November 9, 2021

Outdoor Cookstove Chimney & Secondary Air

Continued from here.

Almost done! For the cookstove chimney, Dan used the old ductwork we saved when our HVAC unit was installed. Why didn't he use double-wall stovepipe? Primarily, because the stove is in an open outdoor space; the chimney doesn't pass through walls, ceiling, or roof. In a house, building codes would require it to keep the house from burning down. Outdoors, we just want the smoke vented away from us as we're cooking. Secondarily, because the design of a masonry stove is such that 90% of the heat is retained in the stove rather than going up and out the chimney. That means that the stovepipe doesn't reach the high temperatures of a conventional woodburning stove.

Part of what helps the efficiency of our design, is the secondary air intake.

Secondary air tube

It fits into a channel built into the bottom of firebox (core). Click here for core construction photos here.

Dan welded an L-bracket to the tube to
keep firewood from hitting the air pipe.

When the door is closed, air is drawn through the tube under the door to the back of the firebox, where smoke and heat exit through a narrow side opening called the "port." This narrow opening slows the exit of the smoke, and when combined with heat from the fire and the extra oxygen from the secondary air, allows wood gases to be burned. The result is increased burn efficiency. (The concept is better explained at this website.)

Next, the chimney.

The insulation is non-combustible rock (aka mineral) wool.

How well did it work?!?!?!?!? More on that here.


Ed said...

Can't wait to read the next post on this.

Hill Top Post said...

I am sure it does work, but look forward to reading more.

Leigh said...

Ed, coming soon!

Mary, if it does work well, we'll be planning to make a masonry stove for the house!

wyomingheart said...

That is information that I hadn’t considered. It makes sense that the stove is using the heat energy, not the chimney. I honestly hadn’t thought of that before. Great post, Leigh, and can’t wait for your next post on it! Thanks!

Leigh said...

Wyomingheart, honestly? I never thought about any of this stuff either until I started reading about rocket stoves on I admit it's taken a lot of reading and some processing time to start understanding the concept. Watching this stove be built helped me wrap my head around how it works. Blogging about it makes me clarify my understanding by trying to explain it clearly to you all!

on Friday, I'll publish the post about our test fire, with a little more about how the stove works.

Nancy In Boise said...

Well that looks so cool!

Renee Nefe said...

So what is/was cooked first? I know, spoiler alert. ;)

Leigh said...

Thanks, Nancy!

Renee, nothing actually cooked yet. Dan wants to finish the top first and with the weather turning cooler, I'm stitching to the kitchen cookstove. Learning how to cook on it will have to wait until next summer! said...

You 2 are amazing! That project is a huge one and congrats to you both for sticking with it considering the confusing directions. What a wonderful outside kitchen and attractive too!

daisy g said...

What a great family project!

M.K. said...

THAT is fancy shmancy!!! My husband once built a pizza oven, but it was nothing to this. Amazing!

Leigh said...

Sam, I don't know about amazing, but it's always fun when something works out!

Daisy, I agree!

M.K., pretty neat, huh? Originally, we talked about a pizza oven, but this one will hold my pizza stone so another oven is off the project list. :)