Using compost for forced-air room heating

Has anyone made a forced-air heating system by running heating duct through ta compost pile? I've read lots of articles on HW heating but nothing on hot air aside from a brief mention of Jean Pain in the 70's running an experiment with an unheated shed.

My neighbour here in near- coastal British Columbia has a detached work studio about 25' x 40' above her barn which she heats with a pellet stove when she wants to use it, but it burns a lot of pellets.  She also has llamas and alpacas and therefore compost material from the old hay and poop. I'm thinking a dedicated compost bin of suitable size could provide 24/7 warm air flow into the studio, tempered by a damper installed on the inlet air line, which would also introduce fresh air into the room.

If it was designed as a closed system, with the warmed room air being returned to the compost pile for reheating, it would also reduce the heating load on the pile, rather than continually introducing cold outside air.  All ductwork would have to be well insulated outside the box and an appropriate circulation fan installed. Potential corrosion of the metal duct in the bin would have to be considered as well, either by using stainless steel duct, heavy spiral duct. or CPVC.

HW radiators or flat panels could be used but the studio is already "furnished" for use and the installation of rads etc. would be awkward and use up a fair bit of floor real estate. A single-inlet hot air system (essentially like the pellet stove) would be relatively simple to install and could be done without permanent alteration to the building. If it worked, an insulated plenum under the floor could be built to allow distribution to floor ducts, which would improve heat distribution.

As long as the compost gasses can't get into the system, it seems like it should work.  Maybe have the fan pushing the air towards the compost so that the pipe is positively pressured when going through the compost to avoid any chance of gasses getting into the room.

Mike Haasl wrote:As long as the compost gasses can't get into the system, it seems like it should work.  Maybe have the fan pushing the air towards the compost so that the pipe is positively pressured when going through the compost to avoid any chance of gasses getting into the room.

Thanks for the quick reply :) Yes, sealing the in-box pipes is important- I would probably use silicone as any kind of tape alone would likely be unreliable. I believe 3M makes a high-temp. aluminum tape but it is very pricey.  

As to the fan, I yes, was thinking of putting it on the exhaust air line, so it would be pushing the air towards the compost bin. Sizing the fan would involve a bit of trial and error (or actual HVAC knowledge, which I don't have) as I estimate the total length of the closed-loop duct would be about 80-90', depending on how many loops are in the compost box, plus perhaps a dozen elbows, all of which add to the static pressure in the system.  I'd think I'd want about 120-150 cfm inlet into the room. A two-tube in-box duct line (coming in at one end, with a 180º bend and a return pipe out the same end) would be good if it could create enough warm  air initially. Once the room begins to warm up, the return air would be warmer and require less boost.

Fortunately this is a project for next summer - if my neighbour lets me go ahead with it - so there is time to ask questions and do some experimenting.  I clear out her barn every couple of weeks with my small excavator, so in the spring I could create a separate compost pile, keep it covered to reduce excessive rain getting in and run a loop of duct through it with a fan at one end to get some idea of the heat transfer.

I have little experience with a compost pile.....but I've worked a lot with sawdust piles, and the heat they generate is notable,, it always seemed to me that a thirty by four by four foot box suspended high enough that a bobcat or a scraper blade could get under it with a couple dozen 4" pipes ran through it would make adequate heat, let it run a couple of years then dump it and wash the old out with a pressure washer and fill it again....

It may be beneficial to insulate the box, sawdust piles are rarely hot until your a foot or so below the surface (to much airflow to encourage decomposition??)

I'm glad you said 80-90 feet, that suggests to me that you have a decent feel for the amount of surface area of pipe that needs to be touching the compost.  Would the black corrugated drain tile (non perforated) be an option?  I think you want a lot of turbulence in the air stream to increase heat transfer.

There are some calculators that have been used to calculate GHAT systems for geothermal air transfer for greenhouses.  I think SunnyJohn was one that isn't available anymore but searching for that and GHAT could get you connected up to a calculator.

Mike Haasl wrote:I'm glad you said 80-90 feet, that suggests to me that you have a decent feel for the amount of surface area of pipe that needs to be touching the compost.  Would the black corrugated drain tile (non perforated) be an option?  I think you want a lot of turbulence in the air stream to increase heat transfer.

