Contemplating making a wood furnace...

Woodcutter_Mo · January 01, 2015, 01:05:25 PM

Hello all, hope everyone had a safe new years 8).

I'm looking into getting a new indoor furnace. In fact like the title says, I wouldn't mind building one myself. This way it would be to my specs and it would just be a good project.

The idea is to base it off the HotBlast type forced air furnaces, but with deminsions to my spec.
If I go through with it, I'll have plenty of questions so thanks in advance for any help.

I'd like the fire box to be:
Height- 28"-30"
Width - 18-20"
Depth- 30-32"+

I would probably use the doors from an old retired Ashley circulator type stove.

First, would 3/16" plate be plenty for the fire box?
And for the flue, would it be best to pipe it directly out or would it be good to make a secondary chamber or some sort?

thecfarm · January 01, 2015, 02:31:33 PM

I sure have no idea. But on my Heatmor the smoke goes up at the front of the firebox into a 4X8 inch chamber that goes the full lenght of the firewood and than out. I also have a water cool feed door. Good luck.

snowstorm · January 01, 2015, 06:28:07 PM

i have built a couple. i used 1/4" plate. first one had a smoke shelf in it. the other i used well casing. ran it almost to the door. and put the air intake in the door worked pretty good. also welded 1/4x2 flat bar on the outside on edge to help it heat and keep the firebox straight

Corley5 · January 01, 2015, 09:07:29 PM

You should check with your insurance company first. A home built furnace no matter how well built may not satisfy them. A UL tag or similar certification may be required.

Woodcutter_Mo · January 01, 2015, 09:50:15 PM

That's a very good point about the insurance companies. I dont think the companies around here even like indoor wood stoves period. I'm not too worried about that part though. If something were to come up I could always convert it to an outdoor unit or something.

Woodcutter_Mo · January 02, 2015, 12:32:55 PM

Quote from: snowstorm on January 01, 2015, 06:28:07 PM
i have built a couple. i used 1/4" plate. first one had a smoke shelf in it. the other i used well casing. ran it almost to the door. and put the air intake in the door worked pretty good. also welded 1/4x2 flat bar on the outside on edge to help it heat and keep the firebox straight

Since 3/16" plate would be a little cheaper and lighter, would it do the trick for the box?

Also I was thinking about welding some steel angle along the box to work as "fins" and to strengthen the box.

hedgerow · January 02, 2015, 07:24:50 PM

The insurance company's are the same around here. They won't insure houses or even out building with home made wood burning devices in them any more. They don't even like legal wood burning devices in houses and charge extra. When we were looking at adding a indoor wood furnance in our house 10 years ago the insurance company said the same as no way. so we did a Garn instead out in a out building and I heat my shop and house with it.

bandmiller2 · January 02, 2015, 08:35:29 PM

Woodcutter, the fire box should be fire bricked at least to over where the fire is, the reflected heat will help getting less smoke and a more efficient burn. Frank C.

Woodcutter_Mo · January 03, 2015, 05:55:21 AM

OK, I was wondering if fire brick would be a good idea. Thanks for clearing that up.

Woodcutter_Mo · January 03, 2015, 06:53:51 AM

Looks like 9x4.5x1.25" fire brick can be bought for under $3 each so lookin at $50+/- just for that if I do a 9" high row around the inside of the box.

I still have to price some steel plate for the box, some heavy 6" steel pipe and some other misc.

dave_dj1 · January 03, 2015, 08:45:47 PM

If I was going to build my own hot air wood burner I surely wouldn't model after a Hotblast. I would use a round fire box, eliminating the need for external re enforcement's. I would also use a round smoke chamber above the firebox to take advantage of all the heat possible. I would also give it a secondary air intake to burn of some of the smoke.
How big of an area are you wanting to heat? My firebox is 38.5" deep and about 18" diameter and the smoke chamber is a little smaller. The jacket is lined with fiberglass insulation and a baffle so the air blows from under up and around but can't get back to the starting point. I use a regular furnace fan and have 10" in and out ducts. My burner is outside my house, love it that way!
dave

Woodcutter_Mo · January 04, 2015, 01:53:00 AM

The house is around 1100sqft I believe. I don't really mean it will be just like the hotblast, just using the basic concept with improvements. ;D

For the smoke chamber, I was thinking of using some 6" heavy steel pipe the length of the fire box with 2 inlets. Sound like that would work?

Woodcutter_Mo · January 04, 2015, 02:02:51 AM

Oh, and for the draft I plan to use 1 or 2 thermo adjustable flaps. I want it to burn clean but I'm going to make it to where I can slow it down to control an over fireing condition if it happens.

I might draw up a picture of what I had in mind.

Ljohnsaw · January 04, 2015, 02:36:58 AM

I'd be interested in seeing what you plan to build.

