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go4haudio 04-20-11 03:49 PM

Compost as a Heat Source
Compost as heat source?
I am starting this thread on the suggestion of A/C Hacker whose thread "Homemade Heatpump Manifesto" inspired me. A well composted mass often reaches temperatures of 150-180 degrees Fahrenheit. With careful planning this heat can be generated and used efficiently even in a suburban setting. With unrest in the middle east, competition from China for energy, and a lack of renewable energy infrastructure energy prices are bound to rise making "compost heat" more attractive. Over the next few months I intend to discuss collecting compost from a variety of sources, composting methods, collecting and using compost heat, using by products of the composting process, and the advantages and disadvantages of compost compared to other fuel sources. I invite bloggers to participate in sharing knowledge and experience. For so long we have been dependent on cheap fossil fuels that we have lost our connection to the green energy that mother earth provides. Therefore few people are aware of the potential energy in compost and even fewer utilize it. I hope together we can make our world a more harmonious and sustainable system.

Piwoslaw 04-21-11 12:02 AM

I once read about someone who made a compost bin against one of the house's walls, thereby helping to heat that wall.
I also think I remember reading about someone using compost to preheat water.
But no links for either:(

EDIT: I just saw where you are. I guess that you can compost for the whole year round. In many places composting happens only during the warmer months, when heating is not needed.

EDIT#2: The more I look at my city's problems with waste management (i.e. wasting trash, instead of reusing it), the more I'd like to start up a large composting set-up. This would reduce the city's wastestream by 30%-40% and give quite a bit of good soil, plus any extra energy (heat + methane) would make the project more feasible. With the composting happening inside a building, the heat would be able to keep it running year round, with a surplus in the summer.

Daox 04-21-11 09:09 AM

Here is some info on a guy who made a fairly large pile. I read it a while ago, don't remember if there were any specifics.

Compost Space Heating Furnace

It looks like I spent about $840 on gas last year.

strider3700 04-21-11 11:08 AM

I always liked this guys setup.

I use about $260 to heat water and about $300 for space heating.

skyl4rk 04-21-11 05:00 PM

YouTube - 500 showers heated from one small compost pile how to tutorial

Angmaar 04-21-11 06:03 PM

There's another example of compost as a heat source here.

EDIT (by Piwoslaw): The forum's software automatically censors the link's address, so to view the page, copy the following lines and paste as one line:

go4haudio 04-23-11 03:06 PM

I am really grateful and for the feedback on this topic. I am also somewhat surprised at how much information is already available on this topic. Compost depends on the action of bacteria. The digestion of organic matter releases heat and CO2. Bacteria need food, water, and air. There are many methods to provide this culture to the bacteria. Simpler methods are most suitable to agrarian lifestyles. One such method can be found here Another Kind of Garden.pdf - File Shared from - Free Online File Storage starting on page 26 I believe Jean Pain outlines methods for using large mounds of compost as a heat source. Here are some video links of his efforts YouTube - Jean Pain - English - Part 1 for part one and YouTube - Jean Pain - English - Part 2 for part two. These methods are simple to implement with access to material and plenty of labor. However these methods are impractical for the average home.

go4haudio 04-23-11 04:30 PM

A more practical approach for the average home must be used. Large scale compost mounds provide plenty of food and water for the bacteria but do a poor job aerating the culture medium. A home scale composter would require effort periodically but its heat output would be much faster with less wasted heat radiating needlessly. The effort to compost would be far less frequent than the effort used in stoking a wood stove. Here is one example High-Performance HeatGreen Home Heating System Version 3a you might enjoy reading. Here is located a picture of his device. One weak link in this device seems to be that it regulates over temperature conditions with increased airflow. I quote "Take this last example, of the maximum performance which I have seen in an HG 3a, around 90,000 Btu/hr. It turns out that with the MINIMUM NEEDED AIRFLOWS we calculated above, the air inside the chamber would get FAR TOO HOT, and you would kill the bacteria in yet another way. Nothing to really worry about, because few people seem to get to that point yet! And installing a thermostat inside the chamber to turn on the blower switch at 150F, will keep you from frying your bacteria!" He goes on to postulate that a heat exchanger to regulate the incoming air temperature would be needed in these conditions. A heat exchanger is needed I admit to increase overall heat retention. However my proposed method to regulate heat excesses involves a heatpump, perhaps even a homemade heatpump such as can be found in this Ecorenevator website The entire process could be monitored and controlled by an arduino type computer device to be most self sufficient. The heat pump would draw heat from a large capacity insulated water tank that is hydronically connected to the composter. Regulating the heat out put is achieved by two things. First the computer chip can regulate the airflow to the bacteria. Second the choice of composting material can regulate the rate of digestion. For instance fresh grass clippings digest far more quickly than wood chips. Full implementation would be complicated and would not be cheap. But once constructed one would merely feed and water ones bacteria and less frequently clean out a little top quality humus. For many people a less complicated set up would be more appropriate. I will out line that in my next blog. Either approach would benefit from a "Bioshed." The Bioshed would be a shed in which The composter, the raw materials, a water tank, and all the necessary plumbing could be housed protected from the environment.

