Introduction: Building a Forced Air Composting System
These build instructions assume you already have a static compost pile that you’re trying to renovate or already have an understanding of composting or understand how to build a static compost pile. There are a ton of very good resources on why aerating your compost is beneficial and they’re complete with graphs and studies from Universities. What I found was that none of them had actual plans on how to build one.
In very basic terms the microbes that decompose the waste feed on oxygen, and they need a lot of it. Tumbling composters are good but studies have shown that the oxygen levels decrease within minutes of the action stopping. Spinning the tumbler once or twice a day does help to make compost decompose faster, but most people aren’t consistent in the frequency require to spin the bin for the weeks or even months required to finish compost. If you search for “compost aerators” you’ll find tools, which look like dirt corkscrews, but honestly if you’re not going to spin a tub twice a day, every day for a month and a half, using a dirt corkscrew for the same time frame is not realistic.
Also, Tumblers are also too small for large home composting, or small farms or businesses. A single horse can produce 50 pounds of waste a day. A tumbler composter may be able to only hold a single day or two of manure. So if it takes says 2 months for a tumbler to fully compost when turned every day. That would be 40-60 tumblers per horse, turned every day. Don't get me wrong, I love tumblers, I have one for kitchen waste, but for horse/chicken manure, grass clippings, and brush clean up it all goes in to the aerated pile.
Forced air systems allow people to compost who would otherwise have to throw the waste away, or simply make their existing systems more efficient and maximize the available space as a well set up forced air composting system can reduce the time to finished compost to 3 – 5 weeks. Yes, you read that correctly, less than or about a month.
If forced air systems are so great why doesn’t everybody have one ? Simply put, they’re unneeded for the average household, and for those who could benefit from an aerated composting system, those systems usually are too expensive (Custom 3 bin aerated system holding 400 cubic feet starting at $5,000) and/or are too large or overboard for what is needed; i.e. a small family hobby farm doesn’t need the smallest industrial composting system, when the smallest system is $100,000 and 5,000 cubic feet of compostable waste. This is a scalable middle ground between a backyard worm bin and a $4,000 custom set up or $100,000 industrial system.
Step 1: Supplies
• PVC piping
• Appropriately size PVC pipe fittings
• PVC pipe primer and cement.
• Electric Leaf blower
• Outdoor extension cord
• Plug-in light timer
• Flexible Rubber Coupling with Hose Clamps - 3" (rough diameter of the blower) x 2" (PVC pipe size)
• Drill & drill bits
• Hand trowel or shovel
• Compost Thermometer
Depending on your set up:
• White wire shelf
• Landscaping fabric
Most of the tools are things an average home would have on hand and the rest of the supplies should cost less than $40 to buy; $70 if you need to buy a cheap, basic $30 leaf blower.
Step 2: Design & Setup
Again these instructions assume you already understand composting and building a compost building. For this project I recommend that your compost bin to be no smaller than 27 cubic feet, or roughly 3 feet wide X 3 feet deep X 3 feet tall and that you have two of these bins that are undercover or can be covered securely with a tarp to control moisture levels.
The set up I am working with up is a 7 foot wide X 15 feet deep X 5 foot tall with a 12 foot roof roughly the same width and depth as the average parking spot . By dividing this space in half, this will allow for a two bin system , each bin roughly 3.5 feet wide X 4 feet tall X 12 feet deep, with a rough capacity of 144 cubic feet per bin, which is generally just enough space for a months’ worth of horse manure for 2 large horses or 3 small-medium horses.
Once you have determined the permanent location and dimensions of the bin, map out the best and most efficient location for pipes to be laid. When filled all of the compost should be within 18 inches of a pipe.
For me this was simple, just single, straight pipe the length of the bin. As the bin was 3 1/2 feet wide, the single pipe down the middle, would be perfect.
Heated air will naturally rise so incorporate vertical chimney pipes help to aerate the upper layers of the pile and help to distribute heat more evenly through the pile. The chimney pipes will also help to direct moisture down deep to the center of the pile instead of lingering in the upper, outer layers.
When designing the system, try to keep the lengths of pipe the minimum distance you can. If your blower is 35 feet from the compost pile the force of the air will not have adequate pressure to push the air though the compost properly.
Design your system so that the pipes are cyclical or capped to help keep the air pressure up and the air directed properly.
Determine the size of PVC pipe you want to use. For small bins, less than 50 cubic feet in volume I recommend using 1 1/2 inch PVC pipe but anything larger I recommend 2” PVC pipe. I am using 2” PVC pipe in my setup.
