I think Guts is thinking of bring the structure up to 55 by just pumping water. The problem is, you want it over 55. As soon as you add heat to make it 70 the underground pipes will rob the heat & send it to the Earth. To try & use it for cooling, without a heat pump all you get is clamy condensation.
Heat pumps! News to most I'll bet.... a company in Bangor, Maine called Honneywell International (A tiny group of people) have invented a heat pump that works well below 30 degrees. It's tightly patented and they're thinking very shortly no one will buy "the old type". Why stop getting heat at 30* when for a few hundred bucks more you can make heat down below zero. Yes, below zero. Not only that, but no drilling wells, no underground pipes. This is an air to air system. Later this year they will have an air to water system for radiant heat.
Here's a link to their site. http://www.gotohallowell.com/index.php