OT: Wood Stove Design

[CalM]

I have thought strongly about an "afterburner" system. Some would call it a "Smoke Eater".
In my mind, I see a secondary combustion "box" that is located between the wood burner and flue pipe. All the "smoke" from the wood stove must pass through this chamber.
A natural gas or LP gas burner is positioned within the box and in full contact with the "smoke", and unburned hydrocarbons (think creosote).
Secondary air and some sort of "catalytic" hot surface may also be incorporated.
Simple automotive type O2 sensors (heated of course ) and thermocouples would provide feed back to control startup of the auxiliary gas burner.

Of course, a heat extraction devise such as the "Magic Heat" would be fitted to be sure not to send all that additional Techno-energy efficiency up the chimney. Or just send it through a "Russian masonry heater" for heat storage and moderation.;-)

If wood were scarce, I might get started..........

Ash borers have that down to a low priority at this time.

[Sparky_NY]

I have been the wood stove route in the past, everything from horrible to highly efficient. My last one was a Napoleon brand, extremely efficient. The efficiency was due to several factors.

1) full firebrick in the stove and ceramic plates in the ceiling, this keeps the combustion temperatures high for efficient burns (including the smoke !)

2) a secondary air system feeding the top area of the firebox, this is for secondary combustion, burning the smoke, there is a lot of heat energy wasted in the smoke.

3) the entire stove (except the front) was enclosed with a sheet metal second wall with a blower on the back. You could put your hand on the side of the stove with a roaring fire. This greatly increased the convection heat from the stove which travels from room to room making the temp more even in the house.

4) Modern high efficiency stoves burn the smoke, its known as secondary combustion and results in much higher efficiency. Outside you cannot smell wood burning and no smoke is visible from the chimney.

5) After a seasons burning the double wall pipe would have just a thin light tan coating inside except for the last foot before the cap.

That stove described cut my firewood consumption by about 1/3 and did a far better job of heating the house evenly. It would also burn all night easily. The EPA put emission requirements on wood stoves years ago and increased the requirements again just a couple years ago, this is directly related to efficiency.

Woodstoves are NOT created equal the technology has improved considerably. Of course, the newer very efficient stoves are not cheap ! The price sends many running to buying the old school type. There are other brands with similar technology, Napoleon is just one brand.

https://www.napoleon.com/en/us/firep...ts/wood-stoves

[MrWhoopee]

Hi everyone.

We heat our home using a wood stove during winter. There is plenty of wood that falls around the house and I am able to chop it up during summer to burn during winter. We have a fairly large Jotul and it can build some serious heat.

However...

I can't help but think that this traditional wood burning design is just horribly inefficient. It takes a lot of wood to heat the house and keep it warm. Most of the heat blows right out the chimey, so the heat going into the room is by way of natural convection and radiation.

One simple improvement could be to blow a fan at the stove to increase the temperature gradient and improve heat transfer efficiency to the room.

But it has got me thinking... could I design my own ultra-efficient heat exchanger wood stove? Essentially, some thick steel plate with heatsink fins, ducting, and a built-in fan. The indoor chimney pipe could have a corrugated shape to increase surface area. This way, the outside of the fireplace itself would be just warm to the touch because the fan was constantly forcing air across the surfaces.

I actually used to own a fireplace with a built-in fan and catalytic converter in the exhaust. It put out so much heat we had to turn off the fan because the house would get too hot.

Why isnt this more common? Cost? Complexity? Are there some safety concerns or something I am missing?

They already build them, high-efficiency EPA stoves. I have a 16 y/o Buck Model 81 and have been heating exclusively with it for all that time. It's double wall with circulation fan and secondary air combustion. After doing my research, I decided that secondary air was more trouble free than catalytic stoves. Load it up with ponderosa pine, white fir and Doug fir, wind it up until it's hot enough for the secondary jets to start firing off then slowly throttle it down.

https://www.youtube.com/watch?v=5ffouzG3DwI

I save the oak for overnight burns, leaves a bed of coals for restarting in the morning. I also have an Ecofan (heat powered) on top of the stove. It is silent, which the stove's fan definitely is not. It moves a small amount of air continuously. I keep it pointed towards the kitchen, which is noticeably colder without it. We start using the circulation fan on the stove when outside temps are below freezing and we start having trouble keeping the house comfortable.

During the winter the stove runs 24/7. If I'm being conscientious about running it, I clean the stovepipe once per season. If I get lazy, I will hear the tinkle, tinkle of creosote falling down the stovepipe and before long the stove is drawing poorly. Then a January cleaning is required. I live at 4000 ft. elevation in the mountains of northern California. Overnight temps in the winter are generally mid-teens to mid 20s.

