That's a very intriguing idea--to use the heat from a compost pile to warm a house. Have you ever made a compost pile? It requires a fair amount of work and care to get it to heat up though microbial action. Normally a pile is turned several times during the composting process, but if you have a water container with tubing running through it that won't be possible.
My first thought would be to use a pump to pass the water through the pile, but if you are considering a passive system it may be possible to use the thermo-siphon effect. The Wiki shows a diagram of how it works:
http://en.wikipedia.org/wiki/Thermosiphon
The idea is that when water is heated it will expand slightly and rise up in a tube, then when it cools off it will sink back down again creating a convection loop. I don't know enough about the physics of it to tell you what size water container to use, what diameter or how long of tubing or how high relative to your pile the model house could be.
This project could turn into something really interesting and fun, but it is going to require some experimental construction work, placement of thermometers, and testing of various diameters and lengths of tubing and locating the house relative to the pile. A properly made compost pile can reach temperatures of 50-60 degrees C
http://compost.css.cornell.edu/physics.html
Unless you are an experienced compost pile builder or have a friend who is, you will have to experiment with building piles to find out the right ingredients, proportions and moisture to get it heating nicely.
I hope you make this work and we will be happy to help you along with working out the details.
Good luck!