I'm new to all this rocket-stove stuff, but I decided to try my hand at building a heater for my small (1200 sq. ft.) house on the Texas prairie.
Does it work? YES
Will it make my entire house toasty warm? Hmmm . . . .
Does it take the chill off the room temperature and hold things somewhere around 68 degrees F even when it freezing cold outside? YES
It is constructed out of welded 3", 14-gauge red iron tube with a standard "J" tube design. It burns oak wood pellets, which sit in a stainless steel punch-plate basket at the base of the angular feed tube. It burns a 15-ounce can full of pellets in half an hour. The flame front is only 2 1/2" wide, but it burns with no smoke (after start up) and only a minuscule amount of ash, which is easily cleaned up with a tiny "rake" I manufactured out of a piece of sheet metal with an "L" bent at the end.
The 3" feed tube is angled, so I can burn long, thin strips of wood, if I want to keep an eye on it or just play in the fire, like all boys delight in doing. I work part time in a cabinet shop and I scrounge hardwood (oak, hickory, etc.) strips out of the trash and burn them in my heater. Free wood is available all over the place, if you just think about it. Hardwood pellets are easier to set on fire and forget. They self-feed really well and a 40 pound bag is $5. Either way, this thing works like a champ.
The part of the "J" tube, which is subject to the most heat is jacketed so as to draw cool room air up along side each of the three hot sides, which cools the sides as well as pushes heat into the room. The flame front is so small, I am not worried about "corrosion" of the metal due to over-heating it.
The stack is 3" red-iron tube and extends upward, through a metal wall, then terminates 12" atop the roof eve with a chimney cap to prevent wind, debris, snow, hail, rain, small birds, bugs -- you know; all the usual crud that wants to get in a 3" square hole -- from getting inside. The chimney stack is welded to my deck awning framework where it passes through the awning then goes up another 7' to the roof eve, where it terminates.
I'm from California and everybody there is all gaga about a mythical creature, called a Cocopelli. It is something, I think, from Australia. Google it. The reason I mention it is, I am reminded of the Cocopelli when I look at my stove, which has heat fins atop itself much the same as the cocopelli has flutes atop its head.
I constructed a corner shelf above the stove and place a small electric fan behind it pointed up. The fan draws air from the room and when the air hits the shelf, it is forced outward into the room THROUGH the heater fins atop the stove and the fins transfer the heat to the air and that's why I call it my 'redneck' FAU (Forced Air Unit).
The use of metal in the combustion zone aside, a 3" system will have less than 1/4 the heating power of a 6" system, which is the smallest ordinarily used for heating a smallish house in a not-too-severe climate.
So your stove is working fine within the reasonable limits of your size and materials. To keep one room modestly comfortable with a stove that size is the best you could expect.