[A.K. Boomer]

MrW, I have an Avalon and it too has an upper secondary combustion area, it's similar looking when in action just a horizontal tube with small and larger jets drilled into it, mine is slightly pressure fed due to it being a double box stove with a forced air fan, it's main air inlet that comes in at the bottom is also strategically placed so that it's preheated before the air gets to the fire, it is not "turbo charged" like the secondary air jets....

This system does help - your vid is about the best iv ever seen mine work, the ports that feed the secondary air are always open --- you cannot shut them down least on my stove, they are not huge but there's always at least a little air getting into the stove to help with secondary combustion...

There is all kinds of different shaped ceramic grid that can be installed into stoves with some insulation putty to seal the area's around it --- mine would take a very long and narrow grid but they do make them and im thinking of springing for one as it would be the icing on the cake to inject oxygen just before the ceramics,,,

Iv had my wood stove going for over a decade and never had to clean the vertical stove-pipe, but I do have to clean the one elbow at the bottom of the vertical run annually or every other year depending how much wood i burn and what type.. my pipe resides inside the chimney for the entire vert run just 3ft in the house and elbow into the chimney and then elbow up for the rest of the vert run....

I love my stove - got it for 200 bucks on craigslist and it was a train wreck,,,

I have two complaints though --- they come with these rattly little duel squirrel cage fans that are junk - they set up harmonics with each other --- sounds like crap almost like those 747's with the engines out of sync,,, except add all kinds of rattles to it, drives you crazy in fact you would kick the unit all the time just to change the sound some, I tried everything before I gave up on them,
so right under the stove in the basement I installed a single commercial grade squirrel cage fan that pressurizes the massive but hollow cement stove block I built for the stove,,, orifices then connect with where the duel gutted out fans left off --- it's on it's same variable speed adjuster - it's so nice and quiet it's wonderful, and also moves air far better...


The other complaint is something I cannot do anything about - it's just a simple brick lined stove and has to be emptied by letting the fire go out and hand scooping the ashes,,, if you ever had a grate type stove you will know how much of a hassle this is in comparison ---- they are so handy, you can shake the grates some to clean them while the stove is burning - pull a pan out and empty in a safe place outside if you want,,,, they are the ultimate stove "workhorse" as they never eve need to take a break,,,

My saving grace is it's very rare to have so many cold days in the row to where I cannot shut down the stove at least once during the day to empty it,,, I have incredible solar gain and over 300 days of sunshine - also NG backup and let it take care of the mornings,,, most winter days I don't even have to fire up the wood stove,
but when the sun don't shine and it's cold - or the nights drop to below 15-20 degree's I fire up the stove...

Some wood is horrible for ash --- and sometimes i have to use it, this creates extreme inefficiency when you are just building a fire on top of a n insulating blanket of ash...

id say it's real close that i heat my house with 1/3 solar 1/3 NG and 1/3 wood....

Another thought I might put into action is using thinner firebrick to get a little more heat out of the unit and increase it's efficiency --- at least on the floor as that's of course where the ash builds up anyways,,, plus I could use a little extra room in there - it's a pretty small stove but gets the job done....

[jdunmyer]

The other complaint is something I cannot do anything about - it's just a simple brick lined stove and has to be emptied by letting the fire go out and hand scooping the ashes,,, if you ever had a grate type stove you will know how much of a hassle this is in comparison ---- they are so handy, you can shake the grates some to clean them while the stove is burning - pull a pan out and empty in a safe place outside if you want,,,, they are the ultimate stove "workhorse" as they never eve need to take a break,,,

When I used the barrel stove, I built a rake and a shovel for cleaning the ashes. Rake the coals towards the rear, shovel out ashes, then rake the coals towards the front and add wood. Mind you, I left a bed of ashes, perhaps 3" or 4" deep, never cleaning it out completely. It wasn't necessary to shut the stove down completely, just do it when the fire is low, like first thing in the morning.

[darryl]

I don't have much to add here, as I am not an expert on anything. But I did burn wood for heat for a lot of years, and I've had my share of 'incidents'. One was when I fired up an old stove in a house I'd just moved into. Long story short, I should have pulled all the pipes for cleaning, and to have access to the chimney also for cleaning. I didn't do that. I used something called Red Devil to clean the pipes, and to save myself a lot of messy work. Nearly burned the house down- I don't know why it didn't.

But that brings up the issue of secondary combustion. The Red Devil obviously caused the build-up in the pipes to combust, and I had tossed a second tablespoon of it into the stove after the first tablespoon didn't seem to be doing anything after an hour or so. At any rate, I suppose you'd have to call that secondary combustion. Whether the products exiting the chimney were any cleaner I can't say- there was a steady stream of sparks and some were still glowing as they fell onto the roof. Had the system been cleared decently to begin with, I would not have had that problem- but I don't know if this compound would have created more pollution than the simple burning of wood would have.

