a friend of mine is experimenting with a muck heap!
he has piled lots of tubing through the heap and turns the water on, the water that comes out the other end is HOT!!
This was just an experiment, he is now working out how to replace the manure, or a top up method that will be ensure continuity, without disturbing the microbes that rot the manure too much, and loosing heat. As once the manure is fully rotted it no longer heats up.
he believes is the pipe is run underground then it wont cool down too much from the yard to the house
He says he will have this sorted by the time he is ready to build his house and will be running all his hot water to the house through his muck heap!
For us this would be a fantastic way to generate hot water as we have tonnes of much every year!