Efficient Wood Stove/Furnace Design Discussion

[chicksdigwagons]

I live in the upper reaches of Michigan and I'm looking for ways to save a few bucks on heating this winter and winters coming. Lets put aside any legal precautions, local building codes or insurance coverage issues. What I'm interested in is the design and construction of an efficient EPA-certified-style non-catalytic Wood stove/furnace.

Has anybody looked into this? I'm somewhat interested in the design constraints for a secondary combustion chamber, heat-output and how to size accordingly, and any other tricks, design theories, etc to increase the efficiency.

I'm not looking for a 55gal barrel or water heater converted, but rather lets start from a clean slate, and go through the motions to design a good compromise of ease of build, cost of materials, efficiency of burn and even consider asthetic properties.

[zapster]

how much space do you want to heat with this ?

...zap!

[chicksdigwagons]

I'm looking for supplementary heat for approximately 1700 sq/ft IIRC. I currently run a 55,000 BTU NG Forced Air Furnace, possibly just used for backup? I guess in theory I'd like it to be big enough to do basically all the heating, granted the efficiency is enough to where I'm not loading wood all the live long day, and I can get enough who-house airflow to be comfortable. At the moment, the forced air furnace has virtually no ductwork, relying on convection and luck to heat the outer reaches of the house. This is something else I'd like to possibly address with a wood fired stove/furnace of some sort.

[zapster]

Originally Posted by chicksdigwagons

I'm looking for supplementary heat for approximately 1700 sq/ft IIRC. I currently run a 55,000 BTU NG Forced Air Furnace, possibly just used for backup? I guess in theory I'd like it to be big enough to do basically all the heating, granted the efficiency is enough to where I'm not loading wood all the live long day, and I can get enough who-house airflow to be comfortable. At the moment, the forced air furnace has virtually no ductwork, relying on convection and luck to heat the outer reaches of the house. This is something else I'd like to possibly address with a wood fired stove/furnace of some sort.

well to tell you the truth...

look in a northern equipmeng catalog...
nice cast iron stoves (where i got mine)
for a real good deal...

$$$$ wise? unless you get your materials for nothing

its a no brainer...

...zap!

[chicksdigwagons]

No offense zap, but this is exactly the type of response I was trying to avoid. Lets say, money was not an issue (but it always is), I had a hearty supply of material for free or cheap, and I was stubborn.

I'm honestly mostly interested in the design theories behind the stove more than the welding involved which isn't a problem. Perhaps this is more of an off-topic subject?

[zapster]

no i think this topic belongs right here..
sorry about my reply but thats just what i would do..

no offense taken

i once made a wood stove out of 2 rear wheel "wells" from a oil delivery truck..

it was huge..heated the whole house for the owners...

...zap!

[chicksdigwagons]

Your opinion has been dually noted

I've been drawing up some basic sketches of what I think would be in the ballpark for the right size for what I'm looking for. I think for total stove materials I can get out under $250 including steel, firebrick, etc. To buy a comparable sized, featured stove at Northern Tool would put me in the $500 range, not including shipping. Haven't looked locally but it might be worth a shot.

I'm still hung up on the concepts of baffling, creating the secondary combustion chamber, and most of all introducing combustion air. I realize this isn't exactly rocket science, but the engineering behind how to best build it is sobering.

I'm also in the early conception stages of an automatic combusion air control based on a thermocouple reading of combustion temp and some other variables.

[Wannabe]

I found this while perusing old threads on AWS:

http://www.motherearthnews.com/top_a..._35__Or_Less__


HTH,

Mike

[chicksdigwagons]

Thanks, I've come accross that several times over the years. And if you notice, I specifically said I didn't want anything involving 55gal barrels or old water heaters But that article was the original inspiration. If I was confident I could find water heaters with steel tanks in good condition I'd at least recycle some of the material in my project.

But to just build it like that... That'd just be too easy.

