Design Principles for Wood

Burning Cook Stoves

Aprovecho Research Center

Shell Foundation

Partnership for Clean Indoor Air
Illustrations: Mike Van, Jayme Vineyard, and Ethan Hughes

The Partnership for Clean Indoor Air was launched by the U.S. Environmental Protection Agency (EPA)

and other leading partners at the World Summit for Sustainable Development in Johannesburg in September

2002 to improve health, livelihood, and quality of life by reducing exposure to indoor air pollution, primarily

among women and children, from household energy use. Over 80 organizations are working together to

increase the use of clean, reliable, affordable, efficient, and safe home cooking and heating practices that

reduce people’s exposure to indoor air pollution in developing countries. For more information, or to join the

Partnership, visit www.PCIAonline.org.

This document was developed by Aprovecho Research Center under a grant from the Shell Foundation to

provide technical support to household energy and health projects to ensure that their designs represent

technical best practice. The principle authors of this booklet include: Dr. Mark Bryden, Dean Still, Peter Scott,

Geoff Hoffa, Damon Ogle, Rob Balis, and Ken Goyer.

Indoor air pollution causes significant health problems for the 2 billion people worldwide that rely on

traditional biomass fuels for their cooking and heating needs. Over the last 30 years, awareness of the

environmental and social costs of using traditional fuels and stoves and knowledge about how to reduce

emissions from these stoves has grown. Yet the improved stoves currently available to poorer customers do

not always represent best practice or an understanding of design based on modern engineering. The

knowledge required to design cleaner burning stoves exists in centers of excellence in several locations

around the world. Providing this information to those involved in promoting improved stoves is a necessary

first step to reducing indoor air pollution exposure for stove users.

Aprovecho is a center for research, experimentation and education on alternative technologies that are

ecologically sustainable and culturally responsive. The Advanced Studies in Appropriate Technology

laboratory at Aprovecho works to develop energy efficient, nonpolluting, renewable technologies that reflect

current research but which are designed to be made in most any country. The center is located on a beautiful

40-acre land trust near Eugene, Oregon. For more information on Aprovecho, visit www.Aprovecho.net.
Introduction .....................................................................................................5-6

Chapter 1 - Stove Theory................................................................................7-11

Chapter 2 - Ten Design Principles .................................................................12-16

Chapter 3 - Designing Stoves with Baldwin and Winiarski ............................17-25

Chapter 4 - Options For Combustion Chambers ..........................................26-29

Chapter 5 - In Field Water Boiling Test ..........................................................30-35

Appendix - Glossary of Terms.......................................................................37-38

Design Principles for Wood Burning

Cook Stoves

Dr. Mark Bryden, Dean Still, Peter Scott, Geoff Hoffa, Damon Ogle,

Rob Bailis, Ken Goyer

Table of Contents
Design Principles for Wood Burning Cook Stoves

4
Design Principles for Wood Burning Cook Stoves

5

Proven Strategies

Indoor air pollution causes significant health

problems for the 2 billion people worldwide who

rely on biomass fuels for their cooking and heating

needs. Over the last 30 years awareness of the

environmental and social costs of using traditional

fuels and stoves has grown. At the same time,

studies of the problem have resulted in proven

strategies to reduce both fuel use and harmful

emissions. Unfortunately, the local stoves currently

available do not always represent the best designs

that modern engineering can offer. This booklet is

an attempt to address the problem by summarizing

some of the advances in stove theory and design.

Understanding these concepts would be useful to

administrators of stove projects, policy makers,

field workers, and cooks alike.

Although open fires are often used wastefully,

carefully operated open fires can be fuel efficient

and clean burning when tested in the lab. In many

situations, cooks are not overly concerned with fuel

use, and studies have shown that when fuel is

plentiful three-stone fires can use an excessive

amount of wood to cook a small amount of food.

But in other places where fuel is scarce, open fires

can be carefully controlled so that fuel efficiency

rivals many first generation improved cook stoves.

How an operator controls the open fire makes the

difference, as in the use of other tools. In the

seventies and early eighties, open fires were

generally characterized as being basically

inefficient. But it was by analyzing the open fire

that researchers were able to develop truly

improved stoves. Dr. Grant Ballard-Tremeer and

Dr. Kirk Smith were foremost among those who

found that the three stone fire could be both more

fuel efficient and cleaner burning than some

“improved” cook stoves.

Respecting that indigenous technologies are

evolved from countless years of experimentation

Introduction

Introduction

and have great worth changes the perspective of

scientists who are trying to address the causes of

human suffering. Watching how experts operated

the open fire has taught engineers how to design

even better stoves. Modern cook stoves are

designed to clean up combustion first. Then the

hot gases can be forced to contact the pot

increasing efficiency without increasing harmful

emissions.

Fires can be clean burning when expert cooks push

the sticks of wood into the fire as they burn,

metering the fuel. The open fire can be a hot fire

useful when food or drink needs to be prepared

quickly. The

energy goes

into the pot,

not into the

cold body of

a stove. The

open fire can

burn wood

without

making a lot

of smoke; hot

fires burn smoke as it is released from the wood.

Unfortunately however, many fires used for

cooking are built emphasizing simplicity of use and

are wasteful and polluting.

Modern stoves score higher when tested than even

the most carefully operated fire in the laboratory.

Good stoves can offer many advantages. Stoves do

much more than save wood and reduce smoke.

How the stove cooks food is usually most

important to the users!

Improved stoves can make cooking with fire easier,

safer, faster, and can add to the beauty of the

kitchen. A good stove is quicker to start, needs little

tending, and can meet the specific needs of a cook.

The successful design is appreciated as an addition

to the quality of life and usually these concerns far

outweigh scores on a test.

Figure 1 - Traditional Wood Fire
Design Principles for Wood Burning Cook Stoves

6

Decades of Investigation

Many investigators have contributed to a

modern understanding of the thermodynamics

of cooking stoves. The scientific study of wood

burning stoves has reached the point where a

great deal of consensus now exists about how

stoves function. Dr. Larry Winiarski has

studied combustion and wood burning cooking

stoves for more than thirty years. He has

helped organizations build thousands of stoves

in countries around the world. Dr. Winiarski is

the Technical Director of the Aprovecho

Research Center, where stoves have been a

major topic of study since 1976. The team at

Eindhoven University, led by Dr. Krishna

Prasad and including Dr. Peter Verhaart and Dr.

Piet Visser, experimented with wood stoves for

more than a decade and wrote pivotal books on the

subject. Dr. Sam Baldwin summarized years of

experience in West Africa and in the lab in his

comprehensive book Biomass Cookstoves:

Engineering Design, Development and

Dissemination (1987).

Chapter One, Stove Theory, outlines the work of

these leading researchers and offers strategies that a

stove designer can use to improve a stove.

Chapter Two, Ten Design Principles, details the

synthesis of design created by Dr. Larry Winiarski.

Chapters Three and Four, Designing Stoves with

Baldwin and Winiarski, and Options for

Combustion Chambers contain technical

information to support the designer in charge of

developing a stove project.

And lastly, chapter Five, In Field Water

Boiling Test, provides designers with an in field

method for measuring the performance of stove

prototypes as they are developed. The test does not

require a computer or complicated calculations for

data analysis.

Respect for local knowledge

We hope that the following design principles add to

a project, highlighting the respect and inclusion of

local knowledge. A sensitivity and appreciation of

local knowledge supports a two-way information

exchange, learning from the expertise of local

people and their technology while sharing

knowledge.

Hopefully, sharing design principles is more

inclusive than promoting a static stove design.

The literature frequently points out that local

inventiveness has a place in every part of a stove

project. Without information from the community

that will be using the stove, a project is starved for

the input needed for success.

All members of a design committee including

cooks, craftspeople, administrators, promoters and

technical advisors can easily learn stove design

principles. The inventiveness and practical

experience of the whole team is essential to create a

product that suits local needs and ‘tastes’.

The empowerment found in the design process can

serve as motivation for locals to become trainers,

promoters, designers, and builders. Technical staff

frequently find valuable input about design,

manufacture, and promotion from the users and

learn just as much as they teach. Perhaps the

conclusion that stove projects are more likely to

succeed when all concerned help to create the

design parallels the hope that better representation

will create solutions to larger problems.

Introduction
Design Principles for Wood Burning Cook Stoves

7

Chapter 1

Stove Theory

Even an open fire is often 90% efficient at the

work of turning wood into energy. But only a

small proportion, from 10% to 40%, of the

released energy makes it into the pot. Improving

combustion efficiency does not appreciably help

the stove to use less fuel. On the other hand,

improving heat transfer efficiency to the pot makes

a large difference.