There are some calculators that have been used to calculate GHAT systems for geothermal air transfer for greenhouses.  I think SunnyJohn was one that isn't available anymore but searching for that and GHAT could get you connected up to a calculator.

I'll take a look online for those GHAT calculators- thanks.  I hadn't thought of (or in fact heard of) black corrugated drain tile.  I'm familiar with what we up here call "Big-O" corrugated (ribbed) plastic drain line, available from 3-6" diameter, but it's not "tile"...  Not sure about its heat tolerance. Agreed about maximizing air-duct contact area/turbilence. I wonder if the heavier, commercial "spiral duct" would do? Easier to support inside the box as well, and I think it will need some rigid support to allow the compost to be dumped in and/or removed.  I can weld up struts for this.

All ducting outside the box would be well insulated. I'm thinking the box would be about 8' x 6' x 6' high, built next to the studio and the air lines run up the wall to the second floor where the studio is (barn & storage is below.) Knauf makes pre-slit semi-rigid fibreglass pipe insulation with a jacket to fit various pipe dlameters, and with insulation thicknesses from 1" to 3". Something like that would be used on the run up to the covered deck, where I'd build an  insulated box along the inside edge of the deck to the inlet point, lined with R-14 Roxul.

@ Bill Haynes: I'm not familiar with sawdust compost piles, but this one will definitely use the available hay + poop. I understand that such compost piles need to be aerated (or the contents turned regularly) and protected from too much rain, so the initial idea is to provide an air space underneath and make the sides of the box of slatted/spaced 1 x 4 or similar to allow some air in. The lid would be light to allow it to be lifted, and also 3 or 4" above the sides to keep rain out while allowing some air in.  Hopefully this would allow the interior of the pile to heat up to 130º -140º, but prevent the contents from going anaerobic at higher temps. As I recall, Jean Pain used a sawdust pile in his hot-air experiment, and the interior of the little shed got up to 51ºC (124ºF).

Mike Haas wrote: I think you want a lot of turbulence in the air stream to increase heat transfer...

Hmmm... i wonder if some sort of vane system at the inlet to the box could help create more turbulence? Something like a very low-angle fan blade or length of twisted sheet metal (like a twisted longitudinal turbine blade)? Or even a lightweight passive fan blade?  I believe any bends will also contribute.  Both would increase static pressure/resistance in the air column a little but this could be accommodated by using a variable-speed fan.

For those who might be interested, I just rediscovered the reference to Jean Pain heating a shed with hot air, which you can read about here: https://www.permaculturenews.org/2011/12/15/the-jean-pain-way/
Scroll down about 1/3 of the way to:

Directly heating air with a compost pile is also possible as Jean Pain shows. By burying a 125mm [5"] air duct in a 50 ton pile, a 12m3, uninsulated forest shed can maintain a constant temperature of 52°C for 8 months. This system used a thermo siphon effect, the hot air coming in at the ceiling of the shed, and the cool air falling to the floor and exiting through a pipe there.

It would appear to be an experimental setup only, although I don't have the book so don't know for sure if he ever went further with it. It's a very small shed, but getting it to 50ºC (125ºF) for 8 months isn't shabby.

Colin Miller wrote: It would appear to be an experimental setup only, although I don't have the book so don't know for sure if he ever went further with it. It's a very small shed, but getting it to 50ºC (125ºF) for 8 months isn't shabby.

I studied scientific experiments on forced aeration of manure. According to the temperature graphs, active blowing with the required amount of air allows the mass to heat up from 10ºC (50ºF) to 60ºC (140ºF) in just two hours. If air treatment continues, then in about 10 to 12 hours, humidity decreases from 75% to 58%, and biochemical reactions stop. I am confident that with additional moisture, thermophilic microbes could produce significantly more heat. Nonetheless, even with this data, calculations show that 1 cubic meter (35 cubic feet) of compost can generate approximately 20 kilowatts of heat over 10 hours. If we consider a pile with a mass of 50 tons (110,000 pounds) and spread the time over 2000 hours (three coldest months), the heat output would be around 5 kilowatts per hour. The size of the house that can be heated with this amount of heat naturally depends on the region.