1) This is a two-line furnace. How is the second (return) fuel line used? Does the furnace constantly recirc the fuel while running? Why?
2) This is near Portland, OR. Typical lows in the winter get down to mid 30's F. I have maybe a 50' run of pipe from the tank to the furnace, most of it outside/above ground. Given the price of copper, what's the minimum diameter tubing I can get away with? Can I use smaller diameter for the return?