Improving the combustion efficiency is necessary

to reduce smoke and harmful emissions that

damage health. Improving heat transfer efficiency

can significantly reduce fuel use. Fire is naturally

good at its job, but pots are not as good at

capturing heat because they are inefficient heat

exchangers. In order to reduce emissions and fuel

use, the stove designer’s job is to first clean up the fire

and then force as much energy into the pot or griddle

as possible. Both of these functions can be

accomplished in a well engineered cooking stove.

It is always best practice to add a chimney to any

wood burning cooking or heating stove.

Additionally, it is preferable to use a cleaner

burning stove to protect air quality in and outside

of the house. Chimneys that take smoke and other

emissions out of the living space protect the family

by reducing exposure to pollutants and health risks.

Even cleaner burning stoves without a chimney can

create unhealthy levels of indoor air pollution.

Unvented stoves should be used outdoors or in

open areas. When chimneys are not affordable or

practical using a hood over the fire, or opening

windows, or making vents in the roof under the

eaves are all ways to decrease the levels of harmful

pollution. The use of a cleaner burning stove can

also be helpful in this regard but, if possible, all

wood burning stoves should always be fitted with a

functional chimney!

How can we design a stove that improves upon the

open fire? First, let’s list the advantages of the threestone fire when compared to some stoves:

f No energy is absorbed into the mass of a stove

body. High-mass stoves can absorb energy that

could have gone into the pot. The three stone

fire can boil water fairly quickly.

f Fire hits the bottom and sometimes the sides of

the pot, exposing a lot of the pot to the hot

gases.

f Sticks can be fed in at the appropriate rate as the

tips burn, assisting complete combustion.

f A hot open fire can burn relatively cleanly.

Every stove suffers because it has some mass that

absorbs heat. But an improved stove can still

achieve better combustion and fuel efficiency

than an open fire.

How to improve combustion

(make less harmful pollution compared to an open

fire)

f Make sure there is good draft into the fire.

f Insulate around the fire to help it burn hotter. A

hotter fire burns up more of the combustible

gases and produces less smoke.

f Avoid using heavy, cold materials like earth and

sand around the combustion chamber.

f Lift the burning sticks up off the ground so that

air can scrape under the sticks and through the

charcoal.

f Placing an insulated short chimney above the

fire helps to increase draft and gives smoke, air,

and fire a place to combine, reducing emissions.

This is a popular strategy used in many stoves

such as the Z stove, the Vesto, the Wood Gas

Camp Stove, the Rocket stove, the Tso-Tso

stove, etc. The Eindhoven group used a

chimney above the fire in their cleanest burning

downdraft stove. Micuta built stoves

incorporating this idea as well (Modern Stoves for

All, 1981). Winiarski developed the concept in

the early 1980s creating a stove that cleaned up

Stove Theory
Design Principles for Wood Burning Cook Stoves

8

combustion and improved heat transfer

efficiency (Capturing Heat One, 1996).

f Meter the sticks of wood into the combustion

chamber to make a hot, fierce, jumpy looking

fire that does not make much charcoal. This type

of fire will make less dangerous emissions,

chimney clogging soot, and creosote. Heat only

the burning part of the wood. Do not encourage

the non-burning wood to make smoke.

f Limit the cold air entering the fire by using as

small an opening as possible. Small openings into

the fire also force the cook to use less wood,

which can be burnt more efficiently.

f A certain amount of excess air is necessary for

complete combustion. Preheating the air helps to

maintain clean combustion.

How to improve fuel

efficiency

(get more heat into the pot)

f Increase the temperature of the gas/flame

contacting the pot, having the hot air scrape

against both the bottom and sides of the pot in a

narrow channel, using a pot skirt.

f Increase the speed of the hot flue gases that scrape

against the pot. The fast gases punch through a

boundary layer of still air that keeps slower

moving gases from scraping against the surface of

the pot (or

griddle.) Air is a

poor heat transfer

medium. It takes

a lot of hot air to

bring heat to the

pot.

f Use metal rather

than clay pots

because metal

conducts heat

better than clay.

f The size of the

fire determines

the size of the

channel gap in the pot skirt and the maximum

efficiency of heat transfer. Smaller fires that can

still please cooks but are not too big will be

considerably more fuel efficient.

f Use wide pots with large diameters. Using a wide

pot creates more surface area to increase the

transfer of heat. Make sure that the top of the

stove slopes up toward the outer perimeter of

the pot, as shown in Figure 2.

Sam Baldwin’s Biomass Stoves: Engineering

Design, Development, and Dissemination (1987) is a

very good summary of how to make improved

stoves. It is highly recommended. Dr. Baldwin

figured out how the channel size between pot and

skirt, firepower and efficiency are related. Here are

a few examples using a family sized pot:

1.) A 1.7 kW fire with a channel gap of 6 mm that

forces hot flue gases to scrape against the pot

for 15 cm will be about 47% efficient.

2.) 4 kW fire with a channel gap of 10 mm that

forces heat to scrape against the pot for 15 cm

will be about 35% efficient.

3.) A 6 kW fire with a channel gap of 12 mm that

forces heat to scrape against the pot for 15 cm

will be about 30% efficient.

4.) A 8 kW fire with a channel gap of 14 mm that

forces heat to scrape against the pot for 15 cm

will be about 26% efficient.

As an approximate, general rule of thumb,

Baldwin recommends that a family sized stove that

burns less than one kg of wood per hour can use a

channel gap between pot skirt and pot of 11 mm.

If the stove burns 1.5 kgs per hour the gap needs to

be 13 mm. If 2 kilos of wood are burnt per hour

then the gap should be 15mm. Please refer to

Biomass Stoves for complete information.

In wood burning stoves a lot of the heat is

transferred to the pot or griddle by convection.

The amount of wood burnt per hour and channel

gap are related. If the pot skirt gap is made too

narrow, there is insufficient draft and smoke backs

up into the room.

Figure 2 - Appropriate Use

of Pot Skirt

Stove Theory
Design Principles for Wood Burning Cook Stoves

9

Increase heat transfer to the pot by keeping the

temperature of the hot flue gases as high as possible.

Insulate everywhere the heat goes except to the pot

or griddle. If there is enough surface area in the

stove for the hot flue gases to scrape against, the

flue gases will be much colder by the time they exit

out of the chimney. If exit temperatures in the

chimney are above 2000

 C, add more surface area

to make use of the heat. Secondary pots or griddles

placed near the chimney may never boil water but

they can help preheat cooking water and warm

food and dishwashing or bathing water.

Using a pot skirt also forces more heat into the pot

by forcing the hot flue gases to continue scraping

against the pot all along its sides in addition to its

bottom.

A haybox makes even more efficient use of

captured heat. Placing the boiling pot of food in an

airtight box filled with insulation holds the heat in

the pot, and food cooks without using added fuel

(See Figure 4).

Once the food has boiled, the fire can be

extinguished. Placing the pot of food in an

insulated cooking box most effectively uses the heat

to accomplish the task of cooking. The haybox

does all simmering without using extra fuel. This

technique saves tremendous amounts of wood. And

using a retained heat cooker saves time for the cook

who lets the haybox do the simmering!

Figure 3 - Top Down

View of Haybox

Figure 4 - Placing

Boiling Pot in the

Insulation

Figure 5 - Putting

Insulating Lid on the

Haybox

Figure 6 - Food

Continues to Cook

Inside the Insulated

Haybox

Stove Theory
Design Principles for Wood Burning Cook Stoves

10

Common Misconceptions

1. Retained energy in the stove body helps to

cook food.

FALSE

Experiments by Baldwin have shown that retained

energy is mostly lost. Leftover charcoal can heat

food after the fire has been extinguished but

retained energy in the stove body is usually too

cold to effectively heat pots.

Note that retained energy in a stove can be

advantageous if the stove is used for space heating.

2. Keeping energy in the stove by decreasing the

draft will help to cook food. Lowering the exit

temperatures in the chimney means that the cook

stove is operating well.

FALSE

As stated, slowing down the draft hurts both

combustion and heat transfer. Hot flue gases need

increased velocity to achieve good heat transfer.

3. Using a damper in the chimney helps to make

a stove work better.

FALSE

Again, slowing down the draft in a cooking stove is

usually detrimental. Dampers should not be used

in a well designed cooking stove.

4. Packed earth or stone acts like insulation.

FALSE

Dense materials absorb energy rather quickly while

insulation slows the passage of heat. Insulation is

made of pockets of air separated by a light weight

less conductive material.

Insulation is light and airy. Heavy materials are

better examples of thermal mass. Insulation helps a

stove to boil water quickly; thermal mass robs

energy from the pot which makes water take longer

to boil.

