Convection Wood Heat

[RogerAS]

Hey All,

Last fall I built an outside wood fired furnace. It wasn't all that tough to do. I already had the steel firebox, a 2' cube made of 3/8" plate steel with 2" pipe about 6" long for legs. I poured a 4' 6" square slab of concrete on which it sits. The initial flue pipe is thick wall 6" steel almost 2' long that runs straight out of the firebox through the blocks. An elbow and 10 feet of stack exhaust the smoke, controlled somewhat by a baffle damper. The firebox also has an air feed damper which consists of a series of 1.5" holes and a slide baffle. I used the narrow concrete blocks to build an enclosure around the firebox. On one side I made the wall one row of blocks taller (8") to provide for rain runoff. That side is about 5' tall. I then filled the ends with broken bits of blocks to fill in the slope to the other side. I made the top out of sheet steel with two layers. Between these layers is fiberglass insulation, and is about 4" thick. This top is secured with althread set with mortar and nuts so it can be removed. Our bathroom floor is 1 foot lower than the main house and the cool air feed leaves the house there. The hot air outlet from the furnace is set on one upper corner and lifts about 5 feet above the bathroom level entering a side wall on our main house. The pipe for both hot and cold are 6" diameter. The hot air pipe passes through the wall via a double wall stainless flue pipe designed for much hotter service. The furnace is about 1.5 feet lower than the bathroom.

Since we live in the south and hardwood forest surrounds us this fuel is readily available, if one can run a chainsaw that is. I try to pick sick or dying trees to harvest. Hickory burns way too hot, post oak is my favorite of the many species of oak we have. Above all others white ash is my favorite. It burns forever and leaves very little ash. I burned about 2.5 chords of wood last year, but we had a relatively mild winter. I would expect that to double in a bad winter.

Last winter the air exchange pipes were not insulated, but they are now. The concrete blocks are just empty, but I've been thinking of filling them with granular insulation. I'm just not too sure I want to expose that stuff to the heat, is it safe? Is it safe? (Think Dustin Hoffman and Marathon Man).

We previously heated with propane, which was far too expensive. Our house has a somewhat ample insulation factor. The walls are 2X6 having R19 fiberglass between the studs, with 1" R5 foam under 7/16" blandex sheathing which gives us an R24 not counting the sheetrock or sheathing. We are saving up for and trying to figure out our siding choice. We just have the Tyvek covering the balndex now. The same insulation is in the floor. Awaiting the blowing machine to pile the attic full we only have the R19 there. That's the weak spot in this setup, but we're saving up our coins to correct that before next summer.

Before this system was working our bathroom was always much cooler than the rest of the house. The cold air settled there. Now it stays the same throughout the house. On the very coldest days we never really have to provide any type of powered ciculation. I do have a small fan from a microwave on the cool air intake. When we've been gone all day and the fire burns low it does speed up the time to get the house warmed up again. I've been thinkin of a thermostat for this function, but it's rarely needed.

I've also considered adding a water preheat system, or even converting the whole system to water transfer, but it works so well now I hate to mess with it much. The air exchange pipes are pretty much on the "off" side of the house and are not too obtrusive.

Anyway that's our heating system and if anyone has questions I'll try to answer them as best I can. I'm very pleased and sortta surprised at how well this works. No mess in the house. No smell of smoke. I figure about 2 gallons of chainsaw gas, one gallon of bar oil, a new chain and 4 weekend days getting the wood gathered and stacked. So for around $50, not counting my time, I keep my house warm and cozy all winter. Winter before last we spent over $400 for less comfort. The materials for the furnace were under $200, but remember I already had the firebox. I'd say one could duplicate my system for around $400.

[stephent]

Well, Roger, like they say--heating with wood heats 3 times--when you cut it--when you split it (lots there) --and when you burn it.

I'm lookin around now for some steel plate to do somewhat the same here on the west side of my house.

Some day I'm going to come visit y'all up there in the hillz--it ain't so far away.

[nothing to lose]

Good way to go.

Maybe you could fill the concrete blocks with expanding foam if they don't get too hot as you have it built. About $54 and shipping, 8 cubic foot, 2lb density.

http://www.shopmaninc.com/foam.html

Just filling voids in hollow blocks you may not need alot. I have thought alot about using this stuff for various items. Just a thought, I expect it would work as well as the foam insulation sheets or that $8 can of expanding foam insulation sold in stores.

 May be good for hard to do areas, mix and pour. Check it out for yourself if you might use it. I think it's fire resistant once cured, non-toxic etc.., but do your own checking to be sure.

I still use my wood burner in the house, planned to build an outdoor unit but haven't yet. Went traveling instead. I geuss I need to get busy here! No fire wood stocked in yet either.

I did cut off some large tree limbs yesterday, good fire wood and they were in the way of the Efka genny, it was not getting any wind! Kid shoulda recorded me climbing the tree 25' cutting limbs as I went, would have been funny to watch.

[Chagrin]

I don't think the foam or any other insulation type would have a whole lot of value (assuming it could withstand the heat). The holes in the blocks should be sealed to make a sort of air chamber that the air cannot easily convect out of though. After that, all you'd be worried about is the convection currents inside the air chamber but that's really quite minor.

[electrondady1]

i would be interested in how you load the wood in.

is the door outside the concrete block?

[powerbuoy]

Roger:

Are there any plans or rough drawings available? If not ... any suggestion on where to obtain some?