Woodcutter_Mo · January 04, 2015, 04:34:10 PM

OK I'll try to get a pic drawn up on what I'd like to make.

bandmiller2 · January 05, 2015, 08:41:37 PM

Woodcutter, insted of expensive firebrick I just used cement brick from Home Depot not mortered but just laid in. They will last a long time and its no trick to replace if one does crack. Frank C.

Southside · January 05, 2015, 08:45:48 PM

Not familiar with the design you are talking about, is that a gassification type of furnace? I have been doing some research into building a gassifier boiler that I plan to set outside. I loved the one we had at our last place but don't want to sink that kind of $$ into another one.

Woodcutter_Mo · January 05, 2015, 09:31:19 PM

Southside logger, I'm not sure what the furnace I want to build is called, but its not a boiler type. The hot air from the furnace is pushed through the ducts.

bandmiller2, thanks for the tip. I'll give that some consideration 8)

Ljohnsaw · January 05, 2015, 10:29:15 PM

Quote from: bandmiller2 on January 05, 2015, 08:41:37 PM
Woodcutter, insted of expensive firebrick I just used cement brick from Home Depot not mortered but just laid in. They will last a long time and its no trick to replace if one does crack. Frank C.

The difference between regular cement brick and fire brick is that fire brick insulates. It will not transmit heat through to the other side nearly as much as cement. Also, as stated, it cement will crack, sometimes with energy! I found fire brick for about 2 bucks each at a rock and stone store. I used them to make a (gas) welding table. They are fairly heavy, probably 2/3 the weight of clay brick. There are also some fire brick that is extremely light weight, but very brittle, like Styrofoam. The prices varied GREATLY, so shop around.

Woodcutter_Mo · January 05, 2015, 10:38:17 PM

Come to think of it I have seen concrete block explode, moisture or air pockets inside I guess

bandmiller2 · January 06, 2015, 07:52:47 AM

John, you speakth the truth but I have used boath fire brick and cement brick the heat transfer is not a bad thing. If the furnace is heated up slowly theirs no problem with the brick as any moisture within is driven off slowly. If you can find cheap or used fire brick use them they are the premium product. Just saying cement brick can be used. Frank C.

DeepWoods · January 06, 2015, 07:49:03 PM

This may be a little off topic, but you mentioned the possibility of having an outdoor wood furnace. I went round and round with my insurance company when I wanted to install a wood furnace in my shop. Not at all the same for an indoor wood furnace for in a house as I found out. The insurance company was concerned with any heating appliance with an open flame that was closer than three feet off the floor. Flamable vapors from gasoline, paint thinners, etc. The only way they would allow an indoor wood furnace, was if I built a separate room to enclose the furnace. This was not feasible in my situation. They would not even consider any type of heater that did not have an appropriate testing agency stamp on it. After making sure it was acceptable to my insurance company, I installed a propane ceiling hung heater. I hated it. Last year we had one of the coldest winters I've seen, and the cost of propane went thru the roof. I used it very little because of the cost to run it. Moving ahead to this past October. I found an outdoor wood furnace that looked like it would be what I really wanted. After another talk with my insurance company, and sending them all the info they required, they approved the installation. I have been warm and toasty every day I have worked in the shop since. I put a load in a couple times a day, and now let the propane heater keep my shop at 45 degrees overnight.

I guess the reason for this post is to make sure you check with your insurance company. Every situation is different, and needs to be approved by them for you to have coverage. At least that is what I found.

Here is a picture of what I installed.

jaygtree · January 07, 2015, 01:26:31 AM

the fire brick on the sides of my firebox are on a slight angle so they do not have a tendency to fall over. the firebrick on the front and back are straight up and down. no problem with the ones in back moving but the ones in front move some if they get hit while loading wood. the piece (angle iron) that held them uprite broke off a few years ago. i'm going to fix that soon. my furnace firebox is made of 1/8 inch. that's what i had at the time. that was 25 years ago. i would go 3/16 or 1/4 if thats what i had. 3/16 if i had to buy the steel. a friend had one built of 1/2 inch and it was a failure according to him. most of the heat went up the chimney instead of through the heavy walls. my furnace is a downdraft design like the old tempwoods.

bandmiller2 · January 07, 2015, 07:31:20 AM

For a furnace to be low smoke and efficient you must have a hot concentrated fire, that cannot be maintained with fire agenst a steel jacket with water on the other side. Masonry to maintain and reflect heat back into the fire then above that your steel (or cast iron) water jacket. Years ago when steam powered our country dutch ovens were used especially with damp low grade fuel. A dutch oven is simply a masonry furnace with the exiting hot gasses passing through a boiler. Frank C.

Woodcutter_Mo · January 07, 2015, 09:36:13 AM

OK so sounds like 3/16" plate steel would work fine then 8)

I planned for the fire bricks to stand pretty much straight up and down. I the plan is to make an angle iron retainer that I can just slide the bricks in from one end.

Going to call and get a ballpark price on 3/16 plate today.

Next thing is I need to figure out what to use for grates on the bottom. I want it to have grates and a decent size ashpan.