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close this book Boiling Point No. 17 - December 1988
View the document Fault finding and fixing
View the document Stove Problems - Causes &: Solutions
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View the document "A Watched Pot Never Boils"
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"A Watched Pot Never Boils"

But An Observed Fire Leads To Greater Efficiency and Control

By Pete Young - ITDG

One of the best methods of studying burning in a stove under real cooking conditions is to watch the fire develop and observe the flame paths. Most stove users do this instinctively as they cook and adjust the position of the sticks to get more or less heat depending on the cooking process.

When using "improved" stoves with shields for better heat transfer to the pot, users complain that they cannot see the fire without removing the pot. This has led to the introduction of observation slits or wider door openings for better view of the fire.

Regretably, not many stove designers or combustion engineers use direct observation as a way to study combustion efficiency and heat transfer. Since 1980 when the ITDG stove project started its in-depth study of stove combustion it has made such observations by placing pyrex glass dishes on the stoves. Easy and continuous study of fire development and flame patterns was possible with this technique under conditions identical to normal cooking.


The effects of changing fuel types or position; fuel to pot distances, primary or secondary air supply, chimney height etc can all be watched and related to temperature changes in the stove and to fuel consumption. See Fig 1. The best conditions for maximum heat transfer can thus be quickly determined.

A small amount of water should be placed in the dish to avoid cracking and to simulate actual cooking. Soot must be removed from the dish from time to time depending on the efficiency of combustion and under ideal conditions, very little soot will be deposited.


Fires can be observed by this technique in single or multi-pot stoves but it is important to ensure that the Pyrex dish sits in exactly the same position as a cooking pot and has approximately the same size base.

Complete combustion is indicated by bright yellow flames. Duller flames indicate less efficient combustion leading to more smoke and higher carbon monoxide emmissions. The effect of introducing secondary air can also be observed this way and can not normally be seen in any other way.