5. Anything is better than an open fire.

FALSE

An open fire can boil water faster than many heavy

stoves. The three stone fire can be clean burning

and relatively fuel efficient. While the open fire can

be wasteful when used carelessly, the early estimates

that any stove was better has been replaced with a

new respect for this ancient technology. Engineers

have learned how to design improved cooking

stoves by learning what is great about the three

stone fire.

Stove Theory
Design Principles for Wood Burning Cook Stoves

11

Testing is essential

Dr. Baldwin includes a remarkably thorough

chapter on stove testing in Biomass Stoves. He

points out that the testing of prototypes is necessary

while the stove is being developed. Testing stoves

also helps determine if the model is marketable,

whether production costs are as low as possible, and

if improvements are needed. Testing should happen

during the entire life of a stove project.

Baldwin includes tests to determine whether

consumers are happy with the product, if firewood

is being saved, and how lifestyle issues are affected.

Without continual testing, a stove project operates

in the dark; it lacks essential technological,

sociological, and business information. Reading the

stove testing chapter in Biomass Cookstoves is highly

recommended.

Careful testing of stoves has resulted in a more

accurate understanding of how to make better

stoves. Without experimentation and testing, the

development of a stove is based on conjecture.

Careful investigation can quickly separate truth

from opinion. Testing has a twofold function: to

identify problems and to point out solutions. It is

an essential ingredient for progress. A simple water

boiling test is included in Chapter 5 on page 30.

Make stoves safe!

Preventing burns is quite possibly one of the

most important functions of an improved stove.

Burns are quite common in homes using fire and

can be fatal or horribly disfiguring. To protect the

family the stove body should not be hot enough to

cause harm. Stoves and pots should be stable.

Surround the fire with the stove body so that

children cannot be burnt. Injuries from fire are a

major problem that stoves can remedy.

Chimneys or smoke hoods can be used to get

smoke out of the kitchen. According to recent

estimates by the World Health Organization, up to

1.6 million women and children die every year

from breathing polluted air in their houses.

Pneumonia and other respiratory diseases in

children are caused by breathing smoke. Unvented

stoves can be used outdoors, under a roof, or at

least near a large window. Operational chimneys

and airtight stoves can remove essentially all

pollution from the indoor environment. Chimneys

are used in industrialized countries and are

required for protecting families from dangerous

emissions. Shouldn’t people in poorer countries be

provided with the same protection?

Stove Theory
Design Principles for Wood Burning Cook Stoves

12

Dr. Larry Winiarski’s design principles have been used by many organizations to create successful stoves. The

HELPS plancha stove in Guatemala, the PROLENA EcoStove in Nicaragua, the Trees, Water and People

Justa stove in Honduras, the ProBec stoves in South Africa, the new generation of GTZ cooking stoves in

Africa, and the famous Rocket stove are all designed using his principles. Winiarski’s design approach

combines both clean burning and optimized heat transfer characteristics. Any type of intermittently fed

wood burning stove can first be designed by locals to meet their needs and then finished by adapting these

principles.

Batch fed and fan assisted stoves operate differently. These alternative stove design methods can be used as

successfully to improve wood burning stoves. While many experts are working on these two approaches,

both Crispin Pemberton-Pigott and Dr. Tom Reed have developed excellent working models, both of which

are for sale. For more information on the Vesto stove please contact: Crispin Pemberton-Pigott at

vesto@newdawn.sz or VESTO, P.O. Box 85274 Emmarentia, Republic of South Africa 2029. Dr. Tom

Reed has spent decades experimenting with wood burning. His fan-assisted stoves are wonderful inventions.

He currently markets them under the name “Wood Gas Camp Stoves.” Dr. Reed can be reached through

the Biomass Energy Foundation Press or at tombreed@comcast.net.

Chapter 2

Ten Design Principles

PRINCIPLE ONE:

Whenever possible, insulate around the fire

using lightweight, heat-resistant materials. If

possible, do not use heavy materials like sand and

clay; insulation should be light and full of small

pockets of air. Natural examples of insulation

include pumice rock, vermiculite, perlite, and

wood ash. Lightweight refractory brick (brick that

has been fired and is resistant to cracking at high

temperatures) can be made from locally available

sources (for recipes see Chapter 4, Option #2:

Insulative Ceramics, page 27).

Insulation around the fire keeps it hot, which helps

to reduce smoke and harmful emissions. Also,

insulation around the fire keeps the heat from

going into the stove body instead of into the pot.

Unfortunately, metal does not last very long

near a hot fire. However, locally made ceramic tiles

can be found that are durable when used as walls for a

combustion chamber. Loose insulation can surround

this type of construction. (See Chapter 4, Option #1:

Floor Tiles, page 26.)

Figure 7 - Insulation around the fire

Insulative brick

Pockets of air which slow the

transfer of heat to the brick

Ten Design Principles
Design Principles for Wood Burning Cook Stoves

13

PRINCIPLE TWO:

Place an insulated short chimney right above the fire. The combustion

chamber chimney should be about three times taller than its diameter.

Placing a short chimney above the fire increases draft and helps the fire

burn hot and fierce. Smoke will contact flame in the chimney and

combust, reducing emissions. Pots or surfaces to be heated are placed above

the short chimney. A taller combustion chamber chimney, more than three

times the width, will clean up more smoke, but a shorter chimney will

bring hotter gases to the pot. The very tall combustion chamber chimney

can develop too much draft bringing in too much cold air that will

decrease heat transfer.

PRINCIPLE THREE:

Heat and burn the tips of the sticks as they enter the fire. If only the wood that is burning is hot there will

be much less smoke. Try to keep the rest of the stick cold enough that it does not smolder and make smoke.

The goal is to make the proper amount of gas so that it can be cleanly burned without making charcoal or

smoke. Smoke is un-burnt gas! It is harmful to breathe. Even cleaner looking combustion contains harmful

emissions.

PRINCIPLE FOUR:

High and low heat are

created by how many sticks

are pushed into the fire.

Adjust the amount of gas made

and fire created to suit the

cooking task. (Wood gets hot

and releases gas. The gas

catches fire and makes heat.)

Figure 8 - An insulated

short chimney above the fire

Figure 9 - Cleaner Burning Figure 10 - Smoldering Wood Makes Smoke

Figure 11 - Low Heat Figure 12 - High Heat

Ten Design Principles
Design Principles for Wood Burning Cook Stoves

14

PRINCIPLE FIVE:

Maintain a good fast draft through the burning

fuel. Just as blowing on a fire and charcoal can

make it hotter, having the proper amount of draft

will help to keep high temperatures in your stove.

A hot fire is a clean fire.

PRINCIPLE SIX:

Too little draft being pulled into the fire will

result in smoke and excess charcoal. But too

much air just cools the fire and is not helpful.

Smaller openings into the fire help to reduce excess

air. Improving heat transfer to the pot or griddle is

the most important factor that will reduce fuel use

in a cooking stove. Improving combustion

efficiency reduces pollution but is less important

when trying to save firewood.

PRINCIPLE SEVEN:

The opening into the fire, the size of the spaces

within the stove through which hot air flows,

and the chimney should all be about the same

size. This is called maintaining constant cross

sectional area, and helps to keep good draft

throughout the stove. Good draft not only keeps

the fire hot; it is also essential so that the hot air

created by the fire can effectively transfer its heat

into the pot. Air does not carry very much energy,

so a lot of it needs to go through the stove in order

to accomplish the task of heating food or water.

The size of the openings is larger in more powerful

stoves that burn more wood and make more heat.

As a general rule, a door into the fire with a

square opening of twelve centimeters per side

and equally sized chimney and tunnels in the

stove will result in a fire suited to family

cooking. Commercial stoves need bigger openings,

tunnels, and chimneys because bigger fires require

more air. For more information, please see the

chapter Designing Stoves with Baldwin and

Winiarski on page 17.

Figure 13 - Maintaining a Good Draft

Figure 14 - Balancing the air flow in a multipot stove

Figure 15 - Maintaining Constant Cross-Sectional Area

Ten Design Principles
Design Principles for Wood Burning Cook Stoves

15

PRINCIPLE EIGHT:

Use a grate under the fire. Do not put the sticks on

the floor of the combustion chamber. Air needs to

pass under the burning sticks, up through the

charcoal, and into the fire. A shelf in the stove

opening also lifts up sticks so air can pass

underneath them. When burning sticks, it is best to

have them close together and flat on the shelf, with

an air space in between each stick. The burning

sticks keep the fire hot, each fire reinforcing the

other to burn more completely. It is optimum if the

air passes under the shelf and through the coals so

that when it reaches the fire it is preheated to help

the gases reach complete combustion. Air that passes

above the sticks is not as helpful because it is colder

and cools the fire. A hot raging fire is clean, but a

cold fire can be very dirty.

