire.  Particularly if you're unsuccessful - the chimney will still draw air, although a bit slower, even if the fire goes out.

5) RTFM.  Read The [Friendly] Manual.
What kind of chimney do you need?
What kind of appliance are you connecting?
Would you possibly want to change out in future, say a little woodstove now but maybe a cookstove or bigger heater later?

Modern manufactured chimneys come with instructions.  They are free - and most are available online.  
Woodstoves also come with manuals, even for older woodstove models in some cases.
For lined masonry chimneys, fireplaces, and site-built masonry heaters, some come with instructions, but more often the "instructions" are the building code, and the training of your local masonry tradespeople.  
If in doubt, have a professional inspect your masonry chimney, and confirm whether it's ready to use or needs work.  If you're going to the trouble of building a full-height masonry chimney, it's common practice to include 2 or more flues.

Manufactured woodstoves, appliances, and chimneys are designed and tested with care, to maintain safe chimney temperatures and meet community expectations for clean air, safe operation, and durability.  
If your stove or chimney is not rated for coal (mixed-use or "solid fuel" vs "wood burning), or for burning trash (most aren't, any more), or for a 'banked' fire longer than 6-8 hours, you could easily damage it or endanger yourself by thinking outside the box.
Sometimes people don't want to bother with expensive or complicated chimney installation instructions, and are tempted by cheaper "ventless" stoves and appliances, or by a makeshift workaround vent in the same general style.  
Gas logs are the classic example of an appliance that vents right into the room.  Propane/natural gas cookstoves are another.  
These charming appliances are designed to operate for a few minutes to a few hours at a time, with good ventilation (the instructions on some ventless gas logs actually call for opening a window at least 2 square feet in the room while operating the gas log). If you treat the wrong gas log as an "automatic fireplace" that you can run 24/7, or run an unvented cookstove to keep warm, you risk poisoning the indoor air and/or breaking the appliance in dangerous ways.

Likewise, many modern stoves are designed to operate with specific air to fuel ratios, to cut costs by using a minimal thickness of metal and materials, and to improve sales (sometimes at the cost of safety) by spending the available money on looks not longevity.  While your friend's cast-iron stove may be able to burn coal, trash, or hold 'banked' coals overnight, trying the same trick in a different stove can cause serious problems.  RTFM.

Some wood stoves are designed as "camp stoves" or "space heaters."  Some "camp stoves" I've seen are so thin-walled that the makers expect them to burn through within 3-4 years.
Some space heaters are super-basic like a little black box; others are designed with beautiful trim, enamel, glass, low-clearance double wall construction, and general showmanship as a "fire TV" for your parlor or rec room.  
But a "space heater" is still, at its core, a parlor heater or one-room heater.  It is designed to heat while you are in the space with it, not to heat a whole building evenly, and not necessarily to perform automatically while you are gone.  
A space heater will not give the same performance as a whole-house heater, furnace, or boiler.  

It pays to read the product specs, not just the price.

Look for the BTU output, not just the generic "square feet." Square feet are a salesman's number; your realtor and your floor showman speak this number, but it is almost meaningless when it comes to heating performance.  It would mean more to look at last winter's heating bills, or run your home's details through an online heating calculator like this one at builditsolar.com. https://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm

If you want to know the max. BTU output you'll need on the coldest night of the year, use your USDA zone (average extreme low).
If you want to do some math and work out the amount of wood or propane you might burn in a "normal" year, use your Heating Degree Days (HDD) for your local area, or compare last year's bills.  Wood has a lot less energy per pound than oil or propane, so you may be surprised how big a woodshed you need to properly dry and store enough wood.

If they have brick or soapstone tiles "to hold heat," look at the actual thickness of material.  
Rule of thumb is maybe 1" per hour - so a brick liner 1" thick might buy you an hour or two of extra heat.

If you want to cook on it, or build around it with masonry, or heat water with it, or install a damper for air control, make sure that's included in the original manufacturer's design specs.  Call them if you need to, before buying or altering your stove.
One of our favorite sources for general info on wood heat, and stoves, and all things chimney-related is the Chimney Sweep's Library, http://www.chimneysweeponline.com/libchimneys.htm
A Historical Interlude:
The whole idea of sending the smoke out a chimney, rather than using it to heat the walls or kill insects, was have been a bit of a novelty originally.  

Look up the original meaning of "curfew", and some of the early fire-prevention regulations in Europe and Japan.  
There was even a rule in Ireland that people had to light a smoky fire once a year, and trap the smoke indoors, to fumigate the thatch.
Viking-era homes, like pirate ships, usually had a central hearth where a box of sand held all the necessary tools, including the tripod or trammels to hang pots over the fire.

