Has anyone ever used room t-stats instead of buffer tank temp to control a hydronic geo system? I've never heard of doing it this way, so tell me why this control scheme wouldn't work. I'm imagining a low mass radiant system for a very tight, well insulated home. I understand most water to water systems use thermostatic control with a buffer tank, sometimes including an outdoor reset to lower tank temperature during mild weather. Has any ever let the tank temperature float? What I'm thinking is a two stage (house/room) thermostat. In first stage, the circulator pump turns on and circulates water at whatever the tank temperature happens to be. If call can't be satisfied, t-stat calls for stage two. At a stage two call, the geo system is engaged. Once stage two is satisfied, t-stat drops to stage one and geo shuts down. Water continues to circulate from tank and may or may not finish the call. When the geo is engaged, the flow would be: 1. From the tank. 2. Through the Geo HX 3. To the floor. 4. Return to tank. Tank temperature is thusly increased gradually by the return flow. (How gradually would depend on tank size and Delta T on the floor loop, you know, engineering..) Benefits I see are these: Geo is always being fed lowest possible temperature on load side. Tank temperature is determined by load (via run time) rather than a set point. Plus, a fairly simple (inexpensive) control system.