PRINCIPLE NINE:

Insulate the heat flow path. Cooks tend to like

stoves that boil water quickly. This can be especially

important in the morning when family members

need to get to work. If heat goes into the body of

the stove, the pot boils less quickly. Why heat up

fifty or one hundred kilograms of stove each

morning when the desired result is to heat up a

When designing a stove, it is possible to decrease

the gap in the channel next to the pot or griddle

until the fire becomes “lazy.” Using trial and error,

open up the gap little by little until the fire stays

hot and vigorous.

PRINCIPLE TEN:

Maximize heat transfer to the pot with properly

sized gaps. Getting heat into pots or griddles is

best done with small channels. The hot flue gases

from the fire are forced through these narrow

channels, or gaps, where it is forced to scrape

against the pot or griddle. If the gap is too large the

hot flue gases mostly stay in the middle of the

channel and do not pass their heat to the desired

cooking surface. If the gaps are too small, the draft

diminishes, causing the fire to be cooler, the

emissions to go up, and less heat to enter the pot.

kilogram of food or a liter of water? Using

insulative materials in the stove keeps the flue gases

hot so that they can more effectively heat the pan

or griddle. Insulation is full of air holes and is very

light. Clay and sand or other dense materials are

not insulation. Dense materials soak up heat and

divert it from cooking food.

Figure 16 - Use of a Grate Under the Fire

Ten Design Principles
Design Principles for Wood Burning Cook Stoves

16

The two most important factors for getting large

amounts of heat into a pot or griddle are: 1) keep

the flue gases that touch the pot or griddle as hot as

possible; and, 2) force the hot gases to scrape against

the surface quickly, not slowly. Air does not hold

much heat. Faster hot flue gases scraping against the

pot or griddle will transfer much more heat than

slow-moving cooler air.

The size of the channel can be estimated by

keeping the cross sectional area constant

throughout the stove. When using an external

chimney that provides greater draft, channel gaps

can be reduced. For more information on gaps,

please see the next chapter.

Figure 17 - A proper sized gap

optimizes heat transfer to the pot

Figure 18 - Too large a gap will reduce

heat transfer to the pot

Ten Design Principles
Design Principles for Wood Burning Cook Stoves

17

Chapter 3

Designing Stoves with Baldwin & Winiarski

Forcing hot flue gases to flow past the surface area of

a pot or griddle in a narrow channel is a stove

design strategy popularized by both Dr. Samuel

Baldwin and Dr. Larry Winiarski. In 1982 Dr.

Winiarski created the pot skirt, a cylinder of sheet

metal that surrounded the pot, which formed a

narrow channel increasing heat transfer efficiency.

Dr. Baldwin studied stoves in Africa and in 1987

wrote his seminal book Biomass Stoves: Engineering

Design, Development, and Dissemination in which he

also stresses the importance of using narrow

channels to deliver more heat to the pot.

In general, there are three ways to increase

convective heat transfer:

f The flue gases scraping the surface to be heated,

should be as hot as possible.

f The surface area of the heat exchanger should be

as large as possible.

f The velocity of the hot flue gases should be

increased as much as possible. A faster flow over

the exterior of the pot disturbs the stagnant

boundary layer of air that slows effective heating.

The narrow channels formed close to the pot by an

insulated skirt (see Figure 19) can help to optimize

the three principles simply and inexpensively.

Although narrowing the gap increases heat transfer

efficiency, doing so also decreases the flow of air

through the stove. The size of the gap must

therefore be in relation to the firepower. As more

wood is burned per minute, more air is needed to

support both the combustion and the necessary

flow to avoid back drafting into the room. If too

small a gap is used the fire may burn well while

simmering but will be short of air when operated at

high power. On the other hand, very large channel

gaps will sustain a large fire but unnecessary

amounts of heat will be lost due to poor heat

transfer.

Design Strategies

The two stove designers approach the problem of

sizing the channel gap differently. Winiarski in

Rocket Stove Design Principles (1997), advises

technicians to start designing stoves by maintaining

constant cross sectional area throughout the stove.

He sets the cross sectional area at the opening into

the fire, or fuel magazine, and then creates

appropriate gaps around the pots based on

maintaining the same cross sectional area. Baldwin’s

method requires a designer to pick a maximum

high power for the stove design. Starting from a

fixed firepower the size of the channel gap is then

determined. In one case, Winiarski chooses the size

of the fuel magazine first while Baldwin uses

firepower as the starting point. The spaces within

the stove are determined by either of these two

primary choices.

Figure 19 - The narrow channel close to the

pot increases convective heat transfer

Figure 20 - Hot flue gases are forced to flow past the

surface of the pots in a narrow channel

Designing Stoves with Baldwin & Winiarski
Design Principles for Wood Burning Cook Stoves

18

Winiarski Method

The following stove diagram and tables (see pages

19-20) show how the size of the channels near to

the pot or griddle change as the opening into the

fire is expanded. Dr. Winiarski suggests that a

12 cm by 12 cm opening is usually sufficient for a

family sized cooking stove. Larger openings that

allow more wood into the fire result in higher

power and larger channel gaps.

Establishing the same cross sectional area

everywhere in a cooking stove ensures sufficient

draft for good combustion while resulting in

Figure 21 - A Typical Winiarki Stove

(Use this diagram along with the calculations found on pages 19-25 to

determine proper gap size)

channel gaps that increase heat transfer efficiency.

This means that the opening into the combustion

chamber, the combustion chamber, the air gap

under the pot or griddle, and the chimney are the

same size (equal number of square centimeters)

while having different shapes. Winiarski advises

designers to create prototype cooking stoves that

maintain the cross sectional area to keep the draft

flowing at an optimal rate. Slowing down the draft

hurts both combustion and heat transfer efficiency

to the pot.

Designing Stoves with Baldwin & Winiarski

Lid

Pot

Pot Skirt

Stove

Top

Short

insulated

chimney

above fire

Fuel

Entrance

Stove

body
Design Principles for Wood Burning Cook Stoves

19

12 cm X 12 cm Square Combustion Chamber

Pot Size (cm) 20 30 40 50

GAP A (cm) 3 3 3 3

GAP B (cm) 2.5 2.5 2.5 2.5

GAP C (cm) 2.3 1.5 1.1 0.9

GAP D (cm) 2.1 1.5 1.1 0.9

14 cm X 14 cm Square Combustion Chamber

Pot Size (cm) 20 30 40 50

GAP A (cm) 3.5 3.5 3.5 3.5

GAP B (cm) 3.1 3.1 3.1 3.1

GAP C (cm) 3.1 2.1 1.6 1.2

GAP D (cm) 2.7 2 1.5 1.2

16 cm X 16 cm Square Combustion Chamber

Pot Size (cm) 20 30 40 50

GAP A (cm) NA 4 4 4

GAP B (cm) NA 3.7 3.7 3.7

GAP C (cm) NA 2.7 2 1.6

GAP D (cm) NA 2.5 1.9 1.6

18 cm X 18 cm Square Combustion Chamber

Pot Size (cm) 20 30 40 50

GAP A (cm) NA 4.5 4.5 4.5

GAP B (cm) NA 4.3 4.3 4.3

GAP C (cm) NA 3.4 2.6 2.1

GAP D (cm) NA 3.1 2.4 2

20 cm X 20 cm Square Combustion Chamber

Pot Size (cm) 20 30 40 50

GAP A (cm) NA 5 5 5

GAP B (cm) NA 4.9 4.9 4.9

GAP C (cm) NA 4.2 3.2 2.5

GAP D (cm) NA 3.7 3 2.4

CROSS SECTIONAL AREA FOR SQUARE COMBUSTION CHAMBERS

Use these tables to create stoves with constant cross sectional area

Table 1

Designing Stoves with Baldwin & Winiarski
Design Principles for Wood Burning Cook Stoves

