FYI: You can actually deplete the ground temp over time, but drilling further down helps as does storing excess heat in summer to raise the temp below a bit (yay! free cooling!). There's a ton of research on this and these kinds of systems are very common in Europe. They go very well with floor/ceiling (i.e. radiant-based) heating systems as opposed to the more common air-based heating systems used in the US. It's also _way_ more energy efficient.