Rocket Stove Experiments – The importance of the chimney

In the last posting, I introduced you to my experiments with Rocket Stoves.  I started with the most basic, 4 brick rocket stove to introduce the concepts of the air and fuel intake chambers, the combustion chamber, and the chimney.  It is a simple burn process, input, combustion, output.

In this posting, I want to talk about the role of the combustion chamber and the chimney, because that is where all the real work gets done.  The combustion chamber is of course where the burning happens, and the chimney is where the smoke and flames go as the air rushes through the system.

It is important to understand at this point that a rocket stove works as efficiently as it does because it allows enough air to go through the system, and burns hot enough, to allow the fuel and the smoke to get burned in the combustion chamber.  The chimney is an extension of the combustion chamber, and it is essential to the process because it gives the fire enough time to consume the energy released in the smoke by burning it.  This process requires a very hot combustion chamber and chimney to be successful.

4 brick rocket stove with chimney
This is an experiment to see how adding a metal chimney affects the burning of the rocket stove.

This video shows what happens in the combustion chamber of the 4 brick rocket stove when the chimney is removed, when it is added back to the system, and when it is improved.

So from the video you can see that the enhanced chimney significantly increases the air flow and allows the smoke to burn off before exiting the chimney.  These are two crucial factors in creating an efficient rocket stove.

Since having a hot combustion chamber and chimney as extension of the combustion chamber is so important, the next thing to consider is finding a way to insulate around the combustion chamber and chimney so that the heat can remain in the chamber instead of leaching off through the surrounding materials.  I will explore this in later posts.

In the next post, I graduate from the 4 brick rocket stove to using a J shaped feed, combustion, chimney approach that is very common with rocket stoves.  The next posting is the last of the articles on testing, then in the article after that I proceed to an actual prototype.

Learning about the Rocket Stove

Burn chamber view

I have been experimenting with Rocket Stoves in an effort to build a rocket stove BBQ grill.  Well actually, I want to use it for a bunch of different types of outdoor cooking.  This post will be the first in a series of posts that walk you through my experimenting with the rocket stove.

A rocket stove is an efficient cooking stove using small diameter wood fuel which is burned in a simple high-temperature combustion chamber containing a vertical chimney, which ensures almost complete combustion prior to the flames reaching the cooking surface. Wikipedia

My ultimate goal is to build a permanent installation with a rocket stove as the heat source, but with a number of different options for cooking.  These options could include a BBQ grill, indirect heat grill, rotisserie, burner, and maybe even a pizza or bread oven.  To do this, I am going to have to start small and build my personal knowledge.  I have done a lot of research along the way, and looked at a lot of other people’s designs.  If you are interested, the Design Principles at is a good place to begin.   I begin with a 4 brick stove that I will discuss in this article, and end up with a relatively flexible system that allows me to adjust bricks to customize the way I use the heat.  Along the way, I will have a handful of articles where I will share what I have done and learned.  I hope you enjoy.

Four Brick Basic Rocket Stove

Let’s start this out by saying that I did not have a budget for experimenting with rocket stoves.  That doesn’t mean I had an unlimited budget, it means I had no budget.  Most of what I do is done with bricks and cinder blocks (concrete blocks) I had around the house.

The most basic design can be built using 4 concrete blocks.  It is a basic design that requires either an “H” block or, as I did, knock one end off of one of the blocks.

4 brick rocket stove design
This shows the basic 4 brick stove design.

The first brick is simply there as a base.  I placed it so the air chambers went from side to side.  On top of this, I placed another brick that I customized by knocking the end off so that I could create a right angle for the burn chamber.  In front of the first two bricks, another brick is placed standing on end so that the openings go from the front, forming the burn chamber with the section of the second brick with the end knocked off.  The fourth brick is used to create the chimney above the burn chamber.  In the picture I have it overlapping the second and third bricks.  I did this so that it stabilized all the bricks together.

