A 1,000 feet of 1/2" OD soft annealed carbon steel hydraulic tubing, 0.049 wall thickness was purchased for the flues.  Made of cold drawn, 1010 steel (UNS G10100), the seamless, soft annealed, low-carbon steel tubing is designed for heat exchanger, condenser, or hydraulic fluid line applications.  Pressure tested to 1000 PSIG, the tubing meets AMS 5050-G, JIC, and SAE J524 specifications.