Victorian cast-iron foundries turned out dozens of "brilliant" heat-saving add-ons for chimneys - from swooping sections of stovepipe designed to snake along the ceiling before exiting the room, to Escher-like heat exchangers with honeycombs of cris-crossing flue pipes.  The biggest problem with all these clever heat-exchangers is they did their job too well: extracted heat from the smoke and, incidentally, helped that smoke to cool and condense into creosote.  Although creosote may not catch on fire for months or years, eventually some cold winter day, someone is going to build a bigger fire, or the last little dribble of creosote is going to reach the wrong part of the stove underneath.
A creosote fire in a curving, long stovepipe section has a distressing tendency to pop it open like a flaming seed-pod, spraying creosote-dripping flames all over supports and ceilings.  

Times change.

If you're going to the trouble of putting in a new chimney, it's worth reading up on the latest specs and recommendations.
There are a couple other home improvements that might be worth considering.

Fire Without Smoke:
- if you need more heat, and particularly overnight heat, have you looked into masonry heaters or rocket mass heaters instead of a conventional wood stove?

They are less well known as yet, but I would love to see more of these tiny masonry heaters become "normal."
Donkey's latest has a lot of charm: https://permies.com/t/71576/tiny-house-rocket-mass-heater
This example heats a 10x30 mini-cabin.  

For bigger structures, you can find gorgeous professional examples on the Masonry Heater Association website (www.mha-net.org).
DIY masonry heater plans are available using with brick (see David Lyle's Book of Masonry Stoves, the back has Russian bell stove plans), with earth and pipe (rocket mass heaters), with combinations of brick and steel (see recent designs called "sidewinder" or "cabin stove", or even with raw adobes and scrap metal.  
Professional installation for any heater or appliance is going to bring the price up, and more so for a site-built project than a quickie installation job.  But there are ways to spend time instead of money, if you really want one.

The main advantage of a masonry heater is that it store heat in masonry mass, so you burn a few hours per day, and you can shut down the chimney draft in between fires.  Saves heat, saves fuel, and saves the worry of "operating" a woodstove while you sleep.  
The main disadvantage is they're huge (in proportion to how much heat you need to store, and for how long).  So it's a bit of trouble to build one in a house that wasn't laid out with masonry heating in mind; more of a remodel project than replacing a standard appliance.

House as Chimney:
- if you are struggling to keep the house warm on a budget, have you considered an audit or inspection that includes the upper part of the house?  
Many people take steps to chase cold drafts at the floor level and windows, but don't realize that a lot of the problem comes from upstairs or ceiling leaks.  
Just like the chimney, a warm house will tend to move air upward and out, through any available openings.  
A good energy audit may recommend sealing attic hatches and behind the ceiling trim, insulating the ceiling (but not the roof, unless it's designed for it), and making sure upstairs windows are just as leak-proofed as downstairs ones.  Occasionally they may find other leaks; hot water leaks are particularly costly, and sometimes electrical appliances can lose significant energy.  
A reputable energy auditor, especially a third-party one who isn't just selling one flavor of weatherstripping or insulation, can help you locate heat leaks with an infrared camera... or you can get your own that attaches to a smart phone for a few hundred bucks, which is a pretty cool Xmas toy for the techie.

The basic concept is, you don't want your house drafting like a chimney.  You want it to trap heat, like a nice winter hat or old-fashioned chafing dish (covered).  
You and your building probably need to breathe a bit, but you don't need to wander the world bare-headed to do so.

House as Oven:
- Some of the original "black stone" dwellings of the Neolithic England did not have chimneys.  Or they were chimneys, depending how you look at it.  The house was built like a classic beehive, with a low door, except that instead of straw it was stone and earth, and instead of bees it was sized for people.  A central hearth fire heated the whole structure, the low doorway helped trap most of the smoke inside, and people presumably spent most of their indoor time sitting low to the ground, breathing the relatively fresh air available below door level.  In warmer climates like Greece the top had a smoke-hole; in some colder areas like the Hebrides, there would be a conscious effort to trap the smoke and not let it escape.  The stone of the house held heat from the fire, radiated it back down into the space, and people presumably figured out how to time the fire and bank it at night so they'd get sufficient fresh air.  A bit like an oven, for people.

- The origin story for the Chinese masonry heaters / platform furnaces known as k'ang says that the earliest cave people heated their caves and food with a fire on a big flat stone, and at night, they would sweep aside the fire and ashes, and sleep on the heated stone.  A bit like a pizza stone for people.

Does this sound dangerous?
Wouldn't you asphyxiate with a smoke-line down to 3' off the floor?  
Wouldn't you burn yourself if you went to bed too soon after cooking dinner?

As David Lyle put it when discussing braziers (like putting a Hibachi under your bathrobe or tablecloth to keep warm), "... much the same as with modern automobiles: It is great if used correctly, mistakes can be fatal.  ...  The ancients knew very well how to use braziers safely.  Nevertheless individuals sometimes failed to do so; they did not bother, or had too much wine and forgot, and so died.  Much the same might be said today of automobile drivers, or wood-stove users."

