Second Generation Compost Heater

Test results from the second generation compost heater MOTHER EARTH NEWS' researchers designed significantly improved on those for the first.

065 compost heater - wrapped core

The internal core was then wrapped in a series of coils of 1"-diameter black semiflexible plastic tubing, which—in the completed pile—carries the to-be-heated water


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As many of you undoubtedly know, MOTHER EARTH NEWS has for some time now been researching the use of large mounds of (primarily) vegetable matter to produce heat. We first introduced our readers to the concept in the story "The Genius of Jean Pain," which described the pioneering work done in France by M. Pain ... a noted energy experimenter and one of Europe's foremost organic gardeners.

Inspired by what we'd seen and heard while visiting Jean, we soon decided to begin doing a little compost-heat experimentation on our own. It wasn't long before MOTHER EARTH NEWS' researchers had a prototype heap "up and running." That initial pile was completed on December 14, 1979 and by the 24th of that same month had reached a core temperature of 120°F.

Our experimenters were somewhat disappointed to note, however, that the heap never got any hotter than 120°F (although Jean Pain's mounds were reportedly able to achieve temperatures of 140°F). Furthermore, MOTHER EARTH NEWS' "decomposition oven" held its peak heat for only 5 1/2 weeks before a long cold snap caused the internal temperature to drop slowly. The French researcher's compost piles, on the other hand, are said to be able to maintain their higher temperatures for as long as nine months!

The discrepancies between M. Pain's results and ours, we decided, could be attributed to several factors. For one thing, the Gallic gardener uses massive 50- to 100-ton heaps, while our initial experimental pile probably weighed no more than five tons. In addition to that, we were sure that the exceptionally cold—for North Carolina—winter temperatures (below freezing, night and day, for over a week) played a part ... as, perhaps, did our compost "recipe." (It seems that Jean Pain uses very thin wood chips, and allows the brush's own foliage to provide the compost's "starter." We, on the other hand, worked with larger chunks of wood ... combined in a four-to-one ratio with manure.)

Try, Try Again

 Spurred on by what was certainly a successful—if not overwhelmingly so—initial test, MOTHER EARTH NEWS' researchers soon set to work putting together a second experimental pile which would, we hoped, produce results more nearly equal to those that M. Pain has been able to achieve. Our first thought was to obtain a shredder which could reduce raw underbrush to the 1/16" slivers that the Frenchman uses for his compost. Unfortunately, reasonably priced chippers capable of such fine work aren't available in this country (at least we couldn't find any), and the cost of importing a machine as massive as those of M. Pain's design was prohibitive.

So, being unable to duplicate Jean's heating structures, we decided to modify our own previous design. To do so, MOTHER EARTH NEWS' researchers first built an inner enclosure of chicken wire that was six feet wide by eight feet tall. The cage was then filled with a 3-to-1 wood-chip/manure mixture (as opposed to the 4-to-1 mix that made up our first heap) and wrapped with coils of 1" black semi-flexible plastic pipe.

In order to increase the heap's ability to maintain and store a supply of hot water, we positioned a 30-gallon water heater tank immediately outside of the inner core, and—around both the central cylinder and the tank—added a second layer of compost three feet thick. Thus the entire cylinder-within-a-cylinder pile measures 8' X 12' ... and weighs in at a hefty nine tons.

How It Works

As "outside" water flows into the pile, it travels through the series of coils encircling the hot inner cylinder, taking on warmth as it goes. The liquid then enters the water heater tank (which is, of course, also surrounded by a "working" mixture of decomposing wood chips and manure). From there, the hot water can be drawn off as needed.

Construction Details

The to-be-composted material in our latest experimental heater, as we mentioned above, consists of three parts wood chips (we made no attempt to shred them further because we felt that, with our mixture augmented with animal droppings, such energy-intensive extra preparation was not necessary) to one part manure. Each layer of compost was thoroughly watered as it was put in place. Then, in order to inhibit evaporation and give the decomposition process a solar "boost," the entire structure was wrapped in black plastic. (The whole job took MOTHER EARTH NEWS' staffers—using a tractor-mounted front-end loader—just slightly over 20 hours to complete.)

Promising Performance

The second-generation compost heater required only a week to raise its internal temperature from 60°F all the way to the 140°F mark (which, of course, is the high claimed by Jean Pain for his compost "ovens").

Furthermore, with the external nighttime lows fluctuating from 46 to 66°F, MOTHER EARTH NEWS' flameless "woodstove" has since maintained its peak internal temperature ... never, in fact, varying by so much as a single degree during the month that it's been up and working.

Practical Applications and Future Plans

Certainly, a period of slightly over four weeks is far too short a trial to either prove or disprove the wood-chip heater's reliability and practicality. But the early results are promising, and—before long—we may be able to demonstrate conclusively that a person can actually "power" a closed-loop heating system (to warm his or her home, workshop, or greenhouse) with free-for-the-gathering raw materials ... and produce valuable soil builders, rather than toxic waste, in the process!

We plan to monitor the performance of our heater throughout this summer. If (as we both hope and expect) the compost pile is still going strong when cold weather hits, we'll encircle the outer cylinder with a second series of coils of black tubing and then add another three-foot layer of compost to enlarge the pile's storage capacity and to provide enough insulation to allow the material to keep up its full rate of decomposition throughout the winter!