20

CROSS SECTIONAL AREA FOR CIRCULAR COMBUSTION CHAMBERS

Table 2

Designing Stoves with Baldwin & Winiarski

12 cm Diameter Circular Combustion Chamber

Pot Size (cm) 20 30 40 50

GAP A (cm) 3 3 3 3

GAP B (cm) 2 2 2 2

GAP C (cm) 1.8 1.2 0.9 0.7

GAP D (cm) 1.6 1.2 0.9 0.7

14 cm Diameter Circular Combustion Chamber

Pot Size (cm) 20 30 40 50

GAP A (cm) 3.5 3.5 3.5 3.5

GAP B (cm) 2.4 2.4 2.4 2.4

GAP C (cm) 2.4 1.6 1.2 0.9

GAP D (cm) 2.2 1.5 1.2 0.9

16 cm Diameter Circular Combustion Chamber

Pot Size (cm) 20 30 40 50

GAP A (cm) NA 4 4 4

GAP B (cm) NA 2.9 2.9 2.9

GAP C (cm) NA 2.1 1.6 1.3

GAP D (cm) NA 2 1.5 1.3

18 cm Diameter Circular Combustion Chamber

Pot Size (cm) 20 30 40 50

GAP A (cm) NA 4.5 4.5 4.5

GAP B (cm) NA 3.4 3.4 3.4

GAP C (cm) NA 2.7 2 1.6

GAP D (cm) NA 2.5 1.9 1.6

20 cm Diameter Circular Combustion Chamber

Pot Size (cm) 20 30 40 50

GAP A (cm) NA 5 5 5

GAP B (cm) NA 3.8 3.8 3.8

GAP C (cm) NA 3.3 2.5 2

GAP D (cm) NA 3 2.4 1.9
Design Principles for Wood Burning Cook Stoves

21

Baldwin: Firepower Determines Channel Size

As can be seen in the chart below, Baldwin and Winiarski’s methods seem to create similar sized gaps. These

values are derived from charts found in Biomass Stoves which summarize Baldwin’s findings. The chart is an

approximation meant to serve as a guide to the relationship between firepower, wood use per hour, length

and width of gap size, and stove efficiency.

Wood burned per Skirt gap (mm) Length of gap (cm) Thermal efficiency Firepower (kW)

hour (kg) of stove (%)

0.50 8 20 40 2.8

0.75 10 20 35 4.1

1.00 11 20 30 5.5

1.25 12 20 28 6.9

1.50 13 20 26 8.3

1.75 14 20 25 9.6

A typical Winiarski designed stove with a square, 12 cm x 12 cm combustion chamber burns wood at

approximately the rate of 1.5 kg/hr at high power. In his computer program Baldwin uses a 30 cm diameter

pot as “family sized.” Given this size of pot, the gap at the perimeter using the Winianski model would be

calculated by dividing the area (A = 12 cm x 12 cm = 144 square cm for a square combustion chamber) by

the perimeter at the edge of the pot (P = pi (d), the circumference, or 3.14 x 30 = 94 cm). The resulting gap

is 144 cm/94 cm = 1.5 cm (15 mm). Following Baldwin’s chart, we see that a stove burning wood at a rate

of 1.5 kg/hr. would call for a gap of 13 mm for maximum efficiency, a difference of 2 mm from Winiarski’s

model.

Table 3 - Baldwin’s Suggested Gap Sizes

Designing Stoves with Baldwin & Winiarski
Design Principles for Wood Burning Cook Stoves

22

Designing Stoves with Baldwin & Winiarski

Calculations

To use Winiarski’s method of maintaining a constant cross sectional area under the pot, you will need to

calculate the correct height of the gap under the pot. This height will vary as you move from the center of

the combustion chamber out to the edge of the pot. To do this, calculate the needed gap at the edge of the

combustion chamber and at the edge of the pot. Although this sounds complicated it is relatively straight

forward. There are 5 steps to make this calculation:

1. Determine the area of the combustion chamber, which will be continued throughout the stove. If

the combustion chamber is cylindrical, the area is calculated using the formula

where Ac is the area, π = 3.14, and rc is the radius. The radius is one-half the diameter. If the combustion chamber is square or rectangular, the area is calculated as

where l is the height and w is the width.

 2. At the edge of the insulated chimney above the fire, the gasses turn and follow the bottom of the pot.

To determine the needed gap at the edge of the combustion chamber, first determine the circumference of the area that the hot gasses will pass through. To do this measure from the center of the

combustion chamber outlet to the farthest edge, rc

. In a circular combustion chamber this will be the

radius. In a square or rectangular chamber this will be from the center to one of the corners. Determine the circumference associated with this distance. This is

3. Next, divide the cross sectional area, Ac, determined in Step 1 by the Cc determined in Step 2. This is

where Gc is the needed gap between the bottom of the pot and the top edge of the combustion

chamber.

4. Now determine the optimal gap at the edge of the pot. Measure the circumference, C

p

, of the pot.

This is the distance all the way around the pot. The circumference can be measured two ways. The

easiest is to take a piece of string, wrap it around the pot and measure the length of the string.

Alternately, you can determine the circumference from the diameter, r

p

.

5. As in Step 3, divide the cross sectional area, Ac

, determined in Step 1 by the Cc determined in Step 4

to calculate the needed gap at the edge of the pot, G

p

. This is
Design Principles for Wood Burning Cook Stoves

23

Designing Stoves with Baldwin & Winiarski

As noted above, the area under the pot will need to be slowly decreased moving from the edge of the combustion chamber to the edge of the pot. Careful readers will note that this thinning of the gap is not linear.

However, using the constant area thumb rule as an approximation is the easiest way to handle this. Smoothly

match the gap distance from the edge of the combustion chamber to the edge of the pot by hand in a linear

fashion.

After creating the prototype with a constant cross sectional area, the cooking stove will need to be fine-tuned

by reducing the channel gap while watching the fire at high power. Set the gap as small as possible while

making sure that the draft is sufficient for clean combustion. It is good practice to remember that the stoves

will often be operated at very high power; therefore, the careful designer does not tighten gaps below the

maximum possible firepower. Widening the distance beyond the theoretical best gap also provides some

degree of protection against clogging by products of incomplete combustion.

Example 1

Consider the case of a stove with a cylindrical combustion chamber 12 cm in diameter with a 30 cm

cooking pot.

The first step is to calculate the cross sectional area of the combustion chamber. This is

Next calculate the gap needed at the edge of the combustion chamber. First we find the circumference of the

area that the hot gasses will pass through. This is

From this you can find the needed gap at the edge of the combustion chamber as

If this space were only two centimeters high, the cross sectional area at Gap A would only be 75.4 cm2

,

reducing the draft and increasing the production of smoke. If the space at Gap A were 5 centimeters, the

cross sectional area would be 188.5 cm2. This area is so large that even though flow rate is maintained, the

velocity of hot gases is decreased and gases are not forced to scrape against the pot and so cannot effectively

deliver their energy to it.

At the edge of the pot, the circumference that the hot gasses need to pass through is

The needed gap at the edge of the pot is

We need to remember that this is an approximation and that the gap will need to be field tuned at the

highest power setting of the stove. In addition, we will need to smoothly thin the gap from 3.0 cm at the

edge of the combustion chamber to 1.2 cm at the edge of the pot.
Design Principles for Wood Burning Cook Stoves

24

Example 2

Often it is less expensive to build square or rectangular combustion chambers. Consider the case of a 12 cm

x 10 cm rectangular combustion chamber with a 30 cm diameter cooking pot.

The first step is to calculate the cross sectional area of the combustion chamber. This is

Next we calculate the gap needed at the edge of the combustion chamber. First we find the circumference of

the area that the hot gasses will pass through. The distance from the center of the combustion chamber to the

corner is

We know this looks complicated but remember in the field you will be using a tape measure not geometry.

With this we can find the circumference of the area at that point. This is

From this we can find the needed gap at the edge of the combustion chamber

At the edge of the pot, the circumference that the hot gasses need to pass through is

The needed gap at the edge of the pot is

Again we need to remember that this is an approximation and that the gap will need to be field tuned at the

highest power setting of the stove. In addition, we will need to smoothly thin the gap from 2.4 cm at the

edge of the combustion chamber to 1.2 cm at the edge of the pot.

Example 3

Another application of the constant area thumb rule is determining the gap needed between the pot and an

insulated pot skirt. An insulated pot skirt is a band of metal insulated on the outside that goes around the

cook pot, forcing the hot gases to run along the sides of the pot. Consider the cook stove with the 12 cm

cylindrical combustion chamber and the 30 cm pot examined in Example 1.

To calculate the gap between the pot and the skirt along the side walls, or Gap D in the diagram on page 18,

start with the area of the cooking chamber found in Example 1.
Design Principles for Wood Burning Cook Stoves

25

Divide this by the circumference around the pot.

The gap needed becomes

Note that this is the same gap as between the edge of the pot and stove surface. Also the careful reader will

have noted that this is an approximation. But it is a very good approximation. Also remember that this is

only a starting point and should be tuned at the high power setting in the field.

Conclusions

Both Winiarski’s and Baldwin’s methods result in workable solutions that seem to be closely related.

Creating small channels to increase heat transfer efficiency is a common strategy engineers use to optimize

heat transfer. Applying the practice to cooking stoves has been shown to effectively improve fuel efficiency.

