Modified Justa Stove

Summary

Fig 1: Metal insert rocket box style combustion chamber of the traditional Justa Stove.

The Modified Justa Stove is a version of the Justa stove. This particular version is being disseminated by Foro Para el Desarollo Sostentable Stove Dissemination Program located in San Cristobal de las Casas Chiapas. The Foro stove team refers to this stove as the Justa Mejorada which in Spanish means the "Improved Justa." This page is titled "Modified Justa" because calling this stove an "Improved Justa" may be misleading given that there have been no laboratory or field tests done on this stove. The main modification that characterizes this Modified Justa from the original Justa is in the design of the combustion chamber. The combustion chamber of the Justa has a classical rocket box shape, made of metal or tile, that has been proven in laboratories to lead to highly efficient transference of energy from the biomass fuel to the cooking surface. In comparison to this the Modified Justa has a larger combustion chamber that is built with bricks. There is no rocket box.

Fig 2: Example of the Modified Justa Stove combustion chamber. Note the bricks in the back that act as baffles to funnel the hot gases towards the cooking surface and exit.

Examples of Modified Justa Stoves in and around San Cristobal de las Casas

Location

Found in Chiapas State of southern Mexico

Organizations using this stove

Foro Para el Desarollo Sostentable Stove Dissemination Program

Building specifications

Materials

Material	Dimensions	Quantity
bricks	6 x 12 x 24 cm	60
mortar mix	-	1-2 buckets
plancha	standard commercial size	1
chimney pipe	10 cm diameter	as many as necessary to direct smoke out of the house

Step by step

This first step is to build a base that can handle the weight of the stove and is at the optimal height for the stove user.
An example of the proper arrangement of bricks that will work.
The combustion chamber aperture dimension are extremely important for the overall functioning of the stove. Foro Para el Desarollo Sostentable Stove Dissemination Program specifies that the stove recipients can opt for aperture dimension of between 15 and 20 cm in width and between 10 and 15 cm in height. Users usually prefer the biggest opening possible in order to have the ability to fit larger pieces of wood in.
The exit aperture from the combustion chamber to the chimney chamber is approximately 6 cm in height and 12 cm in width.
Bricks are used to build up a ramp from the combustion chamber floor to the bottom of the chimney entrance.
The bricks are then covered and filled in with mortar in order to create a ramp that directs the gases up towards the cooking surface and out the chimney exit.
The plancha is installed. The outer edge of the plancha is not sealed with mortar in order to allow for flex without cracking the stove surface.
The chimney is installed either through a prefabricated whole in the back of the plancha. Many times the chimney is not inserted into a prefabricated whole in the plancha. In these cases the plancha stops at the back of the combustion chamber and the chimney chamber is covered and sealed with bricks and mortar.