For this build I used my leaf blower that I bought for $30 at Home Depot. It’s a no name brand that you can probably find for even cheaper on eBay. This blower is great for 18’ – 24’ feet of pipe. If you use more than 24’ feet of PVC pipe you’re going to need a blower with more power. I would suggest looking in to using a hot tub air blower.
Step 3: Setting Up the Blower
Prepare the site by either digging a trench for the pipe from the inside of the compost to the outside of the bin, or if you have a cement floor, create a hole in the wall big enough for your PVC pipe to fit through.
Remove the nozzle of the leaf blower leaving only the base. Place the base of the leaf blower where you would ideally like the unit to live.
IMPORTANT : Because we are dealing with electricity outside, specifically very close to an area where we will be hosing down with water regularly you will want to make sure the blower is dry and safe from rain and pooling puddles and out from underfoot where people or animals won’t disturb it. Also you want to ensure that the air intake fan of the blower has free and unblock access to air.
For most electric leaf blowers the air intake is located on the bottom. The area my blower unit is located was also covered so I expanded the trench I dug for the pipe to form a small pit. You can also create a water proof well ventilated housing out of a large rubber storage container and a bit of ingenuity.
For my bin I’m using a small, leftover, common, white wire shelf and laid it over the pit, and placed the blower base on top of it. The pit allows for both constant, uninterrupted airflow but also directs any unaccounted for moisture away from the unit. The pit is 18 inches wide and long and 6 inches deep with 2 inches of pea gravel.
Once you have determined the placement of the blower, measure the distance from the blowers exhaust valve (the hole where the air comes out) to the midpoint of your first chimney or bend. Don’t forget to subtract the amount of overlay your pipe has with the connector or your pipe and connector will be off.
My examples in the picture below show the Flexible Rubber Coupling with Hose Clamps attached which is actually done in a later step. I included it on and with the pipe attached to show how I've trenched under the walls to connect the exterior blower.
Step 4: Laying Pipe
There are two ways you can determine the pipe measurements
1) Place all your connectors where you would like them to be and measure the distance between each of the connectors remembering to account for the overlap where the pipe and the connectors meet.
or 2) Lay the length of pipe down then place the connectors where you want them and mark the cuts on the pipe.
Which ever is most conducive to your bin and setup.
In my case because my bin is narrow and long, I laid the 12 foot pipe down and set the fittings down where I wanted them and marked those cuts on the pipe.
Cut your PVC pipe in to measured lengths, while keeping in mind the following best practices :
1) When filled all of the compost should be within 18 inches of a pipe.
2) Air and heat will naturally rise so incorporate vertical chimney pipes help to aerate the upper layers of the pile and help to distribute heat more evenly through the pile. The chimney pipes will also help to direct moisture down deep to the center of the pile instead of lingering in the upper, outer layers.
3) When designing the system, try to keep the lengths of pipe the minimum distance you can. If your blower is 35 feet from the compost pile the force of the air will not have adequate pressure to push the air though the compost properly.
4) Design your system so that the pipes are cyclical or capped to help keep the air pressure up and the air directed properly.
Dry fit your piping to verify everything is where you want it and so you can correct any mistakes before pipes and fittings are permanently bonded together.
Step 5: Drilling
When to drill air holes in to your PVC pipe is a personal preference.
Some pieces I drilled holes in to the pipe before I cemented them together because the pipes location would be too awkward to hold on to an maneuver after they' are permanently cemented togeather and some pieces I wanted the ventilation holes in a very specific order and location so I drilled them after the set up was completely cemented. It is up to you and how you’re comfortable building.
Drill holes in your pipe using either a 1/4 inch drill bit, or just the sharp tip of a 1 inch drill bit.
You will want to avoid drilling holes straight up but rather off to each side. Also for the pipes touching the ground, avoid frequently drilling holes on the bottom, but do drill one or two per pipe for water and condensation to drain out of.
I suggest drilling the holes in your pipes roughly 3 to 4 inches apart but the exact placement is up to you and the placement in your system.
- For one of the chimneys my holes are 4 inch apart on 4 sides of the pipe and each set of parallel holes are offset by two inches from the other.
- For another chimney I drilled a double spiral on opposite side of the pipe every 3 inches.
- For the ground air tubes, from the small pipe to the first chimney is about a foot long and I drilled one row of holes, two inches apart.