skyl4rk 04-23-11 07:29 PM

In the city where I live, in the Fall, everyone rakes the leaves from the yard, and the city takes away huge volumes of leaves. This is a free resource. Until now I just thought of the piles of free compost from past years leaf collections, but now you have me thinking about using the heat generated from composting leaves through the winter.

go4haudio 04-23-11 09:29 PM

An elegant solution
I like the way you are thinking skyl4rk! The nice thing about fall leaves is they usually are dried up and store pretty well. You might benefit from shredding the leaves so they take up less space. But you can store them dry for a long time without fear they will compost on their own. You might even try posting flyers around your neighborhood requesting bagged leaves. Or you might use craigslist to request people who live near a part of your town contact you if they want leaves picked up. You would need lots of bags of leaves but they are easy to get and easy to store dry. Neighbors wont complain about bags of leaves piling up if you teach them about your system. They will probably want to contribute. Every one wants to be green these days. I will be talking about collecting and storing fuel for compost really soon.
Computer control is great. A far less complicated set up is more in line with most peoples needs. The least complicated compost heat generator is a large compost pile like the one in the video above. It consists of alternate layers of compost and plastic tubing to capture the heat. But this will not suffice for extreme climates in the dead of winter. An insulated box must be constructed to prevent freezing and control airflow and heat exchange. In its simplest form one must create a box with an insulated floor, walls, and ceiling with provisions for air tubes water tubes, and electrical connections. Your bioshed can be placed indoors or outdoors because the air will come from outside and be expelled outside. However indoor designs must be built "air tight" because if your box is relying on fans to circulate air and you should experience a power outage your compost will begin to decompose anaerobically. Methane gas is produced anaerobically and the process smells bad. If your box is well sealed any gas pressure will exit through the vents rather than into your living space.
So let me start describing the process from the ground up. Insulate the floor. How much insulation will depend on your climate but more is always better. The insulation must be capable of maintaining a temperature between 125 degrees Fahrenheit and 180 degrees Fahrenheit. This heat will be generated by the compost itself. Actually less than 180 degrees is probably better. I read a book which described the " U.C. Berkeley Method" of soil sterilization for commercial growers. It describes 180 degrees as the temp to sterilize most pathogens. I am hoping to find this book or any source that can give me better information on temperature ranges for composting bacteria. So lets set the upper limit at 170 degrees Fahrenheit for now.
It occurs to me people want specifics and pictures for any DIY project. I will be outlining my plans for this fall's compost project. The process of composting is as much an art as it is a science. So if you need a quick proven answer too bad for you. But you can join with me as I plan and build my system and watch as I share my results. So how big to build the floor? Well I have a space on the side of my house near my electric water heater ( in Florida an outdoor water heater is not uncommon) and near an outdoor electrical receptacle. That seems good. Also that wall butts up to the bathroom which is always cold anyhow in winter. So I intend to measure this space and begin designing from there.
Some key points of my design will be a system of perforated tubes to allow for ventilation of the compost, a tube in tube heat exchanger that allows for water to distill back into the compost instead of just spewing out warm moist air, hydronic heat coils, various temp sensors, and monitoring plus motor controls located centrally inside for my own comfort.
There is only one problem with this line of thinking. Perhaps you have already spotted it. A design like this one High-Performance HeatGreen Home Heating System Version 3a is already very simple and effective. The fact that it can aerate the compost and turn it makes it far superior to any design that simply insulates a mound. Insulating a mound requires a far larger working mass of compost, requires more insulation, requires more pipes, etc. There is no good middle ground that I can think of. You must either have a giant mound big enough to insulate itself from the cold and steep enough (or covered) to slough off snow and ice or you need an elegant digestion box. I have few concerns with the design I see here. It may actually be too big and produce more heat than is needed depending on what is being composted. It needs a heat exchanger for the incoming and exhaust air. Also it could benefit from buffer tanks to even out the heat exchange instead of increasing airflow.
I can't think of a more elegant solution than a good digester box. Can you? SO I will focus on adapting a digester to fit my needs for this fall. I had to point this out because I fear too many people will try to make compost mounds heat their homes and give up after achieving less than spectacular results, if any. I have shown you my thought process in its naked form. I hope I have not bored you away from this forum topic. My next blog will be about parameters to measure and control in a digeter box system.

