When we first heard about the idea of a compost heated greenhouse, it was the research Woods Hole had done into this idea. Their concept was too complicated, and too large of a scale for us. So at some point, we got to talking with Liz and Matt of Muddy Fingers Farm about how they use compost. And they don't use it to heat their entire greenhouse, per se, just the propagation beds they start their seeds on in early spring.
The first year, we piled a bunch of a chicken manure and straw in our 10x30 hoophouse, put some pallets on that, and put our flats on top, hoping the heat would protect them from the cold, as we had already germinated them in the house. That failed rather spectacularly: one day we came home to find all the seedlings dead, killed by an excess of ammonia given off by the overly high in nitrogen chicken manure.
So we kind of gave up on it that year. The next year, we got more sophisticated, but we still had some ammonia issues, so we decided not to germinate seeds on the bed, but rather put seedling trays on them after the compost had cooled down quite a bit.
This year, driven by necessity and the allure of one of the most affordable ways to start early seedlings in cold climates, we resolved to get it right. And finally, I figured out what seems to be a pretty foolproof way to give the plants the heat they need to thrive early on.
Here's what we do:
(This all takes place in our 12x80 high tunnel, in one half of it. The other half still has winter greens in it for fresh eating.)
First, I bought 14 bales of straw , and arranged them on edge in a long rectangle, with six bales on each side, and one bale on each end. That gave us a chamber about 20 x 3 for the manure. It took about 3-4 pickup loads of manure to fill the straw bed to the brim.
The manure part is very important. I'm not sure how well other types work, but what seems to work best for ourselves and others is fresh horse manure mixed with sawdust or hay bedding. I think that because it has a relatively low nitrogen content and lots of carbon to soak up the ammonia, it is the most reliable. Basically, it's an easy material to get for free if you live near a horse stables. Just fork in loads of their fresh manure/bedding mix into the truck, and repeat. It should be steaming and hot when you turn it at that point.
After you've filled the compost bed, wait for a day or two until it has settled and released some of the ammonia it has within it. Then, cover it with an inch or two of completely composted soil, etc. It should basically be dirt. I think this works so well because it not only soaks up some of the excess ammonia, but it spreads the heat out nicely.
Finally, top the bed with pallets to give the flats ventilation, and start seeding! To keep the heat in, affix some small 10 ft 3/4 in PVC hoops over the bed and cover the germinating seedlings in a layer of row cover and 4 or 6 mil greenhouse plastic, to keep them warmer on cold nights.
So far this season we've had no problems with this system. I would love to hear others' feedback or experiences with it. We plan on expanding ours next year and start all of our seeds on it!
One note: you may need some supplemental heat on really cold nights, like ones that get down to 15-20 degrees. Use a thermometer and your best judgement.
One last thing that's great about this method: not only is it basically free or less than $100 if have you buy the plastic, row cover, PVC and straw, but in late spring when the seedlings are in the ground you can disassemble the setup and spread well-composted horse manure on your high tunnels beds, and then mulch with the straw bales you surrounded the compost with.