Compost Heat Recovery

Alacrates's picture

I'm writing a series of blog posts on compost heat recovery, that is, using compost to heat a greenhouse, a living space, a workshop, etc.

The new post is about a small working model I put together of a "compost water heater", and just some general thoughts on taking on little projects regarding appropriate technology. I had posted the first one to the old forum, it introduced the subject, and looked at an early pioneer in the technology, the French organic farmer, Jean Pain. I'll post links to both post below.

My main source for this is the book The Compost Powered Water Heater by Gaelen Brown. It describes a number of systems, and focuses on the type of system that was worked out by the Compost Power Network, out of Vermont. Basically, they spiral water piping throughout large compost piles, made mainly of shredded woodchips, with sawdust and manure added. The water is pumped through this coil, and then have been run to structures like greenhouses, concrete slabs, radiators, etc. to heat them.

The book also outlines more complex systems, that have so far been successful in small-to-mid-sized agricultural businesses, especially dairy farms. They draw air down through hot compost that is indoors, move it through channels in the floor into heat exchangers to transfer the heat, then filter the air through soil bio-filters. These systems can be a way for a farm to compost animal manure, reduce bedding costs, generate heat and biogas, and to filter wastes.

I don't like to leave people with so much reading before they post, so if anyone wants to share their ideas on compost heat recovery without reading the blogs, please feel free by all means. If anyone would like to read more, the posts are there. There are also a lot of good videos on youtube.

Compost Water Heater Model:

Compost Water Heater: An Early System:

David Trammel's picture

That's great work you did on your prototype. I am surprised that your apartment manager didn't get on you for your composting pile in the parking garage, lol. I'm about to do a few blog posts on Systems Thinking and will use your experiment as an example once I'm finished.

Keep up the good work, seeing what you did makes me want to get down in the workshop and do a project myself.

Alacrates's picture

Thanks! Yes, haha, it is a very large parkade, I have a corner spot way in the lower level, so I thought I could put my little bin by my car, and whatever small odors would dissipate very quickly. My plumbing van is parked on the street, so I don't go down their very often. When I did go down there, I was shocked by how powerfully it had filled up the entire area with ammonia-type smell, and more so that no one thought to check on it's source! Anyway, all in the name of amateur science experiments.

David Trammel's picture

Do you have an idea how long a typical pile of compost generates heat?

That would go along way to telling if this system would be worth the effort and cost. For me and my proposed green house there is just going to be the compost generated by myself and my sister. That's food scraps and compostables from the house, which i hope to keep at a minimum and then yard waste such as grass clippings. I could probably get a bunch of grass clippings from the guy she hires to mow her lawn beside what he gets from hers. I also want to start using a worm bin in my own kitchen and that would cut out most of the in house compostables.

The green house I have in mind will be small, perhaps 5'x30', and will have a South face and a thick concrete wall painted black as a solar heat source, which should help keep it warm. I'm trying to figure out how I can rig a temperature control on the vents in the green house roof, so when it gets too warm, it vents the heat into the tiny house itself, or perhaps the attic to help lower heating costs.

Sweet Tatorman's picture

> I'm trying to figure out how I can rig a temperature control on the vents in the green house roof, so when it gets too warm, it vents the heat into the tiny house itself, or perhaps the attic to help lower heating costs<
Vented air from a greenhouse generally has high %RH. Venting into a heated dwelling is an excellent use of it as Wintertime heated spaces tend to have low humidity. Venting the same high %RH air into an unheated attic space is asking for problems from the resulting condensation in the attic space.
As far as the actual mechanics of venting, my solution on a smaller scale in the cold frame I use for sweetpotato slip production is to use a DC fan powered by a solar panel controlled by a simple fixed setpoint bimetallic thermostat switch. I have been using this setup for years with zero problems. When there is the highest solar insolation and thus highest need for ventilation you also have the highest available power from the solar panel.

Alacrates's picture

First, I should just say that I've never set one of these up myself (yet), and have only read about them & watched videos, but from what I've seen, everything is fairly consistent from project to project, making me think the principles involved are fairly reliable.

Jean Pain, who lived in the warm climates of southern France, made very large piles of only shredded brushwood, and got 18 months of domestic hot water from them, as well as bio-gas he produced from from tanks he kept warm at the centre of the pile.

The Compost Power Network of Vermont (who put out the book) has been experimenting with these in the colder climates of North America. They make 40 cubic yard piles of mostly shredded brushwood, with up to 30% sawdust, and 10% manure. (Adding nitrogen with the manure causes the pile to get hotter, allowing it to keep hot in colder weather, but it reduces the lifespan of a pile drastically) At these hotter temperatures, and they claim to get 110-130F water flowing from late October to early April.

So really, these could make sense for someone (or a group of people) who were living with plenty of trees/brushwood around them to use -- although, I've noticed in my city, tree removal companies will give people woodchips for free. (Only problem there is you have to have some idea if there is rot-resistant wood in the chips that are on offer, those don't break down at high temperatures.)