go4haudio 04-23-11 10:06 PM

18 months Hot Water in Portland, Oregon
This site claims to have had 18 months continuous hot water from this system. It might be appropriate for some people. I still think a good digester is better. Composting greenhouse provides hot water (original) - Appropedia: The sustainability wiki

skyl4rk 04-24-11 10:49 AM

I spent a few hours reading the site: High-Performance HeatGreen Home Heating System Version 3a and am quite impressed by the thought behind this system. Unfortunately there is no synopsis that explains the system in a few paragraphs. I will try to do so here:

Composting Bioreactor

The HG 3a composting system is a bioreactor designed to maintain composting at temperatures of 125F to 150F, which is the range of thermophiiic bacteria. The bioreactor must be held within that temperature range, requiring cooling to remove heat. A small amount of air must be provided to supply oxygen to the bacteria. One of the outputs of the bioreactor is heat, which can be removed by air cooling and water cooling. The reactor also produces carbon dioxide and water vapor.


The purpose for building a reactor is typically for home heating or greenhouse heating using waste materials. The HG 3a bioreactor has a PVC coil water loop, which can be used for hot water hydronic heating, or potable hot tap water use. There is also an air cooling blower which outputs hot air. Apparently there is little to no odor in the exhaust air, although there is CO2 and water vapor as part of this exhaust air. Other uses for the bioreactor could include water distillation and cooking.

Bioreactor Vessel

The HG 3a system is designed to fit through a doorway, the main part is 2 feet wide by 5 feet high. It is a cylindrical tank made of plywood and designed to be rotated about a 3/4 turn (back and forth) approximately every hour. The tank is insulated with construction foam. Its function is somewhat similar to a front loading washing machine that you would typically see at a laundromat. The vessel is made watertight by lining it with a polytarp. There is an access door and ports for air intake and exhaust, as well as water connections for the loop of PVC tubing inside the tank.


The unit is fed with organic matter, which can be leaves, lawn clippings, chipped wood debris, agricultural waste (corncobs, straw) or agricultural products (hay, corn). The material must be kept dry until added to the bioreactor. Water is added to moisten the material. The amount of heat produced depends on the amount of material added, the fineness of the material, the condition of the bacteria, and the type of material. Once the bacteria are active and the bioreactor is at operating temperature, material is added daily or every few days to maintain the bacteria life cycle. There does not appear to be much waste material produced by the bioreactor, removal of waste material is not mentioned, although it is likely that the vessel must be cleared of waste at least once a season. It is unclear whether the bioreactor can be located outdoors. Examples shown appear to be in a garage or workshop location.

Bacteria Requirements

In order to promote bacterial digestion of the material, the bacteria require the proper temperature, oxygen, moisture and the food source material.

Temperature Control

Maintaining the proper temperature appears to be the most critical issue, because the thermophilic bacteria die when outside of their temperature range. Overcooling of the system by blowing cooling air through the system too long is one way to kill the bacteria. Another way to kill the thermophilic bacteria is to allow the system to overheat above about 170F. Water cooling using the PVC water loop would be another way to cool the vessel, which is not discussed in the article.


Generally the air cooling system provides enough oxygen for the bacteria. Tumbling the digested material by turning the tank hourly assist in getting oxygen to the bacteria.


Water is added initially to the vessel to bring the material to "damp sponge" consistency. It is unclear whether additional water is required with each addition of material, or if water produced as a byproduct of digestion maintains the appropriate moisture level.

Food Source

It appears that the food source can be any compostable material. This would indicate a need for a balance of carbon and nitrogen, often discussed as a mixture of "green and brown". A discussion on composting principles is available here:

Fundamentals of Composting: Why Compost

From the discussion, it would appear that a home could be heated using autumn leaves and grass clippings. The author states that 1 acre could provide enough material to heat a home. The material would need to be dried to preserve it through the winter, which could be done using methods similar to agricultural practices used for hay and straw. For example, cut grass when the weather is predicted to be dry, let it lay on the ground in the sun for a day or two, collect it when dry. It may be convenient to press the dry material into blocks to aid in filling the bioreactor and to preserve the material. I do not have any data to back this up, but the amount of feed material required for one winter might be an uncompressed amount of leaves and grass clippings the volume of one or two mini-vans(?).


A composting bioreactor should be able to heat a home with a homeowner effort similar to a woodstove. Only minor additional energy inputs (blower) are required to operate the bioreactor. In wooded areas, high volumes of autumn leaves as a feed material are available at no cost and are currently collected as waste. Automation of the bioreactor should be fairly simple with an Arduino type controller. The vessel could be constructed for about $200 as shown in the article, or $400 with automation.

