Rocket Stove heater, with heat exchanger and ducting to interrior

I made this rocket stove style heater for my shop. I'm pretty bad at taking progress pictures, so you have to imagine the interior structure. Three lengths of 4-inch square tubing of 1/4" steel. A horizontal draft tube welded to the chimney at 90 degrees using 45 degree miter. A shorter vertical stack for feeding in sticks of wood. The first iteration was based on a design I saw online that had the fuel stack angled at 45 degrees. It was supposed to be more efficient, but the sticks wouldn't fall down the slope as they burned, so it required continuous attention. The vertical fuel stack might be a bit less efficient, but is't still plenty efficient (very little visible smoke at any time after the initial firing) and doesn't require constant attention to keep burning. The old coffee can over the chimney is my temporary solution to keeping rain and critters out when not in use. Likewise the sheet metal cover held in place over the draft tube by a paving stone. The draft tube will get a hinged cover like the fuel stack and the chimney will get a proper chimney cap. I had wanted to use 6-inch tubing, but the local welding shop had this damaged drawbar from an air seeder that he was willing to sell at scrap pricing. As it turns out, it's just fine, although I do have to add fuel more often than if it had been 6-inch.

The mostly-black box is a heat exchanger. It's made out of sheet metal scavenged from old BBQs that were being discarded. Hence all the high temperature (red) caulking to deal with how I had to piece things together. The interior of the heat exchanger has fins attached to the hot parts of the rocket stove and contains about 80 lbs of thermal mass in the form of sand and old bricks. Airflow is currently just the output of my shopvac, but now that I've proved it works, I'll spend the money one an inline duct fan.

The bare stove reached temperatures between 40 degrees and 300 degrees Celsius. The region contained by the heat exchanger reached temperatures of 180 to 300 degrees. After 1 hour of use, the exterior of the heat exchanger varies in temperature between 25 and 120 degrees, depending on whether the sheet metal is shielded by the enclosed thermal mass. I expect the areas shielded by the thermal mass to get much hotter with longer firings.

I hesitate to use the word "design" with respect to this contraption, but my design goals were:

  • Quick heat
  • Continued heating after the fire goes out
  • Enough air flow to distribute heat throughout the shop (16 x 16 x 8 feet)
  • Shop interior heat at least 10 degrees Celsius
  • Enough heat be a season extender. I have a variety of other hobbies, so I don't need the shop to be comfortable year-round.

So how did I do? Well, I think I got the balance between thermal mass and open air about right. I started getting 10 degree heat within about 15 minutes of firing. Within an hour, I was getting 25 degree heat and the shop temperature was about 6 degrees. An hour after the fire went out, I was still getting 10 degree heat and the interior shop temperature had risen to just shy of 10 degrees. After another 2 hours, it was still 8 degrees in the shop, but by then the sun was out and streaming through the windows. Outside temperature was about 0, so 6-8 degrees is actually about right for strictly passive heating.

The real testing will not happen until we actually get some code weather, but I'm quite confident I can get comfortable temperatures at -30C. In fact, I suspect that firing the stove long enough to bring the thermal mass up close to full operating temperature and allowing the shop interior temperature to climb into the 20s might keep the shop above freezing all night. I'm going to go out on a limb and say that I would probably be able to use the shop comfortably year-round if I wanted to.

I'm going to go out on a limb and say that I've met and possibly exceeded my goals.

Total cost so far is about CA$100, but I still have to get that duct fan and maybe some more ducting for better heat distribution. The wood is free, so far, and I expect that to be true for the foreseeable future. There are lots of people taking down trees or doing major trimming every year, so all I have to do is get permission to hit up the piles of wood before they have a bonfire. If that dries up, there are many abandoned farm yards in the area and I know current owners will let me cut what I want.