Some people say "safety obviously comes first, cost is secondary" but absolute safety is hard to actually find.  Roman brasiers were very similar to modern wood stoves (some had water-heating pipes, or air vents underneath, or other features).  All they needed was a chimney.

The chimney doesn't really come into mainstream use until the industrial era - when bricks, chimney flues, and other industrial components become widely available.  You might think that understanding the science and physics involved was a factor - but a lot of modern chimney codes come from hard experience, not scientific forethought.  I think one of the biggest factors in widespread adoption of chimneys is that not only were energy-intensive industrial parts more readily available, but there was more and cheaper energy available for providing the extra heat needed to safely send smoke out the chimney instead of 'saving' it inside the cave/hovel/house.  

I don't know when we started to feel that smoke inhalation was a peril worth the cost of a chimney to avoid.  
Collectively, we still haven't decided that smoke-free lives are worth the cost of a good stove, or a masonry heater, or public funding for wildland forest management.   So you might say we're still not 100% there yet.

Finally, once you have a properly installed chimney, and have checked that it's a good match for your stove and heating goals, there is the question of maintenance.
6) Chimney Maintenance:
Annual inspection, and cleaning if needed, is the industry standard.
Some people recommend monthly inspection during the heating season ... if you have to remove creosote that often, you're probably burning dirtier than you should be.

If you sweep your own chimney, especially if you haven't been trained how to do it, it's easy to think you're fixing the problem when you might be missing the warning signs of a chimney fire.
Creosote is generally a bad sign - and it can be hard for a novice to distinguish between soot, 'crunchy' creosote, and the residue from a chimney fire that burned sticky tar into black bubbly leftovers.

Consider having a professional chimney sweep do an "annual inspection" once or twice, and asking them for a quick inspection lesson.  American chimney sweeps may not be quite as knowledgeable as the ones in the Netherlands who are the legal authority charged with approving new stove installations, but most of them have seen a lot of good and bad installations, and they may be able to recognize an accident waiting to happen just in time to prevent you learning your lesson the hard way.

Be safe.
Warm wishes for many nights of lovely, trouble-free, fireside snuggling over the holidays.

Erica W
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Location: Pacific Northwest
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Thank you so much for this, Erica! It's really helpful in knowing how to operate my woodstove. I've read my friendly manual many times, but it doesn't really answer all of my questions... Maybe you can help?

I have a 5 year old Country Striker woodstove. It's in a manufactured home, so it has an intake from below the crawlspace. How does this affect the draw in the chimney? I know without it our house smokes up like crazy since it's so "airtight," but does this impact how if the chimney will draw warm air from our house up and out?

I also had a question about smoke from a chimney. I generally try to keep my stove going at 400 F, with a little flame coming out of the wood. I can usually do this with the flue dampened all the way down. I've noticed, sometimes, that even when the fire is in the "happy" range between 350 and 600, it'll smoke...probably because the wood got wet because our shed likes to leak. What type of creosote is this putting in my chimney? I try to get the fire hotter, which often makes the smoke go away, but sometimes I'm inside and can't see the smoke, and I worry that I'm creating bad creosote buildup.

Another question I have is whether or not it's okay to leave a fire dampened down when it's just a big ol' piece of hardwood that's half turned into coals and burning at about 375F? I'm often going to bed and the fire is still going. I don't really want to open that up and have it go all raging and go to sleep with a raging fire. But, I worry that I'm making too much creosote by leaving it burning at those lower temperatures. But, if the wood is well seasoned and dry, is there still build up occurring? It's hard to know if it's smoking because it's night and I can't see. But, I've done this and left home to go to church or shopping and come back to just a few coals...and no smoke. Does that mean it's burning cleanly, even at low temperatures?

Thank you so much for your help!
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I burn a lot of coal, in part because anthracite coal which is the most common form of heating coal, cannot start a chimney fire because it has no creosote in it. It has some black soot, but this is NOT creosote and will not catch fire!

But coal stoves are very draft-sensitive because of how the coal burns. Unlike wood that burns from either the top down, or bottom up if the stove has grates, coal burns from the bottom up. In fact, the fire will go out with anthracite coal if air goes over the fire. It has to be drawn up through the coal bed. With bituminous coal (soft) there has to be a tiny bit of air above the coal bed to ignite the coal gas that accumulates there.

Either way burning coal makes for very low chimney temperatures. I might have an incredible amount of heat coming off my stove, yet my chimney may only be operating at 250 degrees, under what most chimney temperature gauges show as safe. With anthracite coal that is safe because there is no creosote to form.

