A chef cooks at a stove with a flaming wok

Turn Down Ratio: Cooking and Heating Stoves

The successful stove delivers the needed amount of heat to perform a task. In Haiti, our little charcoal stove could not bring the big pots of rice and beans to boil although the thermal efficiency at low power (simmering) was above 40%.

In the same way, a tight, well-insulated house requires a low amount of heat to stay warm. A “leaky” house needs higher firepowers to replace the constant flow of hot air lost through cracks under doors, etc.

Generally, a good stove has a minimum three to one turn down ratio. Heating stove experts have suggested a high power of 5 pounds burned per hour and a low power of 1.5 pounds. If a heating stove cannot turn down sufficiently, the tight, well-insulated house gets too warm. On the other hand, a “leaky’ house needs a big fire.  To save fuel, a tight house is more important than a new stove.

To boil 5 liters of water in less than 25 minutes in an uncovered pot, low mass cook stoves with tight pot skirts typically need a high power between 3kW and 2.5kW. However, firepower is often a lot higher when cooks are trying to get food on the table. In our experience, many cooks prefer ~ 5kW. 

Experiments at ARC have shown that with a low mass stove, lid, tight skirt on a 5 liter pot, it takes only ~0.4 kW to maintain a 97°C simmering temperature. But, cooking requirements vary a lot from country to country. Chinese cookstoves tend to use 10-15kW and may not need low power. 

Village cooks in Southern India cooked with many small pots and often did not bring the water to a full boil. For India, Dr. K. K. Prasad proposed, “…an ideal burner design with the power output ranging from 2.64 kW to 0.44 kW… (Prasad and Sangen, 1983, pp. 108-109). Dr. Baldwin adds, “One of the most important factors determining field performance of a stove is the firepower it is run at during the simmering phase. Because simmering times tend to be long, quite modest increases in firepower above the minimum needed can greatly increase fuel consumption.” (Baldwin, 1987)

With careful operation, the heat exchanger efficiency of houses and pots combined with delivering the appropriate firepower largely determines the fuel used per task. 

red sign with white letters reading Wood Burning Prohibited

Clean Burning of Biomass

red sign with white letters saying Wood Burning Prohibited

Scotland has banned the use of climate polluting home heating systems such as oil and gas boilers, and wood burning stoves (except in cases of need) in new construction. Heating homes creates one fifth of Scotland’s CO2e. The plan is to switch to electric heat pumps, hydrogen and tighter, better-insulated homes in an effort to achieve carbon neutrality by 2045. https://www.bbc.com/news/uk-scotland-68778757

Oil and gas (fossil fuel) burners create too much CO2. Burning renewably harvested biomass can emit close to zero CO2, but old stoves make too much black smoke which is ~2000 times worse for climate change compared to CO2 by weight.

Heating stoves that burn sustainable biomass cleanly enough to protect health and climate are starting to become available. Testing the new generation of stoves in use will show whether biomass can join solar, wind, and hydro as a useful renewable energy resource in the post fossil fuel era.

Maybe those red signs will become green?

Green sign saying Sure, Light Up
Three images - Icy tree limbs, a man with a chainsaw, a car full of logs

Ice Storm 2024!

On January 14, ARC staff experienced a catastrophe when ice brought down about one third of the trees in our valley and we had no electrical power for seven days. Depending on the grid for heat and light is great when everything works. However, when the lights go out, a warm house becomes really important. Light is easy to make with photovoltaics, etc. Living for a week in the cold was a great reminder of the importance of a clean burning non-electric biomass heating stove. 

I lived without electricity on a ranch in Mexico for eight years where it was warm pretty much all of the time. Even so, on chilly nights we lit an open fire and sat around it, talking. The old folks pulled blankets around themselves, too.

I do not remember missing refrigeration or lights. It was not a big deal. At the ranch, people had lived for over a hundred years without them and much more energy went into trying to find juice for the little radio.

Going for a week without electricity reminded me that a warm house is important. In response, ARC is building health/climate non- electric pellet heating stoves as winter turns into spring.  If the creek don’t rise, we want to have these stoves in all buildings before next winter.

PS: I am fine reading a book by candlelight as long as my feet are warm.

Biomass Pellets in the USA: Fuel Switching

Total Plants: 107Metric tons/year:11,188,200
The pellets are used for fuel.

