Just remember that for every pound of propane that you burn, you get 1.6 pounds of water.  With that much cold iron laying around you might get condensation, causing rust.  If you are going to burn propane or natural gas, it is best to have a vented system.  

EDIT: I just looked up the stat to verify my memory.  Another way to estimate how much water are pumping into the air is 0.98 gallons per 100,000 BTUH.  

--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.