Even an open fire is often 90% efficient at the work of turning wood into heat. But only a small

proportion, from 10% to 40% of the released heat makes it into the pot. Improving combustion efficiency

can have little appreciable effect on overall system efficiency; i.e., decreased fuel use. On the other hand,

improving heat transfer efficiency to the pot can make a large difference, saving significant amounts of

firewood.

Stoves have to use gaps that are large enough to support the airflow at high power. Much less firepower is

required to simmer food. But the efficiency of heat transfer suffers because the channels are larger than

needed at this reduced rate of flow. For this reason, without adjustable gaps, stoves tend to display better heat

transfer efficiency at high power. A pot skirt with adjustable gaps solves this problem.

It is interesting that Baldwin was impressed by the improvements made possible by placing a short insulated

chimney above the fire, which is the defining characteristic of Winiarski’s Rocket stove. By reconfiguring the

combustion chamber in this way Baldwin reports an increase in velocity of hot flue gases due to the height

of the chimney, which results in clean burning and good fuel efficiency (Page 43, Biomass Stoves). In practice

installing a short insulated chimney above the fire seems to help clean up combustion. Forcing the cleaner

hot flue gasses to scrape against the pot or griddle in narrow spaces can increase heat transfer efficiency

without significantly increasing harmful emissions.
Design Principles for Wood Burning Cook Stoves

26

Multiple tests of the sand and clay Lorena stove,

beginning in 1983, showed that placing materials

with high thermal mass near the fire can have a

negative effect on the responsiveness, fuel

efficiency, and emissions of a cooking stove because

they absorb the heat from the fire. Examples of

high thermal mass materials are mud, sand, and

clay. When stoves are built from high thermal mass

materials, their efficiency (when tested in the

laboratory) can be worse than that of the threestone fire.

So what other materials can be used? Cleaner

burning stoves can produce such high temperatures

in the combustion chamber (where the fire burns)

that metal, even stainless steel, can be destroyed.

Cast iron combustion chambers, though longer

lasting, are expensive.

While mud, sand, and clay are high in thermal

mass, they do have certain benefits. They are

locally available, cheap, easy to work with, and are

often long lasting because they don’t burn out

under the intense heat produced by a fire.

Creativity and good engineering allow a stove

designer to use these materials advantageously

without allowing their high thermal mass to

degrade the quality of the stove.

Stove makers have been using ceramic parts for

many years. The Thai Bucket Stove uses a ceramic

combustion chamber. The Kenyan Jiko Stove also

uses a ceramic liner to protect the sheet metal stove

body. Books have been written describing how to

make clay combustion chambers that will last for

several years.** A women’s co-operative in

Honduras called Nueva Esperansa makes longlasting refractory ceramic stove parts from a

mixture of clay, sand, horse manure, and tree gum.

These combustion chambers are used in the Doña

Justa and Eco Stoves now popular in Central

America.

Chapter 4

Options for Combustion Chambers*

The benefit to using ceramic combustion chambers

in these instances is their longevity. As we shall see

in the example below, the key to minimizing the

drawback of ceramic material, which is its high

thermal mass, is to use the least amount possible

without compromising its strength and by

surrounding it with an insulative material.

Option #1: Floor Tiles

Don O’Neal (HELPS International) and Dr.

Winiarski located an alternative material in

Guatemala, an inexpensive ceramic floor tile called

a baldosa. The baldosa is about an inch thick and

can be cut or molded into appropriate shapes to

make a combustion chamber. Loose insulation fills

in between the combustion chamber and the inside

of the stove body. Wood ash, pumice rock,

vermiculite, and perlite are all good natural heat

resistant sources of loose insulation. The baldosa is

inexpensive and has lasted four years in the

insulated HELPS and Trees, Water and People

stoves built in Central America.

Figure 22 - Ceramic Floor Tile

Options for Combustion Chambers

* First published in Boiling Point #49

**A good book on the subject is The Kenya Ceramic Jiko: A Manual for Stovemakers (Hugh Allen, 1991).
Design Principles for Wood Burning Cook Stoves

27

The baldosa floor tile is tested by placing it in a fire

until it is red hot. Then the tile is removed and

quickly dipped into a bucket of cold water. If the

tile doesn’t crack, it will probably last in the

combustion chamber. Baldosa are usually made

with red clay and are fired in a kiln at around 9000

- 10000C. They are somewhat porous and ring

when struck with a knuckle. Using baldosa in a

combustion chamber surrounded by loose

insulation adds one more material option for the

stove designer.

Option #2: Insulative

Ceramics

These recipes are intended to assist stove promoters

in making insulative ceramics for use in improved

wood burning cook stoves. Each of these materials

incorporates clay, which acts as a binder. The clay

forms a matrix around a filler, which provides

insulation. The filler can be a lightweight fireproof

material (such as pumice, perlite, or vermiculite),

or an organic material (charcoal or sawdust). The

organic material burns away, leaving insulative air

spaces in the clay matrix. In all cases, the clay and

filler are mixed with a predetermined amount of

water and pressed into forms (molds) to create

bricks. The damp bricks are allowed to dry, which

may take several weeks, and then fired at

temperatures commonly obtained in pottery or

brick kilns in Central America.

Our test samples were made using low-fired “raku”

clay obtained from a local potter’s supply store. In

other countries, the best source of clay would be

the kind used by local potters or brick makers.

Almost everywhere, people have discovered clay

mixes and firing techniques, which create sturdy

ceramics. Insulative ceramics need to be lightweight

(low density) to provide insulation and low thermal

mass. At the same time, they need to be physically

durable to resist breakage and abrasion due to

wood being forced into the back of the stove.

These two requirements are in opposition; adding

more filler to the mix will make the brick lighter

and more insulative, but will also make it weaker.

Adding clay will usually increase strength but

makes the brick heavier. We feel that a good

compromise is achieved in a brick having a density

between 0.8 gm/cc and 0.4 gm/cc.

The recipes in Table 4 indicate the proportions, by

weight, of various materials. We recommend these

recipes as a starting point for making insulative

ceramics. Variations in locally available clays and

fillers will probably require adjusting these

proportions to obtain the most desirable results.

Insulative ceramics used in stoves undergo

repeated heating and cooling (thermal cycling),

which may eventually produce tiny cracks that

cause the material to crumble or break. All of these

recipes seem to hold up well to thermal cycling.

The only true test, however, is to install them in a

stove and use them for a long period of time under

actual cooking conditions.

Filler Clay (damp) Water Fired at Density

Type Wt. (Grams) Wt. (Grams) Wt. (Grams) (degrees C) gr/cc

Sawdust 490 900 1300 1050 0.426

Charcoal 500 900 800 1050 0.671

Vermiculite 300 900 740 1050 0.732

Perlite Mix 807 900 1833 1050 0.612

Pumice Mix 1013 480 750 950 0.770

Table 4 - Insulative Ceramics

Options for Combustion Chambers
Design Principles for Wood Burning Cook Stoves

28

Sawdust/Clay:

In this formulation, fine sawdust was obtained by

running coarse sawdust (from a construction site)

through a #8 (2.36-mm) screen. Clay was added to

the water and mixed by hand to form thick mud.

Sawdust was then added, and the resulting material

was pressed into rectangular molds. Excellent

insulative ceramics can be made using sawdust or

other fine organic materials such as ground

coconut husks or horse manure. The problem with

this method is obtaining large volumes of suitable

material for a commercial operation. Crop residues

can be very difficult to break down into particles

small enough to use in brick making.

This method would be a good approach in

locations where there are sawmills or woodworking

shops that produce large amounts of waste sawdust.

Charcoal/Clay:

In this formulation, raw charcoal (not briquettes)

was reduced to a fine powder using a hammer and

grinder. The resulting powder was passed through a

#8 screen. Clay was hand mixed into water and the

charcoal was added last. A rather runny slurry was

poured into molds and allowed to dry. It was

necessary to wait several days before the material

dried enough that the mold could be removed.

Dried bricks were fired at 10500

C. Charcoal can be

found virtually everywhere, and can be used when

and where other filler materials are not available.

Charcoal is much easier to reduce in size than other

organic materials. Most of the charcoal will burn

out of the matrix of the brick. Any charcoal that

remains is both lightweight and insulative.

Charcoal/clay bricks tend to shrink more than

other materials during both drying and firing.

The final product seems to be lightweight and

fairly durable, although full tests have not yet

been run on this material.

Vermiculite/Clay:

In this formulation, commercial vermiculite (a soil

additive), which can pass easily through a #8 (2.36

mm) screen, is mixed directly with water and clay

and pressed into molds. Material is dried and fired

at 10500C.

Vermiculite is a lightweight, cheap, fireproof

material produced from natural mineral deposits in

many parts of the world. It can be made into

strong, lightweight insulative ceramics with very

little effort. The flat, plate-like structure of

vermiculite particles makes them both strong and

very resistant to heat.

