I just had an idea about warming your house during the winter using decomposing vegetation. My first thought went to using grass cuttings and the idea is that you have a hollow wall with about a 1 foot space on the house's outer wall. The grass cuttings are dropped in there and the heat generated from their decomposition aids in keeping the house warm. I'm sure there are many flaws to this plan such as the reduction in size of the grass over the winter will cause it to only heat the lower portion of the wall and once decomposed will not provide further heat. How long does decomposing grass generate heat for? I don't know.
My second idea was to have a standard compost heap and have some heating pipes running through the compost. This would be separate to the house's central heating and these pipes might run under the floor and add a few degrees to the house and supplement the standard heating system.
Why I am thinking about that right now I have no idea. It's already 80 degrees outside and I'm going to go for a swim in a short while.