Compost Water Heater

Here is a report on MOTHER EARTH NEWS' third attempt at building and using a compost water heater.

| July/August 1981

It's been a while—almost a year, in fact—since we last reported on the progress of our ongoing experiments in obtaining usable heat from compost. (Our efforts, as most of you probably recall, were initially inspired by the pioneering work of the French organic gardener and biotechnologist, Jean Pain.) However, our recent silence on the subject doesn't mean that MOTHER EARTH NEWS' research staffers have been idle. On the contrary, they've been hard at work over the past months measuring the heat-generating capacity of our second compost water heater (for details of that mound's design and construction, see "Second Generation Compost Heater"), as well as testing various other modifications on the design of M. Pain's original shredded-brush piles.

MOTHER EARTH NEWS' second compost heap surpassed its predecessor by registering an inner temperature of 140°F for two months and then leveling off again at a plateau of 130°F, which it sustained for another two months before dropping any further. The success of that particular heatmaking cylinder convinced us that we could produce temperatures as high as those claimed by M. Pain for his experimental mounds. Later, when the next pile we built actually registered a high of 155°F for a good portion of its 4 1/2-month life span, we knew we were onto something that was potentially very important!

So, Larry Hollar—one of our horticulturists and our staff compost-heating expert—decided to undertake an even more ambitious project. In order to tap the heat produced in compost piles for home use, Larry decided to hook up our newest heap to a closed-loop circulation system and provide hot water for one of the yurts at the MOTHER EARTH NEWS Eco-Village. His goal is to demonstrate that a decaying compost pile can indeed be a practical source of abundant energy for individuals.

Simpler Construction

In order to avoid the effort that was spent in building chicken-wire-and-bamboo enclosures to support the earlier piles (which were all cylindrical), Larry settled on a lower, rectangular-shaped heap with sloping sides. Such a configuration, he reasoned, would have the advantage of not requiring a wire-and-wood cage to hold it together and could be constructed quite easily with common garden hand tools. (While preparing all his previous humus-heating experiments, Larry had needed to use a cumbersome tractor-mounted loader to shape the tall, cylindrical piles.)

To make the heat-producing mound, our intrepid composter first put down a base layer consisting of about two and one-half feet of "the recipe" mix of three parts wood chips to one part manure. Unlike the previous mounds we've erected, this one makes use of dehydrated cow manure; the nearby dairy where we've obtained fresh droppings in the past now processes the material before passing it on to us. (Our researcher suggests that a potential compost-heap builder use whatever form of manure is most readily available in his or her area and then water the structure thoroughly—at one-foot intervals—as the layers are piled up.)

On top of this foundation, Larry placed two flat coils of one-inch black polyethylene tubing. (Each ring is composed of 100 feet of hose piled in four-foot-diameter loops.) The coils were then connected to separate lengths of half-inch tubing, which carry water into and out of the pile's interior. In addition, three thermocouples were attached to the assembly: One of the heat-sensing devices was placed inside each tubing coil, to measure the temperature of both the incoming and outgoing water, while the third heat detector was positioned between the coils to record the interior temperature of the compost pile itself.

6/30/2018 4:26:32 PM

Is there a preferred wood chip to use? Hard wood verses soft?

2/12/2016 10:28:50 AM

What a great article! I hope Mr Mayes' regularly tests his water for Legionella. It sounds like the warm water would be the idea breeding ground for it. I'd hate for him to lose all his sheep!

9/19/2015 11:13:44 AM

Hi I live near the coast and collect vraic (seaweed) to feed the garden soil, when left in a heap it generates a massive amount of heat - I have not measured the temperatured or timed the heat drop off but am aiming to construct a closed system for heating, similar to what you descibe - can I ask what you do when the heat does drop off - do you add more to the pile or do you dismantle and start again with fresh manure? many thanks

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