Powerbuoy

 

[RogerAS]

ED,

The feed door is exposed, a weak point to be sure. I've thought about adding a second door, but I haven't come up with a good plan for that yet. I was thinking of one of those crawl space doors, steel, with some insulation on the inside of that. The trouble is my air intake is on the same plane as the feed door. I suppose I could make a pass through for that. Still thinking, and man does it hurt! :-)

[RogerAS]

PB,

I am going to make a set of drawings and will post them soon. It's so simple it shouldn't take long. I'll try to get a set of plans posted before the weekend is over.

[RogerAS]

Stephent,

Yep your right on about the work involved! I try to only cut wood in a small diameter and avoid the splitting if possible. Red oak is the only exception as it spits real nice.

Come on up, we'd be proud to have ya!

[RogerAS]

Chagrin,

The blocks are motared and the top row of holes are filled with broken bits of blocks and mortar. That's where I placed the top anchoring allthread. I think I'll just live with it the way it is. The convection between the blocks and the firebox, the heated air, doesn't seem to mind that the blocks are hollow. Besides I'd have to rip out the allthread and try to realign the holes after insulation was added. That might be a pain!

[stephent]

ok--just remembered the name of the "stuff" used in concrete blocks for insulation--

Zone-a-lite  or zone-o-lite

zoneolight?

Somnething similar...most larger hardware stores (or building construction suppliers) carry it.

Looks like vermiculite to me.

Comes in big bags and you can pour it into the bolcks...fireproof, etc.

It's small stuff and runs like water out of a 1/4 in hole (I know as I've drilled into the stuff inside blocks for decades and felt like the little boy with his finger in the dike sometimes).

It's also very lightweight and the dust ain't something I like to breathe much, it makes me sneeze and stuffs me up.

It's not very high priced and works well for insulating concrete blocks as well as fireproofing them (as if a concrete block is gonna burn). But it's better for this usage then much of anything else I can think of at an affordable price.

Actually another way of doing the insulating would be insulating the outside of the blocks and using them for heating "mass" storage--F26 adhesive and some blue styrofoam or somewhat better for fire rating would be polyisouranate (sp?) foam panels.

[powerbuoy]

Thanks a lot, I am still living in the propane heat world ... bad bad ...

Powerbuoy

[jmk]

 If you have a lot of trees in your area try calling the tree trimmers. Where I live they cut, deliver, and some will stack it for you for free. It saves them gas money when cutting in your area. The best time to call them is in the summer. When it gets cold the competition may take you out of the game. Plus they cut a lot of green wood and it will give you time to dry it.

[dbcollen]

What you want to fill the cavities with is vermiculite. It is mica that has been "popped" like popcorn, in an autoclave. it is completely fireproof to 2000 deg F, and doesn't outgas. Available at most larger nurseries as "Horticultural Vermiculite" in several sizes. Here in Nor Cal it costs $18\ 4cu ft.

[electrondady1]

i  need to put some thing together this year . ive got a lot of firewood and no fire !

using an air exchange sounds easyer than  a liquid, as long as the danger of contamination is eliminated.

maybe both could be incorporated eventualy

it sounds as though you fire box is fairly conventional

yes please show us some drawing or plans

i'm wondering what sort of clearance there is beweenn the stove and the  concrete

[pepa]

hi Rodger, very nice work, pepa.

[richhagen]

Isn't that the stuff sold as "Pearlite" for potting and other soil mixtures.  If it is you can get it at most large home improvement and most gardening type stores.  If not, pearlite woudl probably also work well for insulating and fire proofing them.  It is a common ingredient in home made refractories.  Rich

[RogerAS]

I feel the thick steel plate was welded together by a pro. It looks like the firebox is milled from a block, and is almost as heavy! It took a tractor (New Holland TN75) to lower in onto the slab. I was given the firebox a couple years ago or more and it was unused. I hired a guy that's a pro come and replace the flue pipe with one I drove to Heber Springs (27 miles one way) to find. It's a 7/16" thick 6" ID about 34" long. Like I said we (I) ground those welds down and (he) made another pass with a wider "weave" bead. I took a wire wheel and roughed that up and then (I) added a layer of furnace cement by Rutland® on the joint. So from the back of the 2' deep firebox this thick pipe runs through the concrete blocks. It extends well away from the blocks before transitioning into the stack via and elbow. I have a 10' steel pole I have anchord to the stack. There's a in-pipe plate/baffle mannual damper about 4.5" off the ground. There's a weather cap on top of the stack.  Yes I broke several blocks wrong or in bits making the arc segments for the flue exit and air exchange holes but so far so good. I have wet the exterior down and applied a skim coat of very thin mortar. It really looks odd, like a hillbilly attempt at stucco. At the exposed-firebox-to-masonry-joints around the face I left a thin gap, around 1/4", which is filled with ample application of furnace cement. It may not be kosher but it doesn't seem to mind. I let everything cure at least 3 or 4 days before applying any heat. I feel that letting the mortar, concrete and furnace cement cure out real slow makes it a bunch harder. If it hadn't rained on it a little I would have wet the whole thing down. I'm still thinking of hanging 1/4" hardware cloth over the entire exterior and giving it a for real redneck stucco coating. All I know is the thing works. I still have the ability to heat water in the exchange box and use that somehow, I just wanna make sure that's done as safe as possible. As far a smells, like most heating equipment, mine did give off that musty metallic smell for a little while on our first fireup of the season. We anticipated this and made only a small fire and had the windows and doors open. I've refired gas units that stunk on a whole nother level than this little beast.

I've gotta get a few pictures of my experimental life support system so the rest of you can have a good laugh! :-)

Take care,

[RogerAS]

JMK,

I live on 60 acres, and am surrounded by thousands more, of prime hardwood forest. The tree trimmers here grind it all up and haul it away, AFTER the ice storm. The fuel isn't a problem. I actually like gathering wood, for a few hours.