One thing I find exiting about compost powered water heaters is that they kind of pay for the "fire smarting" of wooded areas, which with forest fires becoming an increasing issue (I think) is a good value. The calculations in the book suggest that you can derive as much heat from a compost water heat system as simply burning them (perhaps due to the fact the compost microbes are generating heat themselves over a long period of time.) Plus, one ends up with finished compost instead of ash.

That said though, I still don't think I would rely on compost heat recovery unless I had obtained consistent results from them over several years in a row. I think it would be best for someone interested to start with it as an auxiliary heat source, and see what kind of results they can produce. It definitely would require whoever was attempting one to tinker a fair bit with it, ensuring that the pile stayed hot in cold temperatures and that the heat was being transferred well.

On the other hand, I don't want to make it sound too daunting, here are two fun videos I like to show people to get them interested.

A student group setting up compost hot water under their greenhouse beds:

Compost powered hot-tub in a Vermont winter:

Sweet Tatorman's picture

I recall reading a paper many years ago on sweetpotato slip production by that earlier generation green wizard, George Washington Carver. Fairly warm soil temperatures are required for slip production. Dr Carver's scheme involved digging a trench several feet deep and filling part way with active compost. A layer of straw on top of the compost and then sand or other media for the sweetpotatoes and finally more straw for insulation.

Alacrates's picture

Interesting example. The book I was reading had a little forward regarding some historical examples of using compost heat. It said that in ancient China they would use a layer of manure (and I'm guessing straw) below seedbeds in the spring to warm up the soil. They also said in some parts of Europe there was a practice of heaping composting manure up against the home to use some of the heat. (That method was abandoned happily with the advent of more abundant fuel sources, for obvious reasons!)

lathechuck's picture

... for a garden plot (probably covered by a low greenhouse) gently warmed through the winter with decomposing buried manure. It's a cousin to the "cold frame", which is unheated (except by the sun), but protects the plants inside from wind, sleet, and snow with its transparent roof (which need be no higher than the tallest plant). (As I've written elsewhere on the site, many cold-hardy plants survive freezing, but are damaged by movement while frozen.)

I'm currently experimenting for the first winter with a small Brussels sprout plant protected by a 3-gal water bottle, with the bottom cut out. Think of a bell jar, but the bottle cap is off for ventilation.

(follow-up edit) My Brussels sprout plant survived the winter just fine, but bolted (bloomed and set seed) as soon as the weather warmed up. I didn't get any sprouts to eat. However, a tiny BS plant that I hadn't even noticed looks like it's going to perform, with sprouts about the size of a pea (in July) on a stalk about 2' tall (so far). The plant has had green and striped caterpillars (which I picked off) and a lot of white flies (which I couldn't).

lathechuck's picture

The author of claims to have heated a small house near Chicago for 13 years, on composting grass-clippings. He shows how to build a tumbling compost vessel, well-insulated, that produces up to 10,000 BTU/day. By carefully managing the air supply, he claims that the decomposition is thermophilic (>125F) and aerobic (very little odor). There are actually at least two designs at this web site. One uses a large wheeled trash bin ("toter") with a large auger down the center to mix the compost, and the other is a waterproofed plywood drum that rotates to tumble the compost.
If you're going to grow, cut, and bag lawn grass anyway, this turns out to be a free fuel, but of course you need storage space from summer when it grows to winter when it's composted. Part of the process, though, is to cut the grass, allow it to dry, and then bag it. Bagged fresh clippings get anaerobically hot and smelly in a couple of days!

Since the composter is located inside the house, 100% of the heat produced is contained, without circulating air or water.

I'm intrigued, and thinking about a trial run with a small (but well-insulated) plastic bucket which once held cat-litter. A suppose that I could scale it up by stacking 2-3 buckets.

A UK company,, has a small-scale aerobic thermophilic composting system without scavenging the heat.

lathechuck's picture

Whether it's an aerated vat of composting solids suspended in water, or a damp pile of composting solids, it occurs to me that it's not that much different from a stationary animal in the house! It's going to consume carbohydrates and proteins (in the form of dried grasses and/or wood chips), and produce CO2, H2O, and N2 (hopefully not CH4, H2S, or NH3, but those are possible). So, if I feed the composter one pound of dry matter per day, that's a standard ration for an adult human, so I'll get as much heat from it as if I have an extra resident in the house, which I seem to remember is about like having a 100W (incandescent) light bulb burning continuously. One pound a day doesn't seem like it's going to make much difference.

On the other hand, if I need to store fuel for the composter during the summer and fall to keep warm in the winter, let's say that my heating season runs about four months (depending on how warm my clothing is), or 120 days, or I need 120 lbs of fuel in storage. I have an ample supply of plastic bags that used to hold mulch, 2 cu. ft. each, which hold 5-10 lbs of dry plant matter. (They were a lot heavier when they were full of moist mulch, of course.) Fresh grass clippings about about 60-70% water, so I need to dry them before I bag them. Fallen leaves probably about the same. So, I'll need a couple dozen of these bags just to add the thermal output of one more person, for a season.

Whimsically, it raises the question: how much of my own body is performing exothermic composting as the food I eat passes through? How much of my "metabolism" is "me", and how much is my gut microbiome?