Basic chamber view
The top opening of the brick standing on end creates the opening for the wood and entrance for the burn chamber.

In the image above, you can see the burn chamber.  The  wood is placed through the top hole in the brick placed on end and into the second brick through the space where the end was knocked off that brick.  From there, the heat and smoke go straight up the chimney and out the top of the stove.  In some of the things I have read and seen, they use a shelf that the wood sits on, and allows the air to go underneath the wood into the burn chamber.

Top front view
This image shows the stove with the additional air chamber installed.

In some of the things I have read and seen, they use a shelf that the wood sits on, and allows the air to go underneath the wood into the burn chamber.  This image and the next show this kind of design.  From what I have seen, there is some debate about whether this is the best way to do it.  I haven’t taken a scientific enough approach to this to be able to answer that question.  The important thing is that the air needs to have plenty of room to come into the burn chamber, past the ends of the wood that are burning in the burn chamber, then out through the chimney.

The key is to have enough air going through the burn chamber and chimney to burn very hot, and in the process burn the smoke so that almost no smoke ever comes out of the top of the chimney.  Again, there are a lot of opinions about the best way to do this, but the things that appear to be agreed on are that the burn chamber needs to get hot, and the fire needs to go up into a chimney and burn through the chimney.  In the best designs, most or all of the smoke will be burned, and that is where the efficiency comes from.  Smoke coming out the chimney represents wasted energy since it did not burn.

Burn chamber view
This shows how the shelf allows the air to come in under the wood.

From what I have learned, this L shaped burn chamber with the air coming in under the shelf gives the person running the stove the greatest control of the burn, and subsequently also is the most efficient.  The draw back is that the wood has to be constantly shifted to keep fresh wood in the burn zone.  I think overall, that is one of the greatest issues with the rocket stove.  Not a problem, just something that needs to be considered when you are designing your stove.

Fire coming from chimney
This one shows the fire coming out of the chimney.

One of the things that I found with the four brick stove is that it is hard to get the burn chamber hot enough to get to a point of efficiency.  In the next article, I will start working with the use of a tube inside the chimney to minimize the amount of block being heated up, and hopefully increasing the efficiency of the unit.

So, what I learned here, is that the most basic form of rocket stove will work.  I did not try using it for anything, but this model would be relatively limited, perhaps the best use would be for a burner.  It wouldn’t be big enough for grilling. In a later article I will build a prototype which will be  built around a J shaped intake, combustion chamber, and chimney.  In the next posting, we take a look at the role of the chimney.

Frost advisory

Frost covers on tomato plants

I guess I shouldn’t complain, I mean it is only May 12th after all.  Our last expected frost date is still 3 days away.  But would it have been too much to ask for just one year when we don’t get a late frost.

Anyway, we got the word today that we have a frost advisory tonight.  Temperatures expected to be 35, close enough that I can’t take a chance, so the tomato plants all went under cover tonight.

Frost covers on tomato plants
A frost advisory this late in the season means cover up the plants and protect then in any way you can.

In the bed above I have 26 tomato plants.  They have been in the   ground for several weeks, and they were in the greenhouse many weeks before that.  To lose them now would really be a bummer.  So, I threw empty planters over 9 of them, and covered the other 15 with landscaping cloth.  It isn’t going to take much to keep a light frost at bay.  I hope it is a light frost at worst.

I have 9 more tomato plants in another garden, the one that gave me poison ivy already this season.  So I had to cover those with landscape cloth too.  That is 35 plants tucked in for the night.  Glad I hadn’t put the rest of them or my pepper plants in yet.

The lettuce is getting close to a time when I can start pinching.

Other plants didn’t get covered.  Here there are some lettuce and probably a sunflower seedling or two as well.  There are lots of sunflowers just barely up that may not be able to take the frost, not sure about how hardy they are.

Bean seedlings.
These purple beans are freshly up, they should be alright.

Hopefully this will be the end of it.  Time to get some warm sun on these plants so they can grow up and bear great fruit.