I've thought about adding the secondary burner, utilizing propane, but obviously that has to be reliable or you risk blowing the place up. Think about it- you're using wood smoke as your fuel, and that will contaminate everything below your flame. It would seem that the potential is high for clogging up the propane burner. Perhaps there is a design that keeps every part of the propane (or natural gas) system out of the flue except for the flame jets- that would seem to be almost mandatory to me.

Perhaps secondary burners for this purpose have evolved into efficient and safe devices by now, I don't know. But I'd sure want to look into that- whether there's a design you could copy, or simply purchase. One of my issues is do you have this thing active all night- when the stove is blocked down and set to do little more than smoulder all night?

My last thought for now- is there, or could there be, a device which is a catalyst that would finish combusting the smoke without being consumed itself, something you would build into the stovepipe and basically forget? Perhaps you would pre-heat it electrically, or perhaps use a gas to get it up to temperature so it could then maintain it's temperature after that. Complexity comes back into the system now- and that probably does include maintaining it. Such a catalyst would have to deal with a fairly dirty smoke, and also smoke of different types- including the apparently obligatory burning of milk cartons.

And this does bring up another issue, that of dealing with waste. We either burn waste, or bury it. Either way the final products end up in the atmosphere- either right away as you burn it, or years later as it decomposes underground. The question here is, can you burn something so efficiently that you reduce the polluting component (s) to the absolute minimum- anything you put into such a device gets reduced to the cleanest it could be? Whether wood smoke, or household garbage of every type- the more completely it burns, the more heat you could get to use, and the least pollution you would release to the environment. And on this note- can you arrange for the final gaseous product to include itself in the production of a solid lump of 'waste' instead of being released to the atmosphere? Say, the slow but steady production of another, potentially useful, material?

[CalM]

Well, To blame the Red Devil compound for your chimney fire is misplaced. The Red Devil is merely a chemical conversion to harden, em brittle creosote formations as an aid to either falling down or being brushed out. (Or being caught up in the flue gas and carried away. The "fire" was a result of previous build up and the intensity of the fire you had at the time the compound was dosed into the fire box.

Propane burners are as simple as a metal tube with a few holes in it. The mixing valve and chamber can easily be kept outside of any "dirty area".

https://external-content.duckduckgo....JPG&f=1&nofb=1

On the final point. " flue gas Scrubbers" are well evolved technologies. There are different iterations depending on the fuel.
There is a "biomass" power generating station up state. No real smoke is ever evident. Still, uninvited visitors are not allowed, even in the ash dump.

clean burn tech.
https://www.fao.org/3/t0512e/T0512e0a.htm

[CalM]

I've also considered placing a LP gas burner right in the fire box.

The thinking is that most of the "inefficient" burning is associated with starting and bring a new charge up to temp.

So, Turn up the HEAT, with gas.

Never a frustrated lack of kindling. Works with any sort of fuel wood. and if located at the back of a log wood stove, would serve to put a bit of KICK into what smoke there might be and get it to burn clean. A "multi-fuel Hybrid.

Plus, when a fella was lazy, or run out of wood in the wood box. (Lazy again ;-) just strike up the gas! Not much different than a kitchen oven.;-)

someday.... if I ever really need to.

[A.K. Boomer]

Finally got cold around here lately - at least for a few days, not really cold but cold enough to use the stove some even during the day,

So took a pick tonight of the secondary air burner --- allot of that fuel you see burning at the top part of the stove would normally be going up as unburnt hydrocarbons creating more pollution and also coking up the stove pipes making for unsafe stove pipe fires...

I do like the wood stove - this time of the year look at it as a kind of work horse of sorts...

[J Tiers]

People do scream about "creosote".

Thing of it is, if the combustion is complete, there isn't any. Then you can do things to extract more heat from the flue gas.

You still need to have convection create a "draw" in your chimney, though. I suspect it is cheaper to burn more wood to get the draw, than it is to add some sort of powered forced draft to the system, in terms of total energy cost. But that would need a well controlled study.

I have a shop-related suggestion, though...... Run the forced draft with a small steam engine you make in your shop, and a operate the boiler from the flue gas heat. If nothing else, it would be a good project. I do not suggest any form of Stirling engine, they just are too much trouble and too little power.