Vermiculite appears to be one of the best possible

choices for making insulative ceramics.

Perlite Mix/Clay:

For best results, perlite must be made into a

graded mix before it can be combined with

clay to form a brick. To prepare this mix, first

separate the raw perlite into three component

sizes: 3/8' to #4 (9.5 mm to 4.75 mm), #4 to #8

(4.75 mm to 2.36 mm), and #8 (2.36 mm and

finer). Recombine (by volume) two parts of the

largest size, one part of the midsize, and seven parts

of the smallest size to form the perlite mix. This

mix can now be combined with clay and water and

formed into a brick, which is dried and fired.

Perlite is the mineral obsidian, which has been

heated up until it expands and becomes light. It is

used as a soil additive and insulating material.

Perlite mineral deposits occur in many countries of

the world, but the expanded product is only

available in countries that have commercial

“expanding” plants. Where it is available, it is both

inexpensive and plentiful.

Perlite/clay bricks are some of the lightest usable

ceramic materials we have produced so far.

Options for Combustion Chambers
Design Principles for Wood Burning Cook Stoves

29

Pumice Mix/Clay:

Pumice, like perlite, produces the best results when

it is made into a graded mix. Care should

be taken to obtain the lightest possible pumice for

the mix. Naturally occurring volcanic sand,

which is often found with pumice, may be quite

heavy and unsuitable for use in insulative

ceramics. It may be necessary to crush down larger

pieces of pumice to obtain the necessary

small sizes. The mix is prepared by separating

pumice into three sizes: 0.5 inch to #4 (12.5 mm

to 4.75 mm), #4 to #8 (4.75 mm to 2.36 mm),

and #8 (2.36 mm) and smaller. In this case, the

components are recombined (by volume) in the

proportion of two parts of the largest size, one

part of the midsize, and four parts of the smallest

size. Clay is added to water and mixed to form

thin mud. The pumice mix is then added and the

material is pressed into molds. Considerable

tamping or pressing may be necessary to work out

the air and form a solid brick. The mold can be

removed immediately and the brick allowed to dry

for several days before firing.

Pumice is widely available in many parts of the

world and is cheap and abundant. Close attention

to quality control is required, and this could be a

problem in many locations. It is very easy to turn a

lightweight insulative brick into a heavy noninsulating one through inattention to detail.

Pumice (and perlite as well) is sensitive to high heat

(above 11000

C). Over-firing will cause the pumice

particles to shrink and turn red, resulting in an

inferior product. Despite these concerns, pumice

provides a great opportunity to supply large

numbers of very inexpensive insulative ceramics in

many areas of the world.

There are many viable recipes to make lightweight

refractory ceramic combustion chambers. Using

insulation around the fire helps to boil water more

quickly, makes the stove easier to light, and saves

firewood. It is necessary to create very high

temperatures in a combustion chamber in order to

clean up dangerous emissions. Unfortunately these

high temperatures quickly degrade metals,

including stainless steel. Refractory insulative

ceramics provide a material that is both long lasting

and does not lower combustion temperatures as do

materials with a higher thermal mass.

Options for Combustion Chambers
Design Principles for Wood Burning Cook Stoves

30

This test provides the stove designer with reliable

information about the performance of wood

burning stove models. The test consists of three

phases that determine the stove’s ability to:

(1) bring water to a boil from a cold start;

(2) bring water to a boil when the stove is hot; and,

(3) maintain the water at simmering temperatures.

It is used to evaluate a series of stoves as they are

being developed. The test cannot be used to

compare stoves from different places because the

different pots and wood used change the results.

The test is a simplified version of the University of

California Berkeley (UCB)/Shell Foundation

revision of the 1985 VITA International Standard

Water Boiling Test. The wood used for boiling and

simmering, and the time to boil are found by

simple subtraction. All calculation can be done by

hand in the field.

By using a standard pot, taking into account the

moisture content of the wood, steam generated and

other factors the complete UCB/Shell Foundation

Water Boiling Test makes comparison of stoves

from different places possible.

Before starting the tests...

1. Collect at least 30 kg of air-dried fuel for each

stove to be tested in order to ensure that there

is enough fuel to complete three tests for each

stove. Massive multi-pot stoves may require

more fuel. Use equally dry wood that is the

same size. Do not use green wood.

2. Put 5 liters of water in the testing pot and bring

it to a rolling boil. Make sure that the fire is very

powerful, and that the water is furiously boiling!

Use an accurate digital thermometer, accurate to

1/10 of a degree, to measure the local boiling

temperature. Put the thermometer probe in the

center of the testing pot, 5 cm above the pot

bottom. Record the local boiling point on the

data sheet (see page 34).

Chapter 5

In Field Water Boiling Test (WBT)

3. Do the tests in a place that is completely

protected from the wind.

4. Record all results on the data sheet.

Equipment used for the In Field

Water Boiling Test:

• Scale of at least 6 kg capacity and 1 gram

accuracy

• Heat resistant pad to protect scale

• Digital thermometer, accurate to 1/10 of a

degree, with thermocouple probes that can

be in liquids

• Timer

• Testing pot(s)

• Wood fixture for holding thermometer

probe in water

• Small shovel/spatula to remove charcoal

from stove

• Tongs for handling charcoal

• Dust pan for transferring charcoal

• Metal tray to hold charcoal for weighing

• Heat resistant gloves

• 3 bundles of air-dried fuel wood. One, used

for simmering, weighs around 5 kgs. The

other two bundles, used for cold and hot

start boiling, weigh about 2 kgs each.

In Field Water Boiling Test
Design Principles for Wood Burning Cook Stoves

31

Beginning of Test

a. Record the air temperature.

b. Record weight of commonly used pot without

lid. If more than one pot is used, record the

weight of each pot. If the weights differ, be sure

not to confuse the pots as the test proceeds. Do

not use pot lids for this, or any other phase of

the WBT.

c. Record weight of container for charcoal.

d. Prepare 2 bundles of fuel wood that weigh about

2 kgs each for the cold and hot start high power

tests. Prepare 1 bundle of fuel wood that weighs

about 5 kgs to be used in the simmering test.

Use sticks of wood roughly the same size for all

tests. Record approximate dimensions of the

fuel wood. Weigh and Record weights in spaces

marked # on the attached data sheet. Identify

each bundle and keep them separate.

High Power (Cold Start) Phase:

The stove should be at room temperature.

1. Fill each pot with 5 L of clean water (~200

).

Record the weight of pot(s) plus the water.

2. Using the wooden fixtures, place a thermometer

probe in each pot so that water temperature may

be measured in the center, 5 cm from the

bottom. Make sure a digital thermometer is

used. Record water temperatures.

3. Record the weight of the starting materials.

Always use the same amount and material.

4. Start the fire using the wood from the first 2 kg

bundle.

5. Once the fire has caught, start the timer and

Record “0”. If using a watch Record the starting

time. Bring the first pot rapidly to a boil

without being excessively wasteful of fuel.

6. When the water in the first pot reaches the local

boiling temperature as shown by the digital

thermometer, rapidly do the following:

a. Record the time at which the water in the

primary pot (Pot # 1) reaches the local

boiling point of water. Record the water

temperature for other pots as well.

b. Remove all wood from the stove and put out

the flames. Knock all loose charcoal from the

ends of the wood into the tray for weighing

charcoal.

c. Weigh the unburned wood from the stove

together with the remaining wood from the

pre-weighed bundle. Record the result.

d. Weigh each pot, with its water. Record

weight.

e. Remove all the charcoal from the stove, place

it with the charcoal that was knocked off the

sticks and weigh it. Record the weight of the

charcoal and container.

This completes the high power (cold start) phase.

Continue without pause to the high power (hot

start) portion of the test. Do not allow the stove to

cool.

High Power (Hot Start) Phase

1. Refill the pot(s) with 5 L of fresh cold water.

Weigh pot(s) (with water) and measure the

initial water temperatures; Record both

measurements.

2. Start the fire using kindling and wood from the

second 2 kg bundle. Record weight of any

additional starting materials.

3. Record the time when the fire starts and bring

the first pot rapidly to a boil without being

excessively wasteful of fuel.

4. Record the time at which the first pot reaches

the local boiling point. Record the temperature

of all pots.

In Field Water Boiling Test
Design Principles for Wood Burning Cook Stoves

32

5. After reaching the boiling temperature, rapidly

do the following:

a. Remove all wood from the stove and knock

off any loose charcoal into the charcoal

container. Weigh the wood removed from

the stove, together with the unused wood

from the second bundle. Record the result.

b. Weigh each pot, with its water and Record

these weights.

