Don't worry too much about the size, or even getting your compost hot. You can pile it up higher than the 18" anyway, and a pile that wide and long will heat if everything else is right.

These will work, but expect the cardboard to rot fairly fast, followed in a season or two by the untreated wood (which appears to be softwood). If these are being tossed anyway, you're doing a good deed by at least reusing them for something. Me, I'd salvage the lumber for projects, but that's just me.

Poke some holes in the cardboard for air. After awhile there will be lots of holes. :-]

With the compost up off the ground, and a bit narrow; it could take extra watchfulness to prevent the pile from drying out. The bottom airspace would keep worms from moving in, but if you have sogginess or tree root issues, it could be great. I would love to know how it works out.

In an active compost pile, one that has an internal temperature over 80 degrees, earthworms will not be a part of the digestion process because that material will be too hot for them, so whether the compost pile touches the ground or not is of no concern.
I have not had much success getting an active compost pile with anything less then the minimum 27 cubic feet (a 3 x 3 x 3 volume) and often even that was difficult. There is no real need for a bin, except neatness.

Is the bottom covered with plywood, cardboard or open? If it's solid wood, you might want to drill a few holes for drainage. You do want the pile moist, but not too wet, so it needs drainage.

The volume is 21 cu ft. which is not far off a cubic yard, and if it's piled up I bet you could get more than a yard in there.

If you have room and can get more than one, I'd set up two and divide the second in to 1/3 and 2/3 sections. When the first bin is full and sinks down some, turn it into the 2/3 section of bin 2. Refill the big bin. After a few weeks or months or when your first bin is full again, turn the first pile into the smaller section, turn the big pile into bin 2, etc. The small bin always has compost ready to use, the middle one is cooking, and the new stuff always goes into the big bin.

The crates are slatted on the bottom just like the sides, and lined in cardboard.

I can cut the "legs" off them so they'd sit on the ground. Those are just so fork trucks can pick them up easily. That would probably be better anyway, so the stuff doesn't drop out and run off in the rain once the cardboard is mulchified.

I like the idea of the 2-crate/3 bin rotation system, tox.

I am sure it will work. Will it cook? Not sure. Depends on what you put in it and if you can keep the moisture level right. If it is convenient, just do it. If you want to be more conventional, go for something more squarish in the 3 to 4 foot range. I would take the cardboard out on the bottom but leave it on the sides. It will help keep in moisture and warmth for a while. I advise to take the cardboard out of the bottom to avoid creating airspace which will later collapse when the cardboard gets soggy and fails. Not a big deal.

The important thing about composting is to get started and see how it goes.

I also recommend reading the Humanure Handbook, because even though it is about safe composting of human manure, it is also one of the best books on composting I have seen. It is available free online.

--McKenzie

Why not double the width by joining two together and cut out the center partition?

Compost pile needs a natural aeration to keep going. That is the main purpose of turning it. I scatter a lot of brushes and twigs at the bottom, so that air can get in from all sides. Actually a crated sided bins are better than fully closed. If the openings are too wide, nail something like 1x2 between them, leaving some breathing opening.
JMO

Perhaps this link to a very good composting tutorial might be of some help.

Here is a link that might be useful: Composting Tutorial

"They're not the minimum 3'x3'x3' that's supposedly required for a hot pile"

Depends what you want. And your ingredients. If you want to have the option of making a hot batch if you desire, not much easier or cheaper than a round 4' mesh bin. I like them 30" high, does not have to be 3' high if resting on the ground.

"...so i don't really get how you can just be a batcher)."

I find it very easy to make a batch in a single day in my 1/4 acre lot, but my climate provides fall leaves and spring grass. In the fall, select the right week, shred the colorful leaves (mine are oak, maple and sweet gum), and fill the bin. Core temp of 130+F in three days.

Since I have two bins, I could also shred the brown leaves, and wait until spring starts the grass growing. Then mix in the fresh smelling grass and let the temp shoot up.

When I say shred I mean fairly finely; 4 passes with my mower equal 1 pass thru most chipper/shredders.

Whether I used your structure or my mesh bin would depend on a number of factors. How particular you or your neighbors are, plus any zoning codes.

I put tons of unshredded leaves into wire fence bins and compact them and leave them over the winter. A few get used to layer with kitchen waste during the winter. In spring whatever is left is now slightly composted, and gets mixed with fresh grass clippings and loaded into a bin to cook.

I usually leave the cut grass on the lawn to decompose/mulch/whatever it does. If I instead put it in the compost bin, will my lawn suffer for it? Not that we've got a lawn that has any bragging rights whatsoever, so it's more a theoretical question.

I don't even use a bin to compost I just put vegie scraps,grass clippings,manure,basically anything that breaks down in one big pile under old tree and just turn it every few weeks,works well and its the best plant food that I have ever used in the garden,nothing will beat it,you also get the added bonus of big garden earthworms,they do wonders in your garden as well.