Intro image for YouTube Video

Watch what happens with PM2.5, CO2, Oxygen and more during a wood burning stove test in this real-time video from Apro’s Laboratory Emissions Monitoring System. The LEMS provides a display of what’s being recorded by the various sensors in the stove being tested, and in the emissions hood. In this video, Dean Still gives an overview of what the five lines on screen represent, and how they relate to each other as the fire progresses.

For more info about Aprovecho’s emissions monitoring systems, see

Rocket Stove 2021 - Pot Skirts

In this video, Dean Still explains why a pot skirt – a sheet of metal wrapped around the cooking pot – is a simple yet important way to improve the fuel efficiency of a rocket stove. He also explains how to calculate the appropriate distance between the skirt and the pot. Stay tuned to the end of the video to find out who is causing all the ruckus in the background…

Helpful references:

simplified diagram of constant cross sectional area
Simplified drawing of the concept of constant cross sectional area.

This is a very simplified illustration of what “constant cross-sectional area” means. The top circle represents the cross-sectional area of a stove riser. The bottom ring shows the same area translated into the space around a pot. It’s important to keep the cross-sectional area that the hot gasses flow through consistent, so they don’t slow down. Hot, fast flowing gasses transfer heat most efficiently. 

graph helps calculate proper skirt gap for best heat transfer efficiency
Chart for calculating channel gaps, from Dr. Samuel Baldwin’s “Biomass Stoves: Engineering Design, Development, and Dissemination.” 1987, Volunteers in Technical Assistance.

This is the chart for determining efficient channel gaps, explained towards the end of the video. It was developed by Dr. Samuel Baldwin in 1987.

Here is the Ten Stove Design Principles poster referred to in the video. Many more helpful documents are also linked on the Publications page.

sticks and charcoal start to combust in a rocket stove

The Jet-Flame was developed from combustion concepts used in fluidized beds and TLUDs.

Fluidized Bed

fluidized bed combustion diagrams

“In its most basic form, fuel particles are suspended in a hot, bubbling fluidity bed of ash and other particulate materials (sand, limestone etc.) through which (under air) jets of air are blown to provide the oxygen required for combustion or gasification. The resultant fast and intimate mixing of gas and solids promotes rapid heat transfer and chemical reactions within the bed.”

Top Lit Up Draft

diagram explaining how a top loaded up draft stove works

The TLUD uses under air flowing up through the fuel to transport wood gas into the hot layer of charcoal and flame above the fuel assisting more complete combustion efficiency.

Cleanly Starting the Jet-Flame

High velocity under air jets blow up into the lit charcoal placed on top of small sticks of wood. When the charcoal and wood are on fire, long pieces of wood are pushed into the made charcoal to start a Rocket Jet-Flame without making visible smoke. The sticks of wood are burned at the same rate as the continual production of charcoal creating a cleaner combustion process related to a fluidized bed and the TLUD.

sticks and charcoal start to combust in a rocket stove

Charcoal over wood is lit.

bed of charcoal in rocket stove

The charcoal becomes superheated with jets blowing up into the pile.

sticks burning in rocket stove

After 30 seconds, long sticks of wood are pushed against the burning charcoal creating flame.

Chart showing how more air exchanges reduces indoor air pollution from cooking
Chart describing the influence of air exchange per hour rates on the concentration of PM2.5 in a 30 cubic meter room. Higher air exchanges equal lower PM2.5 concentrations.
Using the ISO box model, Sam Bentson has calculated how increased ventilation helps a classic Rocket stove (around 30 mg/minute of PM2.5) and a modern TLUD burning pellets (about 5mg/minute PM2.5) to protect health.

In the lab, we are used to thinking of the ISO Tiers as static, based on how much pollution enters a 30 cubic foot kitchen during four hours of cooking with 15 air exchanges per hour. However, in 2018 ISO published 19867-3 that further explains how, for example, increasing the air exchange rate (ACH) changes the Tier rating. Generally, doubling the air exchange rate cuts pollution (PM2.5 and CO) in half.

In a low ventilation situation (10 ACH), Tier 4 requires that the emissions of CO are lower than 2.2 grams per megajoule delivered to the pot (g/MJd). But in a higher ventilation condition (30 ACH) the stove can be three times dirtier, emitting up to 7 g/MJd, and still be in Tier 4. Cooking outside is often employed by the cooks we work with because smoke is bothersome and unhealthy.

ISO 19867-3 reports that studies of air exchange rates have found a lot of variation in ventilation, from 4 ACH in very tight buildings to 100 ACH outside in the fresh air. When I lived on a ranch in Mexico, most of the cooking took place outside under a veranda (which also made it easier to smell the wonderful homemade coffee brewing in the early mornings). When Sam Bentson carefully measured the ventilation rate under our veranda in Oregon he also found that when a gentle breeze was blowing (2 MPH) the air exchange rate per hour was around 100.

At 100 ACH, with so much dilution occurring outside, achieving Tier 4 for PM2.5 and CO is easier. In our experience, the most successful and cost effective interventions are situation dependent. We find that a combination of approaches to protecting health enables a welcome adaptability to the actual and interwoven circumstances.