[TubularFab]

20 years ago when I was in gradeschool we lived in NW PA - right in the snowbelt the perfect distance from Lake Erie in Warren. We lived on top of a hill about 1200 feet higher than the town and heating was definitely an issue. The house was built with a water base board heater that had a boiler fired by natural gas. My dad installed a wood fired boiler that was very homebuilt looking - the whole housing was simply fabricated out of 1/4" steel sheet. It stood about 5' tall, 2.5' wide, and 3' deep, and the heat exchanger was a unit that slid into the top from the back. As I recall it looked fabricated out of steel plate with a bunch of tubes going straight up through it for the smoke / heat to rise up through. It had a thermostatically controlled damper on it, and was completely self operating short of stoking once in the morning and once at night each day. It kept the ~1500 sqft house around 80 all winter. He cut the firewood 32" long, and split it fairly thick to about 8 - 10" thick so they would burn longer. Seems like it was a fairly efficient system - specially with the thermostatic damper control. And this was early 80's technology... Not sure it would meet today's epa standards, but I do know it is STILL in use today. Ok, maybe not TODAY, but when winter comes it'll be burning.

BTW - the only problems it ever had were occasional water leaks in the heat exchanger. He pulled it out the first time - lotsa fun - but after that had a friend stick weld it in place. I can still picture that guy with his head arms, and torso squeezed inside the firebox! No thanks!!!

[MicroZone]

Interesting topic, I'd definately like to find an alternative source than spending $2000 this winter on Kerosene for my hydronic base-board heat.

[MicroZone]

Originally Posted by Wannabe

I found this while perusing old threads on AWS:

http://www.motherearthnews.com/top_a..._35__Or_Less__
Mike

Just read that article, I'm wondering if anyone has sent away for their $6 plans. They say they work wonderful and the mother-earth folk have the plans.

[MicroZone]

Hmm, some interesting input from a wood burning source:

Read the last FAQ -

http://www.woodheat.org/contact/faq.htm

[chicksdigwagons]

Originally Posted by MicroZone

Just read that article, I'm wondering if anyone has sent away for their $6 plans. They say they work wonderful and the mother-earth folk have the plans.

I've seen the plans. Its a very basic design. I'm sure it does work sufficiently well. The hard part is most modern water heaters aren't just plain thick steel tanks any longer. I recall many of have glass or thermoplastic inner tanks or glass lined steel, etc.

Originally Posted by MicroZone

Hmm, some interesting input from a wood burning source:

Read the last FAQ -

http://www.woodheat.org/contact/faq.htm

I came accross that and several other advisories against designing/building your own. I personally have a feeling a lot of the Pro-Wood websites have been subsidized by woodstove manufactures which of course don't want you to build your own. While I don't mean to oversimplify the design parameters that are put into a modern woodstove, I really don't think its nearly as complicated as they would like you to beileve.

I liken it to building speaker enclosures. Anybody can build a box and screw a speaker in, but it takes some math and some design to build an ideal and proper enclosure that compliments the speaker. Its not rocket science, just takes knowing the math.

I've been working through some basic conceptual designs on paper, I'll draw them up on the computer and post them. I've also found some rather interesting articles online that I'll compile and post also when I have a moment.

[MicroZone]

That would be great, thanks for sharing.

[littlefuzz]

I guess according to that article, the heater I built is no good. It heats the shop up, why is that no good. I thought it was a good design I came up with. It has a box built onto the back and heat ducts built into the wood chamber and vents out the front. It has a squirrel cage blower on the back to push the hot air through. I'll try to post some pics later.

[awright]

A book that you might find very interesting, but may have difficulty finding, is

"Proceedings
Residential Wood & Coal Combustion
Specialty Conference"

Edited by:
East Central Section
Air pollution Control Association
March, 1982

It is a 300 page collection of papers by scientists involved in optimizing wood and coal burning stoves for maximum efficiency and minimum emission of pollution. Lots of fairly technical stuff, but also discussion of the very beneficial effect of refractory linings of the combustion chamber (firebrick), secondary combustion air, draft control, etc.

I tried several book search engines and came up with nothing. Perhaps the Air Pollution Control Association can help. Since this was the 1982 conference proceedings, I imagine there are proceedings from other years' conventions, also.

I picked my copy up at a used book store for $11, just in case I needed it someday.

awright

[RonL]

I think a triple pass design like modern oil burners might be worth considering. Also, using outside air for combustion would add to the efficiency. There is a point of diminishing returns. If you extract too much heat from the process the stove will not draft properly. Insulating the chimney flue might help here, however, you once again reach a point of diminishing returns. Adding combustion air to the upper chambers would help draft and would assist in burning the gasses.

