Woodchip Compost Heater-Outside Insulation Alternative

Hello Good Permies People,

I am building my first  small scale wood chip compost heater to spec,  based on the The Compost-Powered Water Heater book.

I was planning to use straw bales for the outside insulating wall but at 75 bales times $3-$4 each equals out of my budget.  This is being built at the high school I work at to heat our small greenhouse (8x12feet).  I have unlimited access to wood chips so I was wondering if anybody has used them instead of straw bales.  I was thinking I could create an inside and outside fence and fill it will wood chips. Would this work?

Also I have not ventured into plumbing planning for inside of the greenhouse yet.  Any good resources out there for this area? I have access to 55 gallon plastic and metal drums.  Can I use these with the proper pressure valves to store heated water?

Solar powered water pumps would be the next thing to ask about.  Any suggestions?

If I'm understanding, you would make a fence to hold in the "heat pile" and then build another fence around it that is also full of wood chips as insulation?  

It seems like it should work.  Wood has an approximate R1 but I'm not sure about wood chips.  They'd likely be wetter (less air within the wood chip) and heat/cold would be able to squeeze between the chips more than with straw.  But if it's an unlimited supply, I don't see why it wouldn't work.  It may even compost a bit and make the core hotter.  

I'm not understanding how that would be different than just making a bigger wood chip pile?

Oh Gosh, that will work just fine.

Wood chips are just a slightly bigger derivative of sawdust and that is what they used to cover ice so that it did not melt here in New England forever. it is not only insulation, it is excellent insulation. As the others have said, no need to build a "fence", just berm the outside with solid wood chips instead of wood chips/compost and you will be fine.

As for the plumbing, compost heat does not get past the boiling point, so you need a very meager amount of supplies. In fact you will not need pressure relief valves because it is doubtful you will be using a closed loop pressure system. Just keep a bung open on the drums and at worst, it would boil over; again super doubtful using compost heat.

I think you are on the right track, just remember to keep it simple.

It seems to me that you could have a very slow loop of water going through your compost, and have pipes go deep under your green house bed's soil or into and out of water barrels, which should never overheat (so no need for pressure relief valves), as they would be radiating much more surface area than the pipes radiating within them, but keeping a bit of the barrel open would solve any potential problems anyway.  You would not need to complicate it too much.  As Travis wrote, wood chips make great insulation, much better (less dense) than wood.  If volume is not an option, then I would go with something like 4 or 5 feet thick all around the greenhouse, as deep below grade as you can.  If you mixed these with manure and kept them damp, you would be getting additional heat.  If you kept them dry, you would have better insulation.    

Probably the simplest way to use compost to heat your greenhouse would be to make a bigger structure then you need for the growing beds.  Add a third to the dimensions.  One that you can open the back and build your compost inside the poleward side of your greenhouse.  That way you would have the heat benefits without needing to have the water at all.