Thumbnail from Rocket Stove 2020 video about height and weight

Why is a heavy stove an inefficient stove? A tall combustion chamber makes a lot of draft to keep a fire roaring, how can that be a bad thing? What is TARP-V and how will it improve your stove? Dean Still has the answers for you in the latest Rocket Stove 2020 Video.

Here is the Ten Stove Design Principles poster. Many more helpful documents are also linked on the Publications page.

link to Rocket Stove 2020 YouTube video

How can burning wood, agricultural waste or even cow dung be a carbon neutral energy source? How do you start a fire without making a lot of smoke? How can a metal skirt around a cooking pot help with fuel efficiency? Dean Still has the answers for you in this new video.

Find out more about the Jet-Flame combustion accessory used in this video at

YouTube Video explains the importance of mixing for clean combustion

In this video, Dean Still explains why mixing air into flame is important for cleaner combustion. He uses several Rocket Stoves to demonstrate the effects of both natural draft and forced draft secondary air jets. Which style is more effective? Watch to find out!

For a simple way to add mixing to a Rocket Stove, check out the Jet-Flame.

Dean Still explains time and temperature in a Rocket Stove in a YouTube video

Dean Still and Sam Bentson have started collaborating on a series of videos that explain the basics of how Rocket Stoves work, so that stove designers and stove users can get the best performance out of this popular stove design. In this first installment, “Time and Temperature,” Dean explains the importance of high combustion temperature in a Rocket stove where there is limited time to burn up smoke particles. He demonstrates how the Jet-Flame ( helps to increase combustion temperature by blowing air under the fire.

Be sure and subscribe to Sam’s YouTube channel so you never miss an episode! New videos will be added every other week.

Sad cooking pot on a stove
Two cooking pots
Mind the Gap!

Here are the TLUD (Top-Lit Up Draft Stove) derived heat transfer principles that ARC designers use when designing and improving stoves. They are just as important for Rocket stoves as TLUDs:

T: The temperature of the hot gas contacting the pot or griddle should be as hot as possible.

A: Expose as much of the surface area of the pot or griddle to the hot gases as practical.

R: Increasing heat transfer by radiation is important. Move the zone of combustion as close to the surface to be heated without increasing harmful emissions.

P: Optimize the proximity of the hot gases to the pot or griddle by reducing the channel gap without reducing the velocity of the gases. Reduce the thermal resistance with appropriately sized channel gaps under and at the sides of the pot. Match the firepower to the channel gap size and to the size of the pot or griddle.

V: In convective heat transfer, the primary resistance is in the surface boundary layer of very slowly moving gas immediately adjacent to a wall. Increase the velocity of the hot gas as it flows past the pot without reducing the temperature of the gases. As a rule of thumb, heat transfer efficiency can double when the velocity of the hot gases also doubles (N. MacCarty, et al, 2015).

Sam Bentson and David Evitt with the new Jet-Flame
Sam Bentson and David Evitt with the new Jet-Flame
Sam Bentson, ARC Lab Manager, and David Evitt, ASAT COO, developed the Jet-Flame with Shengzhou Stove Manufacturer and Dr. Dan Lieberman and Dr. Mike Barbour at the Gates funded Global Health Labs

The cast iron Jet-Flame sends 30 jets of pre-heated air up into the burning charcoal and wood in an open fire, sand/clay stove, or in a Rocket stove. It is patterned after industrial burners that position jets of primary air underneath the fuel bed to clean up combustion. Both Underfeed Stokers and Fluidized Bed Boilers use primary air that enters the fuel bed from underneath the fire.

In 2013, with DOE funding, ARC built a bottom-air-only prototype stove and has been experimenting with improving the technique, resulting in the Jet-Flame combustion chamber accessory manufactured by SSM in China. There are several advantages in a bottom-air-only approach. The jets of air flow into the fuel bed from holes in the floor of the combustion chamber. Since the pre-heated air flows vertically, back-drafting out of the fuel door in a Rocket type stove is easier to overcome. The jets of air super-heat the charcoal layer underneath the sticks of wood. The hot jets of air emerge from the charcoal and pierce the laminar flames emitted by the wood creating turbulent eddies that stir up the flames to enhance the speed of mixing and combustion. The turbulent combustion zone creates short, intense flames that burn the fuel more completely before they cool off too much to sustain combustion. The increased velocity of the higher temperature flue gases also improves heat transfer efficiency.

Winiarski sunken pot Rocket stove with chimney
Winiarski sunken pot Rocket stove

When the Winiarski sunken pot Rocket stove with chimney is combined with the Jet-Flame the increase in combustion efficiency results in a truly improved stove with the ability to protect health. Since the stove and chimney do not leak in lab tests the stove does not emit fugitive emissions into the kitchen. The stove achieves all ISO 19867 Tier 5 ratings for both thermal efficiency and emissions of CO and PM2.5.

In 2004, ARC was hired by the Shell Foundation to bring the Rocket stove to India. Protecting health was a component of the project. Unfortunately, the natural draft Rocket stove was not clean enough burning to accomplish the task. Higher temperatures and a lot more mixing were needed. We wish that, when asked for a health protecting stove, we had been this far along. It has taken a while to make some progress. 

Test results of the Jet-Flame with a vented Rocket stove.
Test Results of the vented (with chimney) sunken pot Rocket with Jet-Flame