RonL

[chicksdigwagons]

awright:
Thanks tremendously, you've pointed me in the right direction with your information. With the right wording I've stumbled upon a number of articles that are technical in nature and exactly the type of things I need to study.

RonL:
You bring up a very important point that I've been struggling with for the past several days now. Finding the balance between extracting as much heat from the combustion process as possible and still maintaining proper draft.

It seems secondary combustion is a good step in the right direction, as you're basically burning particles/gasses that would otherwise go up and out the flue. Problem is, in order to start and sustain secondary combustion you need a relatively high temperature, to the tune of 12-1300 degrees from what I've been reading. This requires a hot, fast fire and a good thick heat storing stove. Also, as you mentioned, you need to supply it with a secondary source of combustion air as by the time the flue gasses reach the secondary combustion area the O2 is all used up. This is easy to do in theory, but a major consideration is adding too much air or too cool of air.

I'm not entirely sure what you mean by triple pass design, though the design I sketched up has three levels and might be similar to what the oil burners use.

Pardon the elementary Microsoft Paint drawing, but this is the general concept for starters:


I'm also getting started on high temp ceramic coatings like they use in automotive exhausts. I think they could be effectively applied to this application as well. With the combination of firebrick, strategic use of ceramic insulating coatings, multiple outer walls and baffles I think I could still cheaply build an efficient inexpensive woodburner.

[chicksdigwagons]

Originally Posted by littlefuzz

I guess according to that article, the heater I built is no good. It heats the shop up, why is that no good. I thought it was a good design I came up with. It has a box built onto the back and heat ducts built into the wood chamber and vents out the front. It has a squirrel cage blower on the back to push the hot air through. I'll try to post some pics later.

My sentiments exactly. As far as I'm concerned, if it puts out heat, has a good draft and doesn't pour smoke into the building, its an effective design. If I was trying to heat a workshop or cabin I'd definately not put as much thought into this project as I am. And if I was to believe everything I read on the internet I'd have quit long ago.

Fact is, I'm probably thinking too much here but for the sake of discussion I'm curious about how it all pulls together. Plus, I'm looking into the real world feasability of migrating entirely to a wood heated three bedroom two story home. While I think I will always continue to maintain my natural gas furnace for backup if nothing else, the closer I can become to complete self-dependancy the better.

Not to sound like a weird world apocolypse theorist, but I think things are going to get worse before they get better. If it wasn't for my welding, I'd consider pulling migrating off the grid in the next 2-5years. Solar, wind, geothermal, biomass waste fuel, the technology is out there.

[MicroZone]

That is essentially how my smaller wood stove looks inside. I would like to move up to a little larger unit. The smaller unit heats the house nicely but I'd like to have a larger one to fit bigger pieces of wood.

[chicksdigwagons]

Originally Posted by MicroZone

That is essentially how my smaller wood stove looks inside. I would like to move up to a little larger unit. The smaller unit heats the house nicely but I'd like to have a larger one to fit bigger pieces of wood.

Have you ever monitored the smoke coming out of your flue? Pretty clean burning? If you had any time, would you be willing to measure some of the dimensions? Like, main firebox size, and the baffles and if there are any angles, etc? I think with a little reverse engineering I might be able to get a good handle on proportions.

[RonL]

The design you show is basically a triple pass. The first baffle would be open in the rear of the firebox. The second would be open in the front. Gasses would travel to the rear of the firebox, to the front of the second area, and to the rear and exit the flue pipe. ( Triple pass.) I'm thinking that maybe you could have a small diameter pipe coming up from the bottom rear of the firebox. This would add air to the gasses right at the point were they would be at there hottest. The gasses would then make the second and third passes. The air injection would have to be sized so as not to take away too much from the main draft. If there is too much unburned elements in the exhaust gas and it is too cool you get condensation of the flue gas and creosote buildup. That's the idea of a catalyst. It burns the flue gasses.

[hdwood]

I helped a couple of fellas at work with the plans for a wood-fired boiler. Massive boiled. Here is the web site http://www.deb-design.com/prod01.htm

[billie_]

here is my first one