[MrWhoopee]

I've thought about adding the secondary burner, utilizing propane, but obviously that has to be reliable or you risk blowing the place up. Think about it- you're using wood smoke as your fuel, and that will contaminate everything below your flame. It would seem that the potential is high for clogging up the propane burner. Perhaps there is a design that keeps every part of the propane (or natural gas) system out of the flue except for the flame jets- that would seem to be almost mandatory to me.

Perhaps secondary burners for this purpose have evolved into efficient and safe devices by now, I don't know. But I'd sure want to look into that- whether there's a design you could copy, or simply purchase. One of my issues is do you have this thing active all night- when the stove is blocked down and set to do little more than smoulder all night?

I think you may be misunderstanding secondary combustion, at least as far as the term is used in stove production. There is no additional fuel added, just air. It relies on heating the firebox to a temperature where the unburned gases will spontaneously ignite when oxygen is introduced. My stove relies on natural draw through the secondaries, which are just horizontal pipes with small holes at the top of the firebox. When operating properly, there is no flame at the bottom, the primary air intake control is closed. There is just this ghostly flame floating around the top, with jets of flame emitting from the holes in the pipes. An hour before bed time I load the stove completely and open the primary air. Once the fire is running good and hot, the secondaries will start to fire off with jets of flame. At that point I gradually throttle down the primary air until it is closed. The stove will run all night on the secondaries, producing a hot, clean burn with no smoke.

[alanganes]

My last thought for now- is there, or could there be, a device which is a catalyst that would finish combusting the smoke without being consumed itself, something you would build into the stovepipe and basically forget? Perhaps you would pre-heat it electrically, or perhaps use a gas to get it up to temperature so it could then maintain it's temperature after that. Complexity comes back into the system now- and that probably does include maintaining it. Such a catalyst would have to deal with a fairly dirty smoke, and also smoke of different types- including the apparently obligatory burning of milk cartons.

There is such a catalyst, it is a ceramic honeycomb thing that goes in a chamber between the area where the fire burns and the flue. The catalyst reacts with the flue gas to (gross oversimplification here) make it burn. I believe it works in a manner very similar to an automobile catalytic converter. They have been around for years. They are an alternative to the secondary air type stoves and while I have never owned one I know a few people who have had them and liked them very much.

lots of reading material here:

https://www.google.com/search?client...tic+wood+stove

[A.K. Boomer]

I think you may be misunderstanding secondary combustion, at least as far as the term is used in stove production. There is no additional fuel added, just air. It relies on heating the firebox to a temperature where the unburned gases will spontaneously ignite when oxygen is introduced. My stove relies on natural draw through the secondaries, which are just horizontal pipes with small holes at the top of the firebox. When operating properly, there is no flame at the bottom, the primary air intake control is closed. There is just this ghostly flame floating around the top, with jets of flame emitting from the holes in the pipes. An hour before bed time I load the stove completely and open the primary air. Once the fire is running good and hot, the secondaries will start to fire off with jets of flame. At that point I gradually throttle down the primary air until it is closed. The stove will run all night on the secondaries, producing a hot, clean burn with no smoke.

Well said but I will add - it's kinda a catch 22 thing, Yes you can even get the lions share of combustion out of your secondary air jets but you have to be careful to not totally smother the main fire either otherwise your kinda defeating the purpose,,, You want to use the secondary air jets in conjunction with the main flame, you want to be able to taper down the main burner but not totally smother it and make it inefficient, Ideally using the secondary air as a "clean up" jet system,,,

If you let the main burner diminish too much you also run the risk of the whole ball of wax going out in the middle of the night ...

depends on the type of wood im burning - cedar almost seems to make it's own oxygen lol that's not really the case but it's so burn hungry that it will draw extra hard on the stoves seals and secondary jetting,,, in that case I will do as you describe simply to keep it from getting away from itself... there's never a worry about the main flame getting too low, but even this is dependent on stove design and condition...

On my stove when im burning hardwood I like to see about a 2/3 main burn to 1/3 clean up, I get it rip roaring, then shut it completely down and let it stabilize, then add just a smidge of main burner air...
When I do the above it seems the entire stove is in it's happy place and the only time the secondary burn goes out is when the wood is reduced to red hot coals...

[J Tiers]

Yeah, it seems as if you need enough primary air to continue to generate gas to be burned above, or it could go out.. Maybe the stove leaks enough to keep going, the bottom inlet damper does not allow complete closure, or some of the secondary air gets down to the fuel load.

[Sparky_NY]

Yeah, it seems as if you need enough primary air to continue to generate gas to be burned above, or it could go out.. Maybe the stove leaks enough to keep going, the bottom inlet damper does not allow complete closure, or some of the secondary air gets down to the fuel load.

Yes, newer EPA rated stoves have a stop that limits how far you can close the primary air. I will not close completely.