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Outdoor Wood Cookstove: Testing and Tweaks

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Continued from here.

Once the chimney was in place, it was time for a test fire. Since this build was a first for us, we were curious as to how well it would work! Dan started a fire and put a saucepan of water on the stove. I attached our magnetic woodstove thermometer to the chimney, and we watched and waited.

Typically, a woodstove fire will be smokey at first, until it gets going well. Happily, no smoke came back out the firebox door, (backpuffing) which means we had a good draw.

Where we did get some unexpected smoke, was under the stovetop. You can see it if you look closely at the photo above, with a close-up below. 

The fix for this will be to add more rock wool to seal any air gaps that are there, although once the fire got going the smoking stopped.

The other place we got smoke, was when Dan removed the oven door.

This actually wasn’t a surprise because the oven is what is called a “black” oven. In the world of rocket stoves, baking ovens can be either “white,” (completely enclosed so that the heat and smoke pass around them), or “black,” (where heat and smoke pass through them). Once the fire got going well and the stove heated up, the smoking stopped. Since I won’t be using the oven until it’s completely heated anyway, this isn’t a problem.

The other thing we were curious about was the temperature of the chimney. Batch box and rocket stoves are designed to retain heat in the stove for as long as possible. This is done by 1) how the smoke and heat are routed through the stove, and 2) thermal mass (typically brick or cob).

In a conventional wood stove, the flue (exit from firebox to chimney) is located at the top of the stove. So heat and smoke go directly from the firebox to the outside via the chimney. This is not a very efficient design. In the batch box or rocket stove design, heat and smoke follow an indirect path. Heat is retained in the stove, where it is absorbed by the thermal mass. Only the coolest air exits the stove because the flue is located near the bottom of the stove.

In our stove, the flue is built into the bottom of the oven.

Photo from the core construction phase. Heat enters the
oven on the left, and exits to the chimney on the right.

Heat and smoke follow a convoluted path from the firebox through the core and enter the oven by the archway on the left. The heat circulates in the oven and the air sinks as it cools. The coolest air exits through the opening on the right (the flue). The chimney is directly above it.

If our stove was properly constructed, the chimney temperature should be much cooler than a conventional stove. Here’s what our test run showed us. While water was simmering on the stove, the outside of the chimney pipe measured less than 100°F.

Once the fire got going, there was no smoke from the chimney. 

Smoke from a batch box or j-tube rocket stove is very clean, because its design enables wood gases (primarily carbon monoxide, methane, and hydrogen) to be burned rather than emitted through the chimney. (Detailed explanation here.) The result is a hotter fire with less wood, no soot, and clean exhaust.

Using less wood is one of the appeals of this kind of stove, so that’s something else we were curious about. For the fire, Dan used paper, kindling, and half-a-dozen inch to inch-and-a-half sticks. It burned for about an hour, but the stove remained warm for the rest of the day.

Besides the leaky stovetop, two other tweaks were needed. We decided to reset the firebox door so that it is now flush with the stove front. The second was to fill in the air gap under the door. The only opening there should be the one in the secondary air tube.

Unfortunately, I won’t get much use out of it (yet!), now that cold weather is settling in. Instead, I’ll use my wood cookstove in the kitchen because it helps warm up the house. 

Our future plan is to replace our living room woodstove with a batch box masonry design. Hopefully, that will be next summer’s big project.


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