I like the idea of going deeper than 6', but how much deeper? A lot will depend on the excavating equipment I can gain access to, or spend to hire. I have granite rock down below, not too far (another point to gather data about) but certainly close enough to know that vertical loops would be very expensive. The frost depth varies from year to year. Without have precise data, I think it depends as much on snow cover as it does on air temperatures. Some type of 'dance' between the two. We generally use 6' as a limiting depth, but the frost does not go that deep every year. With the low soil temperatures we have even in summer, topping out at about 50F, the thought of using a passive cooling/active heating GSHP seems very plausible. That is, just circulating ground loop water through a coil in the airflow; no compressor running,no reversing valve required. They seem to make passive cooling systems in Europe and Quebec, but I cannot seem to find a source available to me. Anybody know of one? Or should this go in a different thread?