I'd like to have my oil fired boiler in the system to either back up my heating or augment the geo it if necessary. I'm planning on a heat only water - water system (5 ton) with an 80 gallon buffer tank that will supply a small amount of radiant heat and two hydro air handlers. My plan is to either find a twin coil buffer tank or use two buffer tanks, one for the geo and one for the oil burner. What would the best method be to install the dual tank set up. My thought is to have return water from the heating system enter the boiler buffer tank first then the geo tank then out to the the supply manifold. an aquastat on the geo buffer would call for additional heat if need be and the boiler would fire to heat it's own buffer tank. basically a pre heater for the geo tank MY design parameters call for the geo system to supply 130 degree water to it's load buffer tank. Ultimately, I'd like to have an outdoor thermostat regulating the system SO...is this feasible? smart? I'd like aquastat to vary the call for boiler pre heat (it may NEVER be required) based on OAT. that said, on a 40 degree day 110 supply in the geo buffer might be OK with no preheat but on a minus 20 degree day I might need the preheat to get the 130 degrees and have the aquastat call for the boiler at say 120-125 degrees. Probably thoroughly confused every one by now! So the questions are: 1. dual tanks, twin coil tank, something totally different 2. what do you guys recommend for a temp control system that works with OAT? Thanks, Keith ps I know if the geo system is properly engineered (and it is) the boiler augment won't be needed but I definitely want it as a backup.