6. Remove all remaining charcoal from the stove

and weigh it (including charcoal which was

knocked off the sticks). Record the weight of

the charcoal plus container.

Without pause, proceed directly with the

simmering test.

Low Power (Simmering) Test

This phase is designed to test the ability of the stove

to simmer water using as little wood as possible.

Use the 5 kg bundle of wood to bring the water to

boil. Then record the weight of the remaining

wood and simmer the water for an additional 45

minutes.

Only the primary pot will be tested for

simmering performance.

Start of Low Power test:

1. Record the weight of the 5 kg bundle of fuel.

2. Refill the pot with 5 L of cold water. Weigh the

pot (with water). Record weight. Record

temperature.

3. Rekindle the fire using kindling and wood from

the weighed bundle. Record the weight of any

additional starting materials. Replace the pot on

the stove and Record the start time when the

fire starts.

4. Bring the pot rapidly to a boil without being

excessively wasteful of fuel. As soon as local

boiling temperature is reached, do the following

steps quickly and carefully:

5. Record the boiling time and temperature.

Quickly weigh the water in the primary pot and

return it to the stove. Record the weight of the

pot with water. Record the weight of remaining

wood in 5 kg bundle. Replace the thermometer

in the pot and continue with the simmer test by

reducing the fire. Keep the water as close to 30

C

below the boiling point as possible.

6. Record temperature of the water.

7. Record the time. For the next 45 minutes

maintain the fire at a level that keeps the water

temperature as close as possible to 30C below the

boiling point.

8. After 45 minutes rapidly do the following:

a. Record the finish time of the test (this should

be 45 minutes).

b. Record the temperature of the water at end

of test.

c. Remove all wood from the stove and knock

any loose charcoal into the charcoal weighing

pan. Weigh the remaining wood, including

the unused wood from the preweighed

bundle. Record the weight of wood.

d. Weigh the pot with the remaining water.

Record the weight.

e. Extract all remaining charcoal from the stove

and weigh it (including charcoal which was

knocked off the sticks). Record the weight of

pan plus charcoal.

This completes the full water boiling test. The full

test should be done at least three times for each

stove for accurate results.

In Field Water Boiling Test
Design Principles for Wood Burning Cook Stoves

33

It is ok if temperatures vary up and down, but:

1. The tester must try to keep the simmering water as close as possible to 30C below the local

boiling point.

2. The test is invalid if the temperature in the pot drops more than 60

C below the boiling

temperature.

3. The tester should not further split the fuel wood into smaller pieces to try to reduce power.

Analysis of Results

ANALYSIS of RESULTS:

• Figure out the time to boil for cold start, hot

start, and for the boiling phase of the simmer

test.

• Calculate the wood use by subtracting the wood

left at the end of each phase from the starting

weight. Do this for cold start high power, hot

start high power, boiling phase of the simmer

test, and simmering.

• Calculate the water lost to steam for each of the

four phases by subtracting the remaining weight

from the starting weight of the water.

• Do the same for the charcoal produced.

• Use these numbers to evaluate stove

performance. Change the stove design to reduce

wood use and to create less charcoal. Making a

lot of charcoal indicates poor combustion.

• Calculating the steam lost is a valuable method

to check that performance is similar in all

phases. Usually the hot start high power phase

uses substantially less fuel, and time to boil is

faster compared to the cold start high power

phase. If there are significant differences

between the recorded weights for wood use,

time to boil, and steam lost between phase 2 and

3 it is recommended to repeat the testing

procedure being careful to feed the fire without

as much variation.

• Steam lost during the simmering phase is also a

good indicator of the stoves ability to perform

well during low power use. It is difficult to

design a stove that can boil water quickly and

simmer well without using a lot of fuel.

However, since the majority of cooking time

often occurs at low power (simmering), the

greatest fuel savings can be made with a stove

that saves fuel during this time. Producing large

amounts of steam while simmering is an

indicator that the stove is having a difficult time

transitioning from the high power needed to

boil water quickly to the low power needed for

simmering food efficiently. Try changing the

design so that the stove easily maintains a low

simmer while keeping cooks happy with rapid

boiling.

Remember that results from this test cannot be

used to compare stoves tested in other places. The

complete UCB/Shell Foundation test should be

used for those purposes.

For more information, visit Aprovecho’s web site at

www.Aprovecho.net or contact us at:

Aprovecho Research Center

80574 Hazelton Rd.

Cottage Grove, OR 97424

(541) 942-8198
Design Principles for Wood Burning Cook Stoves

34

Data Sheet

Data Sheet
Design Principles for Wood Burning Cook Stoves

35

Calculation Sheet

Time to Boil:

_________ = B – A = Time to boil for cold start hi power phase

_________ = D – C = Time to boil for hot start hi power phase

_________ = F – E = Time to boil for boiling phase of simmering

Wood Use:

_________ = G – H = Wood use for cold start hi power phase

_________ = I – J = Wood use for hot start hi power phase

_________ = K – L = Wood use for boiling phase of simmering

_________ = L – M = Wood use for simmering phase

Water Converted to Steam:

_________ = N – O = Water lost to steam for cold start hi power phase

_________ = P – Q = Water lost to steam for hot start hi power phase

_________ = R – S = Water lost to steam for boiling phase of simmering

_________ = S – T = Water lost to steam during simmering phase

Charcoal Created:

_________ = U – Y = Charcoal made in cold start hi power phase

_________ = V – Y = Charcoal made in hot start hi power phase

_________ = W – V = Charcoal either made or consumed during the simmering phase.

(If this number is positive, then additional charcoal was created during

simmering, and if negative, then charcoal was consumed during the

simmering phase.)

Calculation Sheet
Design Principles for Wood Burning Cook Stoves

36
Design Principles for Wood Burning Cook Stoves

37

Baldosa—Inexpensive ceramic floor tile about one

inch thick that can be cut or molded into

appropriate shapes to make a combustion

chamber.

Boundary layer—The very thin layer of slow

moving air immediately adjacent to a pot

surface; insulates the pot from hot flue gases

and diminishes the amount of heat that enters

the pot.

Charcoal—The black, porous material that

contains mostly carbon that is produced by

burning of wood or other biomass.

Convection—The heat transfer in a gas or liquid

by movement of the air or water.

Combustion chamber—The region of the stove

where the fuel is burned.

Combustion efficiency—The percentage of the

fuel’s heat energy that is released during

combustion. Combustion efficiency refers to

the amount of the energy from the biomass

that is turned into heat energy.

Draft—The movement of air through a stove and

up a chimney.

Emissions—The byproducts from the combustion

process that are discharged into the air.

Excess air—The amount of air used in excess of

the amount for complete combustion.

Firepower—The rate of fuel consumption, usually

in kg-fuel per hour.

Flue Gas—The hot gases that flow from the

combustion chamber and out the chimney (if a

chimney is present).

Fuel efficiency—The percentage of the fuel’s heat

energy that is utilized to heat food or water.

Grate—A framework of bars or mesh used to hold

fuel or food in a stove, furnace, or fireplace.

Haybox—A relatively airtight insulated enclosure

that maintains the temperature of the pot

enabling food to be cooked to completion after

the pot is removed from the stove.

Heat transfer efficiency—The percentage of heat

released from combustion that enters a pot.

High mass stove—A stove made of uninsulated

earth, clay, cast iron, or other heavy material

that requires significant energy to be warmed

during stove operation.

High power—A mode of stove operation where

the objective is to boil water as quickly as

possible; the highest power at which a stove can

operate.

Low power—A mode of stove operation where the

objective is to simmer the water or food

product; the lowest power at which a stove can

operate and still maintain a flame and simmer

food.

Pot skirt—A tube, usually made of sheet steel, that

surrounds a pot creating a narrow space so that

more of the heat in the flue gases enter the pot.

Retained heat—Heat energy that warms the

enclosures around the fire that does not escape

to the surroundings; can be used for space

heating.

Appendix: Glossary of Terms

Appendix

Glossary of Terms
Design Principles for Wood Burning Cook Stoves

38

Vermiculite—A lightweight, cheap, fireproof

material produced from natural mineral

deposits in many parts of the world.

Vermiculite can be made into strong,

lightweight, insulative ceramics with very little

effort. It is very strong and resistant to heat,

and appears to be one of the best possible

choices for making insulative ceramics.

Appendix: Glossary of Terms

Water Boiling Test (WBT)—A test used to

measure the overall performance of a

cookstove. There are several versions of the

water boiling test. In general the test consists of

three phases. These are: (1) bringing water to a

boil from a cold start; (2) bringing water to a

boil when the stove is hot; and, (3) maintaining

the water at simmering temperatures.
Design Principles for Wood Burning Cook Stoves

39
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