I think I'm pretty much set on a piping and controls plan. After I posted my original piping diagram, I kept finding problems with it. I think as attached, it finally looks sound. Another big concern has been how do I add a wood boiler and heat storage without adding a bunch of expensive to my existing Tekmar tn4 control. I want to use the wood boiler when its firing and use the storage when its temperature is sufficient and only use the oil boiler as a last choice. I think a couple relays as shown in the other attachment will do what I need. For anyone patient enough to follow the diagrams and the logic description that follows, I'd be interested in any input you might have or any problems you see. When Wood Boiler fires, Wood Pump is on, which closes coil on R1. A heat demand will power Primary Loop Pump. This will power R1-6, which directs wood boiler water to the primary loop through a 3-way valve. No heat demand means no power to R1-6, which will direct boiler water to storage tank through same 3-way. When no Wood Boiler fire, Wood Pump is off, which opens coil on R1. A heat demand will power Primary Loop Pump. This will power R1-5, which will initially power Storage Tank Pump through R2-5. Also at this time the Tekmar 422 will be sensing the temperature of the storage tank through R2-4. If 422 sees low temp it will close boiler contacts to fire oil boiler. This will close the coil on R2, which will power the oil boiler pump, R2-6 and switch the 422 temp sensor to the primary loop, R2-3. Thanks, Rob.