Thursday, December 30, 2010

Masonry "hat" for an old box stove

My old cast iron box stove heated up my office quickly, but was hard to regulate, dried out the atmosphere, burned a lot of fuel, and cooled off quickly. Now, with a little masonry "hat" to capture the heat that was previously vented out the chimney, I heat up a few hundred pounds of bricks and mud which radiate a gentle heat into my office for up to 12 hours (depending on how much wood I use).
The surface temperatures are much lower than hot iron and (except briefly, at it's hottest), very huggable. In addition, I lined the firebox with brick, which makes the metal surfaces less dangerous, and increases combustion temperatures for a cleaner burn. Once hot (which takes up to about 10 minutes), there is no visible smoke coming out of the chimney. I typically burn it for less than an hour and have heat all day (I live in western Oregon, so it's not that cold). In my under-insulated cabin, the "hat" usually holds heat until the next morning. The stove was an old cast-off that now provides clean heat with minimal smoke. Oh, it also has a little "white oven" that we can bake in (note the wooden oven door on the lower R).
I don't have the technical apparatus to do the testing, but I do have neighbors with "approved" stoves that they load up with damp fuel that they allow to smolder all night long, clogging their chimneys with creosote and putting clouds of grey smoke into the blue sky.
Here's a quick video that gives you an idea of what's inside the heater. Most of the bits are recycled brick and tile, with a few new firebricks for good measure. The base I cast from special hi-temp cement.

Wednesday, December 8, 2010

efficacy of firing doors

Watch (and listen) how the firing door improves combustion. How it works: the chimney determines how much air can get into the fire. Without the firing door, the air all goes in at once, and the fire burns primarily at the edges, where air meets flame. The firing door, however, limits airflow, forcing the fire to suck in a smaller "jet" of air. Because it's smaller, the jet has to move faster in order to get the same amount of air into the chimney. Increased air speed and pressure create more turbulence and better mixing in the fire -- you can hear it quite clearly, even in a low-quality video. Better combustion makes more heat, more heat heats up your oven faster. (Note that the opening is at the bottom of the door. Letting the air in at the top would interfere with the hot gases trying to get out.) So, to optimize combustion in your wood-fired oven:
1. use bone dry fuel (boiling away any water in wood takes over 500 times as much energy as just heating up the wood itself),
2. burn only a small amount of small pieces (more surface area means better mixing of fuel and oxygen, and more complete combustion; there should always be enough space around the wood for combustible gases to expand and burn completely. I would say roughly 1/2 to 2/3 of the volume of the oven chamber should be free of fuel),
3. maximize turbulence (a firing door: the openings should have an area approximately equal to the cross-sectional area of the chimney -- for most home ovens, that will be somewhere in the range of 20-30 square inches -- or about 4x5 to 5x6 inches.