Coal is also safer than wood in that it can never "run away" on a person. It is like a diesel engine; if the coal bed has air, and it has fuel, it will burn. Centralia PA started a fire at its dump in 1962, it went underground, and today in 2017 is still burning... and that is why a coal stove will burn twice as long as wood. In other words if a wood/coal stove will burn 4 hours with wood, it will burn 8 hours if it it burns coal. But while that seems scary, it should not. As the coal burns and forms ash, it slowly starves the fire of air and fuel, and thus slowly goes out. This is unlike wood that may smoulder for awhile, then an hour after leaving for the grocery stores, roars and takes off! Coal cannot do that, it just slowly goes out. To revive it, you just shake out the ash and add fresh coal. For most coal burners they take pride in the "single match" which is lighting the coal once in a heating season. With 12 hour burns between ash shaking and fresh coal, it is not too hard to do.

BUT coal stoves almost always require masonry or double wall metal chimneys, not because chimney fires...that is not a problem with anthracite coal as a fuel source as previously discussed, but because warm chimney temperatures are needed. Using single wall metal pipe is a huge problem because wind and cold chills the inside temperature of already cool exhaust making it hard to draw air up through the coal bed and keep the fire lit. Double wall metal chimney allows the warm air to stay warm and draft properly. masonry does the same thing, but only because the heat of the fire has thus warmed the inside of the chimney and it too allows proper drafting to take place.

One more piece of advice: NEVER burn charcoal in any wood stove. It is not real charcoal, made like they did in the old days that is, nor even resembles coal. It is a man made product teeming with petroleum to get it to light easily. Not only does it give off copious amounts of carbon monoxide, it is made of tars, oil and toxic chemicals; no one wants to be burning that!

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Erica - Thank you, this is a very informative post. Just FYI - your link to engineeringtoolbox has a typo which breaks the link. There's a superfluous period [.] and space [ ] at the end of the link which need to be removed.

Put another way, the link is currently "https://www.engineeringtoolbox.com/stack-flue-effect-d_1424.html. " and needs to just be "https://www.engineeringtoolbox.com/stack-flue-effect-d_1424.html" (note the extra characters at the end).
Matt Coston
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Erica Wisner wrote:Hopefully you've already sealed drafts near the floor, and insulated or double-lined any cold windows to prevent the cold-air "waterfall" effect (adiabatic flow).

I had not heard that term "adiabatic flow" before so I looked it up. The definition is "Movement of a fluid without heat transfer." It feels wrong to describe the "waterfall" effect as adiabatic flow since it is literally caused by heat transfer. To me, adiabatic flow describes the flow inside the heat riser of a rocket mass heater. The heat riser is insulated so there is (almost) no heat loss as the air rises - hence, movement of a fluid without heat loss.

I'm  beginner at this so I could be wrong.
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I'm a believer in the internal chimney.  

The house I grew up in had a fireplace on the outside wall.  It was a beast to get a draft, always blowing smoke into the room when we lit the wood stove.  

My current house has chimney in the very center of the house, going up three stories and out the peak. Draws perfectly every time I light the stove. Perhaps helps that the attic is finished and usually the warmest room in house. Attic roof has skylights on the south side and is much better insulated than the walls (r40 vs r21 in walls).
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5) RTFM.  Read The [Friendly] Manual.
Thank you, Erica...Best advice anyone could get...I am dumbfounded when I talk to people and ask, did you read, understand and follow the manual?  They look at me like I have a third eyeball...In this day and age of technology, (IMO) there is NO excuse for not knowing
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I had a chimney fire once, the fire brigade came, opened the wood heater, threw in a mug full of water and the steam put the fire out!
They then spent time looking to see if any part of the chimney was broken and had set the roof on fire.
I installed a stainless steel flue all the way through the chimney after that.
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Matt Coston wrote:

Erica Wisner wrote:Hopefully you've already sealed drafts near the floor, and insulated or double-lined any cold windows to prevent the cold-air "waterfall" effect (adiabatic flow).

I had not heard that term "adiabatic flow" before so I looked it up. The definition is "Movement of a fluid without heat transfer." It feels wrong to describe the "waterfall" effect as adiabatic flow since it is literally caused by heat transfer. To me, adiabatic flow describes the flow inside the heat riser of a rocket mass heater. The heat riser is insulated so there is (almost) no heat loss as the air rises - hence, movement of a fluid without heat loss.

I'm  beginner at this so I could be wrong.

Great post Erica!  Thanks.  I am moving into a new to me house, will be making decisions about heating.

Can I ask, if I burn pine, but at a higher temperature, as in a modern technology enhanced wood stove, am I still at risk for creosote?

About adiabatic heating, I was taught that air mass flowing to lower elevations increased in temperature because it was coming under greater atmospheric pressure, and this is the phenomenon responsible for the hot Santa Ana winds.
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Ha ha ha. As I read through the post, wow such a knowledgeable person at the level of Erica too and would you believe it, when I went to thumbs up, it is Erica. Salute.
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