Compost Water Heater: Tabletop Model

Around issues like climate change or economic collapses, I've been aiming to balance out my reading and media-intake. I'm moving away somewhat from general overviews of the predicaments our society finds itself tangled within, to focus more on specific technologies and practices that seem to hold some promise towards making ourselves more resilient to these predicaments, or even being regenerative to the ecological, economic, and social relationships on which we rely.

The idea was to find a series of these technologies, simple or complex, and dial-in on each in detail, one by one, learning about how they work, and how they could possibly be put into practice. And aside from only reading about them I'm trying to get in the habit of finding some way to work with the principles & ideas involved in some sort of physical way, even if it is only making a model or a drawing.

I think that can be valuable in a few different ways. It definitely keeps me thinking about a technology/practice for longer than I probably would've been just reading about it. Trying to implement aspects of an idea in the real world brings up questions that I might have otherwise passed over, sending me back to the books with new questions in mind. As with so many endeavors, the more energy you invest into a subject, from a variety of angles, the more inherently interesting & meaningful it becomes to you.

Not only that, but I think engaging in a physical project sends a signal deeper into our minds that we are serious about an idea: somehow it roots our intentions and shifting thoughts into our daily, physical life. The object itself, sitting around in your living space keeps you coming back to the idea, even if you leave off for a number of months. Objects can become display pieces and act as social beacons to start up discussions with others, maybe sparking an idea in the mind of just the right person.

Materials collected up for a solar oven

A series of these types of projects can initiate a sort of scavenger hunt and leave it running as a background process in your life. The contents of dumpsters, thrift stores, and your parents' garage become much more interesting, as possible sources of cheap supplies to investigate ecotechnic principles.

I think this might be the greatest benefit: building resourcefulness into ones habits, in a way I've always admired in people with a crafty, handy or artistic bent. In times of increasing scarcity, precarity and austerity, I think resourcefulness will be the quality most often called on, as, in Warren Johnson's phrase, we muddle towards frugality.

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Best for this are the skills that can be employed as useful parts of our lives, right now: backyard (or front-yard) gardens and clotheslines in the summer, food storage techniques, bicycle repair, sewing and mending, that sort of thing. They can reduce waste right away, increase our skillfulness, and serve as examples, puncturing the optical veneer of consumerism, populating our cities with hints of some simpler ways of living.

Other technologies can be harder to get involved with, especially if one has limited funds or a lack of land or structures to experiment on. Some are entirely out of reach: too expensive, too difficult, too large scale, not currently practical. But there are often ways to break down a resilient technology into aspects that allow one to get at least a little experience in it. For example, a person interested in natural building, once they've done some reading and watched through some videos, might try out making some small batches of natural plasters, maybe they could be worked into a display piece, or maybe take on a small project like a dog house.

These are changing times, and many people who are concerned with the environment and the economy can feel like they are caught between between two worlds, with full commitments within the status quo, and at the same time feeling a pressure and a pull towards a different way of living. I don't see the tension abating any time soon. But, for those who aren't well positioned to make a dramatic leap towards resilience, I think we can engage some of our free time in imagining and experimenting with specifics of the world we'd like to see. At the very least it might bring some seeds of ideas forwards into times when they'll be needed.

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Reading up on compost water heaters, I thought there was no way to make a project for the subject. These are fairly large projects: even the smaller ones outlined in this book require 40 cubic yards of shredded brushwood + sawdust + manure to compost, with 300ft of pipe coiled within the pile to collect heat, connected to some sort of means to radiate the heat indoors, whether in a concrete floor, the soil of a greenhouse bed, or a cast iron radiator.

It dawned on me that it might be possible to make a working model. It might capture peoples' imagination if they could see a garbage bin of compost heating up small slab of concrete, especially if they could feel the warmth of the concrete with their own hands, maybe there would be available little aquarium pumps that could circulate the water through the model. This could simplify these systems down to some basic components for people attending a presentation, so they could see a whole system at a glance in small scale.

Here are the pieces of the model and some of the real world analogues:


Heat Collecting Coil: For the model, I wrapped the tubing around a wire cylinder, actually for storing coffee K-cup pods, from the dollar store. In the larger systems, they coil 3/4" - 1" polyethylene or PEX pipe within a large compost pile.








Pump: The heat transfer medium, in this case water, has to move through the hot compost, to the area that is to be heated and back again. I found this little pump that I think is for creating a small fountain in an aquarium.

