Quick question about directing heat from compost pile.

I have a room addition on the north side of my house that's always cold. It would be a perfect opportunity to place a large compost pile just outside of the house to bring heat to the room. My question is, copper coils filled with water is most effective? Does this water filled coil pipe in through the wall into a radiator?

If you can get the compost hot enough, I'm thinking circulating water or air would be decent options.  It probably would need to be several cubic yards if you live in a chilly place and more if you live somewhere cold

If water you'd need to be sure it couldn't freeze and you'd be transferring compost warmth to water to a radiator to room air.  

If air you'd need to be sure the compost gasses can't get into the duct that you run through the compost.  Then you'd be transferring compost heat to room air only.  Due to the simplicity of a little fan to move the air, I'd be tempted to do the air approach.  Especially if you have a window in that room that you could use to run the pipes in and out of.

Good luck!

Coils can work good, copper is expensive, I would move towards 3/4 in black poly piping. Also, its important to have a air blower into the pile if you are not going to be turning it.

How large is your compost pile? And what are the general environmental conditions in your area? This determines how much heat you can realistically generate and transport, with losses all the way.

I have followed installations (poly and glycol) in cattle bedding packs, which I recall from my early years. They generate heat, anaerobically, but the cattle love them as long as there is dry bedding added on top daily. At some point, they are dug up and spread on fields, using an industrial articulated front-end loader and two gravel-truck-size spreader trucks. We're talking about massive volumes, with tons of nitrogen.

Look into the Biomeiler technology.