I'm way north at 45�34' and our winters get very cold (-30C is common) but a closed loop geothermal unit to a heat exchanger in my small pond provides all the heat and hot water for our house -- and A/C in the summer where we often get +35C.
The GHP is a Water Furnace E060 and I have a "Slim Jim" stainless steel heat exchanger in the pond. I considered using an open loop but it adds complications for filtering. A heat exchanger made of a loop of coiled plastic tubing was initially proposed, but concern about muskrat damage made us opt for the steel exchanger. It cost a bit more but it was smaller and much easier to install.
I was a bit nervous about this first winter because the top of the exchanger is only ~12" from the bottom of ice (instead of their recommended 48"). However, it worked like a charm, and our energy costs were $1,700 lower than the previous year.
I posted a more detailed description with pictures on
this Pond Boss Aeration forum and would be happy to share my experiences. (The post is in the aeration forum because I initially started there, and learned that aeration and GHPs are not good combinations in the winter!)