In the larger system, Brown recommends a 1/8 HP circulator pump, which are very common, used often in hot water heating sytems and for hot water recirculating lines. These pumps can also include flow restrictors, so that one can set how fast the water is moving through the tubing, which is helpful, as too much cool water being cycled back in to the compost pile too quickly can stress the compost microbes.

1/8 HP Circulator Pump

Hot Water Storage Tank: I was a little unsure of why water tanks were added to these systems. The book mentions that they are useful for hot washing water. I wondered too if maybe they simply added more water to the system, and increased the amount of heat energy that was held within the building at any given time?

To me that sets up two other problems. First, if you're going to be drawing water from the system, then you have to set it up so that it fills automatically from another point, and adding too much cool water too quickly can shock the microbes in the compost pile.

Second, if you are going to be spraying the water, unless the water maintains at (I believe) a temperature around 60C/140F, you can risk Legionella bacteria growing in the tank and becoming air-borne when you use the water, which can cause a deadly type of pneumonia. Compost temperatures can get to 140F and well beyond, but from what I read they also drop to 120F quite often.

I included it in the model anyway, as I think it does increase the amount of heat stored in the system, and it was an easy place to include a thermometer to measure the temperature of the water coming out from the pump.


Concrete Slab: Once the compost pile heats up the water and the water is transferred, you need some way of radiating the heat into the space you've chosen. There are a range of options. Some people have water run into old fashioned cast iron radiators. Others have used it to preheat the water going to their hot water tank. The most common seems to be to run it through PEX pipe that is specifically made to radiate out heat, and to either run this radiant PEX through a concrete slab, or directly through the soil of a greenhouse bed. There are also ways to run it between the joists of a floor.

For the model I choose a concrete slab. Like in a regular radiant slab, I lined the underside of the slab with insulation, so that the heat will radiate upwards. I made a little wire grid to tie the tubing to, then filled the box with cement. I included some powdered biochar into the cement mix, just to include the idea of sequestering some carbon into building materials like concrete. I embedded a thermometer in the concrete to measure the heat transferred into the slab.

Compost Pile: I thought the model would have the most visceral impact if people could feel the warm concrete slab, sitting right beside a container of hot compost. I tried at first a container inside a ceramic flower, but it wasn't a large enough mass of compost to really heat up, so I took an old garbage bin, drilled in air holes and made a inner cylinder out of chicken wire, so that the compost could stay aerated from all sides. I layered in straw to make interstitial air pockets throughout.

It was winter, so I had a problem of how to keep the compost from freezing, so that I could see if it would heat to adequate temperatures. I wheeled the bin down into a corner of the parkade beneath my apartment building, and zip-tied the lid down so that security couldn't check its contents.

It did get hot pretty quickly, getting up to 130F/55C in a few days. The stem of the thermometer was hot to the touch when it was pulled from the bin. It did however smell horribly & powerfully filled up a section of the parkade, I'm very surprised I didn't get in trouble for that! They say that compost, once it is oxygenated and thermophilic bacteria is generating heat, is fairly odourless, but to that should be added the caveat that it be in the open air. I've read since that in the early phases of a pile, at lot of nitrogen gases (ammonia, etc) is pushed out of the mass.

I wheeled it in the middle of the night to the service elevator and back out to my balcony, to freeze until spring. If I ever set this up for demonstration, I think it would probably better to get a bucket of water to drop the heating coil into, adding an immersion circulator to keep some water at a constant temperature common in a compost pile, 120F or so.

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Those are the basic components of a compost-powered water heater system: the compost itself, pipe coiled within it, a pump, a flow-restricting device, thermometers at various points, and something to radiate heat within the space you want to remain warm.

For the next post on this topic, I'd like to look at another one of the early innovative projects in compost heat recovery, the experimental compost-heated greenhouse set up by the New Alchemists in Massachusetts in the early 1980s.

It had an obvious advantage over Jean Pain systems for colder climates, in that the compost was kept indoors, within the structure that was to be heated, so that all that heat from the compost pile was kept from being directly dissipated into the open air. However, this adds the problem of keeping all the gasses & odours produced by compost within the building as well.

The solutions they worked out, including air instead of water as the heat transfer fluid, have been perfected in some contemporary projects that seem like they could have a place within the appropriate technology framework & help increase the resilience of moderate-sized agricultural operations, especially those that keep livestock. They produce heat that can be used in a variety of ways, reduce bedding costs, process wastes, and return nutrients into the soil.

Two New Alchemists scamper across a geodesic dome.