We've been experimenting this year with hot composting, and are now looking at how we can go about using the heat generated in the pile to heat water, either to use directly or to pump around as secondary heating.

My problem is that the hot composting method relies on turning the heap every couple of days, and only gets to a high core temperature for a few weeks before composting is finished. This is a problem if our compost heap is full of water pipes.

Is it possible to maintain a high core temperature in a compost heap so that you can use it long-term for heating water?

  • 3
    If you extract very much heat to heat water, you soon won't have a hot compost heap. – Chris H Dec 22 '15 at 10:01
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    Please let us know your results if you try this. – That Idiot Jan 5 '16 at 19:26
up vote 9 down vote accepted

Consider using a flow through core of some sort. For example:

  • Wrap several layers of chicken wire around about 4 to 8 feet of 24" diameter cardboard concrete forms from the hardware store (Depending on how tall you want to make it).
  • Wrap several layers of black poly tubing around the chicken wire and cardboard tube. Use baling wire to tie the black poly tubing to the chicken wire tube in bundles of coils so that the core maintains integrity once the cardboard form composts away.
  • Arrange some clay blocks or rammed earth so that you can stand the "flow through core" tube on end but still be able to get in underneath it with a shovel.
  • Build a tower of straw bales around the core to insulate it and create a retaining wall for compost to be held against the outside of the core. You will want a couple of feet of compost around the core so make the inside diameter of the retaining wall 5 - 6 feet wide. The thicker you build your wall the faster the more efficient your composting and hot water generation will be.
  • Make sure to leave a hole in the bottom of the wall where you want the shovel access. It is also desirable to fashion a plug for the shovel hole to keep vermin out and to further insulate the compost.
  • Fill up the core one quarter of the way with already composted material, making sure to get a good shovel full of the center of an active compost pile as an inoculum. Then fill up the rest of the core with material to be composted.
  • Fill in between the straw bales and the flow through core about a quarter of the way with already composted material, then add a compost inoculum from the center of an active compost pile. Then fill in the rest of the way with material to compost.
  • Put plywood over the top of the straw bale tower and cut bucket sized holes into it so that you can pour in scraps to the center of the flow through core and edges of the outside of the compost pit.
  • Cover the plywood (and holes) with straw bales in such a way that a few bales can be moved to access the fill holes.
  • Shovel fresh compost out of the bottom to make room for adding new material to the top of the pile.

Note, the straw bales will eventually start to decompose themselves. So the tower would have to be rebuilt periodically, but the same core could be reused until the chicken wire rusts away. Having built similar straw towers just for speeding up normal compost piles I would guess that you might be able to use the same straw bales for at least a couple of years before replacing them.

For everyone to know, the process was developed by and is now known by the name of Jean Pain Composting process. The man used the technique to heat water for house heating and hot water needs.

[...]raw materials of Pain's compost heap were saplings, branches, and underbrush

Question also addressed on our prefered network ;) Would a Jean Pain heater work well for heating in a USDA Zone 4A.

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