Depending on the size of the home, winter heating with a pellet burning stove uses from 2 to 6 tons of wood pellets per year. If the average house burned 3 tons per year, 3,729,400 homes could be heated with pellets currently manufactured in the USA. There are 142,153,010 residences in the USA. biomassmagazine.com/plants/listplants/pellet/US/

Bill Gates has written that the climate crisis can be solved by developing least cost, renewable technologies to replace fossil fuels. (“How to Avoid a Climate Crisis”, 2021)     

How do fuel costs compare?

Fuel Oil #2       Cost per million BTU = $30.19

Electricity         Cost per million BTU = $35.17

Natural Gas      Cost per million BTU = $15.38

Wood Pellets   Cost per million BTU = $19.15

LPG/Propane   Cost per million BTU = $41.13


Fuel switching from natural gas to renewably harvested wood pellets or split logs or dried wood chips (only if they can be burned cleanly enough to meet the Paris Agreements) seems to include a relatively small Green Premium. Replacing LPG/Propane, electricity, and Fuel Oil #2 with wood pellets seems like a good deal. 

Heat Exchangers for Heating Stoves

Man at chalkboard
When analyzing a system, try to improve the least efficient part first. 

There are three types of heat exchangers generally used to capture the heat produced in a combustion chamber.

The hot flue gases can:

  1. Heat mass, like heavy stone or masonry
  2. Heat water which then warms the house or…
  3. The easiest and least expensive route – make the hot stove gases efficiently heat the air inside the room

In modern houses with limited air exchange rates heating the air has become the popular option. High mass heat exchangers were created in the days of drafty houses when heating air was a losing proposition. Old houses had air exchange rates of more than 10 exchanges per hour. All the air in the house was replaced ten times or more every hour! It didn’t make sense to heat air that would quickly be outdoors.

Heat exchangers increase heat transfer to the room by making sure that the hot flue gases leaving through the chimney are as cool as possible. Even a smoldering fire turns about 90% of the wood into heat. But, heat transfer efficiency (heat delivered to the room) can be less than 20% in poorly designed systems. As the cartoon shows, a little improvement in heat transfer equals impressive increases in fuel efficiency.

Retaining Heat is Part of the Equation

We cook beans (and other long simmering foods) at Aprovecho using a “haybox.” The pot of food is boiled for ten minutes on a stove and then placed in a well-insulated, airtight box. The beans inside the pot get soft and palatable because the retained heat is sufficient to finish cooking them. We end up using a great deal less fuel because the haybox has improved the heat transfer into the pot. (It’s also a much easier cooking method!)

How a haybox works
A Haybox cooks beans by keeping the heat in the pot. When cooking on a stove, the heat needs to be constantly replaced, using more fuel.

The reason that beans are usually simmered over a fire for a couple of hours is because the pot constantly loses heat to room air. The reduced flame underneath the pot replaces the lost heat.

In the same way, a furnace or a wood replaces the heat in our houses because the house allows the heat to constantly leak away. The house loses heat and the burning wood replaces it. If the house loses a lot of heat, we use a lot of wood per season. If the house loses less heat, we can save trees and are better stewards of this precious resource. If the house loses very little heat, the stove is frequently not even lit because energy in sunlight and interior sources of heat are now equal to the heating demand.

Watch Integrated Stove Video

Integrated stove for heating, cooking, electricity

Watch a video of the downdraft rocket stove that ASAT Inc. will show in Washington D.C. at the Alliance for Green Heat/DOE sponsored Wood Stove Design Challenge from Nov 9 to 13. ASAT gratefully acknowledges support from the US Environmental Protection Agency under EPA SBIR contract number EPD18009.

See an introduction to the Integrated Stove on Youtube: https://youtu.be/LUH3LMTG1OM

Long sticks are placed vertically in the combustion chamber where only the tips burn. A weight pushes the wood down as the ends turn into soft charcoal. Cooling fins on the top half of the feed tube help keep only the tips of the sticks burning. The room air is heated by the tall heat exchanger and cooking is possible on top of the cylinder.

The stove features a thermoelectric generator near the coals to create 18 W of electricity when the stove is running at its 10 kW high power setting. The electricity is distributed to two USB ports for high speed cell phone charging and LED lighting. Aluminum fins protrude into the combustion chamber to bring heat to the hot side of the generator, and a radiator on the bottom of the stove draws the heat away from the cold side and into the room.

It has been quite nice to have a warm lab as the temperature outside drops. You will certainly enjoy it during our post ETHOS TLUD summit.