AC_Hacker 04-25-11 11:26 PM


Originally Posted by skyl4rk (Post 13163)
...Unfortunately there is no synopsis that explains the system in a few paragraphs. I will try to do so here...

Quite a good synopsis, I'd say.


skyl4rk 04-26-11 11:49 AM

Something interesting...

When using large amounts of leaves to compost, you need to add a material with nitrogen to get the optimum food source for the bacteria. One way to do this would be to add manure.

Composting toilets could better be called Dessicating toilets because they are designed not to compost so much as to limit odors by drying the materials. A basic design principle for composting toilets is urine separation from solids. A simple composting toilet is a pail with a strategically placed funnel to drain urine away while collecting solids. The solids are covered with peat moss or similar dry material, which helps dry them out.

The dry products of a composting toilet would be an advantageous addition to a composter using mainly dry leaves. Since the bioreactor is designed to run hot, it would likely kill pathogens more quickly than a normal manure pile. So you could solve the problem of sewage disposal, heat your home and create a fertilizing compost product for the lawn. Low impact permaculture.

Here are some interesting links I came across...

XACT BioReactor: Composting System | Cuff Farm Services

Earth Tub - Green Mountain Technologies


I don't think any of these bioreactors are as good of a design as the HG 3a system.

strider3700 04-26-11 03:23 PM

I've been gardening for 6 years now and have had a compost pile since the beginning. Not once have I ever got it hot enough to make a difference heating water. the best I've managed to do is get it to feel warm to the touch. Hot compost piles are not trivial to make.

skyl4rk 04-28-11 05:05 PM

Do you think that this bioreactor is a feasible design?

strider3700 04-29-11 05:51 PM

I've seen backyard scale rotating composters for sale now and again. the people always swear that they work great but most quote 2-3 weeks not 4-7 days. I'm happy with my free pallets with a pile in the middle so it hasn't been a big issue.

skyl4rk 04-29-11 08:34 PM

I talked with the city manager, he said I could have as many leaves as I wanted for free. They use a vacuum truck with a container 24 feet by 8 feet, and 6 feet high. The leaves are run through a big squirrel cage blower, thats what vacuums them up from the street. So they are cut up a bit. I looked at the piles from last year, they seemed to be warm and wet, but it rained a lot yesterday.

With an unlimited supply of leaves, I could just do the loops of pvc pipe in a pile of leaves. However, I don't have a lot of room in my yard. I have room next to my garage for maybe one truck load of leaves, and could fence it in so no one would see it.

I'm not sure if I am going to do this as a project yet, but the bioreactor compost bin looks pretty interesting and would be fun to see if I could get it to work.

NeilBlanchard 05-04-11 07:13 AM

Sewer pipes are a great heat source:

IamIan 10-18-20 06:45 AM

Basic videos of my first 3 experimental steps.

Eventually , the goal would be to work up to a supplemental home heat source. Ideally fueled by free to me waste products I already get. Mostly high carbon things like leaves in the fall , junk mail , cereal boxes , etc .. Also ideally with nearly the same input effort from me , just effort applied differently.

Steps 1 & 2 Video
#1 = 4 gallon container in basement (~0.02yr^3)
#2 = 6 containers ~40 gallons each in basement (6 @ ~0.2yr^3)

#3 = Data results how #2 did over time.

#4 = will be roughly ~3yr^3 bin outside .. ~5ft dia ~4ft tall

I think I'll have just enough 'fuel' accumulated .. to fill/fuel step #4 by combining what is left from #2 , a full years worth of paper goods , and the leaves should fall soon here the end of October also.

From the research I've done on this subject .. It seems common , to be able to get up in the 5,000 BTU/hr to 10,000 btu/hr range for 3-5 months , usually requires about a 30 to 40 cubic yard compost pile (6000 to 8000 gallons) , with good aeration , moisture , etc .. I don't know if I'll ever go that big , but well see what the data says along my journey.

Robl 10-30-20 03:37 AM

I’ve looked with interest at the heatgreen compost system 3a. It does look very impressive - I do wish there was an open forum available at its site, for others to discuss builds and issues. It seems as though this aspect is holding back larger scale adoption. I have not built one, but have considered it!

I also notice that there is a simpler smaller 4a device now shown on the same site. It avoids building a watertight inner bag, and rotating the whole system. Instead, it uses a wheelie bin, and a large corkscrew mechanism for stirring. Similar levels of insulation are used for both - 4inches of polystyrene. Below is a link to the 4a page:

It’s not clear to me, but I believe he places the 4a in a basement, and directly uses the hot air From the device as a heatsource for his house.

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