There are different ways of making your own wood burner, but one way is by using two parallel metal tubes that go through a fireproof barrier that holds a chimney flue pipe in place. One tube has holes drilled into it at an angle so that air can flow through it while it's hot and one tube does not have any holes drilled into it so that air cannot flow through it while it's hot. The non-hole side of the tube is then used as a damper that can be adjusted so that more or less air can be let into the firebox when it's hot and air cannot be let into the firebox when it's not hot.