A reader recently asked why the biomeiler has different temperatures in the inner core, outer core, and mantle. I have touched on the topic in past posts, but it seems valuable to address the matter comprehensively.
The biomeiler was built in the form of concentric cylinders. Details are shown in the main image. It has been quite some time since I last used crayons. So, I was pleased to see how well I could stay inside the lines despite having no recent practice.
The inner core has a radius of 18” and is surrounded by a large-mesh fence and a spiral of PEX tubing. The tubing was intended for use in extracting heat by circulating water through the pile. That portion of the chicken coop’s heating system has not been completed. So, the PEX tubing is empty. The yellow wires seen in the image are the thermocouples that measure inner core temperatures.
The outer core has a radius of 3’ and is surrounded by another cylinder of large-mesh fencing and another spiral of PEX tubing. The snow in the pictures is due to the fact that construction for this portion of the biomeiler experiment was completed in November, 2024. The current biomeiler experiment is a modification of the pre-existing pile.
At a radius of about 4’, a corrugated sewer pipe coils around the pile. That pipe carries outdoor air around the mantle of the biomeiler to warm it before it is pulled into the chicken coop. The region between 3’ and 4.5’ in radius is the mantle of the pile.
The remainder of the pile is the crust, which has an outer radius is 6’ and is surrounded by strong pasture fencing. Weed blocking fabric and cardboard line the fence to keep the material inside. In the fall of 2025, each region of the biomeiler was extended upwards by 28″ to reach a final height of 6’4″. A layer of Reflectix insulation now covers the entire surface of the pile.
The inner core, outer core, and mantle were filled with used bedding from the chicken coop, large wood chips, leaves, and thatch from the yards. The material was soaked with water to encourage composting. The crust is composed of wood shavings and sawdust. This material was left dry to improve its value as insulation.
During operation, heat is generated within the inner and outer cores and moves outwards into the mantle and crust. The corrugated ventilation pipe collects heat that would otherwise continue to move past that radius, into the crust, and be lost to the environment. That heat is, instead, carried into the chicken coop to keep the birds warm in the winter. The cold outdoor air chills the material it touches. In consequence, there is little composting taking place in the mantle outside the radius of the ventilation pipe.
The temperature gradient forms because heat is generated in the center of the pile, which is the best insulated region, and conducts outward to regions that are not generating heat and are in contact with the outdoor air. In nature, heat energy always moves in the direction from higher temperatures to lower temperatures.
The crust temperature is always nearly identical to the outdoor air temperature and the inner core has the temperature of the most-protected compost. Therefore, these become the temperature extremes. The outer core is generating heat, but is less insulated. The mantle only generates heat away from the ventilation pipe and its temperature is usually the average of the outer core and crust temperatures.
Wow! There are 4 images and the pie graph. The visuals make it a lot easier to understand, and I thank you.
Good use of using this method for teaching us.