There are, of course, a number of other directions that our compost heat experiments can—and will, as we gradually learn the tricks of this trade—take. We have yet, for example, to test the practicality of using foliage (as does M. Pain) in place of the manure.

Furthermore, should we be able to locate a reasonably priced shredder that can reduce the chips to the Frenchman's preferred 1/16"-thick strips (or if we're able to design such a device on our own), we'll be able to explore a whole new realm of possibilities. At any rate, you can be sure that MOTHER EARTH NEWS will stay on top of this "new" heating technique ... that's based upon a phenomenon as old as life itself!

EDITOR'S NOTE: An English translation is available of the 88-page book Ida Pain wrote about the late Jean Pain's techniques, Another Kind of Garden: The Methods of Jean Pain.

michael lyman
11/15/2012 7:07:39 PM

a.I disagree that the end product is the same. When you burn something, most of the carbons are released into the air as various carbon componds, thus they are lost and are contributing to air pollution. This does not occur in composting. b. I haven't removed the compost yet as this is the first heater I've built, but I am anticipating it to be a fair amount of work, done by hand, back, shovel and the back of my truck. I built the fencing in pieces to make it easier to remove with the material still in it and am hoping this makes it easier to get the layers off. c. I did the majority of the work constructing the pile by myself, I had help from one person for the first third of the construction. In my view, the use of the tractor consumes far less in regards to resources as gathering firewood does, when taking into consideration the fuel spent getting a permit and making multiple trips up the hill to find and buck the logs. It took 3 loads in my dads dumping trailer totaling about 40 miles round trip. This is an experiment through and through. I will still be burning wood, just not as often I hope.

t brandt
11/15/2012 10:50:09 AM

a) Oxidation is oxidation. Long chain carbon molecules are turned to co2. Fire is rapid; bacterial metaboism is slow, but the end product is the same. The heat is stored as energy in the chemical bonds of the fuel. It's only the rate of release that differs. (b) Perhaps I should have phrased the question like this: The spent compost looks like it would be very difficult to evacuate thru all the fencing and coiled tubes. How do you do it? (c) The article clearly states that the system was built by a team of workers using a tractor.

michael lyman
11/15/2012 3:39:10 AM

Well, first of all, they aren't really the same process. Burning is a chemical reaction and composting is a biological process. Also, I have used firewood as my only source of heat for the last ten years and I can honestly say building this pile was much easier, and didn't require a "team of workers". As for the 5 tons of "spent nuclear fuel", we do lots of gardening here and I plan on using it for compost. I also live in an area where compost is a viable commodity and I can always sell it for more than it cost me, which would be all profit since I acquired the material for free. The what do you do with question seems especially odd to me, with a little imagination I'd think most people could come up with something.

michael lyman
11/15/2012 3:31:40 AM

I have an old radiator I got for free to extract the heat. I'm planning on using a fan to pull some of the heat off of the radiator and circulate it throughout the room. I may put a second radiator in another room to utilize more of the heat before it returns to the pile. I'd love to hear about the research you've done! I'll take a pic as soon as I get a chance, probably this weekend. I'm in Oregon.

t brandt
11/13/2012 11:15:15 AM

Burning and rotting are really the same process. Only the time course is different. Either way, the fuel is eventually consumed. What do you do with your 5 tons of "spent nuclear fuel?" It seems that a team of workers could chop & pile a heckuvalot of fire wood in 20 hours of work with no need of a tractor, then use really simple technology (a fire place) to harness the contained energy at appropriate times.

patrick jeanquart
11/12/2012 1:28:47 PM

Hey Michael, How will you be extracting the heat you pick up from the compost pile through your 1/2 inch pex tubing to heat your house? Some type of hot water radiator set up?. If you are interested I can share with you some of the research I've come across.

patrick jeanquart
11/11/2012 11:19:53 PM

I am thinking of doing the same thing with brushwood compost. Your results seem VERY promising. Could you supply a picture? Also what state are you located in?

michael lyman
11/11/2012 1:10:05 AM

Alright, I just hooked up the pump and she's off and running! The core temp of the pile was 100 degrees and the water was 45 at startup. It's been about 3 hours and after a initial drop of six degrees, the pile is back up to a core of 100 and the water is already up to 85!!! I'll continue to post the heaters progress throughout the winter. P.S. Thanks for all your help Dad!!!

michael lyman
10/16/2012 7:00:57 PM

Well if anyone is reading this, I've got it built. It's 9' by 7' by 6' high. I'm using kiln dried wood chips and hemlock/fir barkdust mixed. I poured a bottle of septic tank bacteria on it to help jumpstart the process. It's been going for 3 days and I'm getting about 70 degrees out of it. I used 500 feet of 1/2 inch pex on the inside of it and am going to circulate with a 10gpm pump. I'll update in a couple weeks

michael lyman
8/26/2012 3:15:37 AM

I am about to construct one of these to heat my 40'x40' home! If all goes well, I should be able to all but abandon my woodstove!!! I'm very excite to get this project going.