- The pipes between the other chimneys have two parallel rows that are off set from the top by 3/4 of an inch for a total of an inch and a half between the two rows, and the space between the holes in each of the rows are 3 inches apart.
Step 6: Cementing & Dry Fitting
Dry fit your piping to verify everything is where you want it and so you can correct any mistakes before pipes and fittings are permanently bonded together.
Before you start using the purple PVC primer and cement
Do not permanently connect every pipe and every fitting. Eventually you will want to move something or replace something or change out something and if you seal every connection the only way out is to saw. There are locations where a night tight dry fit will work perfectly fine and will still give you the flexibility you might not currently foresee.
For example: the pipe after the first elbow fitting is dry fitted so if I need to remove it in order to shovel finished compost out it’s an easy tug and it slips out; same with the chimneys. The first (furthest back) chimney is bonded to the long pipe as its way, far back there and will always have compost around it, but the second and third chimneys are dry set so that I can easily walk back a manure bucket all the way to the very back when it’s empty and step over them. However when the bin starts filling up closer to the front I can switch out the short chimneys with no holes, that’s enabling for higher air pressure and switch it out for the tall chimney with lots of holes that will aerate the compost when it is fuller.
For the pieces you have decided to permanently connect:
1) Purple primer goes around the outside of the pipe and the inside of the fitting.
2) Push in while giving it a quarter twist. It sets super-fast so you have about 30 seconds to get the pipe and any drilled holes where you want them.
Step 7: Flexible Rubber Coupling With Hose Clamps
The flexible rubber coupling with hose clamps is the trick that makes this all work.
I’m using a 3” to 2” coupling as 3” is the rough diameter of the blower valve and 2” is the size of my PVC pipe. The flexible rubber is lovely since where the leaf blower nozzle on my leaf blower wasn’t a circle but instead a long oval, almost a rounded rectangle.
The Flexible Rubber Coupling fits around where the blower nozzle was previously was and the hose clamps allow for the oddly shaped nozzle to have a custom, leak free fit. The 2” end fits perfectly, as it should, to the standard PVC pipe. You would never know that this wasn’t the original intent for this part.
The leaf blower connects to the 2” PVC pipe via the Flexible Rubber Coupling with Hose Clamps which you tighten by tightening the screws. The connect PVC pipe fits under or through the compost bin wall, and now connect the remaining pipe system.
Finish by capping off all open pipes.
Step 8: Testing
Time to test the system. Plug the blower in and turn it on and then go in to your bin and verify there is air coming out of all of the holes you want and nowhere else.
There will most likely be some but very little air leaks from dry fit connections but make sure they are as tight as can be. The air should be actively blowing out of your system.
If the air isn’t blowing with some strong pressure you either have a disconnected pipe, too many holes in your pipe, an uncapped chimney or you have too many feet of pipe for the power of blower.
Step 9: Finishing Up
Cover the pipes on the ground with landscaping fabric or screen to keep the bulk of the dirt and bugs out.
Cover the pipes with some compost and retest the system.
Again verify air is flowing as expected, if it is fill the bin up with any remaining compost and shove the compost thermometer deep in the heart of the pile.
Step 10: Setting Up the Automatic Timer
Go back to where the blower is plugged in and attach an automatic timer. Keep your neighbors happy by NOT scheduling your blower to go off in the middle of the night, after it is still a leaf blower.
I recommend starting out with twice a day for 5 to 15 minutes, it depends on the timer you bought and the settings you have available. Originally I had a fully adjustable timer and had it set, to run 10 minutes 4 times a day but I found it dried out quickly and it never maintained a core temp of 140 degrees.
Through trial and error of compost troubleshooting I’ve gotten adjusted to work for twice a day for 15 minutes and it now it keeps a good moisture level, no smells or odors and has a consistent all over temperature of 140 degrees which means good microbe activity.
There are compost troubleshooting steps on the resources page.
So long as the blower runs according to schedule, and you keeps the pile moisture level correctly and the pile has regular temperature of 140 degrees. in a month your pile will be compost!
Good luck and enjoy the savings!
Step 11: Resources
The Science of Composting, Natural Environmental Systems, LLC
Troubleshooting Compost Problems:
Composting Horse Manure
List of Browns & Greens (Notice horse manure is a green and straw and bedding are browns)
Magic Soil: Forced Air Composting R&D (Studies and data why it’s beneficial)
Cornell: Composting Physics:
Aerated static pile composting