Making an outdoor wood furnace can be as complex as you let it be, there are tons of different ways one can build a outdoor wood furnace. This article will cover a simple way to build a DIY outdoor wood furnace.
A homemade outdoor wood furnace can be made very simply and run off a small duck fan that is inside the ducking. It is really just a steel box inside of another steel box. To start there needs to be 3 pieces of 1/2" steel laid out for the bottom of the furnace. The measurement of the bottom needs to be 1-1/2" bigger on the sides and for the back side. Work with a welder to to handle the seams.
Next, you will want to build the side walls. The height of the walls on the inside where the wood will be burning needs to be 8" to 12" lower then the outside walls. The air inside this space around the inside box is what gets heated and pushed/pulled into the home or structure being heated.
When you cut the sheet of steel for the front of the furnace you need to cut out a door hole, cut it as careful as you can so that it can be used for the inside of the door. To build the door take the cut out for the door hole and add another sheet over it that is 1" bigger all the way around. You will also need to cut two 3"x 7", 1/2" steel blocks to put next to the door on the front furnace wall to fur out the wall to be flush to the door where the hinges go.
Finish boxing the furnace in by adding the tops on, the first one on the inside box needs to have one 8" hole and the second one on the top of the outside of the furnace needs to have two 8" holes. Add on the 4 legs to the bottom and get all the seams welded up.
Last, add a 2 1/4" holes on each side wall at the lowest point possible. Run some 8" ducking from the inside box out through and out the top for venting all the smoke, then run a 8" piece of ducking out of the second hole only into the structure. To help move the hot air add an in-line 8" duck fan to the ducking running into the house to pull heat in when needed. Finish by building a rack 2" tall by a few inches smaller then the width of